Workholding Devices of Milling Machine

The usual objective of millilng is to produce flat surfaces. This means that a number of ottributes of the lathe, unimportant when turning, become prime considerations when milling. I have already referred to one- the fact that the lathe is deliberately built to turn concave on the faceplate by a very small amount. This means that a large radius flycutter will “interfere” on the  rearward part of  its revolution by. perhaps,0.001 inch or so. There are other reference planes. too. which should be checked.

REFERENCE PLANES. Clearly, work must be set true in the vertical plane, but this normally presents no problem as the lathe-bed can be used as a base. However, there are cases where a square applied to the work cannot sit on the bed. Tho  truth  of  the  top  of  the  cross-slide relative  to   the bed is  only  of   marginal Importance when turning (in fact, it is only f  any  importance  at all when boring on tho  saddle)  so  this  should  be  checked crosswise  by  setting  a  dial indicator  in your scribing block on the  bed and traver­sing  the  saddle. You  will,  I think, find perhaps 0.001 inch error over the width. Check that this is not affected by the locklng screw – if it is. then your slide gibstrlp is probably not tight enough in normal use. There may be a similar difference between the height of the slide surface above the bed at the BACK of the rear bedway and the FRONT of the front bedway. These errors are within the limits to which you can expect to work. and you can safely square up from the top of the cross-slide. If they are excessive you must seek out the cause and rectify it – unfor­tunately this book is not the place to deal with machinery renovation, or I could suggest something!

We need also to set work square trans­ versely, across the bed. The concave turning alignment referred to prevents us from using the faceplate (or a chuck face) as a reference plane. though the probable error (better than 0.002 inch per foot) is small. On most machines the concavity is attained by setting over the headstock, so that we can be reasonably  sure that the cross-slide moves at right angles to the bed. But in squaring up we would normally use the front face of the slide and this need not necessarily be square to the slides. This should be checked; again, if it is within one or two thou.per foot it is as close as can be expected. At the same time check the squareness of the tee-slots to this face.for these also provide a useful reference plane. I have a piece of parallel strip which fits the slots closely against which to bed my try-square when necessary.

PACKING, CLAMPS, AND BOLTS.  The actual cutting time when milling is a very small proportion of the time taken in setting up the work; I have been at it a long time but even now I still find that the one “gadget” needed is not there. So, a little thought on this aspect may save a lot of time later. Fig. 1 shows a selection of “aids to production”. On the right are four classical tee-bolts, which can be made in any length required. A perfectly legitimate descendant of the type used on a milling machine, but guaranteed to reveal the weakness of the lathe. If used on the cross-slide slots, any reasonable tightness of the nut may cause sufficient distortion of the cross-slide to stiffen up its movement. It may not affect accuracy, but is a nuisance. If pulled up TOO tight there is also a risk of fracturing the edge of the slot. (That is why the two longer bolts. homemade, have thin heads, so that they will give way first)

Some essential accessories. Note the Tee-slot bars and the Picador adjustable packing blocks
Fig. 1. Some essential accessories. Note the Tee-slot bars and the Picador adjustable packing blocks

A preferable arrangement is the TEE ­BAR, seen at the top and left of the photo. These are machined from rectangular strip to be a reasonable fit to the slots, and then tapped, 1/4 or 5/ 16 inch BSW or BSF, to fit set-screws or studs. That on the left, carrying two allen screws. is made to fit a machine vice. These bars distribute the load in the slot much better and cause negligible distortion of the slide. The two “dogs” in the lower half of the photo are familiar enough. I have a set, but I must confess that they turn out to be too clumsy more often than not, and my “bit box” is full of odd clamp bars etc. which are often preferable.

In the centre are shown two pairs of packing blocks. obtainable commercially, which I have found to be great time savers. They replace the odd blocks of this and that necessary under clamps etc. and being of a relatively soft alloy the risk of burring the surface of the slide is diminished.  Fig. 2 shows a pair of PARA LLELS. These are used   under already machined surfaces of the workpiece to support it at the required height. Most of mine are simple rectangular bar, checked (and occasionally filed or scraped) to be parallel, but this pair is slightly different. They are ‘T shaped and have holes through. They are truly parallel both ways, so that they can be laid on their side with a passage through for any securing bolts holding the work above. There is absolutely no need for such to be hardened and ground so long as you take care of them , remove burrs from time to time, and occasionally check that they remain parallel.

