How to change M12 to 1/2" BSW drawbar thread?

I have a MT3 arbor tapped to M12 thread which must be modified to 1/2" BSW 12 tpi to match the drawbar in my milling machine.

The drawbar cannot be changed as it is captive in the machine and it also expels the arbour when loosened. The manual for the machine (a Steinel) specifically states that the drawbar is to be nipped up by finger pressure only and not overtightened, so the new thread shouldn't be too stressed.

Option 1 would be to use a helicoil repair kit, costing near £23 from Chronos. This doesn't seem too bad as M12 is much more common as a drawbar thread so I'll probably use it several times. My concern with this method is that after drilling out for the helicoil tap there won't be much metal left at the top of the taper on the arbor, and the insert would need to be right at the top because the drawbar only engages about half an inch into the arbor. If the helicoil insert was to pop out stuck to the drawbar it would be a catastrophe and necessitate stripping down the whole head to repair. OK maybe not a catastrophe, but certainly a disaster. Everything's relative of course...

Option 2 might be to fill the threaded hole in the arbor with brazing rod, then drill it and tap it. Would the braze be strong enough for the drawbar? Would this even work?

Option 3 would be to MIG weld the threaded hole to then drill and tap. But would this harden the steel of the arbor so that it couldn't be machined?

Your advice and suggestions would be most gratefully received!

Steve

Edited By SteveM on 29/08/2015 00:50:22

I would look into using a Time-Sert thread repair rather than Helicoil. The Time-Sert is stronger and more likely to stay in place because it is a solid threaded sleeve that is in effect swaged into place on installation. A bit of high-strength Loctite on the OD might be helpful too.

See this video here https://www.youtube.com/watch?v=anjDQJtWFc8

Brazing is likely to wear more quickly than a steel Time-Sert. And both brazing and MIG welding pose potential problems with heat distortion and heat treatment of a precision tooling item like an arbor.

Hi Steve, just my beginner input.

Is the MT3 arbor part of a tool already or is it a blank arbor? If it's just a blank arbor it would be cheaper to just buy another with the correct thread rather than threading kits.

Ed.

Hi

I've just completed a mod on my I/2 " BSW draw bar to accept arbours with M12, M10 and 3/8 BSW.

My captive draw bar is easy to remove, so I cut the end off and drilled and tapped it M8, then I made adapters to screw in. The pics show the before and after.

This does require the draw bar to be removed each time a different threaded arbour is used, If that's not acceptable then the same mod could be done but with the adaptors made to screw tight into the arbour, just provide a flange to tighten up against.

I did notice some arbours require a longer draw bar by 3/4"

Nigel

#2 for Timesert & high strength Loctite. Drill is fraction larger than the helical one so gives, to my mind at least, a slightly cleaner fit. The timesert also has a slight flange round the head which fits into a countersunk recess cut by a tool included in the kit which gives a clean end and proper tapered lead in to the thread. Always assuming you can get BSW timeserts.

As a helicoil is basically a spring expanded into the hole by a combination of its own resilience and the component being screwed in rather than a positively fitted device I always feel that it's potentially less reliable when faced with multiple screw in / screw out cycles. Especially if end thrust when loose, as required by the extraction effect, is involved. Probably more prejudice than engineering but thread problems with a tightly inserted arbour and captive drawbar are not pleasant to contemplate. Definitely puts the in in inaccessible!

On reflection I see no reason why you couldn't make your own Timesert type device. Thin "top hat" ended sleeve with any convenient thread on the outside, cut a whisker oversize for a good fit, and a not quite finished thread cut with a taper or second cut tap inside. Fit with the highest strength loctite you can find making sure that the rim of the tophat sits cleanly down on the end of the arbor. Run a well fitting bolt down it to expand the base ensuring that all is stable. I've done similar before now and expanding the not quite finished thread really does help stability. OK I found that out by accident as first time was a desperation, 11 pm and "got to get to work in the morning", vehicle fix. Only a taper tap in the box and not enough clearance to go right through so things just got wound in. Found out latter that there was an official fix using helicoils but reliability was said to be iffy. My fix lasted at least 15 years!

Clive.

If your MT 3 arbor is anything like the ones I have, I think you have a problem, mine are hard.

Ian S C

Hopper - Thanks for the tip and video about Time-Serts. I watched it and straight away could see it would probably do the job. But then I spoke to a UK supplier and it seems Time-Serts are not available in BSW. They just don't make them.
I agree that a braze infill would be softer than the steel, but given that it is never going to be more than gently tightened would it be hard enough for the job?

