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UGE059/1003 ALL ABOUT CUTTER BUSHINGS
APPENDIX C-1
Rotary knife cutter bushings are probably the most ignored
aspect of cutting. Yet, they are probably the most important
ingredient to obtaining clean, square, accurate cuts with mini-
mal jamming and broken blades.
This appendix contains information about several aspects of
cutter bushings:
● bore characteristics
● bushing length
● shear surface characteristics
● the bushing gap
Cutter bushing bore size
The cutter bushing bore size affects both the cutting process
and the overall extrusion process.
Bushings with relatively large bores are often used to facilitate
start-up and minimize bushing inventory. While this practice is
acceptable for start-up, it will lead to premature blade failure
because the bushings do not properly support the blade. For
optimum cut quality, make sure the bore adequately supports
the tube or profile.
When the blade first makes contact with the tube or profile, it
pushes the part until is assumes the size and/or shape of the
bushing bore. In the case of tubes this causes two marks on
the tube (penetration marks) that show where the tube flat-
tened before the blade actually penetrated it. The tighter the
bushing bore size to tube size, the closer the marks become,
making them less obvious.
If the bushing bore is too tight, excessive extrudate interrup-
tion or even jamming may occur. In turn, this can cause inter-
nal air blockage in free extruded flexible materials and thus
extrudate size fluctuations. In the case of rigid profiles or
tubes, belt puller slippage may occur during the cutting if the
bushings are improperly configured. This can cause annular
rings around the extrudate and size fluctuations.
● For rigid profiles or tubes, allow 0.010-0.020 inch clear-
ance over the OD tolerance. Anything tighter than 0.010
inch will be difficult to process. For easier startup, allow
as much as 1/4 inch above a rigid profile because the
blade will force the profile to the bottom of the cutting
bushing where the shearing action occurs. However, if
perfect squareness is required, the clearance above the
profile should be minimized to prevent bowing.
ALL ABOUT
CUTTER
BUSHINGS