This title is given to attachments which provide a means of driving a rotating cutter other than the lathe mandrel. (Those which provide means of indexing the work relative to the headstock will be dealt with in chapter 6.) There are two basic types of “Attachment” – those which are normally carried on the saddle, so that the cutter can be moved relative to work held in the normal chuck; and those which are secured to the lathe bed,so that work on the cross-slide can be moved about beneath them. I call the first type “Milling Spindles” to distinguish them from the “Attachment” proper which, in effect. converts the lathe to a small milling machine.
MILLING SPINDLES. These are descen dants of the “cutting frames” and “drilling instruments·· referred to in the first chapter. Perhaps the best-known is that devised and for decades marketed (both as castings and finished units) by the late Mr. G.P. Potts. Indeed, many model engineers refer to “using the Potts” even though the actual one they own is by Arrand or one of the other makers. (Messrs Woking Precision Models Ltd, of 16 Dovecote Road. Aberdour. Fife KY3 OTA bought the patterns from Mr. Potts ‘ executors. and now market the castings.)
The basic “Potts” is shown in Fig. 1. though there is usually a pair of adjustable jockey pulleys attached to the body below the drive pulley. That illustrated has the mandrel bored for 8mm collets, but a No. 1 Morse female taper is an alternative. The spindle can be used to carry either milling cutters mounted on arbors or a small drill-chuck.
A more versatile version is shown in Fig. 2.Here the spindle is mounted on its own columnar vertical slide, the column being bolted direct to the cross-slide of the lathe,thus saving the need to mount a normal vertical slide. (The jockeys just referred to can be seen). Other makes are· similar except in detail – see Fig. 13. The Westbury combined vertical slide, cutter spindle and indexing head here. A very much more elaborate type is the geared spindle, usually associated with precision lathes and primarily used for gear-cutting, where lower speeds are usually needed. Fig. 3 shows two such, by l.Drch; one with a bevel drive,the other with spur gears. These are relatively costly, though many model engineers have built their own. All these spindles need some form of drive mechanism, and this aspect is dealt with later.
The main uses of such spindles is in such work as fluting columns, cutting keyways, drilling rings of holes, machining flats and hexagons on round stock and, of course, cutting gears, ratchets and detents – though many workers make up simple cutting frames similar to the Holtzapffel vertical frame shown in Figs. 3 & 4 in this article for this purpose. In all cases some form of indexation of the lathe mandrel is needed, and this is dealt with in this article. It is also necessary to have some means of vertical movement of the spindle. Those illustrated in Figs 13. The Westbury combined vertical slide, cutter spindle and indexing head here, 2 and 3 above incorporate this in the design,but in other cases attachment to the vertica l slide is needed. It is of some importance that the spindle be set at the exact angle. For helical flutes or when thread milling (see here) this angle must be calculated from the geometry of the helix.
Fig. 4 shows a number of applications, most of which are self-explanatory taken together with the captions. Some of the photographs have been set up specially for this occasion, and the drive belt has been left off as the driving motor would have obscured either the subject or the background. Fig. 4c is especially interesting. Here the Potts spindle is being used in association with the normal turning operations. The workpiece is a small (5BA) bolt having a collared head. It COULD have been made from hexagon bar with a washer soldered on, but somehow this “did not look right”. The bolt is. therefore. turned from round bar and screwed using the tailstock dieholder.
The Potts (not seen) carries a 1/4 inch endmill, and this is traversed across the stock, the headstock being indexed using 60 holes drilled in the chuck backplate. Once the vertical slide is set this can be locked, and the hexagon takes only a few seconds to cut. The work is then parted off. By the use of the topslide index it is possible to follow the procedure on a “production” basis – it was not found necessary to set up any special stops.
SPINDLE DRIVES. An independent drive is needed for any rotary cutting frame, and most people think in terms of the classical “overhead” which was a feature of most amateurs· lathes in the 19th century. Fig. 5 shows one type with the usual drum and jockey pulleys supported on standards. That by Holtzapffel, Fig. 6, could be worked without jockeys , as the spring in the support was usually sufficient. Both suffer from difficulties of inertia and lack of balance, which inhibits the use of any but moderate speeds. Ornamental turners can get over this by using very small pulleys on the cutting frame , but when working metal this leads to belt slip. The Idea of the drum was,of course. that the driving belt could move along this as the work was traversed. In fact, however, provided that the driven pulley flanges are reasonable a fair degree of out-of-line can be accepted, usually quite sufficient to deal with most model engineers’ applications.
