Tig welding and magnetism

Many of the more experience welders proably know that tig welding and magnetism do not go together, but I thought it might be intersting to relay my recent experience in this area, which was a first for me.

I had to refurbish a bottle capping head which is essentially two stainless steel discs, each containing very powerfull button magnets, approximately 22mm dia by 10mm thick. These magnets have to be handled carefully, as they are powerful enough when they jump together to trap a finger! One disc is driven by the milk bottling machine, and the other disc contains the claws which grip the botle top and screw it onto the bottle. The amount of torque applied is regulated by inserting teflon washers between the two parts.

The magnets are sealed into pockets in the discs with epoxy resin. After a period of time, the expoxy eventually allows moisture (from daily machine cleaning) to get to the magnets, which then corrode and expand and render the magnetic clutch inoperative.

A suit suggested that I should machine out the old epoxy and magnets, and fit new magents into the pockets. However instead of sealing them in with epoxy, if I would make stainless discs to cover the magnet pocket, the discs could be welded in place, thus hermetically sealing the magnet, and dispensing with the corrosion problem for all time.

One experienced welder expressed doubts due to the magnetism, while another said he would give it a go.

I witnessed the attempt at welding, and can only say that St Elmo's Fire would be a poor display in comparison. The arc was completely uncontrollable due to the effect of the magnetic field and after several attempts welder No 2 gave up, so it was back to sealing with epoxy resin.

The problem of tig welding close to a strong magnetic field is probably insurmountable, but I would be interested to hear if any members have been able to overcome this difficulty.

Tig welding near your electric meter can make it spin backwards so it's not all bad..

SND, a member of this site has done some very nice work welding Nd magnets in stainless containers. I think the trick is is to keep the arc as short as possible.

Or you can send them out to have them laser welded.

What if you inserted the magnet from the back? You could plug it with epoxy away from the daily moisture exposure.

Not seeing the whole design makes this idea a crap shoot, but it might work better than sending it out.

seen arc wander when working as a welder years ago, we used ti wrap the lead from the tongs round the girder or whatever a few times to kill the magnetic feild, or at least controll it a bit when the ocv slumped and the current came up, noticable with some steels more than others.
mark

Jim S, You are quite correct, that without pictures it is hard to comment. Managed to get some now. However, CIP (clean in place) of food machinery involves some nasty caustics, peracetic acids (to kill the caustic after cleaning) and tons of water which gets into everything. Just a well, I suppose, as the idea is to kill any bugs!!



This is the part of the capping head which contains the claws which tightens the bottle top. The view is of the sliding side which I have refaced by taken a couple of thou off.


This is the underside view showing the pockets with the new epoxy resin sealing the magnets. The claws are not fitted. You will see that the magnets are contained in blind holes as suggested. However the wall thickness at the blind end of the hole is less than .040", sometimes only .030" due to wear on the sliding surfaces.



This is a view of the driven member. The bottoms of the pockets were so badly distorted by the expanding corroded magnets that I have had to machine it completey away, and will weld a new piece of material into place to re-establish the blind hole, and then remachine. The magnets will then be re-installed and epoxied in place.

There are five capping heads on the bottling machine. I've still got three more to do in due course, so any further ideas are welcome, and thankyou for the suggestions to date.

You might be able to fuse a gas seal weld seal on them. Being stainless steel the magnetism wouldn't pull the moltem metal.
Good luck

Is it possible to stay away from welding them in? Maybe a plug can be pressed into place to hold the magnets in?

Welding around strong magnets, done it. Last job I had we built magnetic couplings, some of which transmit over 4000lbs/ft of torque(say about 1000 very strong magnets put together), down to tiny ones that probably only transmit a few grams of torque. They were/are in sealed systems with barriers. Yours is just a fairly simple face drive coupling. Different but same idea.

Now about your situation. If you're trying to weld right on top of the pole, it likely won't happen. It can blow the arc right back in the torch and blow the cup into little bits.

If it needs to be welded, and I don't know if yours could be designed that way but you'd need 1 cap that covers them all at once and is welded on the outside diameter. It would likely still get a little tricky when going right over the thin wall with a magnet under, but much easier than trying to weld over a pole.

