This How-to explains how to create a beautiful under glass coaster from a HDPE sheet (20mm).
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Step 1 - Cut the squares
Start by cutting perfect equal squares 10 cm apart. The more precise they are, the more beautiful your coasters will.
Make sure that you have a plastic sheet thick enough to be able to hollow out the circle that will accommodate the food of the glass. You need about 5 mm so use a sheet of at least 1cm thick
Step 2 - Hollow the circle (1)
Now that you have your squares, you need à lathe machine to do your hollow circles.
Wedge your square well in the clamping elements and make a first hole in the center of 5 mm deep.
Step 3 - Hollow the circle (2)
Now that you have your center hole you can start to do the rest of the circle.
Make sure to stop digging when you have 5 mm of edge left on each side.
Step 4 - smooth it out
With a polishing machine, round the 4 angles and smooth the faces if necessary.
Also round the edge of the hollow circle
Step 5 - Final result
Now you can drink a glass on a magnificent under glass coaster 
