The action of the tooth of a milling cutter is quite different from that of a lathe tool. Fig. 1 shows that. with the normal rotation relative to the feed, the effective depth of cut is small at the first engagement of the tooth, rising to a maximum at the edge of the workpiece. There will be a sudden release of energy as the chip leaves the parent metal and even if another tooth has started to cut further round the cutter (as it should, if good practice is being followed) there will be a marked “spring back” both of the cutter on its arbor and of the work support. If the rotation is in the opposite direction the shock comes at engagement, but this style of cutting (“down-cut milling”) should never be used on a lathe, as in the absence of backlash eliminators in the feed-nuts the feed is uncontrollable. In Fig. 2 I show the case of the cutter which has tooth engagement over almost the whole diameter, as might be used in light facing work. Here the depth of cut first increases and then diminishes. There will be much less shock loading, but there is still considerable fluctuation of force liable to cause vibration.
Early types of cutters were designed with a large number of teeth – some even like rotary files – in the hope that this would lessen the problem. In this they were effective, but only at the expense of surface finish and accuracy. Finish suffered because there was not enough space between the teeth to accommodate the chips formed during the tooth travel. Accuracy was poor because it was (especially in the early days) difficult to grind every tooth to the same radius of action. Modern cutters have fewer teeth (Fig. 3) and when used properly the tooth spacing will be arranged such that one tooth starts to cut before the previous one has finished. This action is helped by arranging the teeth as a helix. Fine pitch cutters may be necessary, however, especially when cutting thin material or taking fine cuts.
TYPES OF CUTTER TOOTH
Teeth may be of two forms. Fluted Teeth, rig. 4, are designed to be sharpened on the top land. The means that the diameter of the cutter is reduced each time it is sharpened. This does not matter on those used for facing, but must be remembered if it is used for cutting a slot,or if dimensional work is being done using feed screw indexes and allowing for the size of cutter. The clearance angle of the tooth is maintained by setting over the grinding wheel. Relieved Teeth, Fig. 6, are used when the profile of the cut must be maintained, as on a gear-cutter or form tool. In this case the tooth is sharpened by grinding the flat front face ONLY, the tooth profile having been maintained all the way down the “relief’. These cutters are rather expensive, as a special form relieving machine is needed for their manufacture. The “angle of relief’ provides a constant clearance angle throughout.
It should be noted that many fluted tooth cutters are used for accurate dimensional profiling – the slot drill is a case in point. Fortunately the model engineer does not subject his cutters to the amount of wear found in industry and it will often suffice simply to grind the end face of the cutter, as it is the sharp corner which suffers most. However, when such cutters are used it is only prudent to remember the sharpening problem, and to set up cutting conditions which will minimize wear. In which connection it should be noted that this does NOT mean reducing the cut to a “shave”;this will be dealt with later, but it can be said now that milling cutters must CUT. and cuts of “the odd thou” are just those most likely to take theedge off.
TYPES OF CUTTER
1. The Slabbing Cutter Fig. 7. This is illustrated for completeness. as no lathe likely to be used by a model engineer will have sufficient power available to drive it, nor be rigid enough to carry it. As its name implies, it is used to face slabs or large surfaces. It is often misnamed as a “Roller Mill” – quite a different tool, used for reducing billets to bars In a steelworks by rolling!
2. The Side–and–Face Fig. 8. This has teeth on the sides or faces as well as on the cylinder, and when used on an arbor between centres can machine the vertical surface of the work. It can be used for cutting slots but. of course. once the face teeth had been ground would not be dimensionally accurate as to width. A variant is the Slotting Cutter, Fig. 9.This has teeth on the circumference only – it is really a narrow slabbing cutter. Resharpening does not reduce the width.
3. Slitting Saw. Fig. 10. These are relatively thin variants of Fig. 8 and 9; some have teeth on the sides, some not, but the term “saw ” is usually applied only to those without Primarily intended as cutting-off tools they can be used to form narrow slots. Thicknesses vary from as low as 0.004 inch up to 1/4 inch,but it is doubtful whether a 3 inch lathe could cope with much above 1/8 inch.Selection of tooth pitch is important ; there should never be less than three teeth in contact, but on the other hand, with thick work the pitch must be large enough to provide adequate chip clearance. If necessary the work may have to disposed with the cutter almost tangentially (Fig. 11) to ensure adequate tooth contact.
