This section is from the book "Lathe Design, Construction And Operation, With Practical Examples Of The Lathe Work", by Oscar E. Perrigo. Also available from Amazon: Lathe Design: Construction And Operation.
It would be very interesting if we could make a table giving the power required to drive the lathe on all different diameters, for all different kinds and qualities of metal, when turned with all different forms of tools made from all different kinds of tool steels, and on all different designs of lathes.
It would, however, be an almost endless task and would be of very little practical value when it had been accomplished. The conditions as noted above, and which are all practical, every-day conditions, are so many and so various that there would be found very seldom a repetition of them in regular machine shop work.
To construct a table that should give the power required for different tools and metals worked upon in a certain shop, it would be necessary to observe conditions, to collect and record data, and to make calculations from these individual conditions and circumstances, in this particular shop. And this table, while of considerable value in this shop, and interesting to any mechanical engineer or shop economist, would not be a safe guide in any other shop until corrected by the data made by an extended series of observations, the time and expense of which would be nearly equal to those necessary to produce the original table.
These remarks are not intended to discourage the desire to obtain such data. It is always commendable to search, observe, calculate, and "dig out" all these and similar facts relating to the performance of machine tools, and such habits should be encouraged in all who have to do with this work. No labor of this kind is lost, since every item of such work adds to the sum total of our information and enriches the subject for us, and gives us a more secure and confident hold on the important questions involved in it.
A still further reason for such observation and the recording of the data thus obtained is the constant changing of the design of machine tools, the constant changing of material to be worked upon, the infinite number of forms of the parts to be machined, and the thousand-and-one differing circumstances of their manufacture.
A recent writer whose name, unfortunately, is not given, in discussing the question of the power required for taking the cuts in different metals and the pressure in the tool says:
"The most complete information on this subject is contained in Flather's 'Dynamometers and the Transmission of Power,' in which are collected data from many tests upon various kinds of machine tools. Since the introduction of high-speed steel, however, conditions have changed so much that the deductions from the tables mentioned would be to a certain extent incorrect. Probably the most satisfactory way to determine the pressure on a tool is to obtain this from the power required to drive the machine when cutting. Knowing the horse-power, if we multiply this by 33,000 we have the foot pounds per minute; dividing this by the cutting speed in feet per minute will give the pressure on the tool, neglecting the power lost in overcoming the frictional or other resistances in the machine itself. In tests upon high-speed cutting steels at the Manchester Municipal School of Technology, to which we shall presently refer, it was found that the power absorbed by the machine varied greatly with the temperature of the bearings and also with the speed. After the bearings become warm, the oil is more viscous, which makes an appreciable difference; and tests also show that it sometimes requires more power to run a lathe at high speed - as would be the case when filing a piece of work - than when taking a heavy cut at a slow speed. These facts indicate the degree of care necessary in arriving at reliable information upon the subject of your inquiry.
Referring to the tests in Flather's text-book, we find the following formulas deduced from average results, which give the horsepower required to remove a given weight of cast iron, wrought iron or steel:
For cast iron, horse-power equals...............
For wrought iron, horse-power equals...........
For steel, horse-power equals...................
"In each of these W is the weight in pounds of the metal removed per hour.
"The most complete information upon power required with high-speed steels is that obtained by the English tests at the Manchester Municipal School of Technology. These are very elaborate and cannot easily be summarized, but the following statement of results will answer our purpose and throw some light on the subject:
Cutting Soft Steel
Weight per Hour
Cutting Cast Iron
Weight per Hour
"Applying Flather's formulas to these results we find that for steel the horse-power required would be 4.6, instead of 3, for light cutting; and 19.6, instead of 15, for heavy cutting. In the case of cast iron we find the horse-power would be 1.1, instead of 1.7, for light cutting; and 5.15, in place of 5.5, for heavy cutting. This would indicate that Flather's formula for steel allows more power for soft steel than was shown to be actually required by the English tests, and will probably give ample power for a considerably harder grade of steel. In the case of cast iron his formula appears to apply very closely, but giving results slightly too small.
From these comparisons it would seem that the rule to multiply the weight of metal removed per hour by .04 would give a safe value for the horse-power for both steel and iron.
Further examination of the results of the English tests shows that with the steel more metal was removed per horse-power when taking a heavy cut than when running at high speed and taking a light cut; while when cutting cast iron this condition was reversed.
It was found that the cutting force did not vary much with the speed, because at high speed the cuts were light while at low speed the cuts were deeper and taken with a heavier feed. The pressure on the tool increased very rapidly as the tool became dull; but when the tool was in good cutting condition the following pressures were recorded: