I get asked this question quite often, and usually give a fairly brief summary of the process. Recently I wrote a longer response as a result of bing asked how I got from initial picture to this image of the Tadpoles Nebula IC410:
I used a Skywatcher 130P-DS Newtonian scope which is optimised for imaging, with a matching coma corrector to give sharp round stars across the image. Attached to the scope is an electronic focuser, a filter wheel with eight filters and a cooled mono camera (to give lower noise on very long exposures). There is also a smaller guidescope and camera attached to and roughly aligned with the main scope. This is all mounted on a tracking mount, controlled by a computer that also controls the cameras, filter wheel and focuser. As well as taking photos the computer deals with pointing the mount in the right place. It uses the guidescope and camera to take short (1s) exposures in which it tracks a star and uses this to correct the pointing of the scope in real time, typically with a precision around 1 arc-second.
The larger, cooled, camera is used to take multiple long exposures (typically several minutes, but many factors affect the optimum length, in this case it was mostly two minutes but could usefully have been two or four times as long).
Last year I collected a good set of data using a Hydrogen-alpha (Ha) filter which is a deep red light from ionised hydrogen and usually the strongest signal from nebulas like this. My filter has a 7nm bandwidth, so it suppresses pollution and reduces broadband light sources like stars.
A few nights ago I repeated this with oxygen (Oiii) (a blue-green light) and sulphur (Sii) (an even deeper red) filters.
The images from each layer (typically 20-30) are stacked automatically along with special frames to reduce 'dark noise' and distortions like vignetting (flat frames). As is typical all three layers needed to be 'stretched' (using curves, histogram etc.) to bring out faint details, denoised and given some contrast enhancement or sharpening. Some processes remove the stars then add them back in afterwards to highlight the nebula, but I tend not to use these often.
The Sii signal was very week, and needed a particularly strong stretch, I was surprised that it eventually revealed some quite delicate filamentary structure that sadly don't show strongly in the finished image.
I combined these using the 'Hubble' palette which maps the three layers in wavelength order Sii to red, Ha to yellow and Oii to blue. This starts with Ha mapped to green, but after balancing the image selective colour is used to push the green to yellow. As 0iii is weaker than Ha it often looks very turquoise and is usually shifted to be more blue. The Sii signal gives the Ha signal a browner tinge and can be best seen as a patch to the right of the nebula where the is little Ha.
The actual colour shifts are rather arbitrary as there is no 'scientific' right or wrong for the colour balance. Unstretched images can be used for photometric measurements, but are pretty uninspiring to look at, so just as when you process an earthly image you might choose to alter the saturation for a more pleasing result. In this case the objective is to make the colours reflect the relative distributions of different gasses in the nebula.
I finally improved the smoothness of the image by using the Ha layer as luminance, which remove the noise from the oxygen and sulphur data, but I'm hoping to get more data soon.
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The Veil Nebula is a huge supernova remnant in the constellation of Cygnus. It comprises several distinct deep sky objects such as NGC6960, NGC6992, NGC6993 with many twisting filaments glowing with hydrogen and oxygen light. Far bigger than the full Moon, this image is a big mosaic and may take some time to load. It comprises two panes taken using my home-made ED66 telescope which has a focal length of 400mm and a ZWO ASI1600MM PRO camera. I used an OVO field flattener which does not change the focal length. It is a narrowband image using ZWO 7nm filters mapped Ha - red, Sii - green, Oiii - blue.
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Roller and angular contact bearings suitable for mini lathes are available from Arc Euro Trade. These are made by Nachi, who apparently supply bearings for the legendary Japanese bullet trains. Unlike the standard ball bearings, which are shielded units, the roller bearings are open and in two parts – an outer race and the inner race with caged rollers attached. Try to overcome the temptation of taking the bearings out of their packaging before fitting them. If you do, keep them clean and do not run them ‘dry’.
On the face of it replacing the existing ball bearings with taper roller bearings is straightforward - strip down the mandrel, remove the old bearings, lubricate and fit the new ones and re-assemble. In practice there are a few complications; the bearings are tight force fit in their housings and although the roller bearings are diametrically the same, they are longer which creates the need to make a number of adjustments.
The end of the mandrel with locknuts, bull wheel and tumbler reverse plate removed.
First of all unplug and isolate the lathe, and make sure you have plenty of space in which to work. The next operation is to strip the rear of the headstock. Remove the gear cover, any change gears that are in place, the drive belt and pulley, and finally the mounting plate for the tumbler reverse. It makes good sense to have a few small boxes in which to store the various components as they are removed, especially small items such as the drive pulley key. I suggest that you fully remove the control box, after noting how the various connections are arranged. In these days of camera phones I make a photographic record whenever I dismantle a novel device, it’s an excellent way of avoiding that ‘James Bond’ moment when you have to choose which pair of wires to reconnect!
Even in the simplest of cases a simple sketch or a digital photograph is always a help with ensuring that wiring is reconnected correctly and safely.
The locknuts at the end of the mandrel can now be removed using a c-spanner. You might have a suitable one in the form of the old pressed steel multi-spanners from the days when bicycles had built up bottom brackets. Otherwise it is possible to improvise one from steel strip and a suitable bolt – it should not be subject to huge loads. To free the locknuts you will need to immobilise the mandrel, and this can be done by bolting a bar to the front flange of the mandrel, or more simply by clamping a bar in a chuck. Please don’t write in about chuck abuse – the torque I needed to free the nuts was quite modest.
Removing locknuts.jpg – This method of removing or tightening the locknuts is fine as long as only modest force is required. If the nuts are jammed solid bolt a bar to a faceplate or the mandrel flange instead.
When the locknuts came free I was dismayed to find some apparent dampness and localised surface corrosion, but only on the thread and hidden faces of the locknuts. How this came about without affecting the rest of the lathe is beyond me, perhaps it happened when the lathe was stored in a cold garage for seven months? My remedy was to clean the thread and nuts and then treat them with Jenolite.
It is now possible to remove the 45-tooth bull wheel, wangle out its key and remove the long plastic spacer, followed by the plastic bearing shield which is held in place by three cap head screws. The two ends of the mandrel are covered by these plastic bearing shields. I found that I had to recess the inside of the bore of each shield by 2mm to clear the inner races. Unfortunately this only came to light once I had swapped the front bearing. I therefore had to recess the front shield by hand using a burr in a mini drill. An alternative is to recess the rear shield before removing the mandrel, swap it for the front one at the appropriate time, and recess the other shield when the mandrel is re-assembled. Unfortunately, to do this you will have to temporarily replace much of the drive gear. Whichever way you decide to proceed, the front shield will also need to be released before the mandrel is extracted. This is easily done by poking an Allen key through a hole in the mandrel flange.