Steel parallels, "H " shaped, with holes through the web
Fig. 2. Steel parallels, “H ” shaped, with holes through the web

VICES. The machine vice is the most con­venient workholder , subject to being able to hold the vice itself. We have already seen that the arbor carried between centres may interfere with the vice, but in all other cases there is usually some way of securing it. Fig. 3 shows two vices, the larger one having counterbored holes through the slides to accept the allen screws associated with a pair of tee-slot bars.There is also a recess in the side into which conventional clamps can be fitted. The two tapped holes on top of the jaws are for attaching auxiliary vee jaws.

1t inch and 3 inch machine  vices. Note the holding down arrangements
Fig. 3. 1 1/2 inch and 3 inch machine vices. Note the holding down arrangements

The smaller vice has a bar secured to each side – a machine screw at each end and a couple of dowels in between to take the shearing load. Two tee-bars like the longer shown in Fig. 1 are set across the vice in the slots, and the heads of the screws with hefty washers just engage with these bars. Both of these are sold as good class drilling vices, and very little work was needed on either to ensure that the jaw faces were square, both across the vice and to the base. A little more work was needed on both to ensure that work sitting down on the base was parallel to the surface on which the vice sat, but this was only a case of gentle scraper work. The result is that if the vice is set square there is little doubt about the work being square also. Which leads to an essential, but not illustrated, accessory; the rawhide mallet. It is IMPERATIVE that work gripped in the vice should be tapped down into the jaws, as the act of gripping usually displaces it by the odd thou. or so. Packing underneath will usually be found to be loose until this is done.  However, some  caution  must  be  adopted,  as  too heavy a blow may move the vice under its clamps.

ANGLEPLA TES. This is almost an essential,  and  more  than  one  may  well  be needed. In most articles the “open ended” type, machined inside and out. is usually recommended, but  I must  confess  to liking the  added stiffness  which  arises from  the end ribs. Machined ends are, however, essential if the angleplate is to be used for marking out. Fig. 4 shows a typical application,where a steel sub-base has been bolted to the angleplate to carry the work. The ADJUSTABLE ANGLEPLATE , Fig. 5, has many applications, but in the “obvious” setup there is a limit to the thickness of the work which can be carried, as it may come above the centreline of the cutter.

An angle-plate in service with an auxiliary workholding plate
Fig. 4. An angle-plate in service with an auxiliary workholding plate
An  adjustable   angle-plate
Fig. 5. An adjustable angle-plate

VEE-BLOCKS. These  have  more  uses than the natural one holding circular work, as  shown  in  Fig.  6.  Here  an  engine cylinder, already  bored, is carried  on  a mandrel supported in the vees and clamped down whilst the portface is machined. Almost invisible is the toolmakers’s jack (a simple support screw) which obviates any risk of the work rotating on the mandrel. These are tool­makers’ vee-blocks. ground on all surfaces and can be relied upon to be ··matched”, so that they can be used as parallel packing if need be. However, the photo shows one of my mistakes – there should be a piece of paper between the blocks and the top of the cross-slide; desirable at all times. but essential if any accessory is bolted down.

Use of vee-blocks and arbor. Note the jacking screw - see text.
Fig. 6. Use of vee-blocks and arbor. Note the jacking screw – see text.

A few examples of what might be called “plain” workholding are seen in the photos. Fig.7a and b shows the use of two angleplates together to hold a large engine base. This is about as large a piece as can be managed in a 3 inch lathe without  resetting, and even as shown there was slight interference at the end of the cut. Fig. 8 shows two arches simply clamped down to the topslide to machine the ends: both at one setting, whilst Fig. 9 is the facing of a steel block. Fig. 10 shows a machine vice holding a block of cast gunmetal being roughed to square and Fig.11 one of the few applications of the arbor supported cutter. Here the machine vice is carrying four square bars being faced to equal length.

