Feature Stories Archives
Removing the Heat From
Incorrect feed rates, plunging
speeds and spiral directions can cause excessive heat to the
cutting tool, resulting in a poor cut.
BY VAN NISER
Heat is the enemy of cutting tool life. Cutting tools,
including router bits, are made from materials that are
adversely affected by heat. An understanding of the
mechanisms behind heat generation and how to eliminate or
minimize heat build-up in routing applications can lead to
longer tool life and reduced tooling costs.
Heat Generation in Cutting Tools
There are two main problems associated with heat
generation during routing. The first problem is its adverse
effect upon cutting tool structure. The second problem is
melting of the product.
Melting of the product can cause poor surface finish from
chip rewelding. It can load the flute of the tool and cause
catastrophic failure or breakage. It can also cause a chip
to wrap on the cutting tool. Since all of these problems can
produce unacceptable parts, the best prevention would be to
keep the tool cool.
There are many causes of heat build-up in a cutting tool.
The main cause is having too high of a spindle RPM compared
to the rate of material feed. In other words,. the cutting
tool is not cutting a reasonable chip, but rather rubbing
the material. The rubbing causes friction, which causes
heat, which can lead to structural changes in the cutting
tool and the material being cut.
The following is a partial list of some of the causes of
heat buildup in cutting tools:
- Improper tool material for the application (HSS vs.
- The feed rates for the plastic are too low
- The spindle speed for the tool is too high
- Stopping and/or dwelling in the cut path
- Excessive or slow plunging
- Incorrect spiral direction (downcut vs. upcut)
- Feeding the tool in the wrong direction (conventional
vs. climb cut)
- Running the spindle in the wrong direction
- Plunging the tool into the part beyond the cutting
- Using the tools after they become dull
- Running into the steel hold-down clamps
- The chip load on the finish pass is too light
- The plastic material requires multiple tool passes
without a cooling period
There are many causes for heat buildup in a cutting tool.
When using router bits in any type of router, after you
finish the operation, turn off the power and touch the
router bit. If it is hot to the touch, you have not run the
bit correctly and tool life has been degraded.
Minimization of Heat Buildup
In order to minimize or eliminate heat buildup on router
bits, the trick is to feed the material past the router bit
as fast as possible. For example, for routing plastics at a
spindle speed of 18,000 RPM, the feed rate should be between
200 inches per minute and 400 inches per minute. If you
cannot feed that fast, slow down the RPM of the spindle.
When cutting plastics, you want to do develop a distinct
chip, not powder. The larger the chip, the more heat is
carried away from the tool. The two limiting factors of feed
rate are spindle horsepower and deterioration of the quality
of the cut. If this happens, the feed rate needs to be
slowed down. Just remember, the slower the feed speed,
the more heat builds up in the router bit and the shorter
the tool life becomes.
The other considerations in maximizing tool life revolve
around maintaining your router and collets in good condition
by proper fixturing, proper tool selection, using sharp
tools and controlling the machining process. If all of these
considerations are addressed, and you feed at a reasonable
rate, you will get the maximum life out of your router bits.
Real World Examples
In the normal world of routing, router bits perform two
functions when cutting the parts:
- First, they remove the excess material around the
- Second, they smooth the part's edge. Sometimes this
can be accomplished in one pass, although there are times
when a roughing/finishing combination is needed.
Following are two real-world problems associated with
heat generation and the solutions used to solve them.
Problem 1: The first example required a roughing
and finishing pass to produce a finished part. Acrylic was
being cut for an aircraft window application and the parts
needed an excessive amount of clean-up work because of tiny
cracks made during the routing. The cracks would not become
visible until the polishing operation.
The single edge "O" flute solid carbide tool used on the
CNC router could not remove all the heat with the chips and
was causing structural deformities within the acrylic.
Solution: The solution was to use a carbide-tipped
saw with a negative 5-degree rake. The part was cut within
1Ú16-inch of finish size. A solid carbide three-flute
finishing tool was then used on the second pass in a climb
out direction. The resulting finish eliminated two clean-up
operations and there were no more cracks.
Problem 2: Another example of two passes involved
an electronic parts washer. The washer parts were made from
a high-density polypropylene about 2 inches thick. The part
was being cut in two passes with a solid carbide 1Ú2-inch
diameter "O" flute with a 1-inch cutting edge length. The
edge of the finished part would have a line showing where
the first pass stopped and the second pass would leave chips
welded to the part.
Solution: The solution was to use a 1Ú2-inch
diameter solid carbide three-flute finishing tool with a
21Ú8-inch cutting edge length. This tool also required two
passes, 1 inch deep and 21Ú16 inches deep. The normal depth
a tool can cut is two times its cutting diameter, in this
case 1Ú2 inch by 2 equals 1-inch depth. To eliminate the
line made during the first pass, the part was cut 0.04-inch
oversized. Then the second pass was made at the finished
size. This allowed the tool's cutting edge to cut the full
surface and eliminate the line.
This second pass by the spiral cutter would lift the
chips out of the cut path and eliminate the chip welding
problem caused by the straight "O" flute design. An
additional benefit allowed by the spiral was an increase in
the feed rate so both passes were accomplished in the same
amount of time used in the first pass of the "O" flute tool.
The point to remember is that heat is the enemy of tool
life and heat buildup in the tool can be removed by feeding
as fast as possible, making the largest possible chip. Also
keep in mind that your equipment must be kept in cool
conditions and the collets must be clean to be successful in
your routing manufacturing processes. With proper tooling,
fixturing and the correct speeds and feeds, you can maximize
your tool life and considerably reduce your tooling
Van Niser is director of Plastic Application
Engineering at Onsrud Cutter. Based in Libertyville, IL,
Onsrud Cutter is a manufacturer of a wide range of cutting
tools for the plastics industry.
Plastics Machining & Fabricating
F: (847) 362-5028