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ChattJeff 07-10-2010 11:19 AM

Wobbly core drill?
Hello, all. I'm in the middle of a project to install some decorative metal fence posts in an existing concrete patio. I rented a core drill, but I have a question about it. No matter how tightly I try to chuck the bit, there's a huge wobble in it. I'm talking a couple of inches at the bottom of the bit. It seems like either the drill is missing something in the chuck, or maybe that's just the way it's supposed to be? It sure doesn't seem right. Do you have to guide the bit somehow until it gets a bit below grade? It's a Hilti drill with a stand and suction base.

Ron6519 07-10-2010 12:55 PM

If this is a bit with a patterened shaft that goes into the drill, you might not have it set correctly. Bring it back to the rental and have them check it out.

jomama45 07-10-2010 02:03 PM

Do you have the centering bit in the core, or did it get lost or fall out?

ChattJeff 07-11-2010 08:30 AM

solved-sort of
There was no centering bit in the core bit, I don't know whether it was supposed to have one or not. Since it was a low-rpm application, I made a jig to hold the core bit steady where it met the concrete. It worked pretty well, since I only had to drill two holes and the rental place isn't open on the weekend.

I do think it was a problem with the equipment I rented. I think the drill chuck had some damage that kept me from tightening it as much as it should be. I guess rental equipment gets used hard and maybe isn't maintained well. For what I paid for it, though, I wish it had worked better.

Thurman 07-11-2010 09:14 AM

NO centering bits in concrete core drill that I have ever seen. I have drilled probably thousands of holes in concrete with core drills. NO, the core drill should not wobble but maybe 1/4" max. at the bottom IMO. It's a rental unit--take it back for a number of reasons. 1) IF you break that core bit--you WILL pay for it, they are very expensive. 2) The safety factor here, you don't want to get hurt at all, and certainly not at the expense of a faulty core drill. 3) They own it, it's their problem. That's not rude, it's a part of rental equipment practice. I'm betting that the entire unit was dropped on another job bending the core drill shank near the chuck. You stated you are tightening the bit, this must be a small diameter core drill, under 3" or so. IF you still have the core drill, lay just the core drill on a reasonably flat surface and roll it by hand watching the shank which fits into the drill for wobble. The Hilti units are good units. With the vacuum base it may not want to stay "sucked down" to your concrete patio when pressure is applied to the core bit because of the slightly rough surface. One trick we used to use was to put duct tape in a pattern which matched the base of the drill for better suction. That's one reason I prefer the "bolt-down" pads on these. IF the unit is not equipped for attaching a water hose, do use a small trickle of water where the drill enters the concrete for lubrication, to bring the drill dust to the surface, and to keep the core drill cool. Be careful, David

jomama45 07-11-2010 10:56 AM

1 Attachment(s)
I guess I missed the part about the core rig...... :whistling2:

Here's a core bit for a hand held spline drive though......

Attachment 22430

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