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Old 07-03-2014, 23:03   #1
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Broken Off Allen Wrench in the Prop

Last September, I had the boat out of the water and had to tighten the nut on my Brunton prop. I hadn't expected to be doing that and didn't have my good set of Allen wrenchs to hand. The prop nut is secured with a bolt which comes through the side of the prop bus to wedge against a flat of the prop nut.

So what happened? I used a bicycle three-ended Allen wrench to tighten the safety bolt, and the damned thing broke off . It was already time to splash, so I left it like that. Now I'm going to lift the boat out again, and I want to get the broken off bit out. It is conceivable that I will need to tighten the prop nut again, in which case it will be absolutely essential to get the broken off bit out. I will technically have just an hour in the slings, although Cowes Yacht Haven are so nice that they will let me stay a bit longer if needed, but this is their busy season so I don't want to obstruct their operations.

So what's the best way to be sure of getting it out?

Here are a couple of methods I've thought of:

1. Drill into the allen wrench stub with a cobalt drill bit; put in a screw extractor and wiggle and jerk out the stub. The only problem is I'm not sure whether a cobalt drill bit will drill into tool steel, even the obviously inferior tool steel used here. I'm also not sure that the screw extractor will be hard enough to bite in.

2. Drill into the allen wrench stub with a cobalt drill bit, and tap the hole, put in stainless screw, wiggle and jerk out the stub. Again, I'm not sure of being able to drill into it, and I'm not sure that a tap will be hard enough to cut threads into it.

3. Try to pry out the stub with jeweler's screwdrivers. Don't know if there will be enough space. Also don't know how recalcitrant the stub will be after six months under sea water.


That's all I can think of, and none of it seems all that promising!

Anybody have any other ideas? I will be very grateful for all suggestions.
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Old 08-03-2014, 00:08   #2
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Re: Broken Off Allen Wrench in the Prop

Dockhead,

weld another Allen head driver to the broken bit; then pull it out...

If that doesn't work weld a larger rod to the screw head itself and replace the screw.

Oliver
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Old 08-03-2014, 00:57   #3
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Quote:
Originally Posted by Oliver L.
Dockhead,

weld another Allen head driver to the broken bit; then pull it out...

If that doesn't work weld a larger rod to the screw head itself and replace the screw.

Oliver
That would be great, except the screw is recessed in the bronze prop boss
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Old 08-03-2014, 01:11   #4
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Re: Broken Off Allen Wrench in the Prop

#2 with a fresh titanium drill bit and some cutting oil. Use a bottoming tap and use the cutting oil for it as well. Then vice grips on the machine screw, plus heat and/or penetrating oil.
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Old 08-03-2014, 01:18   #5
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Originally Posted by minaret
#2 with a fresh titanium drill bit and some cutting oil. Use a bottoming tap and use the cutting oil for it as well. Then vice grips on the machine screw, plus heat and/or penetrating oil.
Thank you! Titanium rather than cobalt? I thought cobalt was the hardest?
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Old 08-03-2014, 01:30   #6
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Re: Broken Off Allen Wrench in the Prop

The first thing is try and break the corrosion that will have formed. Heat, penetrating fluid and some taps with a hammer and say a punch will be needed.

I think you will have to drill it out, or try an easy out, but with the hard steel that will not be easy.

A very strong magnet might just provide enough grip. I doubt it, but it would be worth a try. Some of the rare earth magnets are amazing.
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Old 08-03-2014, 02:15   #7
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Re: Broken Off Allen Wrench in the Prop

Dockhead,

what you refer to as "cobalt" is most likely Cobalt bound Tungsten Carbide. About the best you can get for this job, but expensive. What is referred to as "Titanium" is just a tiny Titanium Carbide coating on metal drills. Less expensive, but not nearly as wear resistant as the former.

Oliver
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Old 08-03-2014, 02:43   #8
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Re: Broken Off Allen Wrench in the Prop

After 6 months of salt water you may be lucky - the broken stub may not even be there.
Given your time constraints on the day I'd phone Brunton today and ask them to overnight a couple of spare Allen screws and a nut.

Easyouts small enough to grip in, say, a 3mm drilled hole are too flimsy and a waste of time. IMHO.

If the screw isn't too deeply recessed a disc on a Dremel can be used to cut a slot for a flat screwdriver.

My favourite technique needs preparation but is super-quick on site:

Take a cold chisel the diameter of the screw head. Grind the edge in such a way that, once it bites into the head of the screw, it can be used to undo the screw. Think about it for a minute.

Half the "V" edge is ground away on one side, the other half is ground away on the opposite side so that the tip has a vertical face on each side.
Each vertical face leads when rotated anticlockwise.

Look at a flat drill bit (for wood) to get the idea. Those cut in a clockwise rotation. Break off the centre start screw, reverse (and steepen) the cutting edges so that it would cut anticlockwise, and that's what your cold chisel needs to look like.
Hold with mole grips, hammer and twist.

Grind the chisel slowly or it'll be softened.
This isn't some untried theory, I worked this out forty-odd years ago repairing motorcycles.

If you don't manage it you're left with drilling. You might be able to tap a bigger thread but probably better to start fresh 120 degrees away (3-blade) with a clean-drilled hole.
Cobalt drills are great but brittle, so are taps. Suggest taking more than one of each.

Good luck!

Edit: Oliver L may have meant using a large welding rod and welding IT to the stub or the screw head. Anyone who's stick welded has had a rod stick to the work.
That might work but you need either a very skilled - or a very bad - welder...
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Old 08-03-2014, 03:04   #9
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Re: Broken Off Allen Wrench in the Prop

Dockhead

If I had to do what you need to do I might try a plain vanilla HSS (High Speed Steel) drill bit (as sold in engineering supply vendors, rather than hardware stores)

If it's a well used item (which it might as well be): Repoint (resharpen) it and try it - if a standard HSS drill will not make an immediate and proper conical dimple, no amount of "titanium" (a very thin coating of titanium nitride) or cobalt (an alloying constituent which confers toughness more than hardness) will bridge the gap.