Even in the field of ornamental turning the classical overhead is now seldom used. It is found that a simple jockey system associated with a free-standing electric motor on the back shelf of the lathe meets all needs. This makes the setting up very easy and, moreover, does not lock up the use of a motor ;in my case the same motor can be used on three lathes and can also be called upon to drive other plant. The motor Itself (1/4 HP which is more than adequate) is fitted with pads of rubber under the feet, and only rarely is it found necessary to bolt it down.This makes belt tensioning easy- it is only necessary to give the motor a push in the right direction.
Fig. 7 shows a very simple spindle drive, described in detail in Model Engineer 5 Dec 75, page 117 5. and Simple Workshop Devices. Argus Books Ltd. The upright is a piece of 5/8 inch (nominal) bore electr1c conduit, with the bottom couple of inches turned down to fit the socket of the standard Myford handrest bracket The tee-arm is a piece of t inch BDMS brazed into a lump of 1t inch steel. which is drilled to fit the conduit and fitted with a couple of grub screws. The pulley bearing block has two holes at right angles to accept the tee-arm, so that it can be set either way, and carries a pair of 2 inch jockey pulleys retained by split pins. The sketch does not show all the dimensions. as the device can be modified to suit individual needs. The height of the column is relatively unimpor tant,but if it is made more than 100 times the diameter of the belt normally used this will permit movement of (e.g.) a Potts Milling Spindle over six or eight inches. The one problem may be – “How to machine the end of a two-foot tube when the lathe takes no more than 19 inch between centres?” Easy ICut off the tube about an inch longer than is needed. Remove the tailstock. Set up the fixed steady at the end of the bed to carry the tube, which is gripped at the other end in the self-centring chuck. Machine the chuck end of the tube to the required dimensions, and then part off close to the chuck IAs to the pulleys,tl’,.ere is no need for these to be of metal. Hardwood (Boxwood or Lignum) will serve and as a matter of interest the main drive pulleys on Hotlzapffel No. 484, 180 years old almost. are made of mahogany and the machine is still in use. In many ways wood is better than metal for such applications ; but Tufnol is ideal, as it needs no brass bush – it can run directly on the steel spindle.
The group of photographs in Fig. 8 show various arrangements. At (a) the motor sits at the end of the bed and the hand-rest base Is clamped to the bed, the Potts being set as if to drill a register of holes. The motor is clamped in this case, as the lathebed is narrow At 8(b) the pedestal is clamped to the cross-slide itself and the motor is sitting behind the lathe. This arrangement is suitable when most of the movement is across the lathe axis. The full cross-slide travel is possible with.no loss of drive power. 8(c) shows the arrangement when most of the travel is along the lathe axis; again,the pedestal is clamped to the cross-slide, but behind the vertical :slide this time, and the motor sits behind the lathe. The motor has a 4- cone pulley and the speed-range at the spindle runs from well above 3500 rpm down to about 800 rpm.
I use three sorts of belting. Much preferred is 3/16 Inch round leather, which gives a good grip and which is heavy enough to drive even If not really tight . Second, almost as good. 1/8 inch (nominal) cotton rope “long spliced” to an endless band. Third, 1/8 or 3/16 inch braided nylon cord, as used for “pull starters” on chainsaws and the like. This can be “welded” in a match or candle flame . It drives fairly well , but has the disadvantage that there is NO stretch,and it is not easy to keep the drive tight if there is much movement of the driven element. I have tried plastic belting but none of the smaller sizes I have used is very satisfactory – chiefly because of its tendency to “whip about”.
There are many variations to these drives and some practitioners prefer those with an integral motor. Fig. 9 shows a number of views of a Motorised Drive Unit designed by Mr. J. Malcolm Wild, for which constructional details are given in Model Engineer 20 Nov. 81 et seq, starting on page 1382. It will be seen that the motor and an associated countershaft is mounted on the end of a swing arm which can be swivelled into a wide variety of positions. The pillar can either be mounted on the lathe itself or permanently fixed on the bench behind. Toothed vee-belt is used but round leather will serve, at least for the type of application normal for model engineers; Mr. Wild designed the unit primarily for clock-gear cutting. There is a considerable speed range possible due to the countershaft,6 speeds from 145 to 41 50 rpm, though not equally spaced. The motor is 1/8 HP, perhaps a bit on the light side for true milling operations,as there is a fair bit of power loss in the belts and the countershaft and jockey bearings, but quite adequate for its designed purpose. The castings kit is supplied by Mr. J.M. Wild, 12 Norton Green Close,Sheffield S8 8BP.
