Page:Practical Treatise on Milling and Milling Machines.djvu/120

114 cutter teeth are first brought into contact with the softer metal, and as the scale on the surface is reached, it is pried or broken off.

On the other hand, in milling deep slots, or in cutting off stock with a thin cutter, or saw, it is sometimes better to move the work with the cutter, as the cutter is then less likely to crowd sidewise and make a crooked slot.

When the work is moving with the cutter, the table gib screws must be set up rather hard, for the teeth of the cutter tend to draw the work in, and if there is any lost motion in the table, the teeth may catch and injure the cutter or work. A counter- weight to hold back the table is excellent in such milling.

With vertical spindle milling machines, when a cutter is working on a flat surface, it does not matter which way the table is fed, but if the cutter is milling a side of a casting, as well as a flat surface, the table should be fed in the opposite direction to that in which the cutter revolves, for the reasons already mentioned. Limits in Milling to Size. The limit for error in size to which work should be milled depends entirely upon the character of the job. With some work, a limit of one-hundredth of an inch is plenty good enough, while many other pieces must be finished to within one-thousandth of an inch of being exactly parallel or straight, as the case may be.

In milling to a given thickness or size, the most accurate results are ordinarily obtained by straddle mills or side milling cutters; for when only one side is milled at a time, and the piece has to be changed from one side to the other, it is hardly practicable to work to a smaller limit than two -thousandths of an inch. Side milling frequently requires more attention to keep the work smooth than ordinary surface milling.

Very accurate milling may be done and excellent surfaces obtained by small end mills running at high speeds.

In all cases where roughing and finishing cuts are to be taken on work, and precision is required, it is best to first remove most of the stock with a coarse feed, leaving enough for a light finishing cut. At a second operation, finish at a higher speed with a feed that will give the required surface.

Some light work will spring when the scale and a thickness of the metal are removed by the roughing cut. It is, therefore, advisable to loosen the holding clamps and permit the piece to assume a natural form before taking the finishing cut; otherwise, whatever inaccuracy