Notes on Workshop Techniques


INDEX

THE LATHE

8. MILLING IN THE LATHE

The lathe is a practicable method of performing milling operations in the absence of a true milling machine. The most common way of milling in the lathe is to use a vertical slide with a small machine vice attached. The only drawbacks are that the lathe needs to be converted for milling operation each time, and that the slide travel is normally considerably shorter than on a true milling machine so the capacity is less. Also, unless the lathe is particularly large the rigidity is going to be less than that of a milling machine designed for the job. However, using the lathe certainly saves on workshop space if this is of concern to you. Some lathes, my own Super 7 being one, come with an extra-long cross-slide to facilitate milling, and the power cross-feed makes milling longer workpieces even easier. On other lathes a long slide is often available as an option or an add-on.

Most slot and channel milling will be done with small endmills or slotdrills (up to 1/2" diameter or so). Although between-center bars are available for mounting side and face cutters, these are not really convenient to work with as the work needs to be clamped to the cross-slide and requires shimming up to the correct height for machining - difficult to do with the precision necessary. Endmilling of work held in a small machine vice bolted to the vertical slide is a much more practicable solution, and I used this method for several years before buying a milling machine. Another method of machining flat faces is to use a single-point cutter which sweeps through a fairly large arc, otherwise known as 'fly-cutting'.

8.1 Milling Chucks

Osborn Titanic milling chuck with R8 taper shank.

To make best use of endmills and slotdrills really requires the use of a chuck purpose designed to hold them. Illustrated is my Osborn Titanic milling chuck having an R8 taper for my milling machine. These chucks are available with a variety of tapered shanks, and are fairly expensive items because they are accurately machined, hardened, and ground all over (including ground threads). Shown together with the chuck are a set of imperial collets and also three home-made metric collets turned from silver steel and hardened. Each set of 4 collets (either metric or imperial) will accept all sizes of endmills up to 5/8" diameter (or the metric equivalent), the shank sizes being either 1/4", 3/8", 1/2" or 5/8". The chuck is designed to hold threaded-shank mills which screw into the collet and locate against a stub center inside the chuck head. The locking ring then closes the collet holding the cutter concentrically and very securely. With heavy cuts the spiral flute design tends to twist the cutter clockwise and pull it out of an ordinary chuck, the milling chuck is designed to cope with these forces and any such movement merely tightens the chuck's grip on the cutter. The Clarkson AutoLock uses essentially the same design of locking mechanism. Cheaper 'throw-away' milling cutters, those not designed to be re-sharpened, do not have a threaded shank but instead have a flat milled near the end. Special chucks are available to hold these cutters, or you can make a 'false collet' to fit your AutoLock type which has a setscrew in the side to engage the flat on the cutter. You will need to make one to fit each of the four possible shank sizes if you want to cover the whole range.

8.2 Endmills and Slotdrills

Selection of small milling cutters for use in vertical mill or lathe.

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Modern endmills are usually of a four-flute design, and are designed, despite their name, to cut primarily on their sides. In other words, they are profiling cutters. Slotdrills on the other hand are designed to cut either on their sides or can even (in rigid machines) be sunk straight into the work to form counterbores. Again, as their name suggests, the 2-flute slotdrills are optimised for cutting slots without cutting seriously oversize (they will though if you try to cut to final size on the first pass). It's a good plan to use a smaller mill to cut an under-sized pilot slot first and then use the final sized mill to finish. Remember, even with a perfect setup the smallest slot the mill can cut is the diameter of the mill, and more often than not there are small eccentricities that are going to cause the slot to be cut oversize. If it's critical to get the slot exactly to size use a mill smaller than the intended slot width and shave each side until the desired width is achieved. In the picture above are shown (left to right) long-series endmills and slot drills, normal length endmills and slotdrills, a multi-flute endmill (old design), a dovetail cutter, a ball-nosed slotdrill and a woodruff cutter.

