Elementary machine shop practice

The first edition of Elementary Machine Shop Practice was intended as an instruction book for shop use only. The revised edition contains several additional pages of new matter which gives it a wider range of usefulness.

To get the best results from the book the problems described should be made, because the information may then be directly applied while the student is at work in the shop. But in schools where problems of different design are used, or if machine parts are made, it is believed the book will be a great help as a reference. It may also be used to good advantage in classes of technical English.

In case it is considered advisable to devote to the elementary operations less time than would be necessary to complete the problems presented herein, very good results can be obtained if the student will read all of the instructions carefully and then do only such problems as the instructor considers necessary.

The instructions here given are not intended as fixed rules, for it is recognized that some of the operations may be done by other methods with equally good results.

This manual, while it does not cover the whole field of machine shop work, should meet the requirements of beginners in general machine shop practice. Its main object is to reduce as much as possible the time required to bring a student with no previous shop experience to the point where he is able to do some real work. For this purpose the problems have been designed with the view of giving the student the maximum amount of information in the small amount of time usually allowed for this purpose. The repetition of operations has therefore been avoided wherever it was considered advisable and the time lost in simply cutting off metal has been reduced to the minimum.

The instructions given refer mainly to the cutting of metal, since this usually gives the most trouble to beginners. Little attempt has been made to describe the mechanism of the different machines because it varies so much with the type and make, and besides is easily understood by the average student.

It is assumed that beginners will receive oral instruction on the manipulation, such as shifting the belt, handling the feed control, etc., of the different machines. It is suggested that the instructor give a practical demonstration by doing enough work on the problems to show the tools necessary and how they are used.

The machine speeds for the different operations as indicated in this book are only approximately correct, since the actual cutting speed of the tool in feet-per-minute varies with the size and kind of machine used. The instructor is expected to designate the proper speeds, altho the belt connections given herein will generally be close enough for beginners. In learning machine shop work the student goes thru what might be called two stages, i.e., elementary and advanced. In the elementary or beginning stage it will be necessary for him to acquire considerable knowledge or theory. After the fundamentals have been mastered, practice appears to be the more important factor.

Since the progress that a beginner makes depends largely upon the time required to learn the fundamentals, it is important that he study very carefully the directions for making the problems.

Description of Milling Machines. There are several different kinds of milling machines, but the ones most commonly used are the plain miller and the universal miller. Both of these machines have horizontal spindles. The difference between them is that the table of the plain miller can be moved only at right angles or parallel to the spindle, while the table of the universal miller may be moved at different angles. The principal advantage of the universal machine occurs in cutting spiral gears, spiral milling cutters, etc. This class of work, however, will not be treated in this book.

As in the case of lathes, there are several makes of millers, all of which, altho varying in efficiency and utility, employ the same fundamental cutting operations. When one has become familiar with the operation of one machine, he should be able to operate other makes of different design with very little difficulty.

Milling machines like many other machine tools have either a cone pulley or a constant-speed drive. The one shown in Fig. 105 has a constant-speed drive.

The driving pulley is at the left of the machine. The spindle speed changes are obtained by shifting encased gears near the driving pulley. This is done by means of the small pilot wheel and levers on the gear box. The long hand lever at the right of this box operates the clutch for starting and stopping the machine.

The small pilot wheel below, together with the two levers on either side of it, are used for changing the feed. The double-ended V-shaped lever under the table is for starting and stopping the feed. The hand-crank and hand-wheel at the right are for the hand cross feeds and for raising and lowering the table.


Sequence of Operations:
1. Grind off all gates, fins, sand, etc.
2. Clamp in vise and mill side A.
3. Mill sides B and C square with A.
4. Mill side D parallel to A.
5. Mill both ends square with the other sides.

Object of Grinding Rough Casting. The outer surface of cast iron is hard and more or less covered with sand. This would dull milling cutters even more quickly than it would a lathe or planer tool. As milling cutters are expensive and require more time to sharpen, greater care should be taken to protect their cutting edges. It is not necessary, however, to remove all of the scale, but the casting should be thoroughly cleaned.

