Q:
Plastic is notorious for welding
into the cut. What causes
this phenomenon and how is it
resolved?
A:
Plastic material welding into
the cut is caused by heat,
poor chip evacuation (causing a recutting
of chips) and improper chip
load. This problem can be solved by
using a tool with proper geometry
for the plastic being cut (an upcut spiral cutter may evacuate
chips more efficiently) and by finding the proper chip
load through increasing the feed rate and/or decreasing
the spindle speed.
Q:
What chipload should one be looking at with a CNC
router?
A:
The chipload should be the maximum that can be
achieved without adversely affecting the work part
finish or taxing the hold-down system. On plastics, the
chipload will vary from 0.004 to
0.020 depending on the plastic
material. Generally, softer plastics
have a smaller chipload.
Q:
Many times the plastic will
wrap around the router bit
during the plunging process. How
can this be avoided?
A:
Chip wrap can be controlled
by programming techniques
employing a ramping or zigzag plunge into the material. If
you are hole drilling, the chip wrap can be controlled by
using a pecking method whereby the downward feed is
stopped several times, the tool is removed from the hole,
and then plunged again to a greater depth. Tool geometry
can also assist in reducing chip wrap with fish-tail style
points and upcut spiral geometry.
Q:
At times the scrap has a better finish than the work
piece. Why does this happen?
A:
Check the feed direction. In most cases, conventional
cutting (counter-clockwise direction) will give
a better finish than climb cutting (clockwise direction).
Climb cutting, however, can be effective when using a finishing
pass in hard materials.
Q:
What is the best tool for acrylics?
A:
Many people prefer larger diameter three flute finishers
for cast acrylic. Single edge straight or spiral
O flutes for extruded and both single and double edge
spiral O flutes for stretched acrylics are recommended. All
tools should be solid carbide.
Q:
What is the best tool for thicker plastics such as
HDPE?
A:
Thicker plastics such as HDPE are normally in the
softer range of materials. Two flute solid carbide
tools with soft plastic geometry are recommended.
Q:
What is the difference between wood and plastic
tools?
A:
Plastic tools have different geometry. Higher rake
angles, different clearance angles and shape of the
flute all provide for better chip removal required to cut
plastic materials more effectively.
Q:
Why does the workpiece finish decrease in quality
with smaller diameter tools?
A:
There are a couple of reasons this can occur.
Smaller diameter tools may deflect more than larger
diameter tools and this will affect the finish. Also, chips
may not clear as well with smaller diameter tools; the sur-face
footage of a small diameter tool is less. Increasing the
spindle speed may improve finish. Plastic geometry tools
are recommended.
Q:
What are the most important factors for maximum
CNC router production (with acceptable finish)?
A:
The formula to follow is: productivity = solid hold-down
system, proper maintenance, fast feed rate
and the right tool for the job.
Q:
What are the most important factors to obtain
optimum workpiece finish on a CNC router (with
acceptable production)?
A:
To obtain the optimum workpiece finish, the rule of
thumb is: finish = solid hold-down, proper maintenance,
proper chip load and a larger diameter tool with
proper plastic geometry.
For more information, click on the Author Biography link at the top of this page.
