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Welding on the Lathe

06/02/2010 8:37 PM

I want to put a weld around a circular part mounted in a metal lathe. I will use the slowest speed I have. The welder is a MIG welder. My concern is the ground clamp will be attached to a stationary part of the lathe. Welding current will pass across the headstock bearings. Is there any danger of arcing across the bearings or is the metal-to-metal contact adequate to prevent arcing?

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#1

Re: Welding on the lathe

06/02/2010 8:47 PM

If the headstock bearing is lubricated, there will be an oil/grease film on its parts. If the lubricant is electrically conductive, there might be a decent ground; if not, any intermittent metal contact would risk some arcing. Could you instead rig up some sort of rolling follower to engage the work piece, or maybe even a (heavy) motor brush? That would seem safer.

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#2

Re: Welding on the Lathe

06/02/2010 10:30 PM

Unless your lathe is an old tramp, don't do it that way. You'll wear your bearings and afterwards you'll see it in the precision you'll keep. If it goes really bad you have to replace them all. A contact brush or strip that glides is doable. Just get rid of the spatters too because they might groove your sliding parts. Cover everything well up and use anti spatter spray. Of course if it is just "a" lathe by name.....

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#3

Re: Welding on the Lathe

06/03/2010 5:50 AM

Forget about the grounding on the stationary part.

I have seen there are problems even when you earth the actual job. Unless everything is in order, and absolute order, circulating current still flow through system earth.

And at the welding currents they are destructive.

After seeing bearings destroyed, we have strictly banned any welding being carried out on machine. After all it is the cost of taking out and resetting Vs machine repair. And in machine repair, the cost incurment is the least in the bearing. Down time cost being the maximum.

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#4

Re: Welding on the Lathe

06/03/2010 9:25 AM

There is alo the option of spray welding if you are only trying to build up a thin layer (Not attaching two pieces together) The kit may not be cheap, but it may be less expensive than replacing the bearings in the lathe.

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#5

Re: Welding on the Lathe

06/03/2010 11:28 AM

I used to do something similar, except on a fixture. Surely, you are not going to make more than two or three rotations? You may have trouble slowing down the lathe enough; I would think that also a welding quality concern and you might want to do something about the speed control. Don't you have a wife who you can bribe to slowly turn the headstock?

But, any road, if you turn slowly enough, use a flexible ground cable and let it wrap two or three times. Or, do one CW turn, then one CCW turn, and so on.

But, do not run current through those bearings.

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#6

Re: Welding on the Lathe

06/03/2010 12:11 PM

Thanks all. My suspicion is confirmed.

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#9
In reply to #6

Re: Welding on the Lathe

06/04/2010 12:21 AM

I've done this many times for stubbing shafts etc. but the ground is attached to the chuck jaw. I've never had any slag stick to the ways or any other machined surface either.

I usually had a helper turn the piece in the lathe by hand except on a few machines where the low rpm selector could be set to 5 rpm or less depending on the diameter of the shaft or cylinder etc..

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#7

Re: Welding on the Lathe

06/03/2010 11:07 PM

If only 1 or 2 revolutions are needed, I would wrap some of the ground cable the opposite direction on the revolving part if possible. Then if someone else is available they could guide the cable so it dose not get caught in anything.................it should work out all right..........good luck

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#8

Re: Welding on the Lathe

06/03/2010 11:18 PM

are you surface welding on the outside or is it on the inside of a bore? You can use a BoreTec welder http://www.bortech.com/ and put your part in a fixture ,allowing the welder to travel the outside ... you can also use that same welder to do inside work they are fairly light and not overly complicated to use. Don't weld on the lathe .... in over 20yrs of machine rebuilding and repair some of the more interesting crash and burns i've seen have come from the inappropriate combination of welders and machine tools, both manual and CNC :)

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#10
In reply to #8

Re: Welding on the Lathe

06/04/2010 2:02 AM

How about insulating the job and check jaw, such that there is no connectivity between them? Then the earth has to be connected to the rotating job by an arrangement like 'brush and slip ring' arrangement in motors.

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#11
In reply to #10

Re: Welding on the Lathe

06/04/2010 2:09 AM

Hi there,

If the ground is attached to the chuck jaw which is clamping upon the work piece then arcing is not occurring through the bearing, this is the most effective means known to me.