4. Angle–cutters, Fig. 12. These, as their name implies, are cutters for forming vee-grooves or bevels. They may be single or double angle, and in the latter case the two angles can be different if need be. Special cutters of this type can be had for fluting the teeth of home-made milling cutters and, with a radius at the point, for fluting taps and reamers.
5. Formcutters. Fig. 13. As already explained, these must always be of the form-relieved type. That shown is a gear cutter. but the profile can be of any reasonable shape.Accurate setting-up of both work and cutter is important as even slight deviations can distort the shape (Though at times advantage may be taken of this fact, to produce a non-standard form. A semi-circular form cutter could. for example, be offset to cut the gullet shape of a cutter tooth).
6. The End-Mill All the cutters so far mentioned are designed to be mounted on an arbor – either between centres or held in a chuck. The end-mill, Fig.14,is always held in a chuck. That shown has a screwed shank for use In a special collet chuck (by far the most accurate and safest way) but they can be had with plain shanks. They have teeth on the cylinder and on the end, the former usually (and preferably) helical in form. Though often used as facing cutters their prime function is profiling, the design cutting condition being shown in Fig. 15, with the maximum depth of cut equal to the cutter diameter and maximum width one quarter of the diameter. They are somewhat unhappy cutting a width more than half the diameter even if the depth is reduced accordingly. Note that If the machine is Incapable of accepting the full cut shown In Fig. 15 the axial depth of cut should be reduced. not the width, for it is important that at least two teeth should be engaged. THEY ARE NOT INTENDED FOR CUTTING SLOTS and will not cut to dimension if so used. If a slot MUST be cut with an end-mill then it should be of a diameter less than the width of the slot and a cut taken down each side separately with the work-traverse opposing the rotation of the teeth,of course.
7. Shell End-Mills Fig. 16. Large (relatively) diameter end-mills on a solid shank would be expensive, and the shell end-mill is an alternative. It is normally held on a short arbor, and retained with an allen screw and thick washer .The drive is normally taken through a cross-key engaging in a slot in the back face,but for model engineers’ work the friction drive is often sufficient. If need be a small peg can be fitted on the collar of the arbor to take the drive. Some may be threaded for use on a screwed holder. Though they are, precisely, end-mills and intended for use as in Fig. 15, their larger diameter makes them very handy for facing work . However .if they (or, indeed,any end-mill) are to be used exclusively for facing it is worth while to chamfer the tooth corner as shown in Fig. 17. This corner suffers rapid wear and spoils the cutting action, especially on light finishing cuts, and a chamfer will make a considerable improvement. However, it is fairly important to ensure that each tooth is equal, and If a cutter-grinder is not available it is best merely to stone a very small bevel on each tooth. A suitable arbor for such cutters is shown on here.
8. The “Rippa” cutter. Fig. 18. This is, in effect, an end-mill with chip-breakers on the teeth. The ordinary cutter produces a chip which is long in relation to its thickness, and this can lead to difficulties and also limits the depth of cut. The “Rippa” is formed to make a series of overlapping grooves, each having a discrete The rate of metal removal can be much greater – in sizes below about 1 inch diameter the total area of cut can be as much as four times that of a standard end-mill, though the feed-rate must be reduced somewhat. Provided sufficient power is available at the mandrel the cutter can shift metal at about twice the rate. The finish is surprisingly good on the vertical surface as the multiple “teeth” are arranged in a helix,and is equal to that of an end-mill on the face.
End-mills. shell and solid, and Rippa cutters can be used as small slabbing cutters, but care must be taken,first not to overload them and second, not to reduce the load by reducing the depth of cut. 0.003 to 0.005 inch should be the minimum. Judgement must be used; the cutter is projecting as a cantilever and the cutting forces can be high – very high if the cutter is not really sharp. “Long Series” cutters (Fig. 19) are available,but these are not really intended for use as wide slabbing cutters but rather to reach down below the top of a tall workpiece.
