O Flutes For Mechanical Plastics

Mechanical plastics are steadily becoming the material of choice for job shops and machine shops throughout the country because of their durability, machinability, and mechanical and electrical properties.

These plastics have proven to demonstrate excellent performance in gears, bearings, material-handling parts, and other machine components such as spacers and positioning mounts where reduction of vibration is essential. Common mechanical plastics include ABS, Acetal, Delrin, Hydex, UHMW, nylon, polycarbonate, polyurethane, and polyethylene terephthalate (PET). These plastics have consistently demonstrated predictable performance and are replacing metal for parts manufactured to resist wear.

O-Flute Geometry and Chip Formation

In terms of machining, many mechanical plastics can be classified as soft plastics. Soft plastic utilizes "O" flute router tooling that tends to curl a chip during the machining process. This tooling has been designed to attack soft plastics with a high rake and low clearance geometry that actually carves the material.

Tool Configuration and Chip Evacuation

"O" flute tools are manufactured from micro grain solid carbide tool material in straight and spiral configurations. The upcut, or right-hand spiral, is most readily utilized because of the need to evacuate chips in an upward direction in flat sheet or block applications. Upward movement of chips avoids welding, which is a common problem in the machining of plastic.

The tools are available in either single or double edge cutting diameters. A single edge tool is an excellent choice for most machining applications and can accommodate situations requiring smaller diameters. Care should be taken with single-edge tooling above 3/8 inch because of balance issues.

If larger diameters are needed, double-edge tooling can alleviate the balance problem while providing an improved bottom finish for slotting applications. Double-edge tools also provide longer cutting edges for deeper cuts of two to four times the cutting edge diameter at aggressive feedrates.

Router Bits vs. Traditional End Mills

In shops with high feed and speed CNC routers, the use of router bits is common practice. The benefits of the tooling are understood, but this is not always the case in shops with CNC mills. In these environments, the tool of choice has traditionally been the end mill.

These tools are intended for metal removal and do not possess the proper geometry to effectively machine mechanical plastics. End mills have minimal rake and low clearance and were designed as robust cutting tools for heavy loads. Minimal flute area on multi-edged tools can also interfere with chip clearing, aggravating melting and rewelding problems common in mechanical plastic applications.

Machining methodology in many shops also remains based on practices associated with milling metal. Feedrates and spindle speeds tend to be slow relative to the capability of today's CNC machining centers, while climb cutting with multiple passes may be used to enhance finish. These practices can adversely affect productivity and profitability.

High-Speed Machining of Mechanical Plastics

The use of "O" flute router tooling represents a different approach to high-speed machining of mechanical plastics. By selecting a tool properly designed for cutting soft plastics and making a few basic changes in machining methods, the task of producing parts from mechanical plastics can be greatly reduced and the potential of the CNC milling machine can be more fully realized.