Drilling precise holes in concrete without bit wandering

Question

I need to mount some 4" house numbers on a concrete wall (drilling 3/16" holes for studs extending from the numbers). Normally when I drill into concrete, it's hard to get a hole in an exact location (even using a center punch to force the bit to start in the right place). I assume it has to do with the hammer action of the drill and aggregate within the concrete making the bit want to go slightly off center, but usually the final hole is a tiny bit (a few mm?) off from where I originally marked.

This is usually fine since whatever I'm mounting doesn't need to be in an exact position, but with small house numbers, one of the holes being a little off will mean the studs won't fit in right or the numbers will be a little tilted or not aligned right. Is there any tool or technique to help accurately drill holes in precise locations in concrete?

Answer 1

Start with a small bit. (or maybe a diamond dremel bit.)

You still want to use a punch, then use a 1/8"th bit, or smaller if you can find one. Then step up to the size you need for your anchor or screw. The small hole will help guide the larger bit.

Alternatively you could make a jig;

Drill a hole of the size you need to guide your masonry bit in a piece of wood and then use some double sided tape to stick it to the wall. The hole in the wood keeps the bit from wandering.

"Not all those who wander are lost"

If, after drilling, they do not line up you could plug the hole with a wood dowel and adhesive and drill that out.

Answer 2

The technique is to angle the bit after it walks to get it back on track. Doesn't work if you do a pilot hole, and probably not until you've done several hundred Tapcons. But even I could mess this up, and if it was unsatisfactory I'd mount them to a placard.