In a Model Engineer's Workshop
I had my first workshop at about the age of fourteen. It was only a draughty, unheated Victorian conservatory, five feet square with a crude bench, but it was my space to make models, electronics and generally star accumulating bits and pieces. It seemed natural enough, after all my father and grandfather both had workshops. Sadly when I went to college at 18, I had to make do with tables and odd corners for about fifteen years, but eventually I got a proper workshop again.
This section of the website will gradually accumulate some of the tools and techniques I've got to grips with over the years. Some of these may have previously appeared in print in a slightly different form.
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Making a Set of Quick Change Toolholders
The basic toolholder is a 2” length of 1” square mild steel. Each one needs a dovetail cutting in the back. I made a template from aluminium sheet that was a good fit on the toolpost block dovetails, marked with two lines 2” apart, it provides a quick check that the dovetails are properly placed and proportioned.
The dovetail template
If you can get a good long length of steel, I suggest you ‘mass produce’ a quantity of blanks (I cut 9 from about 20” of bar), carrying out one operation at a time. Start off by milling a 1/4” deep slot in the middle of each blank 0.710” wide. This is the width across the narrow base of the dovetails. Use a good-sized slot drill (I used 11.5mm), to ensure you get a consistent width. Aim to get the slot in the centre of the blank, though a rule dimension or measuring from the template is fine. The depth should be at least 0.250” and reasonably accurate (too deep means the lever needs to turn to far, which may be inconvenient, but keep the same depth for each toolholder, so the handle always locks in the same place). Similarly, the width should be as accurate as possible in order to facilitate accurately cutting the dovetail itself.
My procedure was as follows: Cut one side of the slot in two passes to exactly 0.250” deep. Now take further cuts at the full 1/4” depth until the slot is full width. A digital readout will facilitate this, just subtract the cutter width from the slot width, and zero the readout when cutting the first side. Once you have one blank slotted, ditto repeato (to coin a phrase) until you have a pile of prepared blanks.
A toolholder blank, with a plain slot
Now exchange your slot drill for a dovetail cutter. For each blank start by adjusting the cutter to 0.250” depth. Take suitable passes along one side of the slot until the witness of the vertical edge of the slot disappears. Zero the digital readout (or your handwheel index). Now repeat on the opposite edge of the slot – you should find that just removing the witness on the second edge creates a dovetailed slot that is just a bit too tight. Gradually widen the slot until the dovetail is an easy fit on the toolpost block, ensuring that the dovetails are clear of swarf before testing the fit. Make this test without removing the toolholder blank from the mill so you don’t loose the index setting. Once you have a suitable fit, note the DRO or index reading.
A finished toolholder blank with its dovetail
You can now machine the dovetails on the remaining blanks, working to the same reading in confidence that they should all be a good fit on the toolholder (photo 10). In practice, I tested the fit ten thou before reaching the ‘ideal width’ to be on the safe side. With a suitable fine file, tidy up the edges of all the blanks and make sure they are all a good fit on the toolpost.
Height Setting Studs and Buttons
These studs and their nuts or buttons (figure 8) are what give the quick release toolpost its repeatability. Drill and tap the blind M5 holes for the studs in the blanks. Note how close they are to the dovetails – this ensures a good overlap of the buttons and the toolpost and minimises interference between the buttons and tool fixing screws. The studs themselves are just lengths of M5 studding.
How the height setting buttons work
The height setting buttons can be finished to suit you own preferences, all that matters is that they have a suitable flat base that will overlap the toolpost. To make the height setting nuts I knurled a diamond knurl along just over half of a length of 1/2” brass bar. I then reversed it and held it in the chuck, using some aluminium shim to prevent damage to the first section, and then knurled the remainder of the bar. I used a 1/8” parting tool to create the decorative effects, repeated along the bar, and then a 1/16” parting tool to cut of the individual nuts.
Again using the aluminium shim for protection, I faced off the pip left by parting and then drilled and tapped each nut M5. I used ordinary M5 nuts to lock these height setting nuts, you may wish to make a batch of smaller brass lock nuts.
