Author Topic: Strange broken drill  (Read 2937 times)

Offline Roger B

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Strange broken drill
« on: November 29, 2014, 06:07:35 PM »
I was drilling a deep (40mm) 3mm diameter hole in my CI cylinder head using what the local shop call stainless steel drills. This one was quite happy chewing it's way through the lump of P20 mould steel I used for the ER chuck and most other things I had tried it on.

I was regularly withdrawing it to clear the swarf, but when I got to 35mm it suddenly stopped  :o  The shank had twisted off at the end of the chuck  :toilet_claw:

The remaining part was easily removed with a pair of pliers and is still useable ( if slightly shorter  ;) ) I finished the hole off with normal 3mm drill without any problems.

Any ideas what happened?
Best regards

Roger

Online b.lindsey

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Re: Strange broken drill
« Reply #1 on: November 29, 2014, 06:47:31 PM »
Not the usual place to break I agree Roger. Perhaps it was either a faulty blank or prior use had somehow fatigued it in that area. Good that you were able to extract it though.

Bill

Offline Jasonb

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Re: Strange broken drill
« Reply #2 on: November 29, 2014, 08:04:43 PM »
Did the hole wander?

Offline Roger B

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Re: Strange broken drill
« Reply #3 on: November 29, 2014, 08:26:03 PM »
Not significantly. When I replaced the drill it broke through as expected into the water passage at the base of the cylinder head. I would estimate less than 0.2 mm deviation in a 40mm deep hole.

My first thought was that it had jammed in the hole, but I was able to twist it back out easily with a normal pair of pliers. These drills seem to have finer pitch than 'normal' drills (and also cost a little bit more)  :headscratch:
Best regards

Roger

Offline derekwarner_decoy

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Re: Strange broken drill
« Reply #4 on: November 29, 2014, 08:44:15 PM »
This is also a 3.0 mm drill & stamped HSS  :lolb: .... was used in a drill press with a keyless chuck...& drilling through 2 mm sheet steel that was adequately clamped........

My only thought was that they popped out @ lunchtime to get some fried rice   :ChineseBow: & forgot about the heat treatment process...... Derek
« Last Edit: November 29, 2014, 09:02:57 PM by derekwarner_decoy »
Derek Warner - Honorary Secretary [Retired]
Illawarra Live Steamers Co-op - Australia
www.ils.org.au

Online b.lindsey

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Re: Strange broken drill
« Reply #5 on: November 29, 2014, 09:11:06 PM »
Would have to agree Derek, if it bend like that without a clean break then it probably missed heat treating altogether!!

Bill

Offline Oldboy

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Re: Strange broken drill
« Reply #6 on: November 30, 2014, 01:51:35 AM »
Looks like a faulty blank or your drill ate all it's pinch.

Barry

Offline Florian Eberhard

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Re: Strange broken drill
« Reply #7 on: November 30, 2014, 04:55:14 AM »
Hi Roger

Well, probably the drill bit shaft is soft as it is usual on bigger drills.
Then, if you were drilling a deep hole, is it possible that some small chips got stuck betwen the Shaft and the hole?

However, I know that drills with much cobalt are more brittle (though you say the shaft was twisted where it broke ??) Than the usual hss drills.

Cheers Florian

Offline derekwarner_decoy

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Re: Strange broken drill
« Reply #8 on: November 30, 2014, 05:32:49 AM »
If we look closely at the 3 mm diameter drill failure from Rodger....it presents with a necked profile at the point of failure similar to a tensile failure in extension

If the time point of failure had occurred when drilling [downwards]...would we not expect see a compressive failure profile?  :noidea: ......

With my 3 mm diameter drill failure......it is clear that some form of heat treatment occurred as we see an oiled heat treatment blue/grey surface peeling off just at the commencement points of the relief flutes from the drill shank.......Derek
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Offline Roger B

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Re: Strange broken drill
« Reply #9 on: November 30, 2014, 06:54:06 PM »
I think that Fabian my be on the right track. I was regularly pulling the drill out and brushing the chips away, but one could have got caught and jammed. I'm not actually sure if the drill was cutting when it failed or was still on the way in. The failure looks more torsion than extension. However I don't wish to repeat the experiment to be sure ::)
Best regards

Roger