The Elements of Milling
Authors: Boston O. W. , Kraus Charles E. 
Published: April 2023

This paper presents the results of laboratory milling tests on a variety of materials, using a pendulum-actuated single-tooth milling dynamometer. Tools of various widths having front-rake angles of 15 deg., but no side-rake or helix angles, were used. Tests were run both dry and with a number of cutting fluids. Except where deliberately allowed to dull, the cutters were considered sharp at all times.

The cutting was done with the cutter rotating in a plane perpendicular to the surface of the work, and in all cases data for two fundamental methods of cutting are shown: first, milling up in the regular way with the work being fed into the cutter, and second, milling down with the work fed with the cutter. In nearly all cases a difference in energy is found between these two methods of cutting.

Data giving the energy in foot-pounds necessary to remove a single chip are shown for thirteen widely different materials with constant feed and various values of depth, and then with constant depth and various values of feed. Several different cutting fluids were used. These data are plotted and energy equations are derived for each material, of the form E = Cwfxdy, where E is the energy in foot-pounds per chip, C a constant for the specific condition, w the width of the cutter in inches, f the feed in inches, and d the depth of cut in inches. Values of the constants are given. Corresponding formulas and values for the horsepower per cubic inch per minute are also given. No two materials give the same values of the exponents for the feed and depth variables.

Data are also shown for milling with the cutter in the sharp, dull, and very dull conditions, to determine the effect of the sharpness of the tool on the milling-energy formulas. It is found that dulling does change the formula, but has greater effect on feed than on depth of cut.

Tests were run on four materials using eleven different cutting fluids to determine the effect of cutting fluids on milling energy. It is concluded from the nature of the tests that lubrication rather than cooling has greater effect. The data show a different order for the fluids for each material, but in nearly all cases a power saving is made by their use.

Milling-energy data are also presented for a large variety of materials when the cut was held constant at 0.125 in. depth, 0.010 in. feed, and the cutter was 0.250 in. wide and had a 15-deg. front-rake angle, in order to compare these materials from a milling-energy standpoint. It is concluded that there is no way, other than actual test, to determine the milling properties of a material.

A discussion of previous milling papers has been added, and an analysis of the form of a milling chip is presented. It is concluded from this analysis that, theoretically, the coarse-tooth cutter is better than the fine-tooth cutter, when milling up, keeping the linear feed per tooth constant. When milling down, however, the fine-tooth cutter removes a chip of greater average thickness than the coarse-tooth cutter, and consequently is considered to be more efficient from a power standpoint when diameter and feed per tooth are constant.

The fine-tooth cutter cutting down produces the greatest average thickness of chip of all combinations, and is thought to be the most efficient way of milling metal when power is the chief consideration. It is shown that the average thickness of chip when both feed and depth of cut are varied is not a satisfactory basis for an energy equation.