The extreme cases of the bicycle, the turbine engine and the electric motor are not analogous to the changing character of work in the average machine shop, but every one knows that only the most visionary are unmindful of change required in their own product.
It is the intention here to discuss only one of the phases of the subject, but the principles set forth will be found applicable to a wide range.
In order to mention exact conditions we shall consider the lathe and various machines in which a cutting tool is held and controlled by one part of the machine, while the work is held and controlled by another part, the whole object of these machines being to absolutely control the relative motion of work and cutting tool.
So much attention is being given to the convenience of handling in hand-operated machine tools and the development of automatic features, that we have temporarily lost sight of the greatest of all questions ; and that is, the control under which the work and tool are brought together.
We know that a cutting tool will last almost indefinitely under some conditions, and under other conditions the edge seems quickly destroyed.
The destruction of the cutting edge is not wholly due to the real work of separating material; in fact, only a small amount of wear is due to the work. The greatest edge-destroying action is side motion or quivering, which in the most extreme cases may be heard and felt.
It has been generally assumed that in absence of any marked quivering, such as would be called chattering, that the tool was free from this edge-destroying action. That this is erroneous will be clearly shown later on.
Very few of the standard machine tools hold the work and tool with sufficient firmness of control to prevent this lateral trembling or quivering. Before passing this point, it should be explained that the meaning of lateral motion is that motion which is across the cutting edge of the tool, and not in the general direction of the cut.
The cutting edge of a knife would not last long if used for scraping the surface of a stick; in fact, it would not last long if we were to use the knife in the same way that we use a lathe tool in the average lathe. But the knife edge will last in whittling because there is no scrape across its edge. This is just as true of a cutting tool working in metal, for the lateral motion of the metal across the edge of the tool causes a side pressure against the edge where there is no backing, and causes its wearing away, whereas, when the work is moved against the edge firmly in its true path only, the edge is free from side thrust. For the real work of cutting, the extreme edge is braced and sustained by the backing, and has great durability.
Under correct conditions, the action of a heavy chip of steel on a properly proportioned tool occasionally wears a slight hollow just back of the edge instead of wearing away the edge. This action has been occasionally observed in the engine lathe, where it happens as a result of a balance of conditions which includes an amount of clearance of the tool that just equals the feed, and which allows the tool to steady itself by riding on the finished surface. This prevents the quivering which would otherwise have rounded the edge before the chip could have had time to make an impression elsewhere.