American machinists handbook and dictionary of shop terms

Every man engaged in mechanical work of any kind, regardless of his position in the shop or drawing room, frequently requires information that is seldom remembered and is not usually available when wanted.

With this in mind it has been our endeavor to present in convenient form such data as will be of value to practical men in the various branches of machine work. While some of the matter included may seem elementary, it was considered necessary in order to make the work complete. Much of the information has never before been available to the mechanic without tiresome search and consultation.

We believe that the Dictionary section will be found of service to the younger mechanics and in helping to establish standard names for various parts which are now more or less confused in different sections of the country.

Our indebtedness to various manufacturers and individuals is hereby acknowledged, and in the back of the book will be found a list of such authorities with page references to the information furnished by them.

We dare not hope that no errors will be found and we shall be glad to have them pointed out and to receive any suggestions as to additions or other changes which may add to the value of the book.

The Authors.

The determining of the proper feeds of milling cutters in the past was usually a matter of guesswork, or experience, as a good many would term it, no absolute rule of any kind having ever been established.

A guide for determining the proper feed of milling cutters is found in ascertaining the thickness of the chip per tooth of the cutter.

Taking, for example, an average size milling cutter working in cast iron, say 2 1/2 inches diameter, 3 inches long, with eighteen teeth, which is quite commonly used, and it will be found that the thickness of the chip per tooth is quite small, resulting in .0018 inch, with a table feed of 2 inches per minute. This is entirely too slow. Now, comparing this cut of .0018 inch with a lathe tool cut, it will be seen that such a chip in a milling cutter is much smaller and is far more injurious to the cutter than a heavier feed, since the cutting edge of a tool will hold up longer in cutting into the metal instead of scraping it.
 

Cutting Speeds

The Brown & Sharpe Mfg. Co. recommends a cutting speed of 65 feet per minute for carbon and 80 to 100 feet per minute for high speed milling cutters under average conditions. On soft cast iron, having a tensile strength of about 13,000 pounds - the feed recommended is 0.148 inches per revolution or about 9 inches per minute for carbon cutters. With a medium cast iron of about 23,000 pounds tensile strength, the same speed is maintained but the feed reduced about 1/8 or to 6 inches per minute. For high speed cutters the feed can run up to 0.26 inch per revolution on the softer iron.

It is not always advisable to maintain the highest cutter speeds as a slower speed and heavier feed will prevent vibration and chatter. These are not maximum results but. can be attained under regular working conditions. The horsepower required for removing a cubic inch of metal per minute on the milling machine may be safely considered as if horsepower for steel and f horsepower for cast iron.


The Action of a Milling Cutter

Experiments carried on with cutters at the works of the Cincinnati Milling Machine Company and extending over several years, have led to results of general interest. These tests covered milling cutters of various types.

The action of the ordinary milling cutter is not a true cutting action, as it is commonly understood. By a true cutting action is meant the driving of a wedge-shaped tool between the work and the chip and, although this definition is not based on a generally accepted meaning of the term, it is believed that it expresses fairly well what most mechanics understand by cutting. Practically all milling cutters have their teeth radial and this, of course, excludes the possibility of driving a wedge between chip and work. The tooth compresses the metal until it produces a strain great enough to cause a plane of cleavage at some angle with the direction of the cutter. It then begins to compress a new piece, push it off, and so on. This at least seems to be the action of the cutter, judging by the form of the chips. These chips are in the form of needles or small bars.

The chip taken by a milling cutter varies very materially from those taken by a lathe or planer tool. These latter tools make chips of uniform section, whereas the section of a milling chip increases from zero to a maximum.


Form of Tooth in the New Cutter

The foregoing considerations led to a gradual evolution of spiral milling cutters. At first, the number of teeth of spiral mills was only slightly diminished, as it was thought that some element which was not considered might affect the result. Gradually the spacing was increased and the cutters, as now used, have taken the forms shown in Fig. 3.


The Finish of the Work

It is a common belief that better finish can be obtained with teeth closely spaced, but experience with the wide-spaced cutter shows that there is no ground for this belief. The grade of finish may be expressed by the distance between successive marks on the work. These marks are revolution marks and not tooth marks. It is practically impossible to avoid these revolution marks. They are caused by the cutter not being exactly round or quite concentric with the hole, by the hole not being of exactly the same size as the arbor, by the arbor not being round, by the straight part of the arbor not being concentric with the taper shank, by the taper shank not being round or of the same taper exactly as the taper hole in the spindle, by this taper hole being out of line with the spindle, by looseness between the spindle and its bearings, etc. Each of these items is very small
in any good milling machine; yet the accumulation of these little errors is sufficient to cause a mark, and this mark needs to have a depth of only a fraction of a thousandth of an inch to be very plainly visible. As these marks are caused by conditions which return once for every revolution of the cutter, it is plain that the spacing of the teeth can have no effect on the distance between them and, therefore, on the grade of finish. This has been proven by actual tests.


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American machinists handbook and dictionary of shop terms