REAMER DIFFICULTIES
There are certain frequently occurring problems in reaming for which it is necessary to apply remedial measures. These difficulties include the production of oversize holes, bell mouth holes and holes with a poor finish. The following is taken from suggestions for correction of these difficulties by the National Twist Drill and Tool Co.
OVERSIZE HOLES: The cutting of a hole oversize from the start of the reaming operations usually indicates a mechanical defect in the setup or reamer. Thus, the wrong reamer for work-piece may have been used or there may be inadequate work-piece support, inadequate or worn guide bushings, or misalignment of the spindles, bushings or workpiece or run-out of the spindle or reamer holder. The reamer itself may be defective due to chamfer run-out or run-out of the cutting end due to a bent or non-concentric shank.
When reamers gradually start to cut oversize, it is due to pickup or galling, principally on the reamer margins. This condition is partly due to the work-piece material. Mild steels, certain cast irons and some aluminum alloys are particularly troublesome in this respect.
Corrective measures include reducing the reamer margin widths to about 0.005 to 0.010 inch, use of hard case surface treatments on high speed steel reamer, either alone or in combination with black oxide treatments, and the use of a high grade finish on the reamer faces, margins, and chamfer relief surfaces.
BELLMOUTH HOLES: The cutting of a hole that becomes oversize at the entry end with the oversize decreasing gradually along its length always reflects misalignment of the cutting portion of the reamer with respect to the hole. The obvious solution is to provide improved guiding of the reamer by the use of accurate bushings and pilot surfaces. If this is not feasible, and the reamer is cutting in a vertical position, a flexible element may be employed to hold the reamer in such a way that it has both radial and axial float with the hope that the reamer will follow the original hole and prevent the bellmouth condition.
In horizontal setups where the reamer is held fixed and the work-piece rotated, any misalignment exerts a sideways force on the reamer as it is fed to depth, resulting in the formation of a tapered hole. This type of bellmouthing can frequently be reduced by shortening the bearing length of the cutting portion of the reamer. One way to do this is to reduce the reamer diameter by 0.010 to 0.030 inch, depending on size and length, behind a short full-diameter section, 1/8 to 1/2 inch long according to length and size, following the chamfer. The second method is to grind a high back taper, 0.008 to 0.015 inch, behind the short full-diameter section. Either of these modification reduces the length of the reamer tooth which can cause the bellmouth condition.
POOR FINISH: The most obvious step towards producing a good finish is to reduce the reamer feed per revolution. Feeds as low as 0.0002 to 0.0005 inch per tooth have been used successfully. However, reamer life will be better if the maximum feasible feed is used.
The minimum practical amount of reaming stock allowance will often improve finish by reducing the volume of chips and the resulting heat generated on the cutting portion of the chamfer. Too little reamer stock, however, can be troublesome in that the reamer teeth may not cut freely but will actually deflect the work material out of the way. When this happens, excessive heat, poor finish and rapid reamer wear can occur.
Because of their superior abrasion resistance, carbide reamers are often used when fine finishes are required. When properly conditioned, carbide reamers can produce a large number of good quality holes. Careful honing of the carbide reamer edges is very important.