(I'm assuming the sheared face is largely flat; if not, you might as escalate straight to the "tungsten carbide ball end mill" stage below)

More likely you can just use the same bit, choked up so the minimum possible amount protrudes from the chuck, and maybe repoint it once or twice if it's a borderline proposition (ie it keeps losing its edge)

You might get lucky if the allen tool was REALLY sub-par, which is consistent with it breaking off.

If it proves too hard for HSS (high speed steel), I would recommend using a tungsten carbide drill (ideally a stub drill), or a tungsten carbide TIPPED drill (not a masonry drill, unless you can repoint carbide) -- Karat, by Hawera are the industry standard for this sort of work;

- or better still, and what I prefer: a tungsten carbide ball end mill. It may bugger up the cutter (so don't use a borrowed one!), and you will need to run it in either a heavy duty industrial drill press or a milling machine, or the cutting tool will immediately shatter.

A milling machine is preferable because you can easily tweak the position of the workpiece relative to the cutter so that it ends up in the middle of the hex.
This is especially the case with a ball end mill which will happily cut sideways if you have a delicate touch: something no carbide drill will do.


You want a cutter which is maybe 10% smaller in diameter than the across flats dimension of the allen key. You should be able to pick out the remnants of the key, insert an industrial strength key (with a bit of grinding grit in the hex recess - a hoary but effective way of increasing the torque transfer of recessed drive bits, not just hex, but Posidriv, Phillips and the like)

Having used the usual methods to increase the chances of success: a high class penetrating oil, maybe some judicious heat, and a couple of decent whacks on a pin punch inserted in the hex recess, to shock the fastener axially and hopefully create a bit of clearance, particularly if the thread is packed with corrosion product

You will, with any luck, wind the fastener straight out. I agree with those who say FORGET about ezyouts. The spiral type are one of the most egregious examples of a triumph of optimism over accomplishment in common use, although if the recessed hex were to be rounded out, there is an unorthodox way of using them which can be effective.

I realise you may not be in a position to follow the advice in the middle section of this post, but you might find someone who can.
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Old 08-03-2014, 05:44   #10
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Re: Broken Off Allen Wrench in the Prop

Did you throw the broken three-way wrench away? If you still have it you could test any of the proposed drilling solutions in advance of the haul. I've always been big on testing before doing.

Also the use of a center-punch to create a dimple in the broken face might help with bit-wander and make it easier to keep the bit centered when you first start drilling.

Best of luck!
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Old 08-03-2014, 05:45   #11
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Re: Broken Off Allen Wrench in the Prop

Quote:
Originally Posted by bornyesterday View Post
That might work but you need either a very skilled - or a very bad - welder...

Or better still, a very skilled welder and a bad welding rod.

If you can get hold of a welder to assist, use a welding rod which is damp, and with the protective coating on the rod knocked off, that will help it stick first time.

Another suggestion, again involves welding.
You will need a piece of steel tubing with an OD the same size as the OD of the bolt head (or a tad smaller), it has to be able to fit into the recess.
Cut short, so it extends about 1/2" above the recess.
Push the tubing into the recess so it is hard up against the bolt head (use mole (vice) grips to hold the tubing.
Then apply weld into the tube. A good welder will be able to weld the inside of the pipe to the top of the bolt.

I'm hoping that after 6 months, the end of the allen key will have dropped out on its own accord
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Old 08-03-2014, 05:52   #12
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Re: Broken Off Allen Wrench in the Prop

In your situation. , I have been able to, in the past to use a 12volt battery to arc a small steel rod.to the broken piece
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Old 08-03-2014, 05:52   #13
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Re: Broken Off Allen Wrench in the Prop

I have seen some make spot weld connections using high amp 12v batteries. Might not be your first choice, good trick to have in your bag though.
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Old 08-03-2014, 06:10   #14
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Broken Off Allen Wrench in the Prop

Get a new bolt and nut in advance. Then drill the bolt out from the nut end, first a smaller diameter bit, then one slightly larger than the bolt. When the nut falls off, punch the bolt out and replace.
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Old 08-03-2014, 06:27   #15
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Re: Broken Off Allen Wrench in the Prop

DH,

Everyone here has apparently.... Pretty extensive experience with the good old fashioned... buggered, broken, stripped.... etc... fastener...

All excellent advice...

What you didn't give us was the:
A. size and material of the hex key, ie. plated crap, or hardened tool...
B. Size and material of the set screw
c. The depth and ability to "see" said combo of set screw/hex key

1. Hopefully this broken key has simply corroded itself into falling out...
2. Obtain a length of tool steel that exactly fits the diameter of the recess, use your liquid loosening agent of choice, and lightly tap and earth magnet the piece out... Probably won't work, but worth a try....
3. The problem I see with the welding course is that if it doesn't work, you are likely to end up with an extremely "heat hardened" assembly (screw/key) that will be a bugger to drill into, let alone the precision you require...
4. I'm with everybody else here that thinks screw extractors are likely to get you into more trouble... I myself honestly would drill the whole assembly out...
5. Use your "exact fit" tool rod with a ground point and bang it thoroughly into your hex/screw assembly to create a "substantial" center punch divot... Get a set of left handed bits... and start drilling... Hopefully one of your smaller bits bites, and the screw comes out... keep increasing sizes, and hopefully one of these works... In the end... You may end up drilling the OD of the screw... In which case the new larger set screws, drills and taps which you previously purchased for this task will be employed...
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