Customising the Toolholders
It’s now up to you to decide what different types of toolholder you need and how many of each type. Take your time, and enjoy the variety of tasks after all that repetition work!
The standard size HSS tool for a C3 lathe is from 5/16” HSS. The standard toolholder for these bits requires a suitable slot to be milled along the length of the holder (as in the drawings) and threaded holes being made for four holding screws. If you can get M5 grub screws these are a neat solution, but if not Screwfix (usual disclaimer) supply 16mm stainless steel cap screws at a very reasonable price for 50. The ones I obtained have the advantage of ends turned neatly and in such a way that they won’t burr over.
A standard QCTP toolholder
These holders can also be used for 1/4” HSS, but if you prefer this size you may find you can economise and make toolholders from 3/4” bar instead. If so, you will need to use grub screws to make sure they don’t foul the height setting nuts.
The ends of the toolholders can be treated in whatever way you wish. I plan to bevel mine at about 30° for appearance sake – it will not materially affect their strength.
For boring bar holders, you need to drill and, ideally, ream a hole that is a good fit for your boring bar along the length of the holder. I prefer two or three screws bearing on the bar to hold it in position. Others may prefer to slot the holder and make a clamp arrangement. Ideally, the hole for the boring bar should be reamed to size. I drilled undersize and used a hand reamer to finish the hole.
QCTP Boring Bar Holder
Be careful not to overtighten the clamp screws, especially if your boring bar is unhardened steel, as you could damage the surface of the boring bar, making it hard to remove. If this is a concern, use a clamp arrangement instead, or put brass or copper pads down the screw holes. I have a long inserted cutter bar and several smaller HSS boring bars with 3/8” diameter. The former is hardened steel and the latter are HSS, so I am not worried about this issue.
I made two of these holders – the first has a 0.375” hole that fits both my larger boring bar and the other is bored 0.25”. For the smaller one I have also made a sleeve for a very small 3/16” bar.
There are several different styles of parting blade available, some are rectangular in section, others have some relief on the sides. I made my holder (figure 10, 11)to suit 1/16” by 5/16” section HSS from Arc Euro Trade (usual disclaimer). This shape allows the use of a flat-sided holding groove, but you will still need to obtain or make a small dovetail cutter to form the bottom edge of the groove. Such a cutter can be made from 3/8” silver steel by turning a short cone on the end, filling a few teeth and hardening and tempering it. Such a simple tool will be fine for this task, if a bit crude for making proper dovetails.
Non-rectangular blades will require a holding groove to match their shape. This could be considerably more challenging to make, but the same basic clamping mechanism could be used.
The clamping block only extends for half the length of the groove for two reasons – it concentrates the clamping force where it is needed, and it gives it a more positive location. I suggest fitting the block in place before drilling tapping size for the clamp screw right through both components.
As a DTI is not under heavy loads (hopefully!) it can be made from steel or a length of 1/2” thick aluminium alloy. The drawing is almost redundant – this is an over-length toolholder with a 3/8” (or whatever suits your DTI) hole and a clamp screw. This is another opportunity to have a bit of fun, and see if you can make a nice shape, rather than just something blandly functional – one suggestion I would make is to make the clamp screw from brass with a knurled knob, to make sure it doesn’t get overtightened. A right handed version is drawn, suitable for holding a DTI to check the concentricity of work held in a 4-jaw chuck. A left-handed version could be modified to support a ‘Verdict’ type lever indicator for checking the truth inside a bore.
One of the very first workshop tools I made was an unsophisticated, but effective, clamp knurling tool. This was made with a bar that clamped in the four-way toolpost. I simply cut a dovetail in the bar and added a height setting stud. Although this hasn’t made the tool any prettier, it works well (I knurled along length of 1/2” diameter brass with this device to make the height setting buttons) and demonstrates the principle that any tool that can be fixed in the toolpost can potentially be modified in this way.
Simple scissors knurl modified to fit the QCTP
A Few More Thoughts…
Another idea is a holder reamed for an MT1 taper, a second is to make my keyway slotting tool fit the toolpost. Yet another is to modify my toolpost drill. Equally a toolpost grinder, multi-tool or any other device could be modified to fit on the dovetails. I’m sure there are many other possibilities.