Perhaps the best way to illustrate the use of these small mills is to describe a typical use to which the model engineer puts them. The first example might be machining the hornplates in model locomotive frames. This is a profiling operation where the frames would be held in a vice on the vertical slide facing the endmill held in the chuck (A milling chuck would be a much better option for holding the cutter). An endmill of as large a diameter (ranging up to about 3/4") that will conveniently fit the gap between the hornplates should be used. The larger the cutter the stiffer it will be, so reducing any tendency for it to spring away from the work. The direction of cut is important, with normal rotation the side of the slot nearest the operator would be machined by raising the work with the vertical slide, the opposite side by lowering the work past the cutter. To do otherwise would involve what is known as 'climb milling', which is fine for rigid milling machines, but bad news on a lathe as the work might get dragged into the cutter and be ruined. Fine cuts of about 0.01" would be taken equally off each side (the setup won't stand anything more) until near finished dimension, then 1-2 thou shavings would bring the slot to final size. Do not run the cutter too fast - about 3 to 400 rpm, otherwise chatter will occur, but use too slow a speed and the surface finish will suffer. The very last cut should use both side and end cutting edges of the endmill.

The kindest way to treat milling cutter is to take as deep a cut a possible without straining the cutter or causing deflection - or risking dislodging the workpiece. The reason for this is to use as much of the cutting surfaces as possible. The cutter will be happier treated this way until you reach the point where it's distressed and chatter occurs (or it snaps off - not likely in the lathe but I've done it in the milling machine). Small scraping cuts are OK for final sizing, but don't try to remove substantial amounts of metal this way with a large number of passes, only a small fraction of the cutting edge will be utilised and these will get blunt in no time at all.

Another example might be to cut the recess of the boring tool holder I use on my rear toolpost. This recess is about 1/2" deep and 1-1/2" x 1-1/2" section. I used a 1/2" slotdrill for this job, 4 passes at nearly full depth, followed by another 4 passes to remove the last 15-20 thou to reach full depth leaving a good finish, and another couple along the sides to get the width just right. It would have been a bad idea to try to cut this lot off with 0.05" passes, yet many beginners would do just that.

A few comments about sharpening endmills and slotdrills. Ideally, you need a tool and cutter grinder for this job, and something like the Quorn, Stent or Kennet will cope with all your needs and, having made one, you won't need me to tell you how to use it. So, on the assumption that you don't have any of these, you can still make a reasonable job of sharpening endmills and slotdrills using a mill/drill or the lathe - provided you have some means of indexing the cutter. I use a rotary table (I used a dividing head before this) wedged up at the required angle on the table. The grind wheel is an abrasive disk off an angle grinder. It's not at all difficult to sharpen the end teeth this way, and these are the edges most likely to get blunted first. If you have neither dividing head nor rotary table you can still sharpen the cutters if you make holders for them out of square steel, which will serve to index the 4 teeth. Some of my mills are many years old and have been sharpened only 2-3 times in their lives.

If you do not have proper suds equipment on the lathe it is probably best to cut dry, but it really depends on the job in hand. The thing to avoid is to use a small amount of cutting oil thus sticking the chips together around the mill, this only results in the chips being re-cut many times eventually clogging the cutter with a soggy mess of oil and fine chips - which certainly doesn't help the cutting process. A flood of coolant is the best option as this washes away the chips and lubricates the cutter whilst keeping the work cool. If cut dry, at least the chips will fall away from the work rather than sticking to it, but the surface finish will not be as good.

8.3 Side/face Cutters and Slitting Saws

Side and face cutters and slitting saws.

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Side and face cutters really have little application in lathe work (in my opinion). They are excellent tools for removing large amounts of metal quickly but their proper home is in a horizontal mill. There is little or nothing the S/F cutter can do that you can't do with an endmill or slotdrill in the lathe, albeit more slowly. I do have a selection of S/F cutters (see photo above) but they rarely get used, neither on the lathe nor vertical mill. The only secure way of mounting a S/F cutter for use on the lathe is to use a between-centers mandrel with additional support from a fixed steady, a stub mandrel really doesn't provide sufficient support to prevent chatter. Whilst it is possible to use a S/F cutter in this way, by the time you have set the work up on the cross-slide the job would likely have been completed using an endmill.

Slitting saws on the other hand are very useful, and these can be held in a stub mandrel for most purposes. Most saws of 3" or so and above have provision for a key to prevent slipping, my advice is not to use this key with thin saws but rely on friction alone. If the blade does jam for any reason it's safer to have it slip on the mandrel than to risk shattering the saw blade. Saws need care in use, slow feeds and low r.p.m. and plenty of cutting oil for lubrication are the rule. Slitting saws can have either coarse or fine teeth, use fine toothed saws for shallow slitting only. The teeth normally have no side clearance so if they do become clogged with swarf the saw will likely jam, an exception are the staggered-tooth saws which offer extra clearance and are good for deep slotting work (they are thicker than ordinary slitting saws though). Use a line marked on the work and watch the progress of the saw closely, it's not unknown for a saw to wander off line. The last blade I broke was the result of the blade running out of line, I never realised even though I could feel more resistance to feeding the work in. Eventually the blade was twisted so much that it broke. Not a smart move. HSS saws are brittle and very thin saw blades (30 thou or less) are particularly prone to wandering off line and breaking.

