Rotating weld ground on a production fixture
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  1. #1
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    Default Rotating weld ground on a production fixture

    I'm designing a manual weld fixture where the welder will rotate a "bolt head welded to the side of a nut". It will be pre-tacked and then clamped into a rotating fixture. The current design is to use about a 1" shaft that is mounted to the rotating fixture (co-linear of course to make the rotation without wobble) and it will travel through a bushing/mount with a handle on the opposite side. Just to help out, they'll likely have a saddle for the torch to more easily keep it in position.

    I'm not an expert on weld grounding and wondering if I can pass the ground though the rotating shaft, maybe if the shaft is steel and I press a brass bushing into the support? I would like to avoid a special rotating ground that would get in the way of the operator being able to rotate the assembly while welding. Ultimately, if volume increases, we may add a motor drive/belt to rotate it via foot pedal but it will be starting out with manual rotation.

    If you look at the photo, ignore the little piece off to the right.

    Any help is appreciated.

    Thanks,
    The Dude

    bolt-nut-weld-fixture.jpg

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    Roto-Ground Clamps

    Tweco (and IIRC Bernard) makes others without the "c" clamp

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    How many amps? Most movable connections use high current brushes (think of a subway train) so it's mainly a matter of knowing what size to get and then coupling it to your fixture. I would suggest a ring (short piece of tube) as close as possible to where the bolt mounts. For manual rotation you might even get away with using heavy copper braid and conductive grease.

    Edit: I was typing my post and didn't see the one above until after I hit "post". That sounds like the way to go.

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    Awesome, thanks guys. I was looking for somethign like that and only found something complicated but that looks far simpler and likely cheaper.

    Much appreciated!
    The Dude

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    I have seen on the small welding turntables, a brush (like for a motor, not paint) that is made from porous copper (like an compressed air filter)
    and this is spring loaded to bear upon a smooth machined face

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    just a note on the design, I think the pivot is too far from the weld for a single bearing, two would be better, think about using an old arbor, dividing head, or even a motor, its already got the bearings.

    as to the ground, if you have some woven copper webbing, like the lead on a brush for a largish dc motor or generator, you can really just pinch it with a spring clamp around the shaft, thats plenty for 150A or so

    img_0805.jpg

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    I use a pair of large carbon brushes, biggest I could get at motor shop years ago, to ground a manual rotator I made years ago. 200 amp alum tig, rotates while welding. Brushes are something like 3/8" x 1 1/2" or so, spring loaded to push on rotating shaft.
    Also have a Jancy Rotostar rotator that just uses a 5/8 or so round copper plug that is spring loaded to push on the back of the table platen.
    Both work just fine.

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    Don't the carbon brushes act like a resistor ?

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    yes, think of how large carbon brushes would be on a 150 or 200A motor, and they would need the corresponding contact area, so I don't think I'd use them. certainly the high speed capability they have would never be needed in this application.

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    Quote Originally Posted by digger doug View Post
    Don't the carbon brushes act like a resistor ?
    When I first made the rotating fixture it was just a steel bar through a steel pipe in the table, not intending to rotate while welding. Shortly the arcing inside the pipe made the thing not usable so the large motor brushes came into play. Those worked rotating 200 amp welds on aluminum for probably 20 years of production. I have my own here in my shop that has not seen that level of use but there are NO problems with them whatsoever.
    Similar is an electric rotary union.

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    Strip the insulation off 2 feet of welding lead. Wrap the welding lead around the shaft two or three times and put your ground clamp on the bottom where the leads overlap. The old-time woven battery ground straps work good also.

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    Quote Originally Posted by gbent View Post
    Strip the insulation off 2 feet of welding lead. Wrap the welding lead around the shaft two or three times and put your ground clamp on the bottom where the leads overlap. The old-time woven battery ground straps work good also.
    Best to strip that amount not at the end of cable so the end dosn't fray. I have a lead like that for grounding on stainless tube. Also very useful when the ground clamp wont open wide enough to grip the OD of tube.

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    Quote Originally Posted by The Dude View Post
    I'm not an expert on weld grounding and wondering if I can pass the ground though the rotating shaft, maybe if the shaft is steel and I press a brass bushing into the support?

    Any help is appreciated.

    Thanks,
    The Dude
    Yes you can pass the power through the rotating shaft. I’ve built a number of machines for doing roundabout welds. Pandjiris uses large bronze blocks against the back of their positioner faceplate that are electrically isolated from contacting the spindle frame (so all parts need a connection to the faceplate for ground). Gleason & most other production crankshaft & overlay machines use graphite brushes on the spindle itself and isolate the spindle from the bearings and machine frame.

    I wouldn’t