I make my own guards and router
bases from plastic. How can I drill
clean holes without the bit grabbing the
A simple solution is to file a flat edge on a
regular twist bit’s cutting edges (see photos,
below left and center). 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 (see
photo, below right). 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.
Click any image to view a larger version.
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.