Author Topic: A thread for asking questions where you have no real hope of an answer...  (Read 669 times)

.....but still open for ideas.

My Neff gas hob is about 17 years old, and there are plates surrounding each burner that are fastened to the burner casting with 3 x 3mm screws, or should be if they didn't break off flush with the casing in normal use. The hob is close to being rendered u/s as a result.

If I can't work out an alternative, I either have to try to remove those stubs or replace the hob (not straightforward, either, due to the size).

So, any bright ideas for how to remove stubs of 3mm steel screws out of aluminium castings? They are likely seized, too.

T42

  • Tea tank
Tricky to drill out without slipping off the steel and into the alu, otherwise I'd suggest doing that and re-tapping for 4 mm screws.
I've dusted all those old bottles and set them up straight.

ian

  • fatuously disingenuous
    • The Suburban Survival Guide
Small amounts of C4 will do the trick.
!nataS pihsroW

T42, you've got it. And of course, being stainless I can't rig up a magnetic drill clamp.

Ian, is that a little like R2D2, but with attitude?

If it comes to it, my current approach is to have a go with the Dremel.

If I'm reading your OP correctly, and you have a decent quality automatic centre punch, do the following:

Using the centre punch, mark a dot as close as you can to the perimeter of what remains of the screw.
Starting with a gentle punch parallel to the screw. Wind up the pressure and mark, mark and mark again in the same spot, until you have a generous indentation in the screw.
Next, angle the punch tangentially to the circumference of the screw, place it in the indentation and resume the punching in an anticlockwise direction.
I've managed to free a couple of bolts that someone (here's lookin' at you, Pippa) had sheared in the top of a seat post.
I think they were 4mm - it should, with care, be doable with 3mm.
If they are seized, the shock of the punch should be enough to free them.
Failing that, magnets.
G'luck.

this is potentially a very easy job if you have access to a MIG welder with a low current capability.  Just weld to the screws until you have something large enough to hold onto; you know, like little bird poos like you might make if you are not very good at welding, they are ideal for this job. Or lay a thin plate of steel with a 3-4mm hole drilled in it over the stub and weld away; built in spanner.

The heat from welding will break whatever bonds of corrosion there might be, and if the weldment doesn't 'stick', no worries, just have another go.  You can't weld steel to aluminium, so this way  there is practically zero chance of going backwards rather than going forwards.

I have quickly  recovered numerous assemblies (steel screws busted in aluminium housings) this way, and since starting to do this I have hardly ever had to drill anything out.

The only time this approach hasn't worked is when the broken bolt is a 12.9 grade one; these bolts are made of CrMo steel that temper embrittles very easily, so the weldment tends to break off in the HAZ.  Lower strength grade bolts weld OK by comparison.

cheers

Now there's an idea, thanks brucey. It is made a little more complicated by the adjacency of the burner body (5mm), and of course the close confines that I'd be working in (the burner casting in which the screw fixes is below the hob top, with mebbe 20mm access) but maybe drilling through a nut head, cutting that head off and welding through the hole might let me get a spanner or socket on it. I assume I can attach the earth to the ally no problem.

earthing should be OK provided there a) isn't a coating on the aluminium (in which case you will have to breach it) and/or b) the corrosion isn't so severe that the bolt isn't in good electrical contact with the aluminium any more.


If the tapped hole passes through the aluminium piece, often you can make the ground contact on the back of the sheared screw directly.

It is best to make the build-up in very short bursts. If the set has a voltage control, it is often a good scheme to put the first burst in at a high voltage, and then to put successive bursts of weld in at lower voltage; they fuse more easily to the (now hot) substrate.
 If you stick with a high voltage setting, it is difficult to make a build-up; even a short burst of weld tends to do as much melting as anything else.
 
If you are able to get a mole wrench on the build-up PDQ, you can give it a wiggle whilst everything is still hot.  Often the screw will loosen a little bit (so it will move back and forth a tiny bit when you wiggle the wrench, which is difficult to tell apart from a wrench that isn't completely tight BTW) well before it will actually turn. Any irregularities where the blob of weld touches the aluminium will hinder the screw from turning, usually only for the first turn, after which the two surfaces are separated well enough that there won't be a problem.

If you are welding in situ, do be aware of the hazards of the gas pipe and also of welding to anything that is connected to the earth bonding of the household electricity supply.  I cannot say if there is a potential hazard in the latter case (eg with a modern consumer unit/meter) or not. But with most MIG sets the voltage ought to be low even if the current is high.

cheers