Q:
What is the difference
between climb and
conventional cutting and
which is the better method?
A:
Climb and conventional
cutting merely describe
the way in which the cutter
moves around the part in
respect to its direction of rotation.
In regard to a right-hand
rotation spindle (the most
common type), moving around a finished part counter-clockwise
would be considered conventional cutting while a
clockwise part path would be climb cutting.
The terminology will be reversed if
the spindle is of left-hand rotation or a pocket or hole is being cut out of the
finished part.
While climb cutting is the most
prevalent method in metalworking
and conventional cutting is the dominant
method in woodworking, plastics
routing is somewhere in between the
two. There are usually significant differences
between the finish on a climb
cut part and a conventional cut part
and the degree of difference varies by
plastic and cutter geometry. The only
rule of thumb that can be offered is
that most soft plastics (HDPE,
UHMW, polypropylene, etc.) respond
best to conventional cutting, while
some harder materials (acrylic, polycarbonate,
nylon) occasionally respond
better to climb cutting.
Typically, climb cutting will only show
an improved performance in the
smaller diameters (less than 3/8
inch) but of course there are always
exceptions.
The other factors when considering
climb or conventional cutting are the
aggressiveness of the cutter and part
hold down. Climb cutting is a much
more aggressive means of cutting and
from the cut are also more
likely to weld to the climb-cut
side rather than the conventional
cut side. The best
method of approach for most
new materials is to run sample
parts with both methods
of cutting at the same feeds and speeds and make the
determination from there.
Q:
What is the best method for fixturing small parts,
such as letters, that are difficult to hold with vacuum?
A:
Far and away the best method seems to be using
the paper masking as a source for additional holding
power. By precisely setting the depth of the cutter in
relation to the spoilboard, many fabricators are able to
cut completely through the plastic sheet without actually
perforating the bottom masking sheet. This allows both
universal and conventional vacuum systems to act upon a
large surface area and allows the added benefit of “single
sheet on-single sheet off” loading and unloading.
The only drawback to this method is the difficulty in routing
at a precise depth over multiple sections of the spoil-board
without plunging through the paper masking. While
the routers have enough positioning accuracy to repeatedly
locate at the same depth, the spoilboards typically will
have some bow or flex that can overcome the 0.010-inch
thick paper zone. The standard solution is to fly-cut the
board before use with a large diameter cutter to make it
flat and parallel to the router head. There are dedicated
cutters for this purpose, but just about any large carbide-tipped
cutter will work for infrequent surfacing.
Many fabricators will fly-cut before each shift or more
often depending on humidity and warpage factors.
Typically only 0.010 inch to 0.020 inch will need to be
removed, which will give significant life to a 1/2-inch MDF
universal spoilboard.
Q:
How can chip wrap, crazing or “keyhole” slots be
prevented when plunging in plastics?
A:
While these problems are not unique to the
machining of plastics, they can be much more difficult
to solve here than when they occur in metalworking
Chip wrap is perhaps the most confounding
problem, in that it does not always respond well
to the standard peck or chipbreak drilling cycles that are
available in the router controls. In very soft and tough
plastics such as polypropylene, it can take reducing the
peck amounts to as little as 0.010 inch to prevent the long
chip from curling around the plunging cutter. This can add
significant cycle time and Z-axis wear over long runs. A
better method is to program the router to ramp on entry
for all transverse cuts and to helical ramp and interpolate
for any holes that need to be made. This will prevent chip
wrap and typically save cycle time.
Crazing and “keyhole” slots typically require the same
solution method. Crazing, or cracking of the material, is a
result of the flat bottom cutter inducing too much stress
into the part during the plunge. Ramping will ease the cutter
in and prevent the bottom of the part from stress fracturing.
If ramping is not an option, a higher spindle speed
or slower feed rate can also reduce stress but it sometimes
shows a poor edge finish.
Keyhole slots are the result of the router bit “walking”
when it plunges. Drill bits have a sharp centering point and
a cylindrical land around the outer diameter of the body
that help to keep them aligned in the cut. Router bits do
not have any centering action and will aggressively try to
move in a lateral direction. Because of this, the entry hole
is often slightly larger than the bit diameter and it will be
noticeable when the cut transitions from the plunging area
to the routed slot. Once again, ramping will prevent any
noticeable change in slot width throughout the entire cut.
Q:
What causes the inconsistency in hole sizes, particularly
in stacked sheets or thick plastics?
A:
The first cause is explained above. As the cutter
plunges, it has a tendency to “walk” and create a
larger diameter hole than intended. This is another reason
that interpolating the hole as opposed to plunging a
hole is a better idea when using router cutters.
Interpolated holes will exhibit improved diameter consistency
versus plunged holes.
The second factor is typically a heat and material issue.
It is not uncommon for a plunged hole to actually be
smaller than the router bit that created it. This is a result
of heat buildup and thermal expansion of the hole wall. It
is particularly apparent in soft plastics and there are a
number of solution methods that can be tried. Since heat
buildup is the cause of the problem, the best solutions
center around heat reduction. Slower spindle speeds,
faster plunges, air blasts or coolant will all reduce the
effects of wall expansion. Peck or chipbreak drilling cycles
can be a mixed bag depending on the cutter and the plastic.
Sometimes they will eliminate the problem and other
times they will exacerbate it.
For more information, click on the Author Biography link at the top of this page.
