The Design of Plastic Cutting Tools

As CNC routing has become more common in plastic fabrication, cutting-tool geometry has become increasingly specialized. Different plastics produce different chips, respond differently to cutting forces, and require different combinations of rake, clearance, flute geometry, and tool material.

Router tooling was historically developed primarily for wood and aluminum. Both materials can be machined effectively with a relatively limited range of basic cutting geometries.

Plastics are different. Composition, thickness, temperature, reinforcement, and desired edge finish can all influence the cutting process. This has led to the development of application-specific router tooling designed specifically for plastic machining.

A useful starting point is to divide plastics into three broad machining categories: soft plastics, hard plastics, and reinforced plastics.

Soft Plastics

Soft plastics generally produce long, curled chips during routing. These chips can separate cleanly from the workpiece with little burring or fuzzing.

Because soft plastics are typically less abrasive and impact-resistant than wood or aluminum, cutter rake angle can be increased significantly. A higher rake allows the cutting edge to release the chip more easily, which can support faster feed rates while reducing lateral cutting pressure.

Rake and Clearance

The tradeoff of a high rake angle is increased cutter aggressiveness. Dedicated CNC plastic tooling can become difficult to control when used in hand-routing applications.

To manage this aggressiveness, soft-plastic tooling often uses a low-angle radial or eccentric relief grind on the clearance surface.

Chip Evacuation

Chip evacuation is another major consideration in soft-plastic cutter design.

If chips become trapped inside the flute, they can heat rapidly, reweld to the workpiece, degrade surface finish, and accelerate cutter wear.

One solution is to reduce the number of cutting edges, increasing the available flute area for chip evacuation.

Hard Plastics

Hard plastics form chips very differently from soft plastics. Instead of long curled chips, they typically produce small crystalline fragments or dust-like particles as material breaks away from the workpiece.

Hard-plastic tooling also benefits from increased rake, but generally does not require the extremely high rake angles used for softer plastics.

A moderate rake angle helps the material fracture cleanly while maintaining enough cutting-edge strength to resist damage.

Preventing Cratering

Hard plastics can exhibit a machining defect known as cratering. If rake angle becomes too aggressive, chips can pull additional material from beneath the finished surface and leave a dimpled or cratered edge.

Modified O-flute and straight-rake-face geometries are commonly used. Because hard-plastic chips are relatively small, multi-flute spirals can also provide good chip evacuation and high-quality finishes.

Reinforced Plastics

Reinforced plastics commonly use polyester, epoxy, or phenolic resin systems combined with fiberglass or other embedded reinforcement.

The reinforcement adds strength to the finished material but makes it significantly more abrasive and difficult to machine.

Free-Cutting Geometry

One approach is to use a high rake angle and high clearance angle. This allows the cutter to move freely through the material and can reduce heat buildup.

The drawback is reduced wedge strength. The thinner cutting edge can become more susceptible to chipping and premature breakdown when machining abrasive reinforcement.

Strong Cutting-Edge Geometry

The alternative is to reduce rake angle and clearance angle, creating a stronger cutting edge.

This improves resistance to chipping but increases cutting pressure and heat generation.

Lower spindle speeds may be required with stronger cutting-edge designs to control heat, but reduced RPM can increase cutting forces and place greater demands on part fixturing.

Common reinforced-plastic cutters include spiral and straight-rake-face tools using either radial clearance for stronger, lower-RPM cutting or straight clearance for freer cutting at higher spindle speeds.

Tool Selection

Soft, hard, and reinforced plastics provide only broad starting categories. Many applications require additional changes in cutter geometry based on material thickness, temperature, fixturing, or combinations of different materials.

Examples include acrylic/ABS combinations, laminated phenolics, and co-extruded PVC/ABS materials.

As plastic formulations and CNC routing applications continue to develop, cutting-tool designs will continue to become increasingly application-specific.

Conclusion

Plastic cutting-tool design requires substantially more variation than traditional wood or aluminum tooling.

Rake angle, clearance, wedge strength, flute geometry, chip evacuation, cutting-edge count, and spindle-speed requirements all need to be matched to the way a particular plastic responds during machining.

Choosing tooling based on the specific plastic category and application can improve edge finish, feed rate, tool life, and overall process consistency.