This month, we continue our guide to machining cast polyurethane elastomers and will be taking a closer look at knifing, milling, drilling, and grinding:
Knifing: It’s important that the knife tool be as thin and sharp as possible to get the most precise cuts when close tolerances are present. For metals with hardnesses between 70A and 95A we’ll use a High Speed Steel tool which gives us a turning speed of 600-1000 RPM and a rapid feed rate by hand. Below 70A we use a Ground carbide at slower speeds of 600-700 RPM with moderate to rapid feed rate by hand.
Milling: We only recommend milling for 90 Shore A to 80 Shore A sheets (although in some cases we’ll mill as low as 75 Shore D). We use a two fluted end mill tool or single point fly cutter. These range in speed from 900 to 1300 at a feed rate of 15-20 inches per minute.
Drilling: When drilling we usually recommend slow spiral drills to permit free discharge of chips with minimum binding and heat buildup. As with any method, sharp cutting edges are a must. Polished flutes aid in chip clearance, and coolant is often required for maximum performance. Also note that rake angles should be reduced to 0-degree angles with a generous ~16 degree lip clearance, sharp points of 90-110 degrees are recommended for heavy walls with large diameters and 115-130 degrees for thin walls, and close tolerance require feeds of 0.004 to 0.006 ipr.
Grinding: To manufacture smooth surfaces, we grind wheels of carborundum with fine grain, medium hardness, and coarse texture. We’ve found that a 6″ x ¾” Simonds wheel C60-J-7B-3 works best with a RPM of 2,200 to 3,500.
In case you missed it, be sure to read our June article about sawing, turning, facing, and boring Cast Polyurethane elastomers. If you want to talk about your next cast polyurethane project, here’s how to contact Harkness Industries.