7a and b. Two dissimilar angle-plates in use to hold large work
Fig. 7a and b. Two dissimilar angle-plates in use to hold large work. The fly cutter of image here on fig. 27 is in use
A simple setup for squaring the foot of a pair of arches
Fig. 8. A simple setup for squaring the foot of a pair of arches
Facing the edge of a steel plate using simple packing on the cross-slide
Fig. 9. Facing the edge of a steel plate using simple packing on the cross-slide
Use of  the  vice on an angle-plate; squaring up a GM casting
Fig. 10. Use of the vice on an angle-plate; squaring up a GM casting
A side-and-face cutter squaring off the ends of four bars at once
Fig. 11. A side-and-face cutter squaring off the ends of four bars at once

THE VERTICAL SLIDE. While a great deal of milling can be done using the vice, angleplate and vee-block alone there is no doubt that the absence of movement in the third plane – vertically- is a great hindrance. The vertical slide remedies this deficiency and though even the strongest is relatively flimsy it literally “makes all the difference”.  Fig. 12a and 12b show the two Myford variants. That on the left is the ”fixed” slide; movement is possible only in the vertical direction. On the right is the ‘swivelling” type, which can be tilted over so that the slide movement can be at any angle to the cross-slide top face. In addition the who le issue can be swivelled round about its base fixing.

The Myford Fixed or "plain " vertical slide, set on the MYFORD raising block.
Fig. 12(a). The Myford Fixed or “plain ” vertical slide, set on the MYFORD raising block. Fig. 12(bJ. TheMytord swivelling vertical slide

The fixed slide is the more rigid,but the swivelling type has a wider table and facilitates setting up. Both will travel to about 1 inch below the top surface of the cross-slide. The fixed slide has a total travel of about 4 inches, that of the swivelling slide  being 3 inch. There are arguments in favour of each and against each. I have found that occasions when the swivelling feature is essential are rare, but when they are needed then the fixed slide  is very  difficult  to  use  as  a  sub­stitute, sometimes  impossible.  On  the other  hand, the  additional  travel of  the fixed type has more than once saved the day.  I managed with just  the swivelling slide for some 30 years, but since I bought the fixed one as well I seem to have used it the more often.

Fig. 13  shows  the  Edgar  Westbury vertical slide, which also incorporates a dividing  and  milling  spindle  (dealt  with later).  The   castings  for   this  are   still marketed  by Woking  Precision  Models Ltd, and there are no difficult machining operations  involved  in  its  manufacture. The built-in dividing and milling feature is very useful indeed, and though the unit is heavy this is no disadvantage when milling; mass is the most effective vibration damper there is !

The Westbury combined vertical slide, cutter spindle and indexing head. (Photo Woking Precision Models
Fig. 13. The Westbury combined vertical slide, cutter spindle and indexing head.

In setting up all vertical slides it is essential to see that the reference faces are square. and I would advise the checking of any NEW purchase to ensure that the slide itself is square to the base. An error here is very unlikely, though it CAN happen – but do not forget that it will have been made to a tolerance , and better than 0.001 inch per foot is good! The use of a sheet of paper between the slide and the cross-slide of the lathe is advised. This both neutralises the effects of any slight bruises on either (which you really ought to have seen to as soon as they occurred. of course!)  and  gives a better frictional grip.·A similar piece of paper between vice and slide is worth while,too.

Examples of machining operations are given  later  in  the  book.  but  Fig. 14 illustrates the advantage of the vertical slide quite clearly. The portface of this cylinder could easily have been machined by  using  either  an  angle-plate  or  by setting it up on the cross-slide,but in both cases either a flycutter or a fairly large endmill would have been needed. As it is it can be machined across taking two or three traverses  with a small and cheap cutter. Further.at the same setting it will be possible later to mill out the steam ports. The vertical slide does. in effect. convert the lathe to  a milling machine, albeit with the disadvantage that the work supporting table Is vertical instead of horizontal.

Facing a cylinder portface using the vertical slide.
Fig. 14 Facing a cylinder portface using the vertical slide.

BLOCKS OF WOOD. Nobody ever pays any attention to me when I extol the virtues of the “little bit of wood” as a workholding device, but it is one of the most useful expedients when nothing else will serve. The example in Fig. 7 above, for Instance. could have been set up in half the time by drilling two or three holes in a 4 x 4 inch chunk of fencepost, bolting this to the cross-slide, machining the front face with a flycutter (at about 500 ft/min and lots of top rake) and then securing the work with woodscrews . (I have been using thick  double-sided  tape  in  addition  to woodscrews for this type of fixing. with great improvement in the hold). The point here is that you have machined the face dead true to the cutter and the rough casting can be secured just as easily on the wood as on metal. The one limitation is that you must not (if using a normal wood) expect the setup to stay true over a period. as moisture in the air w ill cause the wood to “move”. I am fortunate in having a stock of very closegrained wood Lignum Vitae, Boxwood, etc, – which “stays put” and have even made a perma­nent machining fixture from Lignum. But I have no hesitation in using a piece of old scaffold-plank if the job can be finished within the day.

