Shipwright Alan Campbell shows how to cut rounded corners using a hole saw
The boat we were refitting recently had to have some port lights with tight rounded corners cut into the newly replaced marine ply cabin. I know some people have trouble doing this job – so here is how I achieved it.
Having traced the outline of the pattern we’d cut for the actual opening size, I selected the circular cutters, which would cut the correct radius of hole required to make the corner. So how do you position the guide-drill so that the edge of the cut hole coincides exactly with the traced curve of the opening? Anyone who has tried doing this merely by eye will know that nine times out of 10 you’ll get it wrong.
The answer is to make a paper or card pattern of the hole saw’s outside edge and mark its exact centre. The corners of the mounting hole of this cutter can be joined with straight lines to mark the centre of the circle. You could also fold the pattern in half at right angles – where the folds meet is the centre of the circle. You now have a circular pattern of the hole size the tool will cut, with its centre accurately marked.