9. The Slot-drill. Though having the appearance of an end-mill this cutter is quite different. First it has two (sometimes three) teeth (Fig. 20) instead of the usual four or more.Second,on the end face one tooth extends across the centreline. This means that the cutter can be plunged down straight into a face,which cannot be done with an end-mill. If the cutter is sharpened only on this end face it will cut true to dimension, provided the chuck holding it runs true. The finish on the sides of the slot is far better than can be had from a multi-tooth end-mill. They can be used for facing, but not for profiling. The correct (maximum) cut is of a depth equal to half the cutter diameter though this must, of course, be reduced to suit the power available and the rigidity of both cutter – and work-holder.
10. Tee-slot and Dovetail Cutters. Fig. 21. These serve the purposes which their names imply. In both cases it is necessary to rough out a plain slot first. In the case of the tee-slot cutter the “size” is some times quoted as the size of the bolt for which the slot is needed. One side of the slot is cut at once – they are not slot-drills. The cutters have many applications and inverted dovetail, or “bevelling” cutters are available. Fig. 21 also shows a Corner Rounding cutter.
11. Woodruffe Keyway Fig. 21. This is similar in shape to the tee-slot cutter.Its diameter and width corresponds to the British Standard or Metric Woodruffe key. It has teeth on the circumference only. Model engineers seldom need this type of key, but the cutter can be used for slotting connecting rods and for ordinary keyways. The neck is conveniently reduced in diameter adjacent to the head,to clear the shaft being cut.
12. Ball–ended slotThese have two teeth as a rule, the end being ground to a radius equal to half the cutter diameter. Their main application is the cutting of grooves and flutes, though when set at an angle to the radius of the work they can be used for gulleting. Being slot drills they can be used in a plunging cut to form the seat for a ball-joint. Care must be taken in regrinding, as otherwise the true spherical form will be lost.
13. ‘Throwaway”cutters. Fig. 22.This is a fairly recent introduction, being so cheap (relatively) that in industry they are not worth the time needed to sharpen them. They are three-fluted slot drills which can. within the limitation of length, be used as end-mllls. The cutters are surface treated after hardening and cut remarkably freely. The maximum diameter available is 0.250 Inch and sizes down to 1/16 inch dia can ho had on 1/4-inch shank. Standard and “long series” (Fig.23) are available, as are ball ended cutters. All have the same diameter of shank and they are usually held in a Morse taper adaptor, as seen in Fig.24. This type of cutter is ideal for such work as port-milling and the like.
SINGLE POINT CUTTERS. The cutters so far illustrated are commercial types (though they can be home-made in some cases) and are more or less costly. The single-point milling cutter, often called a FLY-CUTTER , is, for many jobs , an adequate substitute provided its limita tions are realised, and can very quickly be made in the workshop.That shown in Fig. 25. for example, is no more than a piece of 1/4-inch diameter silver steel bent to shape and hardened at the tip. They are particularly applicable to facing work in brass and light alloy, where the cutting speed can be high enough to avoid the need for a low feed-rate.
This is the first limitation. Suppose we have a fly-cutter to· sweep 2 inch diameter. For carbon steel on cast iron a safe cutting speed is about 25ft/min.The mandrel speed will then be about 48 rpm. Even at a feed rate of 0.010 in/rev – which would leave a tolerable, but not a good, finish – the feed-rate can be no more than about_!-inch/minute which with the normal cross-slide feed screw, means a “handle rate” of 5 rpm; not too easy to keep steady. We shall have more to say about the relative merits of fly-cutters and facing cutters or end-mills for facing large areas later.
The second difficulty arises from the design of the lathe itself. All lathes are (or should be) designed to machine a workpiece very slightly concave when carried on the faceplate. This means that the cross-slide is not EXACTLY at right angles to the mandrel axis. This is very slight – perhaps 0.002 to 0.003 in./foot when the tolerance on headstock alignment is allowed for. But it does mean that the tool point will cut on the upwards return stroke at the back of the workpiece. This will not matter if the travel of the cross-slide is sufficient to permit the work to run right past the sweep of the cutter, but as fly-cutting is usually resorted to when the work is too large in area to permit this there will be a discontinuity of cut part way across the surface. The difficulty can at first sight be overcome by reversing the normal traverse direction – i.e. bringing the cross-slide forward so that the tool cuts on the backstroke. This is well enough for light work. but the golden rule when milling should be to ensure that cutting forces are downwards onto the flat shears of the bed. In any case. if the work is too long to clear the cutter completely this will not serve.