GENERAL. In all clamping operations the strength of the workpiece must be kept in mind, and this applies particularly when holding down castings. Even if the base has been filed up a little there is just a risk that the bottom may be concave,convex, or “rockable”. Clamping may well cause distortion which will be reflected in the newly machined surface as soon as the clamps are removed. This is an additional reason for using a layer of paper, and if this is reasonably thick and soft it can ease the effects of the odd pimple.

All forms of clamping other than a vice rely on the friction between work and table to resist cutting forces – even with a vice this can be true also. It is prudent to look at the direction in which these forces will operate and provide a stop-bar or “sprag” to provide a positive location. Sometimes one of the clamp-bolts can serve with a piece of wood between it and the work, but if a tee-slot is conveniently near then a  bar dropped in backed by packing is most effective. Note,however. that you must take care that any such stop does not interfere with squaring up work.

I have already referred to the risk with tee-headed bolts.both in stiffening up the slides and o,f breaking away the edges of the tee-slot. I much prefer the tee-bars, but whichever is to be used it is only reasonable to ensure that as much of the limited  clamping  force  as  possible  is applied  to  the  work, and  least  to  the packing.  Fig. 15  shows  the  principle. though it has to be accepted that tee-slots seldom come in the right place to allow all bolts to be so sited. Note that the packing should be taller. but only a little taller. than the work. If the reverse applies, the act  of  clamping  can  move  the  work endways.

Correct clamping. The bolt should be as close to the work as possible, and the packing slightly taller than the work
Fig. 15. Correct clamping. The bolt should be as close to the work as possible, and the packing slightly taller than the work

Workholding and setting up in milling is often a fairly long job – far longer than the actual cutting – and then is the temptation always present to skimp on this part of the process. This must be resisted,as if the work is not sufficiently rigid the result is seldom less than disastrous. The slight­ est movement of the work  allows the cutter to take charge and it will, at best, chew out great chunks of metal in the wrong place and at worse cause actual damage to the machine. If an awkward workpiece cannot conveniently be held it may be necessary first to make a sub­sidiary workholder. Fig. 16 is a case in point, where a piece of half-inch steel flat has been drillled and tapped on one side to take workholding screws and on the other to secure it to an angleplate. (This is the front view of  Fig. 14) . Incidentally, this photo shows one way of setting up to centrelines. The point is brought to the marking out lines with the machine running slowly (so that the centre-point runout causes no error) and the work adjusted until it is In line. Note the white­ faced backplate which reflects light onto the work.

A "Subsidiary Workholding Plate". The point in the chuck is used to set up the workpiece on lathe centre-height
Fig. 16. A “Subsidiary Workholding Plate”. The point in the chuck is used to set up the workpiece on lathe centre-height

To deal with the multitude of jigs and fixtures which may be used to hold work would take up a very great deal of room. Indeed, those who have been at it a long time often have a job finding room in the workshop for them I But your own ingenuity will enable you to work them out for yourself when you do come up against the “cannot be held” type of workpiece. Keep the main principles in mind-reference lines or planes to locate the work relative to the axis of the cutterand, preferably, a further plane or mark from which to establish the correct cutter depth. As I shall emphasise later. it is far better to work to co-ordinates,using feed­ screw dials, than to work to marking out lines – provided,that is, that your cutter is not running out. lines are VERY difficult to see when milling, and it is an unfortunate fact that the set-up on the lathe usually means that your head has to be just where your hair may get caught up in the works. Take this seriously – a few lost hairs may not matter, but “scalped by moving machinery” is an official entry in the Factory Inspector’s annual returns.If a rotating cutter-chuck does get hold of your hair it can, quite seriously, take the top of your head off. If you find you can’t see properly, keep your head out of the way,and use a mirror. I mean it!

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