Milling-Machine Vise. Fig. 107 shows the vise used in the milling machine. It has a graduated swivel base so that it can be turned at different angles.

Clamping the Work. The work is held in the milling machine vise as shown in Fig. 108. It should rest on two parallels that are wide enough to bring the surface to be milled above the level of the vise. In order to get a tighter grip on it, heavy paper should be placed between it and the jaws of the vise. The paper will also protect the jaws from being marred by the rough sides of the casting.


The Cutter. Work of this sort is milled with a common spiral milling cutter, Fig. 109. As these cutters are made in different lengths, one should be selected that is a little longer than the widest surface to be cut.

The Arbor. All milling-machine arbors are provided with collars, Fig. 108, of different lengths, so that the position of a cutter on them may be varied. These collars slip loosely on over the arbor and, when the nut A is tightened, clamp the cutter in place.

If the cutter is mounted near the main bearing of the milling-machine spindle, it will work better than if located near the outer end.

One end of the arbor is tapered and fits into the spindle B. It may be withdrawn by tightening the nut C. The end D is supported by an out port bearing.

Most arbors are also provided with a keyway, so that when heavy cuts are to be taken or a large cutter used it may be keyed to the arbor.

CAUTION: Beginners are cautioned to be sure that the direction of rotation and speed of the cutter and the direction of the feed are correct before starting a cut.

Direction of Cutter Rotation. Milling cutters are all ground with Clearance A, Fig. 110-a, and if rotated in the wrong direction will not cut. Therefore, before mounting the cutter start the machine and note the direction of rotation of the arbor. If it rotates as indicated by the arrow the cutter should be mounted as shown. If the cutter were mounted as in Fig. 110-b it would not cut.



Cutter Speed. The maximum cutting or surface speed for a milling cutter made of carbon steel is about 50' to 60' per minute when cutting cast iron. Cutting speed is the velocity of a point on the circumference of the cutter. Thus a cutter 2 1/2" in diameter, and turning 90 revolutions per minute, will have a cutting speed of about 60' per minute.

No fixed rule can be given for the proper cutter speed, as too much depends on the character of the work, the hardness of the metal, the size of the machines, etc. Machinists determine the proper speeds by the action of the cutting tool and from previous experiences on work similar in character.

It will be necessary for the instructor to designate the speed of the machine until the student has had sufficient practice to be able to judge fairly well for himself. It is better to run the cutter too slow than too fast, for if it is run too fast it will soon be ruined.

Direction of Feed. The direction of the feed in relation to the cutter rotation is very important. Fig. Ill shows the correct way. In this case the direction of the feed is opposite to the rotation of the cutter. The wrong way is shown in Fig. 112. If the work were fed in this manner, the cutter would act as a feed roller and draw the work in faster than it would cut. This would break the teeth of the cutter.

Roughing Cut. The roughing cut on cast iron should always be deep enough to get under the scale; in this case about 3/32" or 1/8" deep. Cast iron and brass are milled dry but on steel the cutter works better if lard oil or some other lubricant is used.

Rate of Feed. As in the case of the cutter speed, no fixed rule can be given for the rate of feed. This should be determined by the instructor. It may be stated, however, that .018" feed per revolution of the cutter should be safe in this case. If a good machine is used and the iron is soft this feed could be increased.

Finishing Cut. The depth of the finishing cut can be anything up to 1/32". If more than this is taken off, the surface may not be uniform. The same cutter speed as for roughing can be maintained but the feed should be increased. If a large surface is to be finished and the iron is soft, it will pay, in the amount of time saved, to increase both the speed and the feed.

After side A, Fig. 106, is finished, the sides B and C are milled square with A. D is then milled parallel with A. The method of clamping the work in the vise is practically the same as that used for the shaper work.

Milling the Ends. If, in milling the ends, the block extends so far above the jaws of the vise that the action of the cutter has a tendency to tip it, turn the vise through 90 and clamp it edgewise as shown in Fig. 113.