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I need to drill 4 10mm holes through 50mm thick HDPE and then countersink with a larger diameter (ca 20mm) hole to a depth of 20mm. The material in question is in fact a step made out of recycled soft drink bottles. I have often drilled through thin (6mm) sheets of acrylic but this is a while new ball game. Off my own bat I would probably do the following

  • Use a blunted twist drill bit or a step drill bit to make the 10mm hole
  • Use a 20mm forstener bit to make the countersink

However, before I embark on the job and ruin the step for good - or worse - I thought I would seek a few ideas here. I should mention that I do not have a drill press.

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Reverse the order of operations - drill the counterbore (it's not a countersink, as described) first, then drill the through hole starting from the point of the Forstner in the center of the counterbore. If you don't have a piloted counterbore (ie, a 20mm bit with a 10mm guide pin) this order of operations will make a much better counterbore than small hole first.

Have scrap wood under the step when drilling, so you drill through into the wood, not into air. You might even want to clamp the step and the scrap wood together.

  • That worked perfectly. A note for anyone running into this thread - the plastic "springs back" significantly once it has cooled down after drilling. In order to allow for this for the 10mm through hole I had to drill to 11mm . – DroidOS May 28 '17 at 4:32

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