Anyone who follows the discussions on the ME web forum will know how popular tangential toolholders are. I have made one of these for 1/4” HSS and it has proven very useful. I have made a QR toolholder version, and have included a concept drawing. This is something I have not seen elsewhere, but it seems blindingly obvious (perhaps I should have patented it even so). Note that as the toolbit can be adjusted separately for height, the holder needs to be set as low as possible, and maximise the amount of metal supporting the toolbit. This is why the lower part of the dovetailed body will need to be cut away.
As it is now so easy to adjust the height of your tools to be spot on with this new toolpost, the value of a height setting gauge becomes apparent! It also means there is no excuse not to swap tools as often as you need, rather than making do with the same tool for turning and facing, or accepting the finish from roughing tool. Hopefully, this means the standard of my turning will improve!
- Category: In a Model Engineer's Workshop
Making the QCTP Block
This article show you how to make this Quick Change Toolpost which suits popular 7x12 and similar mini-lathes
The design is also suitable for almost any lathe of about 3 1/2" or 80mm centre height including Myford ML7 and Super 7 lathes.
Whenever you tackle a complex turning task, one that involves repeating a series of operations on several parts, you may get frustrated by the limitations of the standard 4-position toolholder.
Four Position Toolholder
One problem is that, in practice, these normally only holds two tools conveniently. One solution is a Quick Change Tool Post, a variety of suitable ‘QCTP’s can be bought for mini lathes, but once you factor in the need for a good set of compatible toolholders (I have twelve, and could usefully have twice as many) the cost starts to spiral. An alternative is to make one yourself, and although this is not a raw beginner’s project, it is a good test of your skills once you are more familiar with the lathe. Note that although it can be made entirely using the lathe itself, milling the dovetails and tool holder slots is much easier if you have access to a milling machine. This is also a project which is made much easier if you have a metal cutting power saw of some kind. There is a lot of quite large section mild steel that needs cutting, and if you have to do this by hand, be prepared for some hard work.
The design is based on the Nakamura Toolpost, but this has a couple of limitations – the piston does not retract automatically and has to be pushed back; it only takes one toolholder; and it overhangs the topslide excessively. The changes here to address the last two of these issues by adding a second dovetail and piston and reducing the overall dimensions of the block. Internal springs cause the pistons to be self-retracting. The toolholder offers ease of use and rigidity, the handle requiring very little pressure to fully lock the holders. Dimensions given allow the sliding and rotating parts to move freely without excessive slop, and will help make sure the dovetail fits are right. In practice, use your experience and judgement bearing in mind the function of each pair of mating surfaces.
The operating principle of the toolpost may not be immediately obvious from the general arrangement (figure 1).
General Arrangement of Quick Change Toolpost
The main body (block) of the toolpost has a circular cavity in which a cylindrical cam is fitted. The cam has a hole through the middle allowing a free central pillar to be threaded onto the toolpost stud and clamped by the original lathe clamping nut. The pillar is sized to clamp only the block, so the cam, with its own handle, is free to rotate. The eccentric portion of the cam bears on two brass ‘pistons’, one in each of the dovetails on the block. Toolholders fit on these dovetails, and when the handle is used to turn the cam, it pushes on the pistons, locking the toolholders in place. Each toolholder has a setting button on top, which allows it to be removed and replaced at exactly the same height. To make using the toolpost easier, wire springs in the pistons engage with a groove in the toolpost block, so that when the cam is turned back, the pistons release automatically.
For the toolpost itself:
- Just over 1 1/4” length of 2” square bright drawn mild steel (BDMS) or continuous cast iron for Toolpost Block
- About 1 1/2” of 1 1/2” diameter round BDMS for the cam
- 2” of 5/8” diameter brass for the pistons
- 3” of springy wire about 1mm/0.04” diameter for piston springs
- 6” if 3/8” diameter BDMS for the handle
- 1 1/4” of 3/4” diameter BDMS for the pillar
For each standard or boring bar toolholder:
- 2” of 1” square BDMS
- 1 1/2” of M5 studding
- 4 M5 cap head screws
- 2” of 1/2” diameter brass
- 1 M5 nut
Note that, in practice you should allow a cutting allowance e.g. 20” of bar will make nine full-length toolholders and leave a useful offcut. Other types of toolholder may need different materials.
