User guide

C-1 l Appendix
All About Cutter Bushings
Rotary knife cutter bushings are probably the most ignored aspect of cutting. Yet,
they are probably the most important ingredient to obtaining clean, square, accu-
rate cuts with minimal 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 extru-
sion process.
Bushings with relatively large bores are often used to facilitate start-up and mini-
mize 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 pro-
file.
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 caus-
es two marks on the tube (penetration marks) that show where the tube flattened
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 interruption or even jamming
may occur. In turn, this can cause internal 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 clearance 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 clear-
ance above the profile should be minimized to prevent bowing. Supportive
bushings become more important if heat is used to minimize whitening
(fracturing).