Machining Cast Urethanes
Generally, harder urethanes [85A Durometer and above] machine more easily than their softer counterparts. Lathe turning, fly-cutting, milling, grinding, contouring and more are possible on conventional metal working equipment by machinists. Cast urethanes can be readily turned, sawed, drilled, ground and milled.
Softer urethanes [80A Durometer and below] are typically machined by knifing, grinding and sanding. Freezing softer urethanes with dry ice or liquid nitrogen can allow them to be worked like higher modulus materials, but adds cost.
- Because of urethane's low thermal conductivity cutting tools tend heat up which can significantly raise urethane temperature rapidly at the point-of-cut. Melting can occur above 350°F. Also, heat generated during machining will cause thermal expansion of the part, so when the part returns to ambient temperature, it could shrink and measure undersized.
- Elastic Memory - Elastic recovery occurs in urethane during and after machining. The cutting tool must provide clearance to compensate for this, otherwise expansion of the urethane as it passes the tool will result in increased friction between the cut surface and the cutting tool aggrevating heat build-up. Elastic recovery after machining can result in smaller internal diameters and larger external diameters than were measured during cutting.
- Urethanes are pliable and can be distorted by too much clamping force, resulting in the final machined shape to be distorted after the cuts have been made and the fixturing pressure released. Grasp parts securely without distorting them.
- Proper tool geometry, feed rates, and cutting speed, in combination with coolants are necessary to avoid gumming, poor finishes, and poor dimensional control due to excess heat otherwise generated. Cool water soluble cutting oils and/or light machining oils are good selections for urethanes.
When machining urethanes, appropriate safety equipment and personal protective devices should be utilized. Eye protection is a must, use approved safety glasses with side shields, full goggles or a full face shield if large chip pieces could fly out and hit the operator in the face. Fixturing is an extremely important consideration when machining urethane. Never start machining unless the fixturing is safe and secure. Improper fixturing can cause serious injury the operator if the work piece were to come out of the machine. Improper machining practices can result in excessive heat build up in the part being machined. If you notice smoke during the cut, stop and correct your process immediately. Avoid breathing smoke or grinding dust from urethane.
Machining tapers, chamfers, grooves, and other surface configurations into round parts are possible. Tools for grooving OD's in harder urethanes [95A to 85D] have the same basic tool geometry. A 10 to 20 degree front rake works best with no top rake. Higher rpm's (depending on the diameter of the work place), light cuts and moderate feed rates produce clean surfaces.
The best tool position is .025" to .075" inches below the center of the part being worked. A continuous ribbon coming off is desirable, but care must be taken to avoid it from wrapping around the work piece. Reducing the feed rate can usually avoid "wrap around."
Contouring softer urethanes than 95A should be done using a tool with 15 to 30 degrees of top rake. Lathe speed depends on the O.D. of the part. Generally, higher rpm works best where possible. Use slower feeds until experience is achieved. When machining urethane, it is recommended a full face mask be worn.
Urethanes 55A to 80A durometer can be ground successfully in a lathe using a grinder. We use low turning speeds [below 150 rpm] with the lathe running in reverse and a feed rate set .005 inches per revolution. Slower feeds improve surface finish or allow the removal of more material by taking a deeper grind. 40 to 50 grit wheels with a slight radius on the leading edge works well. Wheel RPM should be between 2100 to 3500. Polishing is possible with finer abrasives.
Urethanes above 80A usually require some type of coolant, but some can be ground dry. Coolant can be applied with a brush, nozzle or fine spray mist.
The grinder should be equipped with a dust collector and/or the operator wear an approved dust mask when grinding or sanding urethanes.
Spiral drills perform well. Their large flute area permits free discharge of debris, thus reducing binding / heat build-up. Frequent retraction to clear discharge is still necessary though as flutes will readily fill.
Cut with Knife
Close width tolerances can be achieved on 70A to 95A urethane wheels or disks using cutting tools made of high speed steel that are very thin and sharp. Best results are achieved with rapid feeds and turning speeds from 600 to 1200 1000 rpm. Carbide works best for medium and softer grades. This process, called knifing, will tend to pull the urethane into the tool leaving the finished end of the part cupped or dish shaped. This "pulling effect" on the cut edge can be reduced using tools with very thin blades, taking multiple thin cuts and / or using lubricants.
Harder urethanes (90A to 85D) can be milled easily. Tools must be sharp and the part securely held. End mills having large volume chip relief or single point fly cutters utilizing high speed tool bits ground to a round nose work best; 830 to 1350 rpm with feed of 15 to 20 inches per minute.
Parting tools 1/16" to 1/8" wide with 20° to 30° front rake and 0° top rake work well. A small 3/64 radius is ground into the top of the too with side clearance.
Band saws are best machines for sawing urethanes. Longer blades running at 2600 feet per minute having 4 teeth per inch with significant blade set alternating to the left and right of center works best because they stay cooler to prevent melting the urethane. Do not force the part being cut, let the blade cut at its own speed. Cutting thin low durometer sheet stock needs to have support like cardboard under it. Lubricate the blade with silicone or a spray mist of water-soluble oil (50-50) to reduce friction heat and to improve finish.
Face and Turning
Hard urethanes can be Faced and Turned to size using sharp tools, high turning speeds, and slow feeds with either carbide or high speed steel tools. The top of the tool should be very smooth to help chip reduce wrap around on the work piece, reduce heat build up on the tool and urethane. The cut urethane should come off as a continuous ribbon.