A final thought, for small lathes, toolposts are often made from high-strength aluminium alloys. You may wish to use such a material for the block and toolholders, especially if you have a way to get them hard anodised.
I don’t propose to give every detail of construction, but the nature of some of the parts is such that I feel it is useful to at least give a recommended approach that minimises the number of setups require. This is not exactly how I made my holder, but it is how I would make a second.
The toolpost block is made from a single piece of 2” square mild or medium carbon steel. The former is easier to machine, the latter should last a lifetime. Mount it in a 4 jaw chuck, getting it a central as possible, face off the bottom to a good finish and turn the shallow rebate. The rebate is essential to ensure the outer edges of the block make contact with the topslide, maximising grip. Now reverse the block in the chuck and face it to exact length. Drill the central hole in 2 or 3 stages. Use this hole to start your preferred boring tool to make the central 1” diameter recess, noting that it should be parallel sided with a well finished flat base. Now with a suitable recessing tool, put in the internal 1/8” groove – the centre line of the piston holes will align with this groove, so take care to get it in the right place.
The main block at the heart of the QCTP
Remove the block and mark out two faces with centres for the two piston bores. Return the block to the 4-jaw to bore and counterbore each piston hole in turn. You can drill these holes but if you are not sure of the finish your 1/2” drill leaves, then use a reamer or finish them by boring. The same recessing tool you used to cut the internal groove can be used to cut the counterbores. The depth of the counterbores is 1/8” at the shallowest point. This is to allow the rim on the pistons to move fully out of the way when fitting the cam.
We now need to cut the two dovetails. Here is the method I used, which relies on the block being a reasonably accurate 2” across. Mount the block upright in a milling vice, and zero a decent sized slot drill to the top and side of the block. Cutting a full 0.250” depth take successive cuts towards the centre until you have removed a step 0.500” long. Repeat from the other side of the block, and you should be left with a central raised area an inch wide with the piston bore at its centre. Do the same to the face with the other piston bore and check both raised steps are the same width.
To finish the dovetails, use a dovetail cutter, set to cut 0.250” down from the top surface, on one side of a step cutting in stages until the small ‘witness’ between the top face and the sloping face just disappears. Zero your DRO or handwheel index at this point. Now move to the opposite face of the step and machine the other dovetail the same way, noting the final distance reading. When you machine the second pair of dovetails start the same way, zeroing the index/DRO when you finish the first dovetail. When you machine the opposite face of this second dovetail, make sure your finishing cut is at the distance you recorded at the end of the first one. The two pairs of dovetails will now match perfectly, ensuring all your tool holders can be fitted to either of them.
An additional touch is to mill a small pocket on the underside of the block for the catch on the topslide, allowing the block to be reset correctly more rapidly. To add this, scribe a line exactly on the mid-point of the underside of the block. Now mount the block in the mill inclined backwards at 10° from the vertical. Line up a 1/2” (or similar) end mill with the scribed line and mill the pocket about 1/8” deep. The pocket should be behind the ‘front’ dovetail and be milled with the side dovetail facing down. If you are unsure – copy the pockets on the original four-way toolpost. Though I have done this on my toolpost, I am not sure how useful it is – the setting is not particularly repeatable, and unlike the four-way toolpost, you should rarely need to turn the quick release version.
Tidy up the block with fine file to remove any burrs and break the ‘arris’ on machined edges. You can draw file and then use emery paper to make the unmachined faces look a bit brighter. It is up to you if you want to paint any surfaces of the block.
Toolpost Clamp Pillar
This item (figure 3) takes the full force of the toolpost clamping nut and transfers it to the bottom of the block. If possible make it of a medium carbon steel such as EN24T, rather than plain mild steel.
