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This seems like a stupid question, but I cannot figure it out ...

I have a couple of masonry hole saws (carbide tips) that I am using to drill holes through a cinderblock foundation wall for various purposes. I have a mandrel for the hole saws. It accepts a 1/4" pilot bit.

But since I'm using it for masonry, the pilot bit needs to be a masonry bit. So I bought a 1/4" masonry bit. But guess what ? The shank of masonry bits is usually smaller than the carbide tip at the end of the thing that does the actual cutting. So my 1/4" masonry bit drills a 1/4" hole, because the carbide tip is 1/4" wide, but the shank is a bit less.

This means that when I put it into the 1/4" hole in the mandrel, it is actually off-center. Worse, it's at an angle, because of the way the set-screw pushes on the side of the shank. The result is that the carbide tip of the pilot bit wobbles. For the 10" bit I'm using (to get through to the other side of the cinderblock) the wobble is quite bad.

Question: how do I source a masonry bit that has a round shank of exactly 1/4" diameter ? This is not something that is spec'd. Am I missing something ? This seems like a silly problem that many people have to deal with.

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  • Seems to be a good question, especially for people who need to buy a specific length drill bit. Would get expensive fast buying different bits to find one that fits well. I am guessing either a 5/16 or 3/8 would have the 1/4 inch shaft.
    – crip659
    Aug 23, 2022 at 18:46
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    Shopping the wrong places, I think. mcmaster.com/drill-bits/… which supports 5/16" as a good bet, but also lists 1/4 with 1/4 shanks. Beware of the hex shanks as they may not center correctly with a round hole+setscew setup. Just a happy customer - not the cheapest place to shop, but sometimes that's the less expensive way than cheap stuff that ain't right and you have to buy something else; several times... Or buy a masonry hole-saw with arbor and it should have the right drill bit included as part of the package.
    – Ecnerwal
    Aug 23, 2022 at 18:51
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    Drill the 1/4” hole with a real masonry bit (no holesaw), then follow with the standard holesaw bit/holesaw and know that it’ll get destroyed in the process. Aug 23, 2022 at 19:08
  • Yeah, except I'm going through cinderblock (the hollow part). So I've gotta use a long masonry bit to start the far side (after I've drilled the big hole in the near side). And that long masonry bit needs to be in the holesaw/mandrel, so the far side pilot hole is centered on the near side big hole. Or, maybe I drill both pilot holes at once ! Aug 23, 2022 at 21:20
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    pilot drill with a long mansonry bit, fit a piece of 1/4" reinforcing bar (or other inexpensive steel rod) in the hole saw.
    – Jasen
    Aug 23, 2022 at 22:35

1 Answer 1

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Find a copper or steel tube, ~1/4" O.D. and I.D. large enough for the masonry bit's shaft. Cut it so that when the bit is inserted in the chuck, it's not a tight fit lengthwise, so the bit can rotate and act as a sleeve bearing.

You might put some graphite or other lubricant inside the tube to reduce friction.

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  • That'd be some mighty thin-walled copper tube, but I'm going to be messing with a 1/4-1/2" refrigerant lineset soon and that might just do it. I don't understand about "not a tight fit lengthwise". Aug 24, 2022 at 4:17
  • In other words, don't make it so long that it can't turn freely; don't get it jammed between the large carbide bit and the drill chuck. Aug 24, 2022 at 15:38
  • Usually with hole saws they are locked to the drill bit shaft. Can work if a hole is drilled in the tubing so the allen/set screw can lock to the shaft.
    – crip659
    Aug 24, 2022 at 17:43

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