Machinery's Handbook, 31st Edition
Reamers
917
D
d T
Z
a
Fig. 2. For Countersinks and Spot Drills that are a Truncated Cone
a = allowance for truncated countersink tool d = diameter of hole T = diameter of small end of truncated tool
The table below is a quick reference for some predefined angles on countersinks and spot drills that have been sharpened to a point. To calculate the tool travel “Z”, multiply the factor by the countersink diameter. Countersink Angle α , degrees Factor (Imperial or Metric Units) Countersink Angle α , degrees Factor (Imperial or Metric Units) 60 0.8660 118 0.3004 82 0.5752 120 0.2887 90 0.5000 135 0.2071 100 0.4195 142 0.1722 Reamer Difficulties.— Certain frequently occurring problems in reaming require reme dial measures. These difficulties include the production of oversize holes, bellmouth 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. and Winter Brothers 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 the workpiece material may have been used or there may be inadequate workpiece support; inadequate or worn guide bushings; misalignment of the spindles, bushings, or workpiece; or runout of the spindle or reamer holder. The reamer itself may be defective due to chamfer runout or runout of the cutting end due to a bent or nonconcentric 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 workpiece 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 (0.127-0.25 mm), use of hard case surface treatments on high-speed steel reamers, 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 solution 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 workpiece rotated, any mis alignment exerts a sideways force on the reamer as it is fed to depth, resulting in the forma tion of a tapered hole. This type of bellmouthing can frequently be reduced by shortening * “Some Aspects of Reamer Design and Operation,” Metal Cuttings, April 1963.
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