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

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.

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.

With a polishing machine, round the 4 angles and smooth the faces if necessary.

Also round the edge of the hollow circle

Now you can drink a glass on a magnificent under glass coaster :)