Characteristics of Roll Lathes
In its general layout, a roll lathe is similar to a conventional, horizontal engine lathe. However, since the nature of the work to be done is more restricted, roll lathes can be made much more productive than common lathe types. Some of the restrictions and improvements that result are:
- The bed of a roll lathe is much wider than that of a conventional lathe. The four ways on top of the bed are arranged so that the carriage will pass the tailstock and steady rests. Both saddle ways are to one side of both tailstock ways. This means that the top of the bed must have at least three rails to mount the ways, instead of the two found on engine lathes. All this makes the bed of the roll lathe inherently more rigid, especially in the torsional direction.
- The carriage is heavy and blocky to make it as rigid as possible. It cannot have the characteristic H shape found on engine lathes. As a result, the tool will not cut all the way to the chuck, since the headstock is also much larger and covers the bed at its base. Some older roll lathes helped alleviate this problem by setting the tool to one side of the carriage, but this arrangement sacrifices much of what is gained by the heavy construction. In the case of mill work, the ends of the workpiece are seldom cut. In the case of forge and foundry work, the piece must be turned under any circumstance.
- Workpiece thermal expansion compensation for the tailstock quill is necessary, since large work pieces with large amounts of stock removal will heat up beyond the thrust capacity of the bearings. On engine lathes, this seldom happens, and the tailstock is flimsy enough to provide compliance when necessary.
- The tool has limited cross travel and will not cross center without significant overhang. This is due to the general shape of the carriage and its arrangement on the bed. An engine lathe with the H shape can allow the cross slide to go well beyond the center, which is useful in general machining, such as boring. The trade-off for the rigidity of the roll lathe is to limit this type of work.
- The workpiece must be between centers since the chuck jaws are not intended to take quill thrust. Often, the weight of the workpiece is proportionally much greater than what might be found in a conventional lathe of similar size. As a result, special allowance must be considered for taking the required larger thrust at the headstock end if the workpiece does not have a center hole.
- The final characteristic of a roll lathe is its weight: it is very heavy. Machine weight should be double or more than the theoretical largest workpiece. Some manufacturers try to achieve this by filling the bed with concrete, but a proper bed design and weight does not require filling.
In general, it can be said that there really is no comparison between the performance of a roll lathe and that of an engine lathe. The class of work is much different and the productivity rates are not even close. Historically, a number of lathe manufacturers made heavy versions of engine lathes and offered them as roll lathes. All that was accomplished was the creation of what engine lathes should have been in the first place. Some of these machines still exist, but should not be confused with true roll lathes. The careful observer can step back and look at these machines and see the weak, flimsy sections at the tailstock quill, how much it hangs out, and at the cross slide compound, how disproportionate the height and footprint are. Certainly the hard materials of steel mill rolls justify better designs than what is commonly offered.