The trick with welding things that contain magnets is to put steel where to poles are, thick enough to absorb the magnetic field. If you can move the field position away from your weld as much as possible the job will get much easier. If theres enough of it, it won't affect the arc at all. If you have a little pole tester, which is really just a stick with a tiny magnet on the end that rotates and tells you north or south, you can fairly easily see when the pole is losing strength. Given the size of your magnets I'd say at least 3/8 thick, but likely more depending the their strength. Keeping the arc short will help keep the heat on the metal if you still have some arc wandering, but not so much it can't be controlled.

The arc will follow 1 pole. I can't remember if its the north or the south that attracts it. I should check it again sometime.

Now other problems with welding magnets. Heat, not all magnets like heat. There's many grades of magnets, mostly when it comes Nd magnets. There's a big difference between a Nd42, and a Nd42SH. Smco magnets usually handle even more heat but are a bit weaker, although you don't need to worry about those, except I do think they were better than Nd's for corrosion resistance.

Other problem is glues. Heat can/will make them fume and cause weld problems. We normally use Loctite 620 when the heat wasn't too close to the magnets. It's really good at holding them down, they usually never come back off unless smashed. If it got too close and fumed, some 2 part glue/cement that cures at a couple hundred degrees was used, it was less likely to cause problems.


After all this, I'll say the easiest solution in your case I think is to use good nickel plated magnets and glue them in, unless they're already using that and its still somehow corroding ???

And yes, laser welding does work really good on this type of things.

Good luck.

Hi SND,

Thanks for you helpful advice. I will run it past the welders when I take the parts back next week, and let everybody know the outcome.

With the present system, the magnets are nickel plated and sealed in with epoxy resin, but however much care is taken over a period of time the cleaning chemicals and water eventually get to the magnets are start the corrosion process which usually pops out the epoxy from one side and dimples the blind side of the head.

Just an interim update. Took the parts back and the decision was taken to stick with the epoxy resin method of retaining the magnets in case the capping head was irrtreviably damaged. However, the decision was also taken to make up a test piece with a pocket for the magnet and attempt welding a plug using a steel bar to draw the magnetic flux. Unfortunately a breakdown elsewhere put the trial on the back burner and I am now machining a big blind nut in 316 with an internal 64 mm left hand thread. When the trial does take place I will post and let everybody know the results.

Thanks again everybody for all the helpful advice.

Have you had the chance to check with some good magnet manufacturers to see if they may have something that is more resistance to corrosion? I wonder if you could get them with a real thick plating, instead of the thinner decorative looking stuff...

Threading parts possible? "O" ring possible?

SND said:

Have you had the chance to check with some good magnet manufacturers to see if they may have something that is more resistance to corrosion? I wonder if you could get them with a real thick plating, instead of the thinner decorative looking stuff...

Click to expand...

We got the magnets from Eclipse, one of the leading magnet manufacturers in the UK (mag bases, mag tables etc), and they do not list any of the type we require with heavier plating.

I think Mark's idea of threaded plugs with an O ring is worth investigating and when I have time I will make up a test piece. There is not a lot of spare depth in the pocket for a thread, but I think this could work with a fine thread and some Loctitie thread sealant. Thanks for the idea.

How about my idea of pressing a plug in to hold the magnet? Kind of like a like a frost/freeze plug put in backwards. This could center the magnet at the same time as giving more surface to hold the plug in.

Toadjammer,
The idea of a press in plug is a good idea. If the plug was a light interference fit and was pressed in with plenty of loctite or similar is might be a goer.

I have investigated further the possibility of screwed in plugs. Although I could turn up the plugs and thread them easily, the cost of fine thread taps has put a bit of a damper on that idea. Fine thread taps are available but they cost about £100 each (over $200 in your folding negotiable bonds)

In the meantime, due to presssure of work in other areas, new magnets are being fitted using a new type of epoxy resin which we are hoping will hold up better.

Any further developments will be posted. Thanks again to everybody for their help and ideas. Also Very Best Wishes for Christmas and for a Happy and Prosperous New Year to all.