The pillar is drilled clearance for the toolpost stud and is a light push fit in the hole left at the bottom of the recess in the block. Make sure the step on the end is slightly shorter than the thickness of the base of the block, so that when the toolpost clamp is tightened up it grips the block firmly in place. The length of the middle section of the pillar is a critical dimension – it should be the same as the depth of the central recess in the block. Note that this toolpost uses the existing lathe toolpost clamp (although you could make your own copy if you wish).
It is important that the clamp-screw handle points in the right direction when it is tightened up, so start by making the pillar slightly tall. If the handle points the wrong way you can face off the end a little (0.3mm off the end will allow the clamp handle to turn about 90° further).
The cam is a deceptively simple item. There is little worth in making it from medium carbon steel as the wearing surfaces are large and the impact of minor wear on the action of the cam would be negligible. The reference point for all dimensions is its top. The thickness of the upper part should be about 0.002” greater than the depth of the recess in the top – this ensures that when the pillar is locked in place by the toolpost clamp the cam will turn freely without perceptible shake.
I suggest this order or operations: In the 3-jaw chuck face off one end of the bar and drill and bore it 0.760” or greater – a very loose fit on the pillar. Now turn the outer diameter of the top part and counterbore the recess 0.377” deep. Reverse the bar and trim it to length - 1.490”. Turn the bottom section to a close push fit in the toolpost body, perhaps 0.998” and so that the length leaves the top part exactly 0.375” thick (i.e. two thou less than the depth of the recess). Move the bar to the 4-jaw chuck and offset it by 3/64” (this is not a critical dimension) and turn the offset section noting that its top and bottom edges should match with the piston holes in the block.
Now, return the bar to the three jaw in order to turn the 10-degree taper on the top section (photo 5). Do not make the tapped hole for the handle until later, or like me you will end up with an extra hole.
The pistons (figure 5) can be turned from 5/8” or 15mm brass rod if 9/16” is not available. Turn the main part of the pistons to a sliding fit in the block, making sure there is a good smooth finish. The flanges take no load, they only need to be big enough to stop the pistons falling right through the holes.
So you can have sprung pistons that withdraw automatically, you need to drill No.58 (this size is a loose fit for a 1mm diameter spring wire) right through. Perfection isn’t essential, but you need to be fairly accurate with this hole. A good centre pop will ensure the drill doesn’t skid, clamp the vice to the drill table and once the hole is started check the drill isn’t bending off course. Don’t forget to clear the flutes regularly – small drills clog rapidly. You now need to file flats on the piston flanges so they don’t foul the outer diameter of the cam. These can be done by eye, but ensure they are parallel with the cross-drilled hole.
The QCTP locking pistons
Try the pistons in their bores with the cam in place and make sure they are both free moving and don’t interfere with each other, aside from the desired in-out action. The pistons should be just below the surface of the dovetail when fully retracted.
Before final assembly, you should do a trial fitting of a toolholder on each dovetail in turn, to see what is the best position for the handle. At this point I suggest you make the first toolholder, at least as far as cutting the dovetail on the back of a toolholder blank, but I shall finish the description of making the toolpost before moving on to the holders.
Choose the position of the handles so that they do not get in the way of fitting the toolholders when the relevant piston is retracted, and about a 90-degree turn locks the toolholder without the handle getting in the way of lathe operation. Mark the best position for the hole. Do not assume the best position without this test, or, like me, you will end up with an extra hole. If this happens, you can blank the hole with a short stub of M6 thread which will virtually disappear if carefully filed and smoothed with emery paper.
Properly fitted the handles will not interfere with each other
The hole for the handle should ideally be drilled with the cam in an angle vice at 10 degrees, but if you don’t have one, just raising one end of a normal drill vice on a block will suffice if due care is taken. Check the handle will be in the right place by fitting an ordinary M6 bolt and doing a dry run before fitting the springs.