Ed - I agree, but the arbor is a good quality shell mill arbor.

Traction - excellent idea and execution, but not for my mill I'm afraid. It's just too much of a job to strip the drawbar from the head.

Clive - thanks for the interesting idea. I'll think on it. I have another variation on that theme in mind which I'll post later when I get a chance.

Ian - I ran a tap down it to clean the thread and got a couple of littly shavings out so assumed machining would be no problem, but thinking back that could have just been debris from a previously used drawbar. So I'll need to check. If it was to hard to machine as it is, could it be softened without distorting the taper?

Distortion of the taper is obviously a risk with heating for brazing or welding - has anyone out there done something similar on morse tapers without problems?

Cheers,

Steve

If it's MT3 I feel there is another option. Make a new draw bar with an M6 thread on the end. You can then bush arbours what ever size they are with all thread or a piece you have made yourself, loctite it in and drill and tap M6. The draw bar could be the same diameter as the existing one apart from the end. It's easier to make your own piece of all thread as the M6 can be added at the same time rather than trying to hold the arbour.

Being a dore westbury owner I have to do this because it will only take a 1/4in draw bar. I intend to add small hand wheel to it at some point and arrange for the draw bar to eject as well.

John

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Hi Steve,

Why not just run an 11mm drill down the existing threads then re tap 1/2" BSW?

Peter.

Thanks for the suggestions guys.

John - the drawbar is captive, so I can't replace it without stripping the head. Which I really don't want to do!

Peter - The problem with simply drilling and re-tapping is that the tapping drill size quoted for M12 is 10.8mm and for 1/2" BSW it's 10.4mm. So hardly any metal will be removed and after tapping the BSW threads will overlap the existing M12 like crossthreading. Which I'm thinking will mean that none of the threads will be fully formed. Does that make sense?

Is it possible to just shorten the MT3 taper and then fit a homemade conversion into the end to regain the correct length. I'm assuming that there will be enough grip left in the remaining taper and the conversion extension is made smaller od than the existing taper so no need for clever matching up.

Hi pgk, yes I reckon that might be the way to go provided the arbor is soft enough to machine.

1. Cut off say the top say inch of the taper;
2. Drill and tap the arbor;
3. Make a new stub. It'll be just like an Allen bolt with a big head.
4. Loctite the new stub onto the arbor, then drill and tap it to 1/2" BSW. Might even pin it to ensure it never comes out.
5. Trim the new stub to size in the lathe.

This way there'll be no welding or brazing to distort the arbour.

That should work!

Steve

Try putting a file down the thread to see if its hard.

Ian S C

I have to assume you know your own equipment, Steve (I certainly don't), because it would have to be seriously difficult to remove the drawbar to make it worthwhile going to these lengths for each and every arbor.

OK right now I feel like the village idiot.

Turns out 'captive' drawbar is a misnomer - 'gently restrained' would be a more accurate description!
It was held in by a by just a castellated nut that I had thought was part of the spindle. I can't believe that I couldn't see it earlier, and when I look at it now it is so obvious.

So no need to mess about with the arbors, I can just make a new drawbar.

Sorry to have wasted your time.

Steve

Fine Steve were all learners and not immune to the obvious we have all done it.

Every mill ever had have had three drawbars. Top tip don't silver solder or braze it wont hold for any length of time. Bore a hole to accept decent fitting male, cut a V, weld and turn down after.
The off the shelf threads are weak better off using a HT bolt welded to round and or the head on other end readily available.

Remember you don't need to tighten the drawbar very much especially for MT collets. A length of studding will be just fine, more than strong enough.

This has been an interesting and valuable discussion. Pity it didn't conclude with a proven method. Not everyone has the headroom to casually swap drawbars in a vertical head, or wants to leave the clearance behind a horizontal for the purpose.

I agree Bazyle, it was interesting way before it became embarrassing. I suspect several of the methods would work just fine. Machining a new stub on the end of it would certainly work if it was soft enough, and the infilling of the thread with weld would work if it could be accomplished without distorting the arbor taper.

I'm a bit surprised that nobody has chipped in about the welding option because it would be such a quick fix I'd have thought people had tried it already.
But there you go.

Thanks to all who contributed,

Steve

Not a waste of time, it made you think, and look a bit closer, and in the end probably saved you putting your foot in it, there ain't no silly questions, only dumb mistakes.

Ian S C