The lathe Is by far the most versatile of all machine tools. Though developed as a producer of “solids of revolution”. working between centres, the introduction of the headstock – a very long time ago – rendered the production of flat surfaces possible and “surfacing” techniques are mentioned in almost every written record of the lathe. When turning became a leisure occupation – about 400 years ago so far as we know – the wish to apply decoration to plain turned work resulted in the development of a number of auxiliary devices. Some, like the “Rose Engine”, were effectively a different machine, and need not concern us. But one device, introduced very early, is what was known as the “drilling instrument”.
Fig. 1 shows that fitted to a lathe made by Holtzapffel in 1805. The tool holder is removed and the spindle set in its place. Driven by an “overhead” gear from the treadle and used in conjunction with the dividing circles on the mandrel pulley (this. too, being a very early device) patterns of holes could be produced both on the surface and on the cylinder. The drills could be plain or profiled (some are shown in Fig. 2) and quite elaborate patterns could be produced. The “instrument” could also be used to produce flutes, of plain or complex profile, by traversing the spindle using the feed screw of the slide rest. In conjunction with the Ornamental Chucks, already highly developed even at that time.quite remarkable work could be done.
Whilst the origin of the Drilling Instrument is unknown, the introduction of what might be termed “revolving tool cutters” · is certainly due to Holtzapffel. No doubt he was influenced by the ‘Wheel Cutting Engines” (which were not lathes) of the clock-maker. The first two to be applied to the lathe were the “Vertical ” and the ”Horizontal”‘ cutting frames, the adjective in each case referring to the plane of revolution of the cutter,Figs 3 & 4. Again, the cutter itself could be plain or profiled. We need not concern ourselves with the uses for which they were devised,but it is clear that normal slotting and grooving could be done and that the vertical frame could be and was used as a gear-cutter. These two were followed very quickly by the “Universal” cutting frame, of which Fig. 5 is a relatively modern example (Birch, c.19 12). This accepts the same type of tool. but the plane of revolution can be set at any angle. with obvious advantages. (This example is geared, but most were direct driven).
The next development (possibly about 1825?) was perhaps the most ingenious and certainly the most valuable to the ornamental turner, and has its lessons for the model engineer as well. This was the “Eccentric” cutting frame, Fig. 6. It is, in effect, a micrometer adjustable fly cutter, and it speaks much for the work of Charles Holtzapffel that it was calibrated on the index to 0.005 inch. Using cutters identical to those for the other cutting frames (Holtzapffel was a pioneer of such standardization) its powers were very great indeed. Again, we need not be concerned with “Ornamental ” work, but suffice it to say that used in conjunction with the mandrel dividing index it can generate accurate polyhedral solids. and even a perfect hemisphere.
It is interesting to observe that the use of such cutting frames was confined entirely (so far as we can tell) to the amateur ornamental turner. No doubt the needs of the engineering industry of the day were such that there was no call for such devices. However.it is interesting to find that the first application of a rotary tool seems to have been devised for the making of special hexagon nuts – for a modell Fig. 7 shows the arrangement devised in 1829 by James Nasmyth when working with Maudslay on the manufacture of special collar-nuts. These were needed for a model of one of Maud- lay’s large marine engines. The cutter is described as a “circular file” and was carried in the lathe mandrel, while the circular indexing device was mounted on the slide rest. This was so successful that a larger machine was purpose-made for the works, for use on nuts for the full-size engines. Perhaps not ·the first (and certainly not the last) occasion where the “model engineer” has led the way!
The amateur ornamental turner, and the model engineer if he had such a lathe. now had facilities for producing flat surfaces, cutting slots, gear forming and similar,for fluting, and for the generation of accurate prismatic solid shapes. Naturally the ornamental turner was most concerned with “decoration ” but as the practice of making engineering models “for fun” developed more and more use was made of these facilities, but adapted to the small engineer’s lathe. The cutting frames were made more robust (Fig.8 is a device by Britannia of 1880) and the “turner” became accustomed to using milling cutters on his lathe. The dividing circles on the headstock pulley were less. elaborate but the associated index was made strong enough to meet the higher forces involved in cutting metal. But large flat areas were produced by filing,on hand planing or shaping machines, or on the lathe faceplate. The majority of the milling work done on the lathe used relatively small cutters. In Volume 1 of Model Engineer, October 1898. a prize of £2 was awarded to a Mr. R.B.Matthews for an article “The Lathe as a Milling Machine”. His devices included the drilling spindle, a robust eccentric cutting frame, a vertical cutting frame, an arbor for carrying cutters on the lathe mandrel. and an indexing workholder .
LIMITATIONS OF THE MACHINE
Writers often refer to the “disadvantages” of the lathe when used as a miller, but true disadvantages are few. The chief is the fact that the lathe lacks one of the essential movements. Work can be traversed across the axis of the mandrel and along it, but not in the vertical plane. This means that an extra vertical slide must be added if full use is to be made of milling processes. The second disadvantage is that except when carrying cutters on an arbor between centres (a relatively rare operation) all work has to be attached to a vertical surface, which makes for difficulties in setting. On the other hand, most modern lathes do possess one feature found on few small millers – a considerable speed range. Many model engineers’ lathes can be opera ted at speeds between 25 and 2000 rpm, and almost all can be run at 40 or 50 rpm. This, as we shall see later,is a very important attribute.
The machine has. however, considerable LIMITATIONS. The first and most serious is lack of RIGIDITY. All milling involves a continuous sequence of shock loads, and the lathe is not designed to withstand these. Even a small milling machine will have some 20 to 25 sq. ins. of slide way bearing surface, whereas it is a good 3 Inch lathe which provides 10 sq.ins. on the saddle, with less on the cross-slide. (And even less still on any vertical slide). The mandrel housing of a miller is very robust compared with that available in a lathe headstock. and there is no comparison between the rigidity of the bed of a lathe compared with a miller of comparable price.
The power available at the lathe mandrel is small ; it is true that milling machines designed largely for amateur use may have about the same power but this is probably due to “the market” having become accustomed to that limit. But it does mean that many commercial cutters will be far too large for use on the lathe.
The third limitation is the “daylight'” available. Using the word “tall” to indicate length along the axis of cutter rotation,the fact that a lathe can accept perhaps 12 Inches between work and cutter face is irrelevant; the overhang is too great. whereas with light cuts the same workpiece could comfortably be carried on the horizontal table of a miller. Again, there is but limited space between the top of the cross-slide and the lathe axis – perhaps a couple of inches. A comparable milling machine would accept 6 inches. This situation does limit the work which can be done, especially in facing large castings.
These latter circumstances are, perhaps, not “serious”, for much model engineering work will fall within the limit of dimension so imposed. But the limit of power and rigidity does mean that even a half-inch end-mill cannot be used at its full capacity on mild steel or cast iron. The crux of the matter is, then, that rate of metal removal must be adjusted to suit the machine. Provided this is always borne in mind (and assuming that the work can be accommodated in the space) there Is little that can be done on a proper milling machine that cannot be done on a lathe; it will just take a little more time, that is all.