The term mandrel is applied to that class of tools upon which work that is to be machined between centers is usually held. It is frequently called an arbor, although the distinction between the two may be quite clearly define.

A mandrel is designed to carry work that is to be operated upon by a cutting tool, while on the other hand the arbor carries and drives a cutting tool, as with the milling machine and saw arbors.

Mandrels may be classed under two heads, solid and expanding. The solid mandrel is made slightly tapering, in order that it may be forced to a right fit in the bore of the work. The amount of this taper varies with the class of work the mandrel is to be used on, it being but slight at the most.

A bar common round iron or steel centered and turned to the required diameter constitutes the mandrel in its simplest form. Such a tool, as it is usually found in the average jobbing shop, is shown is Fig. 185.

It is hardly worthy the name mandrel, although a solid one might fairly come under the expanding, or rather shrinking class, as it is brought down by turning and filing to fit the bore of every new piece of work that comes along. It has one quality, however, that can always be depended upon, and that is untruth. With mandrels of this class accurate results cannot be expected.

Since a mandrel must be rigid, it should be as short as the nature of the work will permit, and made of as stiff a material as possible. Its centers should be carefully formed, and the body finished cylindrically true upon them. The centers, at least should be tempered or case hardened, to prevent their wearing out of true.

In Fig. 186 is shown the correct construction for the end of a mandrel. The end for a length about equal to the diameter of the tool is reduced slightly in diameter and provided with a flat on one side, against which the screw of the dog or driver is set. As the dog is very apt to mutilate somewhat the ends, this reduction in diameter is quite necessary. Since the accuracy of the mandrel depends so much on its centers, it is necessary to protect them as much as possible from injury while forcing the mandrel into the bore of the work. This is best accomplished by recessing the ends around the center bearing as shown in the figure. The angel of the bearing should be 60 degrees, with a small hole drilled at the bottom. The object of this drilled hole is to prevent strain being thrown onto the delicate point of the machine center, and to form a small oil reservoir to aid in lubricating the bearing.

In Fig. 187 is shown a hardened and ground steel mandrel. These tools are made for general shop work, the length increasing with the diameter from 3 ¼ inches for a ¼ inch mandrel to 17 inches for a 4-inch. These lengths are, of course, arbitrary and may for special uses be materially increased or decreased. As manufactured by the several makers, these mandrels differ but little in length and details of design. They should be made of a good grade of tool steel, carefully hardened with the centers lapped true after the hardening, and the body ground cylindrically true upon these centers, it being rotated upon stationary or dead centers for this last operation.

When the greatest possible accuracy is required it is considered best to make these mandrels of tough, un-annealed tool steel, with the ends only hardened. This arises from the fact that the steel if hardened throughout changes somewhat in form and receives temper strains, which, although relieved in the grinding, does not allow the tools to immediately take its permanent set. For this reason a mandrel that has been hardened throughout should be first rough ground, leaving a small amount for final finishing. This finishing should not be done for some time after the rough grinding, thus allowing the tool to season and to acquire permanent set. The set will not be appreciably altered if only a very small amount is left for the final finish.

Hardening makes the mandrel stiffer and less liable to surface injury than in the case of the unhardened one. It is not, however, for the purpose of allowing careless workmen to run their cutting tools into its surface with the idea that it will not be injured thereby. Cutting tools are usually made of a higher grade steel than the mandrel, and often tempered harder, in which case the mandrel suffers if the tools comes in contact with it.

These mandrels are usually tapered about one-hundredth of an inch to the foot, the diameter being exact at the center. The size is stamped on the flat at the larger end. They will fit holes reamed with standard reamers, although the taper prevents uniform grip on the work at the two ends of the bore. In forcing these mandrels into the bore, good judgment must be exercised, as they constitute a wedge, which will produce enormous pressure if forced too hard, resulting in bursting the work if hard and brittle, or if soft in permanently enlarging the bore and giving it a taper corresponding to that of the mandrel.

The use of the hardened and ground mandrel does much toward the preserving of uniformity in the size of holes, in the work of shops, where these tools are used. A hole only a few thousandths of an inch under or over size prevents, in the first case, the mandrel from entering and in the latter allows it to fall through.