These are used for holes of an irregular shape, or rather of a shape neither straight nor tapering. They are used chiefly by gun-makers in reaming the end of the gun barrel for the shell, and are termed, when used for this class of work, chambering reamers.
These have a sleeve on one end as shown at A, Fig. 72. This sleeve is a nice running fit on a pilot, and also fits closely in the hole in a gun barrel. Teeth are cut on the end next to the cutting portion of the reamer. When the reamer is cutting, the sleeve does not revolve in the barrel, but the pilot turns in it. When the reamer is drawn out of the barrel, the semicircular slot at the end engages with the pin passing through the pilot, and the sleeve revolves and cuts away any burr that may have been thrown up when the reamer was cutting, thus preventing the burr from tearing the inside of the barrel.
Fig. 72. Chambered Reamer for Gun Barrels with Sleeve Shown at A.
It is essential that the stock be rough-turned a little above finish size and then annealed. As reamers of this form must be accurate in size and shape, it is customary to use a gage; this is generally a piece of steel in which a hole of the proper form has been reamed, and the stock cut away on one side, so that a trifle more than one-half of the hole is left, as shown in Fig, 73. To make the reamer blank fit the gage, the operator must understand the use of hand-turning tools, as most shapes must be made with these tools. Cutting Teeth. The teeth must be cut with a milling cutter of small diameter, following the different shapes of the reamer in order that the top of the land may be of as uniform a width as possible. After cutting, the teeth may be backed off for clearance with a file, care being taken not to remove any stock at the cutting edge.
Fig. 73. Gage Used for Formed Reamer.
When hardening, the reamer should be heated very carefully in a tube until it is of a low uniform red heat; it should then be plunged into a bath of lukewarm brine. It may be brightened and the temper drawn to a light straw. After hardening, it should be tried in the gage, and any high spots removed by oil-stoning.
If a large number of reamers of one form are to be made, the grinding machine may be rigged with a form which makes it possible to grind many of the shapes in common use. It is found quite impracticable, however, to grind some shapes, and consequently the method just described of fitting before hardening must be adopted. Excellent results are obtained with the pack-hardening process.