Metalwork ...HELP!

[Aragorn]

Hi folks
Apologies in advance to the purists and pedants amongst you who would consign a metalwork related topic to the "Off-topic" section, but I really need help so I'm posting it here. So there!

I'm drilling holes into the ends of some ½" metal rods and then threading them to accept an M4 bolt.
Problem is the tap (or is it the die :roll: you can tell I'm not a metalworker) has broken off in the hole :? and I cannot get it out. I've now ground it flush hoping to re-drill it, but I can't get a drill bit to even dent it! I'm using a power drill. I did try rigging up a jig to hold it under the drill press, but it didn't help. I'm pretty sure the noise the drill press made was laughter!
Any advice very greatly appreciated! How can I drill out the hole???
How can something so simple be so difficult?!

 

[DaveL]

Aragorn,

You have snapped a tap off in the end of your piece of work this is not good because as you have found out its as hard if not harder than the drill you have tried against it :shock:. Removing this it quite trickly, don't suppose you have a spare length of metal?
OK thought not. :twisted:
One way of doing this is to soften the tap by heating it. Do you know anyone with a welding set? One of my friends uses his arc set to heat broken taps or drills by holding the rod on the broken end until its red hot, then letting it cool. After this with a good sharp drill you should be able to drill out the now softer tap . Of course if the metal has a finnish on it, it may be affected by the heat.
In a machine tool shop they would use a spark eroder to remove the tap, something I am quite sure you don't have under the bench :wink:

 

[frank]

aragon it will be quicker and cheaper to get another piece of rod , as dave said it will be a @@@@@ to get the tap out

frank

 

[Aragorn]

Thanks for your ideas!
Hmmmmmmmmm looks like I'm b@$%£$d!!
Unfortunately I can't replace the rod. It's part of a finished welded assembly that was made up for me. Threading the holes was the last job before installation

 

[Midnight]

Aragon...

been there.....done that... believe me, I sympathise. The core of your prob is that you're trying to drill into hardened High Speed Steel with a normal HSS drillbit; the drill is always going to struggle. If I remember right, Sandvik produce a series of specially hardened bits for this kinda work; calling them drills is really streaching their description a bit far. In practice, they're designed to bludgeon their way through the remnants of the tap, exploiting the brittleness of the hardened HSS. This said, I wouldn't hold out much hope of recovering the hole, certainly not to the extent that you'll be able to re-drill it to suit M4. The extractor tends to migrate all over the end of the broken piece as these never seem to break with a flat, flush face. Any rag end will serve to deflect the extractor into the much softer material you're tapping into. If at all possible, the best set up would be to lock the workpiece into suitable vice on an upright drill, bolt the vice to the table, align the extractor as precicely as you can and pray that it works. The drill should run at its highest speed; the feed rate should be dead slow and stop, real gentle to ensure the extractor gets the chance to cleave the ragged end of the tap.

IF it manages to clear all the remnants, you'd be best to try to go up a thread size; a 4.2mm drill is best for an M5 thread. Next time, use plenty cutting compound on the tap, and back it out of the hole frequently to clear the swarf.

I hope you get something that works for you.... it's not the first time I've had to bite the bullet and start over because of this...

 

[Anonymous]

if you can at least get a start of a hole in the body of the tap could you try using a small screw extractor that has a left hand thread? if that bites you MIGHT be able to carefully unscrew the body of the tap. try lots of penetrating oil on it as well

another extreme possibility would be to see if you can find some web plans for a DIY spark eroder - years back I vaguely remember one of the motorcycle magazines had an article on making one from an old spark plug, car battery and coil and using a distributor cap turned by hand to trigger the sparks. Snapping off steel bolts and studs in soft alloy engine housings was/is a common problems and has much in common with what you face - getting a hard piece or metal out of a much softer, easily damaged and generally very expensive component

Or phone round a few local light engineering workshops, steel fabricators or motorcycle workshops and see if anyone near you has one and could try removing it for you - would be at your own risk though.

If you can get the tap out would you be able to go upto an m5 or m6 hole and sleeve it back down to m4 - again, common practise on motorcycle engine blocks.

m

 

[Keith Smith]

Aragorn, one possible solution would be to cut a slot across the end of the rod and the offending tap with a thin metal cutting disc in an angle grinder, and then use a flat blade screwdriver to unscrew it. Heat the rod to expand it before trying to unscrew.

It may work, and I can't think of anything else.

Keith

 

[desmoengine]

hi aragon
kieths method is one that i would try cutting slit with likes of dremel.
another way i have used in the past is to use 3 small drill shanks (if its a 3 flute tap) that fit down the sides of the flutes ,heat the rod with blow torch of some kind ,grip dril pins with pair of pliars and try to ease out the tap.
the chances are it may have bottomed out in the hole, the end pressure could be relieved by drilling from other side.
there id=s no way you will drill out the tap as they are tyically made of HSS and hardened.
best of luck.

Dave W

 

[Aragorn]

Many thanks for your suggestions. I'll be giving it a go today.
Cutting a slot across the top is a great idea - so simple but I would never have thought of it!
If that doesn't work I'll try the three drill bits method.

Thanks again

 

[Adam]

I've done the same thing before , like you, on something I didn't have time to "re-machine". Just spent 5 weeks in the machine shop manufacturing it.

