I make my own guards and router bases from plastic. How can I drill clean holes without the bit grabbing the plastic?
A simple solution is to file a flat edge on a regular twist bit’s cutting. Use a small diamond paddle or ceramic stone to file its cutting edges flat.
It is also best to drill at a higher speed, reducing the time the bit is drilling and causing heat to build up. Even then, a burr of melted plastic may form, which can be easily removed with a deburring tool or a bit of sandpaper.
As you’ve discovered, a standard twist bit is too aggressive for plastic, so it tends to grab and sometimes crack the work as it comes through the bottom. Also, plastic easily melts when drilled, causing molten plastic to adhere to the bit and make a gooey mess that quickly hardens.
Specialized plastic-cutting drill bits are available. They are ground to a sharp point and have a flattened cutting edge that scrapes rather than cuts the plastic. This design serves to reduce grabbing. These bits work great but can be difficult to find in your local hardware store and cost twice as much as your typical twist bit.
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File a flat on the face of the cutting edge using a diamond paddle or a ceramic stone. The flat should be in the same plane as the bit shank.
The modified edge produces a scraping cut that results in smooth cuts in plastic without the grabbing and cracking common when using regular twist bits.