The brush and slip ring as you describe; is it capable to handle high amperage loads?

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#25
In reply to #11

Re: Welding on the Lathe

06/04/2010 4:21 PM

"The brush and slip ring as you describe; is it capable to handle high amperage loads?"

If the contact between the (rotating) job and earth is perfect and ample,it should handle the high amperage. If the job is repetitive, even copper ring could be attached to the job and earth can improved with copper brush.

The suggested arrangement could be some thing like this:

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#12

Re: Welding on the Lathe

06/04/2010 2:22 AM

All the concerns above are valid - but in a development/prototyping exercise, I bypassed them by utilizing a copper braid strap laid over an adjacent section of the rotating work and weighted on the free end.

This was welding 4" x 1/8" cold drawn tube to 4" x 1/16" cold drawn tube, which had to receive both a base weld plus a capping run - so two rotations - end product to remain straight and test gas tight.

I stripped then flat braided 100 amp welding cable to do this. It looks like an automotive earth strap. You could use one. Do not use a strip of copper sheet as spatter, or just dirt, will lift it off and you get sparking and welding going on under it. The braid has space for rubbish, so stays in good contact.

Electrically - the important thing is the strap is not ground to the machine - only to the MIG's earth. This prevents any current passing through any other path to the weld. This was initially done simply by clamping the still insulated part of the cable, but is now done a bit more professionally.

Obviously the ways and chuck were covered or heavily coated in anti-spatter.

However having proved in principal the process was moved to a purpose built rig, using a "rubbish" 3 jaw chuck, driven by a variable speed gear motor, and having an adjustable "V" set of two idler wheels to serve as tail-stock support, as the finished products ranged from 4 to 10 feet in length.

That rig is still running ~16 years on with no degradation of the drive gears or bearings. It's had a few new straps.

From memory - on the 4" diameter tube it ran at about 15 rpm (8 seconds full weld cycle). Power was via a 350 amp 3 phase MIG, delivering around 225 A, using .9 mm wire.

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#13
In reply to #12

Re: Welding on the Lathe

06/04/2010 2:40 AM

GA...I've used the strap technique but still prefer the solid contact at the chuck jaw, though have grounded to a junk center at times. Though I did use a jury rigged pulse generator if no helper available to drive the rotation when joining 4140 & 4240 shafts to drive splines etc.. Shaft diameters from 1.500" to 14" x 5' length to 26'. The remainder of the setup was essentially similar except for using the tail-stock and center to maintain concentric alignments.

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#15
In reply to #13

Re: Welding on the Lathe

06/04/2010 5:03 AM

Pulse generator is a good idea. 55 rpm was the lowest the 'test lathe' would go. Wish I'd had one (or thought of it)

I thought about the center idea, using a disc, given it was tube, and played with using a 3 point 'steady', using both sprung brass and carbon 'brushes' - but any increase in ohms (ovality or grime), meant current went through the head stock.

In the finished machine, an adaptor of chuck to drive was needed, so it got a groove to fit the strap. It means the current goes through jaw slides and/or scroll, but hasn't proved a problem. Or the chuck is not any more rubbish than it was - after 10000's of welds.

It also means the job is not marked/polished/copper contaminated, by the strap and the operator does not have to do "extra things" with each setup (i.e. think)

Pulse generator - oh to have it over - good idea. GA

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#14

Re: Welding on the Lathe

06/04/2010 4:32 AM

Hello, many years ago we did the same thing.

The headstock of a lathe is in most cases a tube so that long parts can be slipped through. We were leading the cable to the welder through the headstock and between the chuck's brackets to the back of the chuck and used one fixing screw of the chuck to connect the cable. As a slipring on the opposite end of the headstock a modified slipring from an old motor was used. The headstock of the particular lathe allowed to fix that slipring there. The holder for the brushes was temporary fixed to the machine casing. the current was 80 to 90 Ampere and the welds were excellent no damage done to anything and no limitations for the number of rotations - it was running 20 to 25 turns for one complete weld.

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#16
In reply to #14

Re: Welding on the Lathe

06/04/2010 5:14 AM

So you drilled through the spindle??????

What takes "20 to 25 turns for one complete weld" ?

Please explain.