8.4 Vertical Slides

Without some form of separate milling machine you are going to need a vertical slide for the lathe in order to perform milling work. As stated earlier, milling requires particularly rigid machinery and milling machines are made to be rigid as possible. The capacity of the average lathe in terms of slide travel and depth of cut that can be taken is going to be less than that of even a modest vertical mill (unless the lathe is significantly larger than the average home machine, say, 5-6" C/H). Another drawback is that the lathe needs to be converted to milling use each time by removal of the top-slide and fixing the vertical slide in place, and this leaves it unable to perform turning operations until it's converted back again. With the slide travel being so much less jobs like fluting long connecting rods can be problematical in a small lathe. I well remember on my old ML10 I had to flute a pair of rods in 2 stages, shuffling it along the angle-plate. The myford range of lathes can be fitted with longer cross-slides offering a total of 6 or 7 inches usable travel (the Super 7 comes with a long slide as standard), but this is still a lot less than the 18" or so in a small mill. One advantage of the Super 7 (and Boxford AUD amongst many others) is that it has a power cross-feed, little use for normal turning work but a real help for milling.

Myford swivelling vertical slide.

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The basic vertical slide is made to bolt onto the cross-slide facing the chuck, and no adjustments are provided for altering the angle of cut. In fact, this does result in the most rigid setup possible under these conditions and copes with the vast majority of milling jobs. With the Myford fixed slide it is possible to remove one of the clamping bolts and set the slide at an angle to the lathe axis, it's sturdy enough setup like this for milling small parts or drilling angled holes (like steam passages in cylinders). The photo shows my own Myford slide which is of the swivelling type, able to swivel both vertically and horizontally, and supplied with engraved indexes on both axes marked in degrees. This type of vertical slide is useful for cutting bevel gears where it's necessary to mill at an angle, and also (in combination with a raising block) to mill larger diameter gear wheels using the Myford dividing head. Attached to the slide can be seen a small vice, again a myford factory design. The vice whilst of limited capacity is quite accurate, the only problem I have with it is that the two clamp bolts near the vice jaws interfere with work that overhangs the jaws, and I usually end up leaving these two bolts out. Of more use are larger vices which have jaws the full width of the slide itself (about 5"), these make it much easier (for example) to mill out the slots for hornblocks in model locomotive frames. My slide was originally supplied with a fixed micrometer collar, cast in alloy. This has since been replaced by the adjustable collar seen in the photograph.

An angle plate can be fixed to the vertical slide thus providing a horizontal surface movable in the vertical plane. This is the method used to flute connecting rods, to cut keyways, and so on. It has to be said though, that with the inevitable increased amount of overhang due to the angle plate, that this setup is less than rigid. It does work - just - but light cuts are required and chatter is difficult to eradicate. I would normally recommend leaving the cross-slide gib strips an easy fit (gives the best accuracy for turning work), but in this one case it's worth tightening them just a little. Also, wherever possible, slides that are not actually in use should be locked with their clamp screws.

8.5 Flycutting

Two home-made flycutters.

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Flycutting is a process whereby a single point cutting tool is swept across the workpiece forming a flat machined face. There are some advantages to flycutting compared to the other forms of milling described so far. Firstly, the cutter is far easier to sharpen than a multi-tooth cutter such as an endmill. Secondly, the cutting action is very easy, requires less power and puts less strain on the lathe. In addition, large areas can be flycut leaving an attractive finish. The drawbacks are that metal removal is actually slower than when using an endmill, cuts must be fine (10-20 thou) and the surface, though appearing perfectly flat, is probably less flat than the equivalent surface generated by an endmill (to face large surfaces with an endmill it is best to chamfer the corners off the 4 teeth, this will leave a better finish - a bit like adding a chamfer to the corner of a knife tool really). It is difficult to believe when you compare the surfaces generated by each method, the endmill leaves a striped finish, and the flycutter a smooth finish. The reason lies in the fact that, to get a flat surface by flycutting, the slide travel must be at exactly 90 degrees to the axis of the cutter, and in lathes the cross-slide is usually set to turn slightly concave. The long sweeping arc of the flycutter exaggerates any off-square angle and so it will cut concave. This concavity is very much reduced with an endmill because of the much shorter cutting arc. The difference is visible if the flycut surface is rubbed on a faceplate with a touch of marking blue on it, and may be significant if the surface in question is (for example) the bolting face of a cylinder head. Another way it shows up is when taking a cut across a large casting it may be found that the trailing arc of the cutter will take a thou or so more off than the leading arc. For many other applications this effect is not significant, but it is as well to be aware of it.

Cutter bits are best ground up from HSS, the interrupted cuts typical of flycutting is not very kind to carbide tipped tools and they will likely chip. Having said that, I frequently use carbide tipped cutters on cast iron, and the incidence of broken tips is very low. I made the two simple cutter holders shown at the beginning of this section and they are used for 90% of the flycutting I do. For very large castings I use a tool mounted in a holder bolted to the catch plate (or it can be mounted on even the largest faceplate). The method is clearly shown in the photo below:

Cutter and holder mounted on a catchplate.

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An improvement over the single point tool is to mount two tools 180 degrees apart, with one tool sweeping a fractionally wider arc, and the other cutting 10 thou deeper. This way, the speed of metal removal is virtually doubled. Commercial 'facing cutters' for the mill are no more than multi-tooth flycutters, often using 3 or 4 inserted bits.

Flycutting a large casting.

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8.6 Gear cutting

Gear cutting is a specialised form of milling, and the lathe is quite suited to this job for smaller work (6" diameter or so). There are two basic methods involved. The work may be clamped in the chuck and some form of headstock dividing utilised for the purpose of indexing the work around (perhaps making use of changewheels, or a more sophisticated device like the Headstock Dividing Attachment designed by Geo. H. Thomas), the actual tooth profiling being done with a cutter (either commercial multi-tooth or home-made single tooth form tool) held in a milling spindle. The alternative is to mount the gear blank onto a stand-alone dividing head, such as the Myford or the Versatile Dividing Head (again designed by Geo. H. Thomas), with the cutter mounted in the headstock chuck or (better) a collet chuck. Gear cutting is a fundamental aspect of clock making, and many specialised tools have been developed as a result. Some of these tools associated with horology (such as the gear-cutting frame) are also of more general interest to model engineers for small scale work. However, the person making a 3" scale Burrell is going to need something a bit more substantial!

Cutting a gear with a multi-tooth cutter mounted between centers.

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This is another important use for flycutters in the home workshop. Commercial multi-tooth gear cutters are expensive, and to cover a useful range of gear sizes demands the purchase of a large number of them. Fine if you are in the gear cutting business, but if it's simply a matter of producing a set of gears for a traction engine the amount of use will probably not justify the high cost. It would be cheaper to buy the gears ready made. Clock makers on the other hand frequently make use of small gears for every job and their purchase may be a more realistic proposition. However, despite the large amount of work involved it is still quite common for keen horologists to make their own set of cutters. For the infrequent user one way out is to cut or grind a single tooth cutter of exact form and size and use this to cut the gear teeth. There are many methods of producing the form tool, a gear wheel with flat teeth and square bottom will need a simple tool which can be ground on a tool and cutter grinder (or the bench grinder with the aid of a jig). For cycloidal gears it is necessary to make a form tool to make the form tool! The process involves making the first form tool consisting of two discs of hardened silver steel held side by side which is then fed into the work to produce the correct shape for the final form tool. A similar method is used with a circular blank to produce multi-tooth cutters, but additional processes are involved where the blank is set eccentrically on a stub mandrel to provide relief to each cutting edge - this process being repeated for each tooth of the cutter. For the simple single-tooth cutter a rectangular piece of silver steel is simply set in the 4-jaw slightly above-centre (in the longitudinal axis), the back-relief so produced serves the same purpose. The process of generating cutters in this way requires careful setting up and adherence to precise dimensions, and tables of data are available to help. For those interested in producing their own cutters several books are available, and perhaps 'Gears and Gearcutting' by Tubal Cain (TEE Publishing) is one of the best known and can be recommended. continued.......

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(c) Chris Heapy 1996.


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