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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.

RustyShackleford
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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 '22 at 18:46
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    Shopping the wrong places, I think. https://www.mcmaster.com/drill-bits/round-and-hex-shank-masonry-drill-bits-for-hammer-drills/ 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 '22 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. – Aloysius Defenestrate Aug 23 '22 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 ! – RustyShackleford Aug 23 '22 at 21:20
  • Two good answers in these comments guys. Believe I'd have to pick @Ecnerwal , if you care about such things. – RustyShackleford Aug 23 '22 at 21:24
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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 '22 at 22:35
  • McMaster-Carr doesn't actually list a bit with 1/4" shank that's long enough to get through a cinderblock. I think I'll go with the idea of drilling all the way through with the long masonry bit I have (the one with less than 1/4" shank diameter) and then using a sacrificial regular 1/4" bit with the hole saw. – RustyShackleford Aug 23 '22 at 23:11
  • Another option is to just rent an SDS drill with the correct bits and get those holes drilled in 5 minutes. – whatsisname Aug 24 '22 at 06:20
  • @whatsisname Don't the same issues exist with an SDS drill ? – RustyShackleford Aug 24 '22 at 22:42
  • This is hilarious: I "asked a question" on Amazon about a Bosch 5/16" masonry bit, asking what the shank diameter is. The answer: 0.3125" :-) – RustyShackleford Aug 25 '22 at 00:33
  • @RustyShackleford there are SDS bits big enough to bore your hole the "normal" way, as in not using a hole saw style. – whatsisname Aug 26 '22 at 02:39

1 Answers1

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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.

DrMoishe Pippik
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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". – RustyShackleford Aug 24 '22 at 04:17
  • 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. – DrMoishe Pippik Aug 24 '22 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 '22 at 17:43