Among wood-working machines there are many that cannot be built on the single rectangular box plan—rested on three points of support. Fortunately, the requirements are not such as demand absolute straight and flat work, because in part from the fact that the material dealt with will not remain straight and flat even if once made so, and in the design of wood-working machinery it is of more importance to so design that one section or element shall remain true within itself, than that the various elements should remain true with one another.
The lathe, the planing machine, the drilling machine, and many others of the now standard machine tools will never be superseded, and will for a long time to come remain subjects of alteration and attempted improvement in every detail. The head stock of a lathe—the back gear in particular—is about as hard a thing to improve as the link motion of a locomotive. Some arrangement by which a single motion would change from fast to slow, and a substitute for the flanges on the pulleys, which are intended to keep the belt out of the gear, but never do, might be improvements. If the flanges were cast on the head stock itself, and stand still, rather than on the pulley, where they keep turning, the belt would keep out from between the gear for a certainty. One motion should fasten a foot stock, and as secure as it is possible to secure it, and a single motion free it so it could be moved from end to end of the bed. The reason any lathe takes more than a single motion is because of elasticity in the parts, imperfection in the planing, and from another cause, infinitely greater than the others, the swinging of the hold-down bolts.
Should not the propelling powers of a lathe slide be as near the point of greatest resistance as possible, as is the case in a Sellers lathe, and the guiding ways as close to the greatest resistance and propelling power as possible, and all other necessary guiding surfaces made to run as free as possible?
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.
The tool post or tool holder that permits of a tool being raised or lowered and turned around after the tool is set, without any sacrifice of absolute stability, will be better than one in which either one of these features is sacrificed. Handiness becomes the more desirable as the machines are smaller, but handiness is not to be despised even in a large machine, except where solidity is sacrificed to obtain it.
The weak point in nearly all (and so nearly all that I feel pretty safe in saying all) small planing machines is their absolute weakness as regards their ability to resist torsional strain in the bed, and both torsional and bending strain in the table. Is it an uncommon thing to see the ways of a planer that has run any length of time cut? In fact, is it not a pretty difficult thing to find one that is not cut, and is this because they are overloaded? Not at all. Figure up at even fifty pounds to the square inch of wearing surface what any planer ought to carry, and you will find that it is not from overloading. Twist the bed upon the floor (and any of them will twist as easy as two basswood boards), and your table will rest the hardest on two corners. Strap, or bolt, or wedge a casting upon the table, or tighten up a piece between a pair of centers eight or ten inches above the table, and bend the table to an extent only equal to the thickness of the film of oil between the surface of the ways, and the large wearing surface is reduced to two wearing points. In designing it should always be kept in mind, or, in fact, it is found many times to be the correct thing to do, to consider the piece as a stiff spring, and the stiffer the better. The tooth of a gear wheel is a cast iron spring, and if only treated as would be a spring, many less would be broken. A point in evidence:
The pinions in a train of rolls, which compel the two or more rolls to travel in unison, are necessarily about as small at the pitch line as the rolls themselves; they are subject to considerable strain and a terrible hammering by back lash, and break discouragingly frequent, or do when made of cast iron, if not of very coarse pitch, that is, with very few teeth—eleven or twelve sometimes.