Experience has demonstrated that accurate control of work and tools not only adds to the durability of the tool and accuracy of the product, but this condition makes it possible to leave not only a true surface, but a smooth surface when taking a relatively large cut.
Now, this is not a point at which to throw down the book and say that this is theory, that the finishing cut must be a light cut, and that the writer is now leaving the bounds of reason and practice. Experience with the former types of lathes has proven that finishing cuts must be light cuts, or, in other words, a cut that is lighter than the heaviest cut that we can take with the same machine.
Now, this statement is true, although the light cut for the ideal machine would be considered a heavy cut in the old machine, for in the ideal machine both the roughing cut and the finishing cut should be larger than the respective cuts in the former types of machines; the only practical limitation should be in that class of work where, owing to the distortion of the metal on removing a large proportion of its surface, it is necessary to take a light cut for finishing.
Firm control makes it possible to use sharp cutting tools ; that is, tools with slight clearance and plenty of rake, the rake being principally in the direction to make easy the flow of the metal from the largest part of the chip. This tool, when used without back rests,should have the corner slightly rounded, and the part of the tool that leaves the final surface should have sufficient width to cover two or more times the width of the feed, and the proper shape to gradually reduce the thickness of chip as it approaches the finishing part.
We all know the shape of the ideal tool for removing plenty of metal, and at the same time leaving a smooth metal surface.
We have all seen the other extreme where a blunt tool has been made to tear off metal in a powerful machine, in which the finished surface looked as if the metal had been pulled out by the roots.
Former practice seemed to establish that a comparatively blunt tool should be used in taking a heavy cut. Since this point is more fully explained further on, it is only necessary here to say that the blunt tool is undoubtedly the best for the machines in which it is used, on account of the excessive side quivering, which would quickly destroy the edge of a properly shaped cutting tool.
Other tool shapes are more fully discussed elsewhere; in order to bring out the importance of correct design in the machine we are here considering only those tools that must depend on the firmness of control of the machine without aid of back rests or steady rests.
For the purpose of setting forth the common method of controlling both the work and tools, it is necessary to call attention to the present scheme of work and tool carriages. In doing so it will be necessary to say some unkind things about everybody's friend, the standard engine lathe; not that it is the standard engine lathe alone that is borne in mind, for remarks regarding this machine may be readily applied to other tool mountings. We recognize that the engine lathe has been the machine from which all of the wonderful mechanism of the age has come, and although in some respects it is felt that there are other machines of higher development, there is no machine designer so near the top of the ladder at present that he has any occasion to "holler for more ladder."