—Use a band saw with a 4- or 6-teeth-per-inch blade with raker set. Operate at higher speed settings. Apply oil or water as cutting proceeds. Do not force the work.
Milling—Not recommended for grades below 50D in hardness. The harder grades machine similar to aluminum. Tools must be sharp and the work must be securely clamped. Use high operating speeds and low feed rates. Two-flute end mills and single-point fly cutters are preferred.
Grinding—In the lathe, turn the work at low speed. Set the tool post grinder for a feed rate of 0.005 inch per revolution (ipr) and use a 20-80 grit grinding wheel at a surface speed of 6,000–8,000 fpm. Finish the stock removal per pass to a dust-type grind. Use air blasting to clean the wheel and cool the stock. Too much stock removal at one pass results in melting or smearing of the surface. Fine abrasives can be used for final polishing.
In cases where larger amounts of stock must be removed, turn the piece down with a cutting tool to within 0.020 inches of the finish dimension before grinding. We recommend that the grinder be equipped with a dust collector, or the operator wear an approved dust mask when grinding or sanding polyurethane elastomers.
Lathe turning and boring—In general, use sharp tools, a high turning speed and a slow rate of feed. Water or plastic lubricants can be used as coolants. Cutting tools for polyurethane elastomers must have sharp, carefully honed cutting edges. Clearances must be greater than those used for metal. High-speed steel or carbide tools are recommended.
Smooth surfaces can be achieved on heavy as well as light cuts, so roughing cuts are seldom required. In turning large-diameter polyurethane parts, light cuts of 1/10 to 1/8 inches deep and a light feed of 0.003 to 0.007 ipr are recommended. To boost production, heavy cuts of up to 3/8 inch and feeds of 0.15 ipr can be used if care is taken.
Drilling—Slow, spiral drills perform best because the large flute area permits free discharge of chips with a minimum of binding and heat buildup.
Retraction of the drill frequently aids in eliminating chip blockage of the flutes. Breakout tearing at the exit side can be reduced by slowing the drill speed at the bottom of the cut or by backing with another material.
When drilling a series of small holes, inserting a pin in each completed hole prevents the force of the drill from pushing material into adjacent holes and causing subsequent distortion.
Sharp cutting edges will minimize elastic deformation as the chip is formed. Polished flutes should be used to aid in chip clearance, and a coolant is required for good drilling performance. The rake angle should be reduced to 0° or a negative angle, and a generous lip clearance (approx. 16°) should be provided for proper relief.
The point angle is governed by the final wall thickness. Sharp points of 90 to 110° are best for heavy walls and large diameters, while blunt angles of 115 to 130° are better for thin walls. Close tolerances call for feed ranges of 0.004 to 0.006 ipr. However, a feed rate of 0.015 ipr can be used where tolerances permit.
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