Milling out a broken tap
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  1. #1
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    Default Milling out a broken tap

    I am having some tapping problems which is another subject. I now have an M6 tap stuck in a 316 hole, broken off. It isn't that expensive a piece, but I'd like to try to mill the tap out of the hole. I've tried this before and broke the mill off in the tap ;(.

    So I'm wondering what the tricks are if any. I can center on the hole as the part is fixtured. I think what happened the last time was the end mill started to break up the 3 flute tap, and the bits jammed the end mill. Tap is HHS. Since the pilot hole is 5mm (but I have no 5mm end mills) I was going to try a 4 flute 3/16 end mill. It seems like there is still the possibility of the tap flutes breaking up as the end mill eats the middle, causing chip jamming and breakage.

    What are the secrets to this?

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    I've had my best luck doing them on the manual knee mill running the spindle as fast as I can and hand pecking with the quill. An old hand regashed 5mm or a 3/16 stub em in this case and then picking out the pieces after busting them with a punch. I take my time and not pushing the quill too hard when it starts cutting you will feel it smoothly cut. If it stops cutting I hand regash again the face of the flutes with a diamond wheel and go at it again. Near 100% success after you get use to doing it.

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    Quote Originally Posted by swarf_rat View Post
    I am having some tapping problems which is another subject. I now have an M6 tap stuck in a 316 hole, broken off. It isn't that expensive a piece, but I'd like to try to mill the tap out of the hole. I've tried this before and broke the mill off in the tap ;(.

    So I'm wondering what the tricks are if any. I can center on the hole as the part is fixtured. I think what happened the last time was the end mill started to break up the 3 flute tap, and the bits jammed the end mill. Tap is HHS. Since the pilot hole is 5mm (but I have no 5mm end mills) I was going to try a 4 flute 3/16 end mill. It seems like there is still the possibility of the tap flutes breaking up as the end mill eats the middle, causing chip jamming and breakage.

    What are the secrets to this?
    I have had good luck removing a broken HSS tap this way:
    Start with a 90 degree carbide spotting drill / chamfer tool that is as short as possible and get a spot going on that broken tap.
    Next, drill out 99% of the tap with a straight flute carbide die-drill.

    Feed with the hand-wheel set to the smallest increment and don't blow air or coolant.
    Feed in slow, back out, clear the chips with air, but don't hit the drill with cold air.
    Listen carefully for a change in sound. You don't want to break through if you can avoid it.

    If you break through, you're likely to have the flutes of the tap snag the flutes of the drill and make a big mess of the whole works.

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    Grab a carbide ball end mill of appropriate diameter (3/16), wind it up as fast as you can, .002” pecks 2 ipm. Same thing can be done with a manual winding the depth stop down a couple thou at a time. Don’t try and “only” peck .002” by hand ask me how I know.

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    I usually take a broken off Endmill or a carbide blank, grind what looks like a flat head screwdriver on the end and whittle it out. Try to be .02 to .05 diameter smaller than the tap drill I used.

    Do it with the hand wheel or the pulse feed, with dickloads of air blast.

    I've also done it with a pencil grinder and free hand it with a carbide burr.

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    We just had a 1/4 snap in aluminum just under the surface in a blind hole.

    Used a high speed hand piece with some dental burrs the dentist gave me.

    Best one was an ugly looking one that was diamond covered and it was slow and required lots of dripping ATF as coolant and watching the bits come out like flower.

    It took time but was able to cut in 1/2 at the web then pick it out.

    Tap extractor did not work as the tap shattered under and jammed up.

    Never had much luck with end mills as our taps usually harder and ruin the mill.

    Good carbide with proper holding may work but others can offer better advise there.

    Sent from my SAMSUNG-SGH-I337Z using Tapatalk

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    Interesting suggestions.

    Unfortunately I lost my test subject - I casually tried the tap extractor which has never even come close to working in anything, and the tap screwed right out.

    It sounds as though others share my belief that it is the tap flutes coming apart that cause the problems when trying to mill one out. I wonder if a slow helical interpolation right at the minor diameter of the thread would work better? The pieces shed by the tap body would be smaller than just taking out the center.

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    In the shop I was working we were removing broken taps with a sinker EDM. I am using now a waterjet. A short blast in tap middle brakes it pieces without affecting the work-piece. Seldom did I have a success by milling.

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    Quote Originally Posted by swarf_rat View Post
    Interesting suggestions.

    Unfortunately I lost my test subject - I casually tried the tap extractor which has never even come close to working in anything, and the tap screwed right out.
    Christmass time of miracles, low value replaceable work piece and a serious back up plan is enough to make the impossible possible then, you gotta be the first ever reported success story with a tap extractor!