The bearing shields can be recessed neatly in this way, however, a fair amount of assembly and reassembly is required if both are to be dealt with in this way.
To remove the mandrel you will need a suitable a puller. I used a length of 12mm studding, threaded through the mandrel bore, a section of angle iron and two 10mm bolts. The mandrel should come free under steady pressure, bringing the front race with it. In the unlikely event that the front race remains in the headstock, rather than staying in place on the mandrel, you will need to fabricate a longer puller as described on the Arc Euro Trade website.
This fairly robust arrangement was needed to pull the mandrel. There is a nut and thick washer at the far end of the mandrel bore.
Once the mandrel comes free it will leave the gears and a spacer inside the headstock. Another spacer will probably come out with the mandrel, remove this and the long key (that engages the internal gears) and put them to one side.
The ideal way to remove the front bearing is with a proper bearing splitter and an hydraulic press. (Photo courtesy Arc Euro Trade)
Lacking a press I had to resort to driving to remove the old bearing from the mandrel and replace it with the new inner race. I used two pieces of 3/8” square bar to make a support the old bearing as close to the mandrel as possible. After applying some light oil I drove the mandrel most of the way out, using an aluminium alloy drift to protect its end. Once the bearing was far enough up, I was able to use a proper Picador puller to finish the job. If I was to do this again I would probably either make extensions for the proper puller, or make a custom puller from scratch. With the bearing removed it is now possible to swap over the previously recessed bearing shield.
Unfortunately, the mandrel was too long for this Picador puller to be used the whole way.
I followed the advice on the Arc website and drove the new inner race into place using a tubular drift – a section of 30mm light alloy scaffold pole, probably the ideal material for the job. Whatever you use, make sure that it fits the inner race without overlapping the caged rollers. It is also important to protect the face of the mandrel by resting it on a block of softwood.
With the mandrel removed, I decided to remove the headstock from the lathe, partly out of curiosity, partly out of convenience. To do this requires the removal of the leadscrew, the motor and their respective guards. The headstock itself is retained by three 8mm cap screws. Once the headstock casting was removed the lathe bed beneath was covered in a layer of ‘plastic fluff’, the result of several year’s wear on the internal gears. Fortunately the gears are of generous dimensions and they did not appear badly worn. Even so it was obvious that it would be worth cleaning out the inside of the headstock and applying a generous amount of grease to all the moving parts before replacing it.
The inside of the headstock was devoid of any lubricant, the ‘dust’ on the lathe bed is the result of wear of the gears.
With the headstock removed it was a straightforward job to pull the rear bearing, using the home made puller and a short piece of 3/16” bar across the inside of the bearing. To fit the outer races to the headstock another suitable length of angle iron was required.
The rear bearing can be pulled in situ, but it is easier to carry out with the headstock removed from the lathe.
The front race went in with no trouble, but the rear one started to go in skewed an I had to adjust the puller. When I came to test the lathe it was apparent that one or both of these races was not completely pulled home, so watch for the puller contacting the headstock rather than the race.
A blind puller with a sliding hammer can be used to remove the rear bearing. (Photo courtesy Arc Euro Trade)
Refitting the mandrel to the headstock is best done after refitting it to the bed of the lathe. Grease the internal gears and clean the bed before doing so. Fit the long key and one of the short spacers, then load the front bearing with suitable grease. I used a heavy duty moly-grease. Ideally you shoudl fill no more than 2/3 of the space in the bearing to avoid it slipping rather than rolling. Fortunately, there is plenty of space for excess grease to escape into the void of the headstock, so don’t worry to much about slightly overpacking the bearing.
The front bearing and its race liberally dosed with molybdenum sulphide grease.
The mandrel should slip easily into the headstock, threading into the internal gears and the second small spacer. Now load the rear bearing with grease and slip it onto the lightly oiled mandrel. You can now use the long spacer and one of the locknuts to draw it into place until there is no play, locking the mandrel with a bar in a chuck as before. Check the headstock gears engage properly and refit the tumbler reverse plate, motor, drive belt, leadscrew, control box and any other miscellaneous parts removed earlier. Check there are no obstructions and everything turns freely, then start up the lathe in low gear, gradually accelerating to full speed over a few minutes, then stop and check the spindle has not come loose. Repeat this procedure in high gear.
The bull wheel spacer before being shortened.
Now remove the locknut and thread on the bull wheel, it will be apparent that the bull wheel no longer lines up with the tumbler reverse. I had to shorten the spacer by 4mm, which I did in the lathe, relying on friction to hold the rear race in place against the light load. Once shortened I had to use a rat-tailed file to enlarge the notch in the spacer for the end of the bull-wheel key, before replacing the rear bearing shield, fitting the key and bull wheel. I then refitted both lock nuts and re-tightened the mandrel. Appropriate preload for a roller bearing this size and precision application is very small, of the order of zero to four thous. The pitch of the mandrel thread is 1.5mm or 60 thou so, once all shake has been removed with the first locknut, tighten it by no more than a further 24 degrees and lock it securely with the second nut.
The long bull wheel spacer was the only one I found it necessary to modify, shortening it by about 4mm.
The first task I undertook was deliberately a light one, turning two 1 1/2” light alloy rings for a tailstock micrometer index. For the record the pitch of the tailstock leadscrew is 1.5mm, so 60 divisions for the index is 0.025mm, a close approximation to 1 thou per division. This did not go without incident, as one of the inner races shifted a little and I had to readjust the bearings. If the outer races snug in a bit further when you first use the lathe, it will become apparent as noise when first putting on a cut. Should this happen stop the lathe and retighten the bearing and no harm will be done.
Could I replicate George Thomas’ achivement of parting off 1 1/8” FCMS at 600rpm?
