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QUICK FACT GUIDES FOR MACHINISTS
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GUIDES
FUNDAMENTALS OF
DRILLING & TAPPING
FUNDAMENTALS
OF THE LATHE
FUNDAMENTALS
OF MILLING
FUNDAMENTALS
OF SAWING
METALWORKING
SHOP TERMS |
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. |
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Copyright © 2002 QUICK FACT GUIDES FOR MACHINISTS
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