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Problem - when cutting holes for fans using a dremel tool the holes often come out pretty ragged looking.

Solution - use a pattern. A thick piece of plastic with a precut hole sized to the dremel tool. You clamp the plastic pattern to the work piece and then cut out the hole with the dremel tool by just pressing against the pattern and it should make a perfect hole every time.

Method - Dremel tools are standard sized. a manufacturer could make a set of patterns for 60mm, 80mm, 120mm out of 1/4" plexiglass and sell them as a set for a couple of dollars.

Explanation - This is an old wood workers trick to make complex shapes in wood using a router (oversized dremel tool) The pattern takes in to account the size of the router base and duplicates the original shape, the wood worker just presses the tool against the pattern as he cuts the wood and produces a perfect copy every time.

Grep