MILLING ATTACHMENTS. These accessories, either with an integral motor or taking a drive from the lathe headstock, convert the lathe to a true vertical milling machine. The normal movements of the saddle provide transit of the workpiece in two axes, whilst the movement of the quill or spindle of the attachment adjusts the position of the cutter. The full travel of the cross-slide is available but. unfortunately , the travel possible along the lathe axis is limited by the distance between the quill centreline and the body of the attachment. However. it is possible to apply power feed.and this is a considerable advantage; not so much because it saves effort but rather because the cutter very much prefers power feeding!
Fig. 10 shows a fit-up arranged by Mr. (now Professor) Dennis Chaddock some 40-odd years ago. in which he is using his sensitive drill to carry a milling cutter. taking the drive from the lathe headstock. Now. it is true that a drilling machine spindle is not designed to take the side thrust and it is an unfortunate fact that drill-chucks seldom run dead true. Nevertheless the arrangement is quite practical. and especially for the profile milling job shown in the photo. where the travel of the cutter is limited by a guide or template. (This process is dealt with in more detail on page 109). The main problem with such an arrangement is control of the depth swing of the spindle. and if such a device is used for normal milling great care must be taken to ensure that the re can be no inadvertent movement of the quill vertically. Naturally, only very light cuts can be taken.
SELF–POWERED ATTACHMENTS. Fig.11 shows a milling anachment by Amolco (A.N. Mole & Co. Ltd) intended for use on the tailstock end of the lathe bed.This has a substantial spindle. the nose of which matches that of the Myford lathes. so that Autolock or similar chucks, collets, and the usual lathe chucks can be fitted as desired. The spindle is bored for a drawbar. With a 1/4 hp motor and four spindle speeds from 325 up to 1600 rpm reasonable cuts with the normal model engineers· stock of cutters are possible. The limitation is that the “daylight” from spindle centre to column is 5 inches, but this is not all that much less than is found on the smaller ranges of true milling machines. The maximum clearance between the spindle nose and the lathe cross-slide is 8 inches. Vertical travel, by moving the whole of the head on the column, is 6-! inches. One advantage of this type of attachment is that it can be left in place whilst normal turning is in progress provided that the tailstock is not needed. (It is not normaly possible to remove a tailstock other than by sliding it off the end of the bed.)
Fig. 12 is the Myford-Rodney milling attachment. This takes its drive from the lathe headstock, so that the cutter speed can be varied over a very wide range by using the back-gear. There are two vertical movements; the main spindle housing or headstock can be moved up and down by about 3t inches and locked and the spindle or quill carrying the cutter can also be moved about 3 inches. This latter movement can be controlled either by a feed lever for drilling etc, or by a handwhee l/worm device for precise setting of milling cutters. The “daylight” between cutter centre and the face of the attachment is 4t inches and the maximum distance between the spindle nose and the top of the lathe cross-slide is 6 inches. This last dimension will, of course, be reduced by the projection of the cutter. As might be expected, the spindle nose is identical to that on the lathe.A simpler version is available for the ML10 lathe.
Attachments of this type permit milling operations up to the limit of the rigidity of the lathe bed and saddle – and, of course. the available travel of the latter.The motor power available is more than adequate and is not the limiting factor. It is. however, important to keep in mind that the slides are NOT designed for milling work. All movements save that in use for traversing (including the downfeed) MUST be locked whilst cutting and whether using hand or power feed (the latter is strongly recommended) the direction of feed must always be in opposite ion to the cutting forces. This applies particularly when using the cross-slide feed screw, as there is always some backlash in the nut. When plunge-cutting (e.g.with a slot-drill) both slides should always be locked. These limitations apart, the use of such attachments does provide an adequate milling facility for the model engineer and will cover almost all his requirements.
GENERAL. It is impossible to detail all the various types of milling “devices” which have been made by model engineers over the years. Their number is legion,and it is difficult to find a volume of the magazine which does not show some example , ranging from the simplest to the fantastic. Excellent work can be done with simple home-made spindle-s – if cone bearings or even pointed pivots are used there is no difficulty over “slack “, Fig. 12 shows a home-made cutting frame with 2 ¼/1 reduction intended for gear-cutting. perhaps 100 years old, and made by a clergyman. This may give some idea of what can be rigged up – they are no less “proper” than the more elaborate devices so far shown.