I have successfully parted off 1 1/2” mild steel to make the parts in photograph X and 1 1/8” alloy steel, to make the 0.025” thick disk. It took a little practice to develop the right technique. The tool was 3/32” wide with the cutting section 1/2” long, set dead on centre height. I got the best results starting at 220-250 rpm and then speeding up as the work progressed. It was essential to force the tool in positively at the start of the cut, then to maintain a steady feed rate, but being careful to watch for the work slowing down too much. Lubrication, using neat cutting oil on a small paintbrush helped keep the speed of the lathe up. Chatter was only evident if the cut was not aggressive enough. Feeding the tool in too fast did not cause dig-in, instead the ‘motor slowed and the fused blowed’. This suggests that the limiting factor is now the motor power, not the rigidity of the lathe.
Examples of items parted off with the modified lathe. The disc is 0.025” thick, and could easily have been made thinner. The swarf is 3/32” wide by about 0.004” thick, a flatter topped tool would have made less tightly wound swarf.
A final and more extreme test was to cut a 3/32” groove in 2” mild steel. Running the lathe at about 310 rpm I set up my digital camera to video the event. Remarkably the tool went in like a knife into butter – no chatter and short coils of swarf flying out like chips from brass!
Parting off.jpg – After cutting a 3/32” wide groove in 2” diameter mild steel. The lathe was run at over 300 rpm, and no chatter was experienced.
I had not expected such a dramatic improvement in parting off, particularly from the point of view of being able to speed up and increase the depth of cut so markedly. I certainly did not expect any other changes, but there were two. The first came to light when running in the new bearings – the reduced friction meant the lathe ran far more smoothly at low speed – it would now turn at just 4 rpm without stutter. This could be very useful if I decide to mill or grind helical threads in the future.
The second and welcome change was the improvement in surface finish on tougher materials. I was not expecting this, yet there was a noticeable improvement in the surface finish when turning both mild and medium carbon steels. So far I have seen none of the ripples or waviness that troubled LBSC over eighty years ago. I wonder if his problem was poor bearing quality or just too much preload?
I did have two reservations about the change. One is the apparent 4mm of increased mandrel overhang, however because the new bearings have a linear contact patch instead of a point, the new bearings actually support the mandrel with /less/ overhang. The second is that the new bearings are unshielded. Provided the plastic shield are undamaged and the bearings well packed with grease this ought not to be an issue. Assuming normal levels of use it would be a good idea to draw the mandrel and inspect and repack the bearings every couple of years to ensure long life.
I had considered changing the bearings a last resort to rescue a worn or otherwise unsatisfactory machine, would I be able to detect any difference? In practice, for a day’s effort and at very modest cost, the results really surprised me. Even with a lathe that appears to be performing well with the standard ball bearings, consider changing to roller bearings and taking full advantage of the basic precision and rigidity of the mini lathe design.
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Over recent years various 3 1/2” centre height “mini-lathes” have become increasingly popular as an entry-point into model engineering. Personally, I don’t like the term ‘mini lathe’ – it suggests that they are toy-like or unsuited to ‘real model engineering’. Fifty or sixty years ago, if you could not afford a large, full-featured lathe one choice you had was a true lathe such as the Adept, Flexspeed or Centrix Micro – each of which could truly be described as ‘mini’ and lacked any screwcutting capability. Even so these lathes were capable of producing superb results in the right hands, and in the hands of some owners were modified to become full featured and true precision instruments.
My mini lathe, a Clarke CL300M, taken before the bearing change. At least eight other modifications can be seen in this view!
The modern ‘mini-lathe’ is technically far advanced from these historic examples, though it fills the same niche. It has much greater capacity, a built in variable-speed motor, screwcutting and fine feed, is far more rigid and, with the benefit of modern mass production, is probably more accurate. The one issue that does get raised by users of these lathes, however, is difficulty in parting off. Parting is one of the most demanding tasks for any lathe, as it involves taking a broad cut with an overhung tool, often at the bottom of a deep groove.
You may ask what all this has to do with roller bearings and angular contact? Well there are several requirements for successful parting, and the quality and adjustment of the lathe mandrel bearings are one. Mini lathes are supplied with the mandrel fitted with ball bearings as standard, and for some users these have proven to be a source of trouble when parting off or even, in extreme cases, when trying to achieve good surface finishes. In lathes where these bearings are not up to scratch, changing to roller bearings is an economic and realistic way to address the problem.
See also: Changing the Bearings on a Mini Lathe
But there are many other pitfalls for the careless parter-offer, and unless these are addressed changing your mandrel bearings is pointless, and if they are, you may not find the need to change them anyway. So in this article (sorry!) I will explore the mysteries of parting off and explore the various solutions to allowing our mandrels to rotate.
Perhaps every model engineer wishes to attain LBSC’s nirvana of ‘parting steel with a sound like frying bacon’, but it was the greatly respected George Thomas who made parting off a fine art. He combined a well-adjusted lathe, a rear toolpost, the right tool and the right speed. With a special tool shape he reached a point where he could part off 1 1/2” mild steel at 615 rpm! By his own admission this was a piece of showmanship reserved for exhibitions, and more usually he found a suitable speed (for mild steel) of 200 rpm per inch of diameter.
A well adjusted lathe means ensuring the saddle, cross-slide and top slide all move smoothly, but without any trace of shake or movement. Rock the toolpost and watch anywhere two slides rub together – if you see a bead of oil moving in and out, then they are probably too loose. Make sure there is no play in the mandrel in its bearings. If there is, cautiously tighten the locknuts at the rear of the mandrel.
A rear toolpost, with upside down tool, is widely recognised as improving parting off, by reducing the chance of ‘digging in’. Apparently the rear-mounted tool will tend to spring out of, not into, the work and for materials which produce short chips, rather than curls of swarf, these will tend to fall clear of the cut. I have never seen a completely convincing explanation of the geometry involved, but several respected model engineers have testified to the effectiveness of this approach, and it has been a popular technique since the 1940’s. Mini-lathes do not mount chucks on a screwed mandrel nose and have reversing at the flick of a switch. This means to explore the benefits of this geometry one simply has to invert and pack up a parting tool in the normal toolpost, and reverse the lathe rotation.
George Thomas’ ideal tool had a v-groove on top and the end ground to a v-point, to encourage the formation of curled, narrow swarf (Duplex advised a rounded convex-ended tool for the same reason). I’m sure that both would have admitted that this is a counsel of perfection and that what really matters is a sharp tool with appropriate top rake and adequate front and side clearance. Two essential pieces of George Thomas’ advice, however, are to match the size of the tool to the job and to keep the end of the tool square to the job. Small tools are not rigid enough to tackle large diameters and the angled tool end that avoids making an end pip produces sideways forces that can bend the tool off line and jam the job.
