Q: Plastic is notorious for welding into the cut. What causes this phenomenon and how is it resolved?

A: Plastic material welding in the cut is caused by heat, poor chip evacuation (causing a recutting of chips) and improper chip load. This problem can be solved by using a tool with proper geometry for the plastic being cut (an upcut spiral cutter may evacuate chips more efficiently) and by finding the proper chip load through increasing the feed rate and/or decreasing the spindle speed.

Q: What chipload should one be looking at with a CNC router?

A: The chipload should be the maximum that can be achieved without adversely affecting the work part finish or taxing the hold-down system. On plastics the chipload will vary from 0.004 to 0.020 depending on the plastic material. Generally, softer plastics have a smaller chip load.

Q: Many times the plastic will wrap around the router bit during the plunging process. How can this be avoided?

A: Chip wrap can be controlled by programming techniques employing a ramping or zigzag plunge into the material. If you are hole drilling, the chip wrap can be controlled by using a pecking method whereby the downward feed is stopped several times, the tool is removed from the hole, and then plunged again to a greater depth. Tool geometry can also assist in reducing chip wrap with fish tail style points and upcut spiral geometry.

Q: At times the scrap has a better finish than the work piece. Why does this happen?

A: Check the feed direction. In most cases, conventional cutting (counter-clockwise direction) will give a better finish than climb cutting (clockwise direction). Climb cutting, however, can be effective when using a finishing pass in hard materials.

Q: What is the best tool for acrylics?

A: Many people prefer larger diameter three flute finishers for cast acrylic. Single edge straight or spiral O flutes for extruded and both single and double edge spiral O flutes for stretched acrylics are recommended. All tools should be solid carbide.

Q: What is the best tool for thicker plastics such as HDPE?

A: Thicker plastics such as HDPE are normally in the softer range of materials. Two flute solid carbide tools with soft plastic geometry are recommended.

Q: What is the difference between wood and plastic tools?

A: Plastic tools have different geometry. Higher rake angles, different clearance angles and shape of the flute all provide for better chip removal required to cut plastic materials more effectively.

Q: Why does the workpiece finish decrease in quality with smaller diameter tools?

A: There are a couple of reasons this can occur. Smaller diameter tools may deflect more than larger diameter tools and this will affect the finish. Also, chips may not clear as well with smaller diameter tools; the surface footage of a small diameter tool is less. Increasing the spindle speed may improve finish. Plastic geometry tools are recommended.

Q: What are the most important factors for maximum CNC router production (with acceptable finish)?

A: The formula to follow is: productivity = solid hold-down system, proper maintenance, fast feed rate and the right tool for the job.

Q: What are the most important factors to obtain optimum workpiece finish on a CNC router (with acceptable production)?

A: To obtain the optimum workpiece finish, the rule of thumb is: finish = solid hold-down, proper maintenance, proper chip load and a larger diameter tool with proper plastic geometry.

Van Niser is director of Plastic Application Engineering at Onsrud Cutter. Readers are invited to send questions to Van Niser at Onsrud Cutter, 800 Liberty Drive, Libertyville, Il 60048; e-mail: vanniser@onsrud.com. 

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