Thread: How to drill concentric circles

Get your 2.5mm bit and tightly wrap paper or Al foil around the end of the bit until it just fits into the 5mm hole.
Put some wax onto the outside of the foil/paper so that it slips easily inside the the 5mm hole.
The direction of the wrap should be such that the outside edge of the paper does not catch on the edge of the 5mm hole.
You only just need to start the 2.5 mm hole then remove the paper/foil.

The way I would do it, is using the lathe to make a guide bush.

If pretty close is good enough, I would just get a felt tipped marker, an run it around the outside of the hole, this should leave a small dot in the middle where the marker didn’t touch. Then just close one eye, and line up the 2.5 drill bit on the dot,and drill away

If you have a drill press, place a 5mm drill bit in the chuck. Drop the drill bit into the 5mm hole and clamp the workpiece to the drill press table. Replace the 5mm drill bit with a 2.5mm bit. Drill 2.5mm hole. Given the 5mm hole is only 6mm deep, a 5mm diameter pin would be better than the drill bit if you have one. Or maybe you could chuck the 5mm bit upside down?

These are handy to have - Transfer Punch Set (mcjing.com.au)

What's the substrate? (object and material).

If you had them on hand a 5mm wall plug would make a reasonable guide bush

What's the substrate? (object and material).

Get a small offcut of clear Perspex from a plastic sheet supplier, cut 2 pieces in one drill a 5mm hole in the other drill a 2.5mm hole and then stick them together, line the 5mm hole up with your existing hole and then drill through the 2.5mm hole with the drills hammer action turned off,

BobL, the substrate is PVC

Many thanks for all the suggestions above, they were real eye openers. The things you learn on this Forum

BobL's suggestion of using 5mm wall plugs proved very successful. They were long thin white 5mm diameter plugs (about 20 years old, cannot recall what I used them for) and the 2.5mm drill bit fitted snugly inside it. The plugs were long enough and strong enough to keep the drill bit straight