This drawing shows ideal angles for a normal parting tool as recommended by Duplex.
Getting the right speed for any parting job depends on so many variables (the material, its diameter, the tool and the state of the lathe, and some would say the phase of the moon) that there has to be an element of trial and error. The same applies to getting the correct rate to feed in the tool. Fortunately the variable speed control of mini lathes comes into its own for parting – you can gently vary the speed to get the best results, and also speed up as the diameter drops.
Finally, don’t be afraid of the job. A jam up and a broken tool-tip can make you understandably cautious. George Thomas was adamant that a confident and positive approach was essential, and that over-cautious pecking at the work will never bring good results. Don’t just watch either – let your ears and your fingertips tell you what is happening.
A 1940’s article by LBSC advocated a useful tool, not for parting off, but for the similar task of making deep, wide grooves in large diameters. The tool has the shape of a fish’s tail in plan view, the opposite of George Thomas’ tool. I ground a shallow vee in the end of a normal parting tool using a mini drill mounted stone, in order to turn a wide groove in 3” diameter cast iron. I found this shape of cutter was less liable to chatter than a normal parting tool. In use it is fed in five to ten thou at one side of the groove, moved across to the other side of the groove and fed in again before the return journey. The ‘double pronged’ design cuts the full width of the slot to the full depth with ease.
Producing a series of gear blanks in a medium-tensile steel was essentially a multiple parting off operation. Note chatter marks at the bottom of some diameters.
At first I had all the parting off problems recounted by many mini-lathe owners, and had my fair share of ‘dig ins’ and chatter. In time I was able to overcome these by following the above advice. I also found the huge difference made by a brush full of neat cutting oil in the groove. Most of all, I found the benefit of having confidence to feed in and get a positive cut. After a fair share of jams and scrappers, I eventually reached the point where I could reliably part off 1 1/8” alloy steel. With standard bearings I even used a parting tool to cut a deep groove in some (freely machining) 2 1/4” diameter cast iron.
Using a broad form tool, such as when making a gear cutter in silver steel, is possibly one of the few tasks even more demanding than parting off.
Let us assume that you are in the position of some mini-lathe owners, having tried every tip and wrinkle and still not getting successful parting off. The final option is to replace the mandrel bearings in the headstock. There are a number of requirements for lathe bearings. They need concentric accuracy, they must be able to take both high axial loads and thrust loads, they must have low friction and ideally they must have a long life.
Roller bearings are the ideal choice when turning the barrels of a battleship’s 16” guns, simply because of the loads involved, but for small lathes the choice is less clear as almost any type of bearing could be used. Once, many had mandrels running in a plain cast-iron headstock, which is a perfectly good solution with adequate lubrication. Before the Second World War plain and taper bushes and sometimes ball bearings were the rule. Roller bearings were scarce during the first half of the twentieth century, but their use took off during the Second World War. Even so many top quality post-war lathes, notably Myford’s ML7, used plain bushed bearings and a ball thrust bearing, whilst the Super 7 used a hand scraped tapered bronze bush at the head of the mandrel. There is no doubt that these bearings could handle any task within their lathe’s capacity. On the other hand slightly larger lathes, such as those by Boxford and Colchester, typically had roller bearings.
The mandrel of a Milnes Type R lathe, fitted with two taper roller bearings at the front of the mandrel and two further ordinary roller bearings.
At a slightly earlier time LBSC was in no doubt – anything but roller bearings! In one of his “Lobby Chats” he told the tale of new “Type R” lathe he had received in 1923 from Henry Milnes, of Bradford, to replace his 3 1/2” Drummond. If I dare paraphrase a tale he told in his inimitable fashion, it went something like this: He didn’t care for the specified roller bearings, but obtained a written guarantee that, if unsatisfactory, they would be replaced with plain ones. Within three weeks he had detected ‘fine lines” and got onto the maker, who replaced the front bearing with a bronze cone. Another three weeks and the problem arose again, and this time it was agreed to be the fault of the tail-end roller bearing. The patient Mr Milne agreed to supply a specially made roller bearing, but there would be a few weeks delay. Now LBSC depended on his lathe for his living, so the manufacturer sent him a temporary bronze bush, which he fitted, immediately curing the trouble. In due course the new ball bearing arrived, but as the bush was doing fine, he thought he’d keep it for the time being. Some time later, while turning a wheel for ‘Bantam Cock’ on the self-same lathe he started getting chatter marks; inspection revealed a little play in the bronze bush, it had worn just slightly oval. The replacement ball bearings were found to be a perfect fit. The bush was put into ‘honourable retirement’, with due reverence. This story was told in 1946 – the temporary fix had done its duty for over twenty-three years.
A Milnes Type-R 3 3/4” lathe, fitted with lubricators for the mandrel bearings, as used by LBSC for around a quarter of a century (see www.lathes.co.uk for more details of this and other veteran lathes).
So Curly wouldn’t touch a roller bearing with a barge pole, but a year later in 1947 they were stoutly defended in the Editor’s correspondence. E. H. Doughty, British Timken Ltd.’s Chief Technical Engineer, challenged some of the magzine’s responses to queries concerning the rigidity and finish achievable with roller bearings earlier in 1947. The detailed letter included arrangements for four-, three- and two-bearing spindles. The two-bearing arrangement is exactly the type that applies to mini-lathes, and is stated to be ideal for smaller lathes where temperature variations are small. He claimed high precision, robustness, long life, low friction and the potential for high speeds as the benefits of their bearings. Mr Doughty did not agree that plain bearings were preferred by most users, claiming that the use of roller bearings was increasing rapidly both before and after the Second World War. He added that during the war tens of thousands of units gave “every satisfaction” in machine tools of all kinds.
This design for a two-bearing work spindle by British Timken is comparable with that applying to mini lathes.
Another advantage of ball or roller bearings is ease of lubrication. A plain bearing mandrel demands a slow but uninterrupted flow of oil, otherwise rapid wear will soon be apparent. A sealed ball bearing may not expect any lubrication during its normal service life, and even ‘open’ grease-packed bearings need only occasional attention (how often do you repack the axle bearings on your family car?)
