The ordinary taper arbor, known as the mandrel, is in common use in most machine shops. Up to and including a diameter of 1 1/2 inches, mandrels are made of tool steel, hardened all over and ground to size. Some tool-makers advocate making all mandrels up to a diameter of 4 inches in this way; others prefer hardening the ends BB, Fig. 76, leaving the center A soft, while others maintain that for mandrels above 1 1/2 inches in diameter, machine steel is most satisfactory if thoroughly casehardened.

When making mandrels of tool steel that are to be hardened the entire length, it is not necessary to use the best quality of steel; a lower grade will do, if it hardens well. Select stock somewhat larger than finish diameter, say 1/16 inch for sizes up to 1/2 inch, 1/8 inch for sizes up to 1 inch, 3/16 inch for sizes up to 1 1/2 inches, and 1/4 inch for larger sizes. Take a chip off the outside, sufficiently heavy to remove all scale, yet leave 1/32 inch for a finish cut on sizes up to 1/2 inch, and correspondingly more for the larger sizes. The mandrel should now be annealed, preferably in the annealing box. The ends should be countersunk deeper in mandrels than in tools where the centers are not used after they are completed. In order that the centers may not be mutilated when driven in or out of the work, they should have an extra countersink, as at A in Fig. 77, or else the cut should be recessed as at B in Fig. 78. This operation is known as cupping the centers.

Ordinary Form of Tool Steel Mandrel.

Fig. 76. Ordinary Form of Tool-Steel Mandrel.

The ends BB, Fig. 76, should be turned to size (standard dimensions up to 1-inch diameter are given in Table IV), the corners slightly rounded, and the flat spots for the dog screw milled or planed. The body of the mandrel should be turned somewhat larger than finish size; those smaller than 1/2 inch should have an allowance of .015 inch; from 1/2 to 1 inch, an allowance of .020 to .025 inch; over 1 inch an allowance of .025 to .030 inch. As the length of a mandrel larger than 2 inches in diameter does not increase in proportion with the diameter, the amount given will generally be sufficient if proper care is used when hardening. The size should be stamped on the end next to the large end of the body.

Fig. 77. Extra Countersink on Mandrel.

Fig. 78. Recessed Center on Mudrel.

Before hardening, the centers should be re-countersunk to true them; for this operation, it is best to use a special countersink having an angle of 59 degrees instead of the regular 60-degree tool, as the former facilitates the lapping of the centers to a 60-degree angle after hardening. This is necessary on account of the unequal amount of grinding caused by the shape of the countersink.

Hardening

If a blacksmith's forge must be used when heating the mandrel for hardening, the fire should be large enough to heat the piece evenly; it is advisable to heat it in a tube. Results more nearly uniform can be obtained from a muffle furnace than from the open fire. In either case the piece should be turned frequently, to insure an even heat.

Table IV. Dimensions Of Mandrels

(Diameters up to 1 inch)

Table IV Dimensions Of Mandrels 30068

A

(in).

B

(in).

(in).

D

(in).

E (in).

F (in).

1/4

3 3/4

7/25

1/2

1/2

1/2

5/16

4

9/32

1/2

9/16

5/32

3/8

4 1/4

11/32

1/2

5/8

3/16

7/16

4 1/2

11/32

9/16

11/16

7/32

1/2

5

7/16

5/8

3/4

1

9/16

5 1/4

1/2

11/16

12/16

6/32

5/8

5 1/2

1/16

3/4

7/8

5/16

11/14

5 3/4

4/5

13/16

15/16

11/32

3/4

6

11/14

7/8

l

2/3

13/16

6 1/4

3/4

7/8

1 1/16

12/32

7/8

6 1/2

12/14

15/16

1 1/2

7/16

13/16

6 3/4

7/8

15/16

13/14

15/22

l

7

15/16

1

1 1/4

1/2

Best results follow if the kind of bath shown in Fig. 79 is used. Perforated pipes, which may be moved toward the center for small pieces, are used. These pipes - six in number - extend up the sides as shown. Small holes are drilled in them in such location that the water is projected toward the center of the bath. The bath is also provided with a pipe which throws a jet of water upward from the bottom, thus insuring the hardening of the center at the lower end of the mandrel. A stream must also be provided at the top as shown, to insure the hardening of the upper center hole.

The form of tongs shown at the left of bath should be used, as with these the water has free access to the upper center, which would not be the case with ordinary tongs. If a still bath is used, it should be of strong brine, and the mandrel should be worked up and down violently to insure the liquid coming in contact with both centers.

A mandrel of a diameter larger than 1 inch should be removed from the bath as soon as it ceases "singing", and held in a tank of oil until cold. The ends should be brightened and drawn to a deep straw color, to toughen them so that they will not break or chip off when driven. Mandrels smaller than 3/4 inch should have the temper drawn to a light straw color the entire length of the body. After hardening, the body of the mandrel should be cleaned with a coarse emery cloth to remove the scale or grease which would glaze the emery wheel.

Finishing Centers

The mandrel should then be tested between centers to see if it has sprung more than will grind out before it reaches the proper size. The centers should now be lapped, to insure proper shape and alignment. The lap may be a piece of copper of the proper shape - 60 degrees - charged with diamond dust or emery. After lapping, the centers should be thoroughly cleaned with benzine. (When using benzine, do not allow it to get near a flame of any kind).

Grinding

Examine very carefully the condition of the centers of the grinder, as the trueness of the mandrel depends in a great measure on their condition. A mandrel may be ground in a lathe having a grinding attachment, or in any universal grinder. Better results can be obtained, however, with some form of grinder having a stream of water playing on the work to prevent heating, as heat is likely to spring the piece, especially if it does not run true, and thus to make the grinding heavier on one side than on the other. If a dry grinder must be used, do not force the work fast enough to heat the piece. The mandrel should be ground to within about .005 inch of size with a coarse wheel free from glaze, and then finished with a fine wheel.

Grinding 30069

Tapering

The amount of taper varies. Most manufacturers prefer a .0005-inch taper per inch of length, while others make mandrels with a .001-inch taper, maintaining that if a piece having a long hole is to be held on any taper mandrel, it will not fit at the part nearest the small end of the mandrel, and that consequently the turned surface will not be true with the hole; for such work, they say, a mandrel should be made for the job, having a body nearly or quite straight. They advise that the mandrel be made to taper .001 inch for every inch of length in order that it may be adapted to a greater range of work. However, a .0005-inch taper seems better for most work.

Mandrels With Hardened Ends

When making a mandrel the ends of which are to be hard, and the body soft, the general instructions given for hardening mandrels hold, except that a larger amount of stock should be left on the body. The ends should be hardened for a distance that insures the centers being hard; this can be accomplished by heating one end at a time to a red heat, and inverting under a faucet of running water. As the center is uppermost, the water can readily enter it, forcing the steam away. After drawing the temper of the ends and lapping the centers, the body may be turned and filed to size. The centers of the lathe should be carefully trued before starting this operation. If the body of the mandrel is left .008 inch to .010 inch larger after turning, and then ground to size, the results will be surer; but with extreme care a very satisfactory job may be done by the method described.

Machine Steel Mandrels

With the exception of hardening, the instructions given for making mandrels of tool steel apply to those made of machine steel. Machine steel mandrels must be casehardened. The work should be run in the fire from 7 to 10 hours after the box is red hot throughout; then it should be dipped into a bath having a jet of water coming up from the bottom, to force the steam away from the work and avoid soft spots. It is not necessary to draw the temper, as the hardening does not extend far below the surface.