On final assembly, fit the 1 1/14” lengths of spring wire in the cross holes. Make sure everything is clean and free of swarf, then lightly grease the pistons and fit them into their bores, aligning the springs with the groove in the block. I suggest using molybdenum-loaded or teflon grease as my experience is that these work well for parts that do not continually rotate. Grease the cam and line up the bevels on its base with the pistons, and it should be possible to insert the cam with a light click as it goes home. It will now be very difficult to take the cam back out and disassemble the toolpost. If you need to, drill and tap the pistons, say 8BA, for screws that can be used to pull them forwards so the cam can retract. The flanges will stop the pistons rotating and allow this to be done, or if you are super-cautious you can make these holes prior to parting the pistons off.
The toolpost is now complete, aside from the handle. This is simply made from a length of 3/8” bar, with an M6 thread on one end. The tapered section should be turned at about 1° (2° included angle) to match the standard mini-lathe handles. The handle can now be screwed into place, the pillar popped into the centre hole of the cam, and the toolpost fitted to the lathe.
The QCTP in use
- Category: In a Model Engineer's Workshop
Of all precision tools d-bits are the ones easiest to manufacture from the scrapbox. They are the true and proper destiny for the final two inches of any self-respecting bar of silver steel.
It is possible to take the manufacture of d-bits to the level of high craftsmanship. George Thomas made a special jig to hold his d-bits while he milled them to the correct thickness. Peter Wright has a design with a reduced shank, rather like a boring tool, to ease the flow of swarf. Nevertheless, for us ordinary mortals the simplest of designs will work.
A d-bit cuts only on its end. The name comes from the cross section of the tool, which must be marginally more than a semicircle to ensure that it does not cut on its edges. Because the d-bit cuts its own precision guide as it goes, it makes accurate sized holes with a good surface finish. Many writers claim that a d-bit will produce better results than a reamer, which is not bad considering that they cost a few pence to make.
A selection of home-made d-bits
Manufacture is straightforward, simply grind, mill or file away a few thou less than half of the diameter of a piece of round silver steel. Shape the end of the bar to provide clearance and form the cutting edge. An angle right across the end cuts a little more freely, but if the end is partially straight it will allow you to finish holes with a square bottom.
The tool should be hardened by heating to a good red heat , then quench it end-first in clean water, with a gentle stirring action. Don’t just grab it with pliers, and shake it around in the water if it is of thick section it may well crack or distort (how do I know?) Don’t worry about soak-heating the tool as it is not critical to harden it to the core, indeed prolonged heating will burn out the vital carbon from the
cutting edges and diminish its hardenability. If you coat the tool in washing up liquid first it will be easier to clean it up, using wire wool or scotchbrite.
The tool will now be glass hard, brittle enough to shatter if dropped on a hard surface, (yes, it has happened to me). To temper it, apply gentle heat to the shank until the metal starts to form a coloured oxide film. The colours will run down the shank to the tip, and just as a ‘dark straw’ colour reaches the end, quench the tool again. Finally put a good finish on the end and the flat side of the bit with a slip stone – do not take any metal off the curved surface.
As it has no top rake heavy cuts are out and a d-bit is best used to finish a pre-existing hole. It does no harm if a short pilot the same nominal diameter as the bit is made to get everything off to a good start. Beware overheating the tip, as this will draw its temper and it will rapidly become blunt. The secret is to keep the speed modest and use plenty of lubricant. Swarf soon builds up on the face of the tool, so regularly withdraw it, and slap on a dab of cutting oil.
You can make d-bits for other purposes as well. Tapered d-bits can make accurate tapered holes. Turn up a handful of matching taper plugs with the lathe at the same setting, and they can be used to make taper cocks. A round ended d-bit can be used to make round sockets and rivet sets. One which is flat across the end can be used to cut or renew valve seats.
D-bit style taper reamers
One final thought – the so-called toolmaker’s reamer is an even simpler tool. In this case the blank is cut across at about 20°, hardened, tempered and stoned to a good finish. Less robust than a d-bit, such reamers will take a very accurate final skim from an undersize hole.
- Category: In a Model Engineer's Workshop
Looking back through literally hundreds of model engineering magazines, few things have exercised writers more than describing how to judge the hardening temperature for silver steel.
