Introduction to the Lathe as a Milling Machine

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.

Drilling spindle of a lathe made in 1805, fluting with a form-drill
Fig. 1. Drilling spindle of a lathe made in 1805, fluting with a form-drill.
Ornamental drills
Fig. 2. Ornamental drills, as used in Fig. 1

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 "Vertical'" cutting frame
Fig. 3. The “Vertical'” cutting frame
 "Horizontal" cutting frame
Fig. 4. “Horizontal” cutting frame

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.

A geared "Universal" cutting frame, by Birch
Fig. 5. A geared “Universal” cutting frame, by Birch
The "Eccentric" cutting frame
Fig. 6. The “Eccentric” cutting frame

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!

A collar-nut milling machine devised by James Nasmyth, c 1829. The forerunner of the milling machine.
Fig. 7.A collar-nut milling machine devised by James Nasmyth, c 1829. The forerunner of the milling machine.

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  ModelEngineer, 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 .

A Britannia milling spindle of about 1880
Fig . 8. A Britannia milling spindle of about 1880.

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.

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