Reamers used for finishing a long hole that must be very smooth, are often made of the form shown in Fig. 74. This reamer is drawn through the hole by means of the shank B, the cutting portion being at A. It should cut but a very small amount at each passage through the hole. A piece of hardwood is placed on one side of the reamer, as shown at C. After the reamer has passed once through the hole, a piece of tissue paper is placed between the reamer and the chip, and another cut is taken, this being repeated each time the reamer passes through. Several passages of the reamer and repeated blocking between the chip and reamer, result in a beautifully finished hole of the desired size.
Long reamers and similar tools, made from high-speed steel, are very likely to warp and bend unless heated for hardening in a vertical position. To accomplish this, they should be suspended by their shanks in a specially designed vertical furnace, as shown in Fig. 22. The shanks project through holes in the top of the furnace, and are held by suitable holders. As the temperature in high-speed furnaces is very great, the reamers should be pre-heated to a low red before being placed in the furnace. This pre-heating should be done in an open fire, or in a furnace where the process can be carried on slowly.
Reamers should not be heated to so high a temperature as tools that have no projecting portions. The limit of temperature for tools of this class is about 2300° F. When this temperature is reached, the reamers should be plunged vertically into a bath of cottonseed oil and worked vertically until they have cooled below a red.
As the process of hardening makes the tool extremely brittle, it is necessary to draw the temper of most forms of reamers to a full straw color (460° F.). If the reamer is slender, and is to be subjected to considerable strain, the temper may be drawn to 480° F. or 500° F. (brown color).
On account of the uncertainty of exact alignment of every part of a screw machine or turret lathe, it is desirable to use a holder that allows each part properly to align itself. The form shown in Fig. 75 is common and gives good results. It consists of the body A, which has a hole drilled and reamed its entire length. The hole must be somewhat larger than the shank of the reamer, 1/16 inch being considered sufficient. The center B, of tool steel, which has the point only hardened should be, after hardening, .010 to .015 inch larger than the hole in the holder; the point should be ground to a 60-degree angle, and the straight part ground to a forcing fit in the holder. After being forced to position, a hole is drilled through the holder and center, and the pin C driven in to keep the center from being pressed back by the reamer when in operation. A pin should be put through the holder at D and a hole 1/14 inch larger than the pin should be put through the reamer shank at this point; this pin is simply to prevent the reamer from turning when it comes in contact with the work. The coil springs EE hold the reamer in position to enter the hole, and the proper tension is given by means of the screws FF.
Fig. 75. Typical Reamer Holder.