Now Curly’s problem was not just chatter, it was also fine surface patterns on the work. Unsatisfactory finish was clearly a concern among likely users of roller bearings. Mr Doughty claimed very fine finishes, even at high production rates. Perhaps a clue to where poor finishes arose is given by his explanation of the need for no or minimal preload. Apparently the popular impression was that a fair degree of preload was necessary, generating excessive friction. I imagine the ‘wedging’ action of taper roller bearings makes them easier to overtighten than angular contact ball bearings, and that one symptom of this might be a poor finish. Another one-time Timken employee - ME Technical Editor, Neil Read, pointed out that in 1923 LBSC’s roller bearings would have been made to standards of fit and finish far from what we might expect today.
Whilst there is no doubt that plain bearings can give excellent results in small lathes, most modern small lathes don’t use them because of cost - ironically the relatively complex ball bearing is a mass-produced article. It is considerably cheaper than a tapered phosphor bronze bush – particularly a skilfully hand-fitted one. Our economically priced mini lathes use this cheapest option – ball-races.
Mini Lathe Bearings
Ball races offer only point contact, and in theory even if perfectly adjusted they are inferior to taper bearings (whether plain or roller), both of which have a far greater bearing area, promising greater rigidity and accuracy. Because of the arrangement of the headstock a plain bearing is out of the question for us, but roller bearings are only about 50% longer than an equivalent ball raced bearing, and we can squeeze them into the space available. The standard (ball raced) bearings fitted to mini-lathes are pretty hefty units, and able to take a considerable load. I understand, however, that depending on the source of the bearings on any particular unit, you may get ones made to different specifications. Certainly a proportion of mini-lathe owners report persistent problems with surface finish, chatter and particularly with parting off. If we accept Mr Doughty at face value, those with these problems should be able to solve them by fitting taper roller bearings and possibly improve the rigidity and accuracy of their machine to boot. Certainly there are an increasing number of mini lathe owners who have either carried out the ball to roller conversion, or who are contemplating it.
The original rear ball raced bearing removed from a mini lathe.
For a mini lathe where loads are much lighter and the motor is less powerful, another option is to fit angular contact ball bearings. These share the benefits of better control of end loading while being better suited to high-speed use; they also can by exchyanged for standard bearings with no dimensional changes making coinverion rather simpler.. They have become increasingly popular as a choice of mini lathes.
Replacement front inner race with its caged rollers fitted to the mandrel of a mini lathe (together with the front bearing shield and a short spacer).
All of which begs the obvious question – if it ain’t broke, why fix it? I am another traveller on the endless quest for the ‘ideal lathe’. Adding roller bearings would not harm my lathe, and unlike roller bearings, I gain some real control over the bearing adjustment. If they gradually wear, I can take it up properly.
In a second article I will describe my experience in fitting roller bearings to a Mini Lathe, and give my assessment of the effect they have had on the lathe.
See also: Changing the Bearings on a Mini Lathe
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First, a caution! These tips were written for the Clarke CL300M, as it was supplied around the turn of the century, and after I had been using it for three or four years. Since the CL300M came out, huge improvements have been made to the basic Mini-Lathe design with everything from quick-release tailstocks and brushless motors. Nonetheless, many of these tips still apply and others will be of use to people who have acquired second-hand mini lathes.
The lathe has given me 20 years of service, and is probably one of the most modified mini-lathes on the planet. It now plays backup to my much larger Arc Euro Trade SC4-500, but still gets used. My thoughts on some of the issues below have moved on, see my comments in square brackets.
For a fuller guide to mini lathes see my book The Mini Lathe.
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You might imagine that the limiting factors on cross-slide travel are adequate engagement of the feed nut and the dovetails. This is true when feeding the slide inwards ‘away from the operator’, but it is possible to increase the outwards travel significantly. In fact, you can increase the usable travel from 2 1/2” to over 3” with two minor modifications. Firstly, the slide itself butts up against the index. Milling a shallow recess to clear the index is the first step. The second is to file the end of the feed nut to a radius of 3/8”, so that it can feed right up into the end of the slot in the saddle. Chamfering the bottom edge of the nut should ensure you get the last thou of full travel. This is quite a bonus, as it enables you to face work a full 6” in diameter, as against 5”. I have only made this modification to my new t-slotted slide [do this mod! It's the best one there is for any mini lathe].
The toolpost of the lathe is such that 5/16” square HHS toolbits can be used with no, or very little, packing (depending on how they are ground). This is a common size, but surprisingly, no tools of this size are to be found in the set marketed for the lathe by Clarke.
The set of tools supplied for use with the CL300M comprises a set of small 6mm square tools and a holder, and one big tool - a hefty double-ended bit in a robust holder. The holders are such that the tools come to centre height when in the toolpost. The larger tool is the only one in the set capable of cutting into a square corner. It can also be used for making 45-degree chamfers, but it is rather peculiar, apparently suited to very heavy work, and one I have hardly used.
Of the smaller tools, the finishing tool is acutely pointed and needs the end stoning to get a good finish, but its shape is such that one sharpening and it’s almost useless. It’s too acute to serve as a proper thread cutting tool. The handed roughing tools are better, as they can be sharpened by grinding the flat side of the tip, but they cannot cut into a corner. Finally, the parting tool works well, but is difficult to sharpen and keep the right top rake. The best part of the set is the holder that allows one to use 6mm square tools without packing. Pre-ground 6mm tools in various styles are readily available, and can be very useful for smaller work –especially very narrow parting tools. The smallest ground HSS boring bars fit neatly into the holder. In the end, however, I have turned all my tools around and ground the blunt ends into new tools – thread cutting, knife and parting.
Aside from a 1/16”-thin parting tool, most of my machining is done with very simple tools ground from 5/16” square HSS [these days I prefer indexable tungsten carbide tooling, but HSS is the easiest option for beginners being easier to use]. This generally arrives with a chamfer on the end that is a good starting point. I use a simple rest on the grinding wheel and put on top-rake and front clearance aiming to grind right up to the corner of the bar, then take an angled skim off the end to leave a sharp point. For general use I polish up the edges and round the tip off with a slipstone. These tools cut up to a shoulder and can be used for both roughing cuts and facing at the same setting. For finishing cuts I put a small flat parallel to the cut on the end. I also have a tool of this kind ground to a about 1/16” radius, for rounding internal corners.
In use, the tool is set at an angle, to give suitable clearances front and side. Though tools of this design are often recommended, it is rarely pointed out that, though meant for left hand cutting they can be serviceably used for facing without being reset. I find I have to use a thin alloy shim to pack up the tool to get it exactly on centre height.
