The process of reaming has a number of applications, the chief of which are: to enlarge existing drilled holes; to make a parallel hole into a tapered hole; to bring existing holes accurately to size. It will be clear from this that there must be two main types of reamers, one of which is parallel and the other tapered. These are shown in Fig. 1.
In some respects, hand reamers are similar to screw-cutting taps, for they cut away metal from the inside of a hole. Additionally, they are held in a wrench of similar type to that used for taps. When buying reamers, care should be taken that the correct pattern is ordered, since many of them have a tapered, or Morse, shank; this is for fitting in a drilling machine or lathe. Reamers for hand use have a cylindrical shank with square end, as shown in Fig. 1. Reamers to be used for removing the burr from the inside of plumbing pipes after cutting can be purchased with a square shank for use in a bit brace.
Some reamers have straight flutes, others have spiral flutes, rather resembling those of a drill, but often of much greater pitch. In general the spiral flute type is to be preferred for accurate work, since there is 1ess tendency for it to chatter when in use, and consequently a smoother finish is produced. It is also better when a good deal of metal has to be removed. It should be noted that the spiral flutes are left-handed, although the reamer is turned in a right-hand or clockwise direction.
There are reamers which are made to a taper for their full length, and others which are tapered for only about half the length of the flutes, the remaining being parallel. There is yet another important type of reamer, of the expanding type; an example is shown in Fig. 2. In this case there are five or more cutting blades carried in cellars mounted on a central threaded shank. By means of a cone adjuster the diameter over the cutting edges can be varied over a certain limited range.
When an existing hole is to be slightly enlarged to accurate size – to receive an axle or pin, for example – it is generally best to use a reamer of the type which is tapered towards the tip and parallel nearer the shank, since this makes easier working. The size should be such that the reamer will enter the hole for a distance at least equal to half the length of the tapered portion, or one-quarter of the length of the flutes. The work must be held absolutely rigid, and the reamer turned with a tap wrench, applying a steady pressure and keeping the turning speed as uniform as possible. A few drops of light machine oil may be used as lubricant.