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Incorrect feed rates, plunging speeds and spiral directions can cause excessive heat to the cutting tool, resulting in a poor cut.
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 affect on 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 is 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. carbide)
- 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 (down cut 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 edge
- 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 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 parts. First, they remove the excess material around the part perimeter. Second, they smooth the part 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:
- 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 ?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 1/16?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:
- 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 1/2- inch diameter ?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 1/2-inch diameter solid carbide three-flute finishing tool with a 2 1/8-inch cutting edge length. This tool also required two passes, 1 inch deep and 2 1/16 inches deep. The normal depth a tool can cut is two times its cutting diameter, in this case 1/2 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 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 ?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 ? 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 investment.
ref: http://www.plasticsmag.com/routing.asp?fIssue=Mar/Apr-05&aid=4174
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