Parting tools are best ground by copying a bought one. Sharpness and getting the tip accurately at centre height are the critical issues, as long as there is clearance on all sides and some top rake the exact amounts are less important.
3 The Boring Bit
Say you need a small boring tool, one that will enter a 5/16” hole. One solution is a d-bit style cutter held in a boring bar holder, but I chose to use the rather scruffy device shown in the photographs. This tool is the cranked shank-end of a broken drill bit, held in a drilled-out and squashed piece of aluminium shelf support. Not very glamorous, but effective. In theory the shank of twist drills is rather soft, but I find that this tool happily keeps its edge cutting ordinary steels and cast iron. A more elegant solution would be to make a similar tool from silver steel, fitted in a ‘proper’ holder [I still have and use this ridiculously crude tool!]
Cut a two-inch length of 3/16” diameter silver steel. Smear it with washing up liquid (WUL), then heat it red-hot and allow it to cool slowly. A gas blowtorch is fine for this job, but placing the piece on a small insulating tile will make this much easier. Using a vice and a large pair of pliers, or even a hammer, put a 20-degree crank near the centre of the bar. Now grind away the tip to the profile shown in the drawings. It is important to grind under the cutting edge so that the cutter does not rub when being used in very small holes.
Smear with WUL once more, and heat to red-hot for a few minutes, then quench in lukewarm water. The regularly recommended ‘cherry red’ is hard to judge, Peter Wright’s ‘boiled carrot’ is less ambiguous, I think this is the point when the glow starts to appear to come from within the metal, rather than on it. The mediaeval Japanese artisans who forged a Samurai’s katana matched the colour of the metal to that of the setting sun.
Clean up the piece with emery paper or scotchbrite pad (this is so much easier if you use the WUL!), then temper it by heating the shank until the tip becomes a straw colour. With a small item like this, it is easiest to place it where it can rapidly be poked into a water filled container using pliers, as soon as the colour gets near the tip.
Make a simple holder from piece of scrap square bar that will fit the toolpost. If you drill a 3/16” hole in the holder when it is in the toolpost, then the tool will automatically set itself at centre height. Make another, threaded, cross hole for a suitable screw to hold the bit in place.
4 To Mount or Not?
Large lathes are normally rigidly mounted; either to the bench or even physically bolted to the floor. Small lathes such as the Peatol or Unimat 4 may be free standing or mounted on a baseboard solely for convenience.
With a mini-lathe we have a genuine choice. There is no absolute reason why the lathe should be securely mounted, as long as it is in no danger of moving when in use. If the lathe is used in part of the home, it will be a lot quieter sitting on its built in rubber feet, than bolted to a rigid bench. Furthermore, as long as the bench below is reasonably flat, the lathe be should retain its accuracy. The need for careful levelling and adjustment necessitated when the lathe is bolted down is avoided. I have never bolted down my lathe, and have never had any problems with movement or noise [I never did get around to mounting the mini lathe].
5 The First Cut is the Deepest
Faced with my first lathe, I could only twiddle for so long! I bought a set of hss tools with the machine, so I had to cut metal. This was before I had such useful books as Tubal Cain’s Model Engineers Handbook and Peter Wright’s Model Engineering – A Foundation Course. The only clue I had was the handbook which suggests limiting cuts to a depth of just 10 thou (0.25 mm). Experience has shown that you can take significantly heavier cuts, easily up to 80 thou in soft metals, and 20 in mild steel. The potential of the CL300M is even greater, after fitting a t-slotted table so that the top slide could be eliminated, I found that I could take 1/16” (over 60 thou, 1.5 mm) deep cuts in mild steel, though the finish was a bit rough. The load on the motor for this is considerable, and ought really be reserved for lathes which have had a larger, variable speed AC motor fitted [since fitting taper roller bearings and fitting a3-phase motor with an inverter, finish is fine at 1.5mm deep and I can take 3mm deep cuts]. At this point I ought to recount a cautionary tale!
6 Keep Your Cool
When making the spindle for a rotary table out of a chunk of 3 ½” cast iron bar, the lathe was happily making cuts that just got deeper and deeper (I had an inch of metal to remove), at modest rpm. The spray of iron filings got higher, and I grew dirtier. Unfortunately the power needed for the job made the motor run hot, but the low speed limited the effectiveness of its cooling fan. A loud pop followed by a cloud of smoke announced the failure of two windings in the armature. I found myself inside a Chuck cartoon!
The quotes I had for rewinds were astronomical. Having wound transformers in the past, I considered rewinding the motor myself, but a 240V DC motor has a lot of potential for disaster if wound wrongly, so I decided to seek a replacement.
I won’t tell the whole story of how I tracked down a replacement motor. Suffice to say that one from Clarkes/Machine Mart would have involved a very long wait for a slow boat from China. I could not track down an off the shelf replacement – some washing machines used to use similar motors, but these seem to be obsolete. The various other suppliers of the CL300M and its look-alikes were very helpful, but reluctant to sell me one, as they all had only one or two spares which they (reasonably) wanted to keep for their own customers. Eventually one relented, ‘against his better judgement’.
This exercise revealed some of the differences between the various mini-lathe lookalikes! It also highlighted the excellent service from several of the machinery suppliers advertising in ME – every one I contacted was helpful, despite the fact I had bought my lathe elsewhere. Perhaps the most gratifying aspect of taking up model engineering has been discovering that old-fashioned service and courtesy aren’t dead! Whether I have been buying an expensive gadget, a set of castings or just bits and pieces, I have yet to find a specialist supplier, large or small, who has not spent some time making sure I get what I need and avoid expensive mistakes. Of course the side effects are that a) I usually buy something else as well, and b) I go back to them. If you are in the market for workshop equipment, even ‘cheap’ models, my advice is to contact a selection of ME advertisers and seek their advice, you won’t regret it.
One interesting pieces of advice was the suggestion that I fitted an ordinary AC motor, on the grounds that the lathe was robust enough to take a much more powerful motor. As far as I can see there are a couple of problems with this, but they could be solved. The first is linking into the drive train. This would have to be by belt to a pulley on an intermediate shaft set where the motor is. Secondly, the back gear has roughly a 2:1 ratio, much smaller than most lathes, so a conventional set of coned pulleys would not give a very broad speed range. There would have to be a decidedly Heath-Robinson collection of belts and pulleys hanging off the back of the headstock, and speed selection could be a bit of a nightmare. The easiest solution, would therefore appear to be using a larger AC motor, say ½hp with electronic speed control. This would give a powerful and flexible machine, while remaining reasonably compact [In the end I fitted a 1/2 HP motor and eventually moved on to a three-phase motor and inverter package, which made the lathe an absolute joy to use].