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    Quote Originally Posted by Hazzert View Post
    Grab a carbide ball end mill of appropriate diameter (3/16), wind it up as fast as you can, .002” pecks 2 ipm. Same thing can be done with a manual winding the depth stop down a couple thou at a time. Don’t try and “only” peck .002” by hand ask me how I know.
    Agreed, ball end mill works the best so far as a milling tool solution. It will center on the often rough, angular surface of the broken tool. Because the 'corners' are absent already, there are none to break off so it will stand quite a bit of abuse. Once you get a nice dimple, it's pretty easy progress after that.

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    Default Milling broken tap

    When it's worked at all, my best success is exactly as you said, helical interpolation. Lots of rpm, low forward and ramp down feed, make dust.

    When it fails, sinker edm.

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    I would helical interpolate with a 1/8" end mill with corner radii or ball and mill. High RPM, low feed, and .001-.002" depth of cuts. The end mill is trashed after but it gets the tap out enough you can pick out the remaining pieces.

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    Depending on how deep, If its less than an inch i would do as followed: Helical down wish small ramp angle and a square carbide cutter. Lots of air to blow the chips out. If its deeper I would peck with an old ball end mill, Its gona get beat up so you might need to use a few.

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    Quote Originally Posted by swarf_rat View Post
    Interesting suggestions.

    Unfortunately I lost my test subject - I casually tried the tap extractor which has never even come close to working in anything, and the tap screwed right out.

    It sounds as though others share my belief that it is the tap flutes coming apart that cause the problems when trying to mill one out. I wonder if a slow helical interpolation right at the minor diameter of the thread would work better? The pieces shed by the tap body would be smaller than just taking out the center.

    Slow helical interpolation with an old beat up endmill is the best way I've personally found to remove a broken tap. .002" depth, retract, blow the dust out, do it again. I used to do this on a prototrak knee mill to save my own goof ups when I was an apprentice.

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    Getting a busted tap out is never fun... My last memorable one was 362 days ago.

    3/8 fine 1.6" deep in a $300 piece of material..

    My method has always been moderate speed, light feed.. 1/4" endmillS. 200sfm or
    so, couple tenths of feed. Clean it out after every peck.. I got it out, took damn near
    half the day, but I didn't even ding the threads up.. I really took my time on that one,
    there was a big pile of $$$$ on the line.

    MY observations from removing busted taps.. It sucks.. You need to be PATIENT..

    You are much better off if the tap didn't crack into a million pieces, so don't try shattering
    it with a punch before you try milling it out.

    The second you hear *crunch*, that means another piece broke off and your endmill is f'd..

    Stop OFTEN, and clean everything out, small picks (dental or Harbor Freight) work really
    well to check if anything is loose.. If you can pull out a piece or break it off.. Do it.
    Those long skinny needles that you can put on your air nozzle are great for getting down deep
    in there.. I think McMaster Carr is where I got mine from, they come in an assortment of 3 or
    4 different sizes.

    Beat up endmills with chipped corners that can't be used for anything else are the perfect tools
    for getting taps out, you're going to destroy the endmill anyways, so might as well start with
    one that is already a mess.

    The biggest tap removal tip I have.. Don't break the damn thing in the first place.

    Shop rule #1) If you break a tap, you get to clean a sump.

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    Quote Originally Posted by scadvice View Post
    I've had my best luck doing them on the manual knee mill running the spindle as fast as I can and hand pecking with the quill.
    This is how I do it too.
    Use an old shitty end mill, it can even be chipped. Use air blast. The carbide will glow red but it will still cut.
    Last edited by Mtndew; 12-20-2017 at 03:21 PM.

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    I would just get the tap sparked out and save a lot of wasted time trying to do a bodge job, milling the tap out. Too many occasions of causing even more damage to the thread by attempting this for it to be worth doing.

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    Quote Originally Posted by Bobw View Post
    Getting a busted tap out is never fun... My last memorable one was 362 days ago.

    3/8 fine 1.6" deep in a $300 piece of material..
    ...The biggest tap removal tip I have.. Don't break the damn thing in the first place.
    And that's why anything critical I don't tap deeper than 1 or 2x D and hand tap it the rest of the way.

    On the bright side, a 5/16 or 8mm BN is the perfect size to split a 3/8

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    Endmill smaller than the web...spinning the piss out of it down the center...air and a steady feed. Then knock the flutes in

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    I have done a few with success on mills with a hand feed quill, others were a learning curve. The best thing I've found is the flat drills that are pointed, highest RPM and enough pressure to heat things up and get the chips to start. Problem is when drilled deep enough the flutes will collapse and break the carbide drill if you don't catch it soon enough. What I found is to stop early and try to break the flutes off with a small punch and get them out as I go, then continue on. All I can say is give it a try and good luck. Did I forget to mention the chips look more like powder than chips.
    Dan


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