In my case I was able to use the spark eroder at Brighton college of technology. I still know someone (very vaguely) down there..... if you were to grease a few palms.....

I'll check if we've got a spark eroder here also.

Alternatively, you could try ringing a few engineering firms - how about the one opposite PC World?

Adam

As an aside, were you using cutting compound, - and the correct side drill bit to a deep enough depth? I can't remember the pilot hole size for a tap of that size but you need to get the exact (no matter how obscure) size bit. And clearing the flutes by regularly unwinding/winding and letting the cut waste away?

 

[Adam]

I had a chat with the chaps in the machine shop - they suggested grinding a centre punch to a sharp point and carefully tapping it with hammer, at an angle against the tap in an attempt to "unwind" it. Sometimes bits break off, and if they do, you need to get them out and carry on. (hence the punch needs to be quite slender as you will probably be below the surface of the metal) It has not been unknown for the entire tap to be removed by breaking bits off until the very remaining bit shatters. (and can be removed).

They suggested spark erosion as the #1 best way to remove them. They also mentioned you can buy specialist tap removing spark eroders - as this is something that is quite common in industry.

Adam

 

[Aragorn]

Thanks Adam
I've tried the slot grinding method above. The Dremel cut a slot just fine, but the screwdriver bits just snapped when I tried to unscrew it. Couldn't budge it at all. I've tried tapping it as you suggest to try to unwind it. It just ain't moving.
I've given up now and have a solution (bodge) that doesn't involve removing the tap.
Next time I need to tap a hole I'll be posting here first to get the right method!!

 

[Anonymous]

Aragorn

Sorry t ohear of your probs.

I thought I'd pass on some advice (after the event!!) given to me 20+ years ago as an apprentice engineer.
When tapping holes, always use cutting compound and only turn the tap 1/2 turn by hand before backing it off a little to break off the swarf. This is especially true for small (<6mm) taps. I would not advise placing the tap in a battery drill either.

Cheers

Tony

 

[Anonymous]

further tips for future reference:

check the depth of hole then mark up the tap so it won't bottom out - masking tape/insulation tape wrapped around it usually fine.

If possible make the hole far deeper than you need for the bolts to go in so you always have plenty of clearance - if the bolt bottoms out and you overtighten you can strip the whole thread out or shear the bolt off and have the same problem all over again

If you are tapping a blind hole and need maximum depth of thread start with a taper tap, then finish of with a bottom or plug (think those are the terms - its been a while) tap to cut the threads right to the bottom. If you start with the plug tap its much harder to keep straight and its a much more aggressive cut needing more force.

Alway tap by hand - even if you hold the tap in the pillar drill vice to keep it square, turn it by hand

As Tony says - cutting compound, keep emptying the swarf out and small cuts - with a 3 flute tap max 1/2 a turn, back 3/4 to snap the swarf off, take up the slack and repeat ad nauseum

don't you just hate wise-a#*e advice after the event

 

[Aragorn]

don't you just hate wise-a#*e advice after the event

LOL
I'm rather enjoying it actually! I've managed to patch the problem up so to speak, so I can relax a little bit now.
But I never knew there was so much I didn't know about tapping holes. I'm grateful for all the pointers and suggestions.

 

[Anonymous]

and the learning point I'll take from your misfortune - do all your machining etc of components before putting the final thing together - its a lot easier to remake a component if something goes wrong than after its fixed to everything else.

what was the workaround in the end?

M

 

[frank]

aragorn what are you making :? ??? and will you post a piccy (curious from piddle farting)

frank

 

[Aragorn]

Well it's nothing particularly exciting....
I've been making up oak frames to house a couple of display menus for a restaurant. They needed to be displayed on the outside of the restaurant which is mostly glass and the only solution I could find was to make up a metal frame like a bracket which screwed to the only available wooden bit of the exterior.
The menu boards are held to this bracket on eye hooks that ride along the metal rods that I've been talking about before. One end of the rods needed tapping to hold a washer with a security bolt so that the menu boards can't be removed by lager louts.
I had the metal brackets made up for me because I don't have any welding equipment, but I intended to do the drilling/counterboring and tapping myself. Everything went fine until the very last tap :roll:
When I realised I wasn't going to solve the tapping problem easily I installed the frames and menus and found the fit to be nice and snug. The frame with the broken tap has its other rod threaded so there's no chance of it being removed by lager louts. I've just superglued a false washer/security bolt-head to the "broken" rod just for looks.
There - told you it wasn't very interesting, and I don't know how to post photos otherwise I could have saved these 1000 words...!

 

[Bean]

Aragon
I know I'm a little late on this thread, and it may seem over the top but always tap small holes with a Small Tap Wrench. When I was a toolmaker I made a small one for just that sort of job.
As for getting them out everybody has come up with all the best advice, although I have had some success with a pair of dividers or small circlip pliers, points into the tap flutes and turned as a screwdriver.

Best of luck

Bean

 

[Alf]

Aragorn, sorry to hear of your troubles. I'm rather late to this (internet deprived for the last couple of days) but for my own curiosity would a cobalt drill bit have been any good? I presume not, as I don't think anyone mentioned them, but as this may well happen to me one day I thought I might as well ask. :roll:

Cheers, Alf