7 Keep Under Cover
The gear cover of the CL300M is held in place by two bolts. Continually removing and replacing these bolts is tedious, and eventually the temptation is to operate the lathe with the cover removed. It takes only a few minutes to drill and tap a pair of holes in the reversing gear mounting plate, fit two screws in place and cut two matching slots in the cover. It now takes only a second to fit or remove the cover. For full details see Mini Lathe Gear Cover Mod [again, this is a mod that every mini-lathe owner should make].
8 Do Blow a Fuse
Buy a few packets of 1.6 Amp quick blow fuses, a pack of ten is pence from Maplin or similar. It is easy to damage the motor of the CL300M if it is stalled for any length of time and a packet or three of fuses are a lot cheaper than an armature rewind or a new motor. One reason long, slow jobs with heavy cuts can overheat the motor, is the reliance on a fan on the armature for cooling, which is ineffective at slow speeds. The constant torque circuitry does a good job of keeping a steady motor speed when running slow on heavy loads, but this creates the illusion the machine can cope with jobs strictly beyond its capability, or at least that of the motor. Always use ‘low’ gear unless you want an extra fast cut as this doubles the motor speed, reduces the load and makes more torque available for cutting. The correct rating of fuse is your best protection against overloading the motor in this way; when I blew my motor, I had fitted a larger fuse.
Treat the electrics of the CL300M with extra respect. The motor runs off some 350 volts direct current (essentially rectified mains). DC shocks are more severe than AC shocks, so it is more dangerous than the mains. Unless you really know your electrics, don’t go exploring in here, and get any problems which arise sorted out by a competent electrician.
9 Sudden Halts
A sudden loss of power accompanied by a ‘screeching’ sound probably means the motor has shifted on its mounts, loosening the toothed belt. A milder scraping sound may mean the toothed wheel has shifted on the shaft, and the belt is rubbing on the guard. In both cases switch off and adjust and retension the belt.
A complete loss of power without the dramatic sound effects could be the shearing of the key in the glass-filled nylon toothed wheel on the end of the motor. This happened to me early on, after leaving the auto-feed in gear when parting (OK that’s pretty stupid, but let him who is without sin cast the first stone!). I filed a groove inside the damaged wheel, to match that on the motor shaft. I then fitted a small brass key between wheel and shaft and have had no more problems [keep an eye on this wheel - if you get a lot of slippage it can wear badly but they are cheap enough to replace].
Since writing the previous paragraph I have discovered another failure mode! The sudden onset of rather rough running with a tendency for the motor to stall and blow a fuse perplexed me, as the job in hand was well inside the lathe’s capacity. All was explained when the drive belt, which had split and was skipping a few teeth, stalling the cutter, finally gave way completely. The only answer is a replacement belt, but these are cheap and off the shelf items from the lathe supplier’s spares department.
There do not appear to be any points for oiling the mandrel and other bearings inside the headstock. Does this mean they are ‘sealed for life’, just like the suspension ball joint that seized with the result that the front wheel disappeared into the wing of my car going around a roundabout. An advance over the screw and thread arrangement on my old Marina, I suppose? I worry about this, and fancy the headstock gets a bit warm at times. Several sources suggest replacing the headstock bearings with taper roller bearings. As I have no problems at the moment, I shall leave this job until I do [I did indeed fit roller bearings to the lathe and felt it greatly improved the lathe. Looking back I suspect some of the improvement was the extra confidence it gave me].
An important task is keeping the lathe bed and the various slides oiled. I think a major coolant system for the CL300M is a bit over the top, so I just use neat cutting oil for drilling and reaming, and whenever it seems it may help. The nylon change gears ought to run dry, but as some are metal I slop a little oil about here as well. Keep the leadscrew’s end bearings oiled. There is a nipple at the tailstock end, and a simple hole at the headstock end, but the latter is usually covered by the end cover. Don’t forget that the split nut on the leadscrew takes some of the biggest loads, so keep the leadscrew clear of swarf and well lubricated. Finally, chuck scrolls and adjusting screws deserve a little attention too!
11 Leadscrew Handwheel
In Model Engineer’s Workshop (issue 91, July 2003) Alastair Sinclair suggested fitting a leadscrew handwheel to the similar Chester Conquest. By using a matching wheel to that fitted to the saddle and tailstock, his approach gives the appearance of an ‘original part’.
I essentially followed his design, thinking the addition would be useful. In fact, I have found it to be indispensable, using the leadscrew instead of the top-slide or saddle handwheel for almost all manual adjustments or cuts. My major difference was that I have an Imperial 16 tpi leadscrew, he has a 1.5mm metric one. I considered various divisions for the index wheel and settled for 64. This gives evenly spaced divisions for fractions down to 1/128” and beyond, and each single division equals 0.001024”, close enough to a ‘thou’ for most purposes. I was tempted by 60 divisions, which would have allowed the divisions to be grouped in fives, while allowing accurate fractions down to 1/64”, but this way each division would be just 0.000960”, over twice the error of 64 divisions. Both 62 and 63 divisions would have been more accurate, but they are awkward to subdivide. Alastair pointed out to me that a metric leadscrew with an index using 60 divisions would give 1 division equals 0.001016”!
12 Power Cross Feed?
Alastair also suggested using a hexagon bit in an electric screwdriver to give independent power feed to the lathe’s leadscrew. This idea can be extended to the top and cross-slides. Even if you can’t feel any slack in either of the slides, try this. If you keep the speed down you may be surprised at the improvement in surface finish.
13 Ball Handles
The ball handles have thick chrome plate. To avoid blisters I often use a short section of copper or brass tube as a sleeve, when continually winding the cross-slide in and out. I don’t know If this was a contributory factor, but the chrome plating lifted and I ended up with a nasty, deep cut in both thumb and forefinger. The lifted plate, being harder than the underlying steel, wouldn’t file to a smooth finish, so I had to order a replacement handle I have it in mind to replace the ball handles with either solid brass or sleeve and pivot handles [I ditched the copper tube and the replacement handles are still going strong].