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#17

Re: Welding on the Lathe

06/04/2010 5:53 AM

Couldn't you earth the set-up through the tail-stock? That would avoid risking the main machine bearings.

We use a set-up as described in an earlier post with a vertical 3 jaw chuck on a bench mount. We also only use about 5A in our welds.

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#18

Re: Welding on the Lathe

06/04/2010 8:10 AM

Not many words are need here. Put the ground clamp securely on the piece to be welded. I suggest removal from the lathe for any welding as my first choice.

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#19

Re: Welding on the Lathe

06/04/2010 9:13 AM

Go to your store where you buy consumables and get a round magnet with a ground stud on it. Attach it to the end of the tube,rod and ground clamp of the welder. If it is a rod you can weld a bolt to the end and cut it off when you are done. Take a manilla folder and cut three "shims" for your jaws to insulate, resist marring your piece. I assume you already have way-oil on there already! That should prevent splatter from sticking. Don't forget to clean it after! The copper straps are a bad idea if your cylindrical surface needs to be uncompromised from arcing. Buy four rubber casters and mount them to a flat surface in pairs making a cradle the length of your peice will be safer than using the lathe. If you are critical for concentricity, tack in the lathe and weld in the jig, recheck with a dial indicator.

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#20

Re: Welding on the Lathe

06/04/2010 9:19 AM

You don't want current going through those bearings. Period. The oil/grease is supposed to provide a small amount of space and lube between the rolling elements and the races. However the small gap it does provide is not enough to overcome the voltage. So when you get arcing it kinda spot welds the rolling element and the race together for an instant. This forms an inclusion on the rolling plane of the race and element and accelerates wear and damage. At that point your grease becomes useless as it cannot overcome the high metal. That said, since you're welding, and you're going slow, why not just put a peice of 1/4" plywood inbetween the jaws and the part? Do you not have the room? That would serve well to electrically isolate the peice from the lathe, unless you're using the tail stock, but you could rig a peice of wood there as well. The trouble with trying to ground the peice while it's moving is either the electrical connection is intermittent or the cable wraps around the peice, requiring constant resetting.

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#21

Re: Welding on the Lathe

06/04/2010 9:24 AM

If the Lathe uses some kind of electronics to control motors (I mean drives / PLCs). You might end up roasting them, rare to well done, no matter how you connect the MIG welder ground.

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#22

Re: Welding on the Lathe

06/04/2010 11:22 AM

I won't risk damaging my lathe. I'll do the welding manually. Thanks all.

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#24
In reply to #22

Re: Welding on the Lathe

06/04/2010 3:54 PM

I used a couple of all steel roller wheels from a skate conveyor system, attached to two fixtures,two rollers each, one at each end of the work.The round stock sat into the gap between the rollers, and ground was attached to one fixture.

Rotated by hand.Very effective.

HTRN

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#23

Re: Welding on the Lathe

06/04/2010 12:09 PM

Dear Mr. Ronseto,

No welding to be done on lathe unless you support the job on a steady, and push the Tailstock out and earth it at the end of the job through A BOLT WELDED TO THE END OF THE SHAFT (WHICH CAN BE REMOVED LATER and GRIND) TO THIS BOLT PROVIDE MS plate with holes to freely rotate and at the other end fix the earth cable.

The quality of the weld will be poor if earth current flows through the bearing of the lathe which has an oil film.

DHAYANANDHAN.S

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#26

Re: Welding on the Lathe

06/04/2010 6:26 PM

You might consider swiping your wife's sewing machine and running a new belt from it to your headstock. You can control that really well. Send her to the mall with your VISA card and she may never find out.

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#27

Re: Welding on the Lathe

06/04/2010 7:54 PM

I have done this in the past with a special turn table welding up cones on flanges for header reducers for automotive applications and the original design had the current passing through the bearing with sporadic welding. I solved the problem using spring loaded brushes (carbon) found in GM starters or generator about mid 1960's and fashioned a bracket to hold the brushes against barre metal of the turntable with the ground strap on the bracket. Worked 100% and you could do the same on a lathe.

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#28

Re: Welding on the Lathe

06/05/2010 1:25 AM

you can use a graphite electrode to adsted in contact with the moving headstock just like its application in electric motors, this way you ensure proper electric conductivity and as graphite is used as a lubricant in some of its applications it wont wear the head stock like copper would do.

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