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Materials Selection
and
Machining Parameters

If you do not find the answer to your question below, try our Reference Library or write to support@PreciseBits.com

Select the Material you are interested in below
Hardwoods
Softwoods
Ivory / horn
Mother-of-Pearl / Shell
Non-ferrous metals
Ferrous metals
Plastics
Composites
Reconstituted / stabilized products 

Hardwoods

When cutting, the wood appears to be burning. Should I reduce my spindle speed (RPMs)?
How can I tell how hard a particular wood is?

Softwoods

When cutting, the wood appears to be burning. Should I reduce my spindle speed (RPMs)?
How can I tell how hard a particular wood is?

Ivory / horn

Should I use a 2 flute or a 3 flute cutter to machine ivory?

Mother-of-Pearl / Shell

Which cutter should I use for shaping abalone or mother-of-pearl (MOP)?

Non-ferrous metals

I am cutting gold, silver, and sterling.  What kind of bits should I use?

Ferrous metals

Plastics

How can I tell the difference between polymers and ceramics?
What is the best kind of bit for machining thermoplastics like polyethylene, polycarbonate and PVC?

Composites

What kind of bit should I use to cut composites like fiberglass, and glass filled polymers? 

Reconstituted / stabilized products 


When cutting, the wood appears to be burning. Should I reduce my spindle speed (RPMs)?

You can either reduce your spindle RPM or increase the feed rate. Burning on the side walls during cutting is usually a sure sign that you are moving the cutter too slowly for the the RPMs of the spindle.  If you are moving too slowly (feedrate too low) , the bit is taking such a small bite on each revolution that virtually no debris is coming up out of the kerf.  Since this "sawdust" is the only thing cooling the tool,  the cutter overheats and burns the wood.  This overheating can significantly reduce tool life by weakening the tungsten - carbide - cobalt bond, leading to rapid tool erosion and edge deterioration.  A good way to think of this is to imagine a tire spinning on the pavement.  You don't get anywhere and your tires wear out in no time.

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How can I tell how hard a particular wood is?

The most objective measure of wood hardness is found in the Janka Scale. The Janka (or side) hardness test measures the force required to embed a .444 inch steel ball to half its diameter in the wood being tested. This is one of the best measures of the ability of wood species to withstand denting and wear. It is also a good indicator of how hard a species is to saw, nail, or machine. The higher the assigned number, the harder the wood, and generally, the harder it is to cut.  To determine a specific woods' hardness, go to our Relative Hardness Table.

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How can I tell the difference between polymers and ceramics?

The easiest test to perform is a "tap" test.  Polymers (filled and un-filled) thud when tapped.  Fired ceramics, being quasi-crystalline will ring or "clink" when tapped. Try gently tapping a piece of plastic against your front teeth.   Then try tapping your teeth against the rim of a drinking glass.  The difference is very apparent.  Or at least it used to be.  The introduction of reconstituted stone and stabilized minerals (ground up stone mixed with 3 - 10% polymer resin) has rendered this test almost worthless for selecting between the two.  However, if a material "thuds" like a polymer, it will probably cut well using the same bits, feeds, and speeds that work for other plastics.  If it "clinks" like a ceramic, you will probably need coated carbide or diamond cutting tools and reduced feeds and speeds. To prevent burning and accelerated bit wear, it may also be necessary to flood the tool with a liquid coolant while cutting.

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Which cutter should I use for shaping abalone or mother-of-pearl (MOP)?

I am cutting abalone and MOP for pool cue inlays. What would you suggest in a 1/32 inch cutter: a 2 flute cutter or a 3 flute cutter?  
All shells are made of a natural composite of calcium carbonate in a protein matrix. When cutting shell of any thickness, we always use a 3 flute cutter designed specifically to break up the carbonate and cut the matrix.  It's chip-breaker flute geometry pulverizes the composite as it cuts allowing the debris to be extracted from the kerf without packing.  For a 1/32" cutter, we recommend our Shell cutter M3S8-0313-013F.

Also, would you plunge the full .125 inches (I am guessing not) or would you cut it in increments? 
First of all, we never cut natural pearl (or abalone) that is more than 0.080" think.  The thickest
laminated shell that we cut is 0.100". All of our tools are designed for full plunge cutting.  With the variability of natural shell, it is always a good idea to run a few tests to determine the optimum feeds and speeds for the particular material you are cutting.  In our shop, when cutting at 30,000 RPM we use a feed rate of 15 in./min. to cut 0.080" MOP. 0.100" thick laminated shell can be cut at 18 in./min.

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Should I use a 2 flute or a 3 flute cutter to machine ivory?

Almost without exception, a properly designed 2 flute cutter is superior to a 3 flute bit when cutting ivory or horn. Both ivory and horn products can be very oily which tends to make the cutting debris clump together and cake.  To insure proper "chip" extraction, a large flute volume coupled with the proper helix angle is required to prevent caking and bit breakage.  Our 1/32" horn cutter (M2H8-0313-015F) has proven to be very effective, and popular, for both pocketing and part shaping in the custom pool cue industry.

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What kind of bit should I use to cut composites like fiberglass, and glass filled polymers? 

Glass filled composites are most effectively cut using a grinding action that breaks the glass and cuts the polymer binder without melting or caking.  In terms of bit life and edge quality, chip-breaker and diamond cut router bits excel when machining  glass filled polymers and phenolic. The limitation of this style of cutter is that they are very difficult to make to precise diameters. Typical tolerances run 0.002". For more precise shaping, a 3 flute cutter will give acceptable performance and an excellent edge finish with somewhat reduced life (50 - 75%).

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I am cutting gold, silver, and sterling.  What kind of bits should I use?

All three of the metals you are using machine well with cutters ground with a medium rake on the cutting edge.  Both our stub mills and stub ball mills are designed to cut precious metals as well as aluminum, brass, and copper.  Gold, silver, brass, aluminum and copper present special problems when cutting. See 

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What is the best kind of bit for machining thermoplastics like polyethylene, polycarbonate and PVC?

When machining thermoplastics, there can be a significant difference between cutting cast and extruded products. Cast materials which are usually a bit harder and have a higher melting point than their extruded counterparts, machine easier, with a cleaner edge and less deposition of melt on the cutting tool. In both cases, however, you need a VERY sharp tool designed specifically for cutting thermoplastics.  Our M208 PreciseBIT family of cutters has proven to work very well in these materials if you use tool diameters 0.0625" (1.59 mm) or  greater.

PVC presents an additional challenge in that it is much more abrasive than most of the other members of this family.  Cutting both cast and extruded products will result in accelerated bit wear with an associated deterioration in the quality of machined edges and surfaces.  

The key to cutting all of these materials is to insure that you select a combination of RPM and feedrate that produces high enough chip loads.  For example. If you are cutting cast polyethylene 0.25" (6.35 mm) thick in a single pass with a 0.125" (3.18 mm) diameter cutter at 27,000 RPM you will need a feedrate of AT LEAST 12 in./min. to  prevent burning and melting.  25in./min. would give even better results.  The challenge is that your spindle has to be able to produce the horsepower to support this kind of cutting.  From our experience, small air turbines, laminate trimmers, and low horsepower DC motor based spindles simply do not produce enough torque to keep from slowing down when cutting this deep with a tool this big.  Multi-pass cutting will be required.

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