14 Chuck the Guard?
The chuck guard only obscures the part of the chuck furthest from stray fingers. I confess I have removed mine, as it constantly rattled on the chuck, but sensible advice is to replace it with one that reaches around the front of the chuck, like those fitted to many larger lathes. An alternative is a large sheet of acrylic or (ideally) polycarbonate some 3-4mm thick, suitably mounted, which can act as both safety screen and swarf deflector, arranged to swing in front of the lathe.
15 Fitting Chucks and Faceplates
If you want to fit a faceplate or a four jaw chuck, the fitting is for a standard 63mm register with three or four bolt holes. No separate backplate is required, and the arrangement has the advantage that reversing the lathe won’t loosen the chuck. The typical diameters are 80mm for the chuck and 7” for the faceplate, but I understand that some 100mm chucks will fit the lathe without an adapter. Don’t be surprised when you find that these are supplied with 6mm bolts that you can’t fit behind the backplate! All you have to do is a quick hacksaw job and convert them into 6mm studs. Loctite these ‘studs’ into the threaded holes in chuck or faceplate. I used the bolts supplied, as they were of good quality. They have not suffered from repeated retightening, though I have replaced the relatively soft nuts several times.
Never use the reversing switch when the motor is turning. The best possible result is a blown fuse [more modern versions have more robust electronics and motors, but please treat them with some respect!]
16 Parting is such Sweet Sorrow.
My top tips for easy partings are:
- A sharp tool at exactly centre height.
- Part as close to the chuck as possible, and minimise overhang of the tool.
- Start with a slow speed and feed the tool in gently, but firmly.
The variable speed control of the CL300M comes into its own for parting – you can gently vary the speed to get the best results, and speed up as the diameter drops. Don’t just watch – let your ears and your fingertips tell you what is happening.
Returning to that rotary table spindle for a moment, it required a very deep, wide groove. I cut this using a special design of parting tool, gleaned from a 1940’s article by LBSC. The tool has the shape of a fish’s tail in plan view. I ground the shallow vee in the end using a mini drill mounted stone. This shape of cutter is less liable to chatter than a normal broad parting tool, and is also capable of taking light cuts on the side. In use it is fed in five to ten thou at one side of the groove, moved across to the other side of the groove and fed in again for the return journey. The ‘double pronged’ design means that the tool cuts the full width of the slot to the full depth, with much less tendency to chatter.
17 Fix that Sticky Index
Does the index on your top-slide stick when you wind it out? Well take it apart and you’ll see why – it’s used as a thrust bearing against the stationary index! (I understand this may not be the case on some other ‘brands’ of mini lathe. I solved this problem by making a thrust block of 7/8” of 1/4” by 3/4” mild steel bar, bored 10mm to take the leadscrew, with the hole suitably chamfered, and screwed to the back of the stationary index wheel with two 4BA screws. I then skimmed a 1/64” counterbore on the index wheel, and put it all back together. This simple arrangement also allows considerable reduction of backlash, just leaving that of the leadscrew in the topslide block itself, for the loss of perhaps 1/8” of travel [this 'design error' certainly seems to have been addressed in most modern mini lathes].
18 The Last Splash.
The large black splash guard on the back of the lathe can be both a help and a hindrance. It reduces the travel of oil and swarf across the workshop, but also reduces the travel of the cross-slide and gets in the way if you need to remove it. The guard appears to be held on by 2BA, rather than M5 bolts. I cut short sections of 2BA studding, and fitted knurled brass thumbwheels on them with a drop of cyanoacrylate. Instead of trying to poke an allen key into a hidden screwhead, I can now fit or remove the guard in seconds [oh yes! Do make this simple time saving mod].
First a brief digression – these pages have seen many argue the case for metric or Imperial. Well, my generation was in school during metrication, so I was taught Imperial and CGS metric. Since then ISO metric has taken over, and those who decry Imperial measure ought to contemplate how the inch has outlived the centimetre! Having the whole lot turned upside down while young enough to cope leads to a sort of dimensional bilingualism. Even so my mother tongue is Imperial and if something scares me I run a mile, not 1.6 kilometres, thank you. (How many readers remember the red ‘Sylvine’ exercise books, with a wonderful table of rods, poles, perches and the whole heavenly host of Her Majesty’s measures?) But to get to the point, tools are expensive, so I get the most cost effective solution. This typically means BA threads for small sizes and metric for larger ones.
If a completely separate, low voltage, supply is available it is possible to add a small fan, such as you find on computers, set to blow cooling air through the motor. This will help considerably at low speeds. Make sure you arrange it to assist, not fight, the airflow generated by the motors built in fan when it runs forwards. I assume that you will make very little use of the reverse direction of the lathe – just running a thread back out of a die or similar. If not, you will need to switch the fan off when you run the motor backwards. It has also recently been suggested that a fan could be fitted to cool the motor control circuitry. I am considering moving the control circuitry into a separate box, and adding various other items, including a tachometer, temperature monitoring and an ammeter to monitor the motor load [I never added teh fan as I changed motor drive, some have found adding one useful].
19 Fly Cutter.
This simple fly-cutter is copied from an example by John Rinaldi, and is turned from a section of 1” bar. The shank is turned down to an accurate 1/2”, and the end cut across and filed flat at an angle of about 10°. Of course, if you had a fly cutter, you could use it to do this job… Mount the embryo cutter in the toolpost, by gripping the shank (use
some shim to protect the shank), it will be at the correct height for the slot to be milled out using a 1/4” end mill.
Once you have the slot finished, rotate the tool body and ‘mill’ two rebates for the holding screws using a drill press. If you use a slot-drill or FC3 ‘throwaway’ cutter you will find that, as long as the work is held rigidly, you can gently plunge in the tool this way, as you are not feeding the work from the side. Drill and tap the holes for the screws, and tidy up the job with a fine file.
The bit I use is just a simple knife tool ground on the reversed end of a commercial 6mm square HSS toolbit. Never forget when buying a ready ground tool, that it comes with a ‘free’ do-it-yourself tool at the other end! I grind exactly the same shape and clearances as I would for a general-purpose lathe tool, and it seems to work perfectly well.
The mini lathe of 20 years ago was an affordable introduction to turning, but had a number of weak points - chiefly the vulnerability of the motor and control system to abuse. If you go for a top of the range modern mini-lathe you are getting a remarkably capable machine that addresses many, if not all, of the issues I mention above.
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