Center Drill Tip Broken Off In Hole

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Center Drill Tip Broken Off In Hole

Post by btvarner » Sun Feb 02, 2020 12:09 pm

Hoping for some feedback before Super Bowl starts today..............

I just broke off the tip of a small Center Drill. Got as far as about half way up the counter bore area & snap. I am sure that I was feeding it too quickly. Carbide center drill in 4140.

Before I chuck this already fully milled to size piece, any suggestions on how I remove the tip, or drill through it. Final reamed hole is going to be 3/8" and must be centered on where the broken tip now is. I am afraid that trying to drill it out will force the hole off center as the HSS drill bits encounter the carbide piece of tip.

Any ideas? Thanks!

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Re: Center Drill Tip Broken Off In Hole

Post by Magicniner » Sun Feb 02, 2020 12:36 pm

Drill under size from the opposite side and punch it out.

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Re: Center Drill Tip Broken Off In Hole

Post by btvarner » Sun Feb 02, 2020 12:53 pm

I thought about that. So I am clear, you mean undersized from the final diameter, not undersize from the size of the center drill tip, correct? Plus, I assume the since one is coming in from the opposite side, the drill will just force out the broken tip?

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Re: Center Drill Tip Broken Off In Hole

Post by BadDog » Sun Feb 02, 2020 2:24 pm

To try to answer, undersize from final is the limit, but anything big enough to get a pin punch through should do. But expect to waste the point on the punch drill, and the angle of the point, unless the web is big enough) is going to make a ring that will resist punching through.

I haven't ever dealt with that, but some random thoughts.

You may be able to shatter the carbide tip with a punch. Sometimes works for broken taps, but they aren't completely encased, not sure how it will work in your case.

Hit it with a TIG arc. Thermal shock may shatter it.

If the part has enough value (time, material, etc), it may be worth making a custom coring drill, assuming suitable material is at hand. Doesn't have to work great, or last long, just enough to get the broken tip out. If that's PH 4140, which seems likely given a carbide center drill was used, this becomes much less likely.

Assuming you weren't actually creating a center socket, but rather locating a drill start (based on "fully milled"), consider getting spot drills for future efforts. I don't like using center drills unless I actually need a center socket, too easy to break, particularly in smaller sizes. Center drills don't like ANY misalignment under penalty of breakage, and carbide makes that far worse. That could be the cause of your problem as opposed to too aggressive feed.
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Re: Center Drill Tip Broken Off In Hole

Post by btvarner » Sun Feb 02, 2020 4:22 pm

Thanks for the response. This helps a lot. I am still learning the in's and out's of machining. I have now studied several videos on center drills & on spot drills. Think I understand the usage for each. I have now ordered some spot drills. I also have a larger #5 center drill. I was told that if you only make a dimple using a center drill, that will be the same 120 degree angle as the subsequent twist drills and will work. Will measure and try that on the opposite side later today or tomorrow.

Will drill through the opposite side until it just reaches the front of the broken tip. Then I guess use a punch to see if I can push back (dislodge) the broken piece back through the original side. Let me know if my approach could be better.

Will respond back when completed. Thanks!

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Re: Center Drill Tip Broken Off In Hole

Post by whateg0 » Sun Feb 02, 2020 5:35 pm

You don't have a sinker EDM for just such an occasion?


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Re: Center Drill Tip Broken Off In Hole

Post by pete » Sun Feb 02, 2020 6:17 pm

I broke a few tips off my center drills and then a few more when I bought a new, larger and seriously misaligned lathe before I figured out how to use them and what was going on with the lathe. Russ is also more than right about about that misalignment problem. But even when correctly aligned you have to consider how fragile the tips are on the smaller center drills. Feed just a bit too fast or for too long and the flutes choke up on the cuttings and that tip will get twisted off. You have to feed them as gently as you'd do so with a comparable sized jobber length twist drill and clear the flutes even more often. A standard carbide drill at that size in a commercial operation would almost for sure be run in an extremely accurate collet and at much higher rpms than our manual machines can hope to do. The body OD of those center drills has nothing to do with how durable the tips are. And once the 60 degree portion starts to take a chip it's then even easier to load up the flutes on the smaller tip area. I also make sure to use the correct cutting oil when using them, not so much to help them cut, but to lubricate and help the chips exit a bit easier. If you have enough excess part length left then cutting your losses and parting off the material deep enough to get below where the tip is lodged might be the easiest method. I've drilled through from the opposite side as Russ mentioned, and yeah it won't do much good to that drill tip either. And mine were only HSS center drill tips.

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Re: Center Drill Tip Broken Off In Hole

Post by choprboy » Sun Feb 02, 2020 6:20 pm

Another thing you can try (if the hole still needs to be enlarged) is to use a small cutter-cutting endmill (or a stubby stout drill with pretty flat face) and drill 3 holes around the perimeter of the broken off carbide tip. Then you can usually use a small punch or screwdriver blade to loosen the tip and work it out.

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Re: Center Drill Tip Broken Off In Hole

Post by mcostello » Sun Feb 02, 2020 8:05 pm

If You could slot the tip of the broken off center drill and make the slot a little bigger than the broken off piece it will core the broken piece out.

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Re: Center Drill Tip Broken Off In Hole

Post by Harold_V » Mon Feb 03, 2020 2:35 am

What mcostello said. A HSS centerdrill can be slotted (larger slot than the diameter of the broken tip), then plunged on location. The slot will leave the center untouched, but the centerdrill will create a circular slot around the broken segment. The broken tip can then be removed with a small punch. This method allows for the broken tip to be removed when a hole can't be drilled from the opposite side.

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Re: Center Drill Tip Broken Off In Hole

Post by Russ Hanscom » Mon Feb 03, 2020 9:39 am

Thanks, A one of those why did I not think of that concepts. Might even have a broken center drill to test with.

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Re: Center Drill Tip Broken Off In Hole

Post by spro » Mon Feb 03, 2020 10:08 am

Concepts are so cool. If it could be drilled from the other side, a bum end mill of four flute. They are more hollow in the center and can be ground in the center for just this one job. Coming from the opposite side, it would unwind the tip.

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