The fashion is to compare this to various food items – notably cherries or boiled carrots. The Japanese swordmakers would heat their work to the colour of the setting sun, just touching the horizon, and that rich orange-red colour – where the light seems almost to come from just under the surface is what I aim for. But the truth is that colours are subjective, and until you know what works for you, you will never be sure. So here is how to be SURE you have the right temperature.
Keep a magnet to hand, and a pair of pliers. As the metal starts to glow, offer it up to the magnet. If the magnet jumps up to the hot metal, quickly remove it with the pliers and heat the metal some more. When the structure of the steel changes to its harder form, the same change will also cause it to lose any attraction for the magnet. Take a good look – whatever colour red it is now is the right one for hardening.
If hardening silver steel you are supposed hold it at this temperature for five minutes for every quarter inch of thickness. If you don't 'soak' the metal for this long, it may not harden right through - in most cases this is not a bad thing, as it means the object will be less brittle, so I rarely keep the work hot for more than five minutes, however thick it is.
A last thought, gauge plate often requires a higher temperature - but the magnet test will still work.
So which of these colours is 'cherry red'?
- Category: In a Model Engineer's Workshop
A fly cutter is a single point tool, usually used for machining plane surfaces. It is an inherently accurate tool when used in this way, and if the toolbit is a good one, rigidly held, it will produce smooth, flat surfaces with the most basic of set ups - as long as the mill is correctly trammed (i.e. the column is at right angles to the table). It is also possible to flycut convex surfaces, but the lack of fine adjustment of the typical flycutter imposes limits on accuracy.
There are innumerable ways of making a fly cutter. The basic requirement is simply a toolbit holder, which is easy to attach to a rotary machine such as a lathe or a mill. This straightforward but useful example was inspired by a photo, though many similar ones are manufactured. Stan Bray has described another useful style, which bolts directly to the faceplate. Tubal Cain was unashamedly used a square-section boring bar held crosswise in a chuck!
The body of the cutter is just a piece of 1” mild steel bar, turned down to 1/2” to make a shank. When you hold the cutter in a chuck, the body can be held flush against the jaws, preventing any rearward movement. The thick end of the bar can be sawn and then filed to a 10° angle. This angle gives the toolbit its top relief.
For use with 1/4” HSS tool steel, a slot offset from the centre line of the body is needed. On my mini-lathe I was able to achieve this by holding the shank in the lathe’s tool holder, suitably angled. I cut the slot with a 1/4” FC3 mini-mill, which gave a slot that was good fit for the tool steel. By using the same mill in a pillar drill and ‘plunging’ it into the thick side of the body, two recesses with 4BA tapped holes for the securing screws were easily made.
A simple Fly Cutter
The tricky bit is making a good toolbit. You will need a roughly 2” long piece of 1/4” square HSS. If you need to shorten a longer piece, grind a shallow groove all round with a mini-drill and carborundum wheel. Hold the bar in a vice, covered with a cloth, then tap the end sharply with a hammer. To visualise the shape of the tool, think how it will contact the work as it rotates; with the bit in the holder identify the corner that will do the cutting. The angle of the holder provides one clearance angle, but you need to grind further clearance on the end, ‘front’ and ‘top’ of the bar. Aim for 5-10°, and a little more on the end of the bar. Effectively you need to create a left-handed knife tool.
In use the cutter should be held firmly in a chuck. In the case of milling machines supplied with heavy-duty drill chucks, you can probably get away with holding the cutter in it, but don’t try using the fly cutter in a pillar drill. Fly cutters need to be run slowly; this one has a maximum of a 3” cut and you should run it no faster than you would turn a 6” diameter cylinder - pretty slowly. The work should be moved past the cutter; on smaller lathes it can be a challenge to achieve a set-up where the whole area to be worked passes the cutter. The depth of cut and feedrate should be less than usual, because of the significant leverage between the end of the cutter and the shank. Be patient and keep your cutters sharp and you will get excellent results.
Finally, a version of this tool that is not angled can be used to hold form tools, such as those used for cutting gearteeth or special profiles.
The finished fly cutter fitted with a toolbit
- Category: In a Model Engineer's Workshop
Reference Information Article Count: 4
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