Cutting a circle out of a sleeper to a certain depth

Question

I have been looking about online but cannot find anything that actually covers what I need to achieve, my mission is to cut a hole in a sleeper, for solar lights. The sleeper is deeper than the hole needs to be, the hole needs to actually go down about half the depth of the sleeper. Is there a tool I can use to achieve this?

I was thinking about a hole saw attachment, however I do not think this will extract the wood if the depth does not exceed the total depth of the sleeper. I hope this makes sense.

Will appreciate the help, thanks.

The circle is 64mm.

Answer 1

A forstner bit will result in a clean bottomed hole. Although 64mm is a slightly obscure size, you could expect a 64th of slop from a 2-1/2" bit. (Alternate choices include self-feeding bits, though you'll have to be careful they don't go too deep.)

A hole saw would work as well, if you were willing to chisel out the waste. You could achieve a flat bottom in the hole, but it would be time-consuming.

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Minor edit to cover a couple of points by @graphus ...

• a large forstner bit could 'walk' away on starting the hole if used with a hand-held drill, but a bit of practice goes a long way. A workaround would be to drill a hole first in plywood, and clamp that plywood onto the sleeper as a guide for the forstner. FWIW, I could find vendors (Lee Valley, Woodcraft) that talked about hand-held use, but none that actually suggested otherwise. This wouldn't apply to angled holes, where a drill press is a must.

• Amazon and Woodcraft, to name a couple, have 2-1/2" forstners for between 20 and 30 dollars. I can't vouch for their quality, but I expect you'd get quite a few clean holes out of them, especially in softwood.

• self-feed bits have a spur in the middle, which makes them much less prone to walking, but the only sizes I could find near your needs were 2-9/16, plus they're likely to create a rougher edge, so you'd need a big flange on your lights.

• one last thing that I learned today is that a big bit like this wants to be run slowly in order to keep the rim speed down to prevent overheating: ~450 rpm or so...

Answer 2

I was thinking about a hole saw attachment, however I do not think this will extract the wood if the depth does not exceed the total depth of the sleeper.

That's correct, and in addition you could easily find it nearly impossible to withdraw the hole saw from the wood (for the same reason that it can be difficult to remove the plug from a hole saw when it is used to cut through a board).

Probably the ideal tool for this is a Forstner bit (or saw-tooth bit, similar to a Forstner and often mistaken for one):

^{Source: About Forstner/Saw Tooth Bits on Lee Valley.}

There are two buts though, and unfortunately they're both biggies.

The first is a safety issue: in general Forstner bits (and bits of a similar design) are not intended to be used freehand, but instead with a drill mounted in a drill stand, or in a drill press/pillar drill. Although many people do use them freehand chucked up in their power drills this is not good practice and the manufacturers of the bits themselves will often warn against it — "For use in a drill press only" or words to that effect. The greater the diameter of the bit the more caution you want to take on this front, and 64mm is a very large bit.

The second is that a 60-70mm Forstner bit is fairly specialised and as such is not cheap. Expect to pay around $50 or the equivalent for a good one.