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Machining Polyurethanes

To most people, “machining” means transforming raw metal into a specific shape and size. However, wood, ceramic and plastic can all be machined as well.

Our clients in oil and gas, mining, manufacturing and food processing have all discovered cast polyurethane parts can also be machined into excellent products with a wide range of uses.

Since urethane compounds exhibit a variety of physical characteristics, they can be turned, sawed, drilled, tapped, ground and milled. Machinists who haven’t yet worked with polyurethane should experiment to get a feel for the speeds, feed rates and tools that work best for each method. If you have experience machining plastics, you shouldn’t have any problem lathe turning, fly-cutting, grinding or contouring urethanes.

At Mark Tool, our production machining services include custom-machined urethane rollers and related accessories (a few examples of which can you can see here) are less expensive than metal to produce and are more durable. Our polyurethane rollers are used in the automotive, food, agriculture, textile and oil and gas industries. They come in a wide assortment of sizes and shapes, from hourglass to V-shaped.

Elastomeric Roller

Elastomeric roller machining and grinding

Things to Remember

In general, machining urethanes calls for sharp tools, high turning speeds and slow to moderate feeds. Cutting tools for urethane must have sharp, carefully honed edges. High-speed steel and carbide tools will produce the best results. One of the best machines for sawing urethanes is a band saw. A cutting edge that might be considered sharp enough for metal might be entirely unsatisfactory for urethane.

If you’re used to working with metal, you’ll find there are some quirks when machining PU components. Harder urethanes (90A and up) work well with conventional metal-working equipment. Urethanes of 80A durometer and lower usually call for knifing, grinding or sanding. In some cases, machinability can be improved if the material is first “frozen” in dry ice or liquid nitrogen.

Machinists must control heat to avoid gumming and poor dimensional control. The heat generated by machining can cause your part to expand, then contract when it cools. This can give you the wrong shape. Urethanes have much lower thermal conductively than metals so that they may melt above 400 degrees Fahrenheit. Proper tool geometry, feed rates and cutting speed usually overcome these problems. It also helps to use coolants (water-soluble cutting oils or light machining oils work great).

Parts must be secured while you’re working them. However, urethanes can easily be distorted, so avoid clamping with too much force. Proper fixturing and chucking of the workpiece are critical. Tool clearances must be greater than those used for metal.

You want little or no resistance as the tool travels through the urethane. The chip (material that is being cut away) should come off as a continuous strip or ribbon. High durometer urethanes (95A and up) can be turned very easily. Remember also that centrifugal forces on large, low hardness parts can cause “flaring” (deformation) at high turning speeds.

Three Polyurethane Machining Processes

  • Milling is used when parts that can’t be machined in a lathe or when close tolerances and a good surface finish are required. Urethane ranging from 90A to 75D durometer can be readily milled with two-fluted end mills, single-point fly cutters and high-speed tool bits ground to a round nose. The cutter speed should be 900 to 1300 rpm; start with a feed rate of 15 to 20 inches per minute.
  • When grinding 55A to 80A durometer polyurethanes, use a tool post grinder in an engine lathe. Low turning speeds (below 150 rpm), with the lathe running in reverse will yield the best results. If you’re removing lots of material, first turn the piece down with a cutting tool to within .020 of the finished dimension before grinding. Urethane above 80A durometer may require coolant, such as water applied with a brush or with a fine spray mist. Fine abrasives are great for polishing the end product.
  • When drilling polyurethanes, slow, spiral drills perform best. The larger flute area means chips can fall away quickly without a lot of heat build-up. Retract the drill often, and you’ll avoid having chips block the flutes. Slow the drill at the bottom of the hole to reduce break-out tearing at the exit side. Here again, sharp cutting edges are vital to minimizing distortion.

Safety

Grinding or sanding urethane generates a lot of dust, so make sure you use a dust collector and an approved dust mask with a good face seal. Wear safety glasses with side shields or full goggles. Whenever chip pieces or workpieces could fly out, a full face shield is recommended. Another thing to remember: Improper machining can generate smoke, so avoid breathing these fumes if something goes wrong and

From conveyor belt rollers to sheave rollers, Mark Tool can strip, cast, cure and machine polyurethane parts to meet your custom specifications. We can also resurface coated rollers and help you find the right coating for any roller application. Get in touch with us today and let us know how we can help. 

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