A common expression to be found among the description of new lathes is the one that says “the carriage has a long bearing on the ways.”  Long is a relative word, and the only place I have seen any long slides among the lathes in the market is in the advertisements.  But if any one has the courage to make a long one, they will need something besides material to make a success of it.  It needs only that the guiding side that should be long, and that must be as rigid as possible—­nothing short of casting the apron in the same piece will be strong enough, because with a long, elastic guide heavy work will spring it down and wear it away at the center, and then with light work it will ride at the ends, with a chattering cut as a consequence.

An almost endless and likely profitless discussion has been indulged in as to the proper way to guide a slide rest, and different opinions exist.  It is a question that, so far as principle is concerned, there ought to be some way to settle which should not only govern the question in regard to the slide rest of a lathe, but all slides that work against a torsional resistance, as it may be called—­that is, a resistance that does not directly oppose the propelling power.  In other words, in a lathe the cutting point of the tool is not in line with the lead screw or rack, and a twisting strain has to be resisted by the slides, whereas in an upright drill the sliding sleeve is directly over and in line with the drill, and subject to no side strain.

Does not the foregoing statement that “the propelling power should be as near the resistance as possible, and the guide be as near in line with the two as possible,” embody the true principle?  Neither of the two methods in common use meets this requirement to its fullest extent.  The two-V New England plan seems like sending two men to do what one can do much better alone; and the inconsistency of guiding by the back edge of a flat bed is prominently shown by considering what the result would be if carried to an extreme.  If a slide such as is used on a twenty inch lathe were placed upon a bed or shears twenty feet wide, it would work badly, and that which is bad when carried to an extreme cannot well be less than half bad when carried half way.

The ease with which a cast iron bar can be sprung is many times overlooked.  There is another peculiarity about cast iron, and likely other metals, which an exaggerated example renders more apparent than can be done by direct statement.  Cast iron, when subject to a bending strain, acts like a stiff spring, but when subject to compression it dents like a plastic substance.  What I mean is this:  If some plastic substance, say a thick coating of mud in the street, be leveled off true, and a board be laid upon it, it will fit, but if two heavy weights be placed on the ends, the center will be thrown up in the air far away from the mud; so, too, will the same thing occur if a perfectly straight bar of cast iron be placed on a perfectly straight planer bed—­the two will fit; but when the ends of the bar are bolted down, the center of the bar will be up to a surprising degree.  And so with sliding surfaces when working on oil.  If to any extent elastic, they will, when unequally loaded, settle through the oil where the load exists and spring away where it is not.