The first difficulty can be mitigated by using high-speed steel (or even carbide) but this cannot be manipulated in the form shown in Fig. 25. However, Fig. 26 shows an ordinary BORING HEAD (the photo shows the “A.B.C.”) set up as a facing cutter. The tool is a 3/8 dia HSS tool-bit and the radius can be adjusted both by setting over the tool at an angle and by using the micrometer setting slide. This arrangement can be used for “milling” large diameter circular facings, by adjusting the micrometer after each 2 or 3 revolutions; tedious, perhaps, but effective.
Fig. 27 shows another alternative. Here a normal (though rather large section!) boring bar is held in the 4-jaw chuck to provide a flycutter of very large sweep. It might be thought that the ordinary “bent” boring tool could be used. but unfortunately the cutting edge faces the wrong way. It is NOT advisable to reverse the rotation of the lathe to meet this, as the interrupted cut could cause the chuck to unscrew from the mandrel nose. Frankly, I hesitated before using a fly-cutter of such large sweep, but on one occasion when I needed one I applied the device similar to that shown in Fig. 28. Here a bolt has been drilled to accept a 3/ 16 inch dia HSS toolbit and this can be secured at any desired radius on the lathe faceplate.
FORM FLYCUTTING. Shaped flycutters are, of course, in common use as gear cutters for sma ll pitch clock gears, but this Is not the only application. Fig. 29 shows the profile of a flat cutter made to’ decorate the entablature of a model beam onglne. It is carried in a cutting frame shown in Fig. 5 of previous article here. It cannot. of course, be reground except on the top flat foce,but as it w as needed only to machine about half a dozen 6-inch lengths of brass this did not arise. Naturally, the work was roughed out to a more or less triangular ahape first, and the feed-rate was kept to a low figure, as much to achieve a good finish as to avoid wear.
ODD CUTTERS. All model engineers have experienced the situation where the odd job arises and there is no alternative but to make a special tool. Fig. 30 is a small vee cutter made to cut the vee grooves in a brass slide for an old piece of machinery. The cone was first turned , the material being silver steel, and then three flutes milled out with a slot-drill fed endways.
The profiles were backed off slightly with a fine file (Fig. 31) and the piece then hardened and tempered.It may be noticed that the three teeth are not of uniform thickness. but this is not important – it just was not worth while setting up an indexing device for a “one-off ‘ job.
Fig. 32 is a fly-cutter rigged up to a machine a semicircular slot of 1 inch radius and about 1 inch long. At the time I had no bo,ring head (the obvious tool to use) and the casting prevented the use of the normal between-centres arbor carrying a side-and-face cutter. The toolbit is, in fact , made from the shank of a broken HSS end-mill. Finally, Fig. 33 shows a very quickly made slot miller, using the shank of a broken drill – fortuitously available in the correct and unusual size. The end was first ground flat and then both backed off and the centre slightly hollow ground. Very crude but despite the received opinion that the shanks of drills are relatively soft the cutter performed its office perfectly, though I doubt if its life would be very long if used frequently.
The actual manufacture of special cutters would need a book to itself,but fortunately when such are needed the writers of articles in model magazines usually give full instructions. A few notes on clearances and the like are given in the next chapter and if these are followed little difficultly should be experienced in devising the odd “one off” needed now and then.The hardening and tempering of the cutter also needs a book* and in this connection I would suggest that thought be given to the use of casehardened teeth. There is really no need for solid tool-steel when a tool is to be used only the once and casehardened teeth will actually be somewhat harder than tool-steel which has been tempered (and certainly harder than HSS). This is a point often overlooked. A properly hardened carbon steel tool is actually HARDER than a high speed steel equivalent. and will remain so provided that it is not allowed to get hot. Carbon steel also gives a somewhat better finish if the edge is properly honed. The ONLY virtue of HSS is that it can cut much faster -perhaps twice as fast. But if it can be kept cool, both by using a low cutting speed and, where appropriate, a water-soluble oil cutting fluid. the carbon steel tool will take as deep a cut and last longer. A casehardened tool is harder still. as of course, it does not need tempering.