Fig. 1100.
Instead of having upon a reamer a flat tooth top to provide clearance, very accurate and smooth work may be produced by letting the back of the tooth, as a in [Fig. 1100], proceed in a straight line to b, leaving the reamer, when soft, too large, so that after hardening it may be ground by an emery-wheel to size; and the clearance may be given by simply oilstoning the top of each tooth lengthwise, the oilstone marks barely effacing the emery marks at the cutting edge and removing slightly more as the back of the tooth is approached from the cutting edge. This produces cutting edges that are very easily fed to the cut, which must obviously, however, be a light one, as should always be the case for finishing, so that the wear of the teeth may be a minimum, and the reamer may therefore maintain its standard diameter as long as possible.
When a solid reamer has worn below its required diameter, the same may be restored by upsetting the teeth with a set chisel, by driving it against the front face; and in determining the proper diameter for a reamer for work to be made to gauge under the interchangeable system the following considerations occur.
Obviously the diameter of a reamer reduces as it wears; hence there must be determined a limit to which the reamer may wear before being restored to its original diameter. Suppose that this limit be determined as 1⁄1000 inch, then as the reamer wears less in diameter the bolts to fit the holes it reams must also be made less as the reamer wear proceeds, or otherwise they will not enter the reamed holes. But it is to be observed that while the reamer wears smaller, the standard gauges to which the pins or bolts are turned wear larger, and the wear is here again in a direction to prevent the work from fitting together. It is better then to make the reamer when new too large to the amount that has been determined upon as the limit of wear, so that when the work begins to go together too tight, the reamer requires resharpening and restoring.
A still better plan, however, is to use reamers adjustable for diameter, so that the wear may be taken up, and also the reamer sharpened, without being softened, which always deteriorates the quality of the steel.
Fig. 1101.
Reamers that are too small to be made adjustable for size by a combination of parts may be constructed as in [Fig. 1101], in which the reamer is drilled and threaded, and countersunk at the end to receive a taper-headed screw s, which may be screwed in to expand the reamer, which contains three longitudinal splits to allow it to open. To cause s to become locked in its adjusted position a plug screw p is inserted for the end of s to abut against. It is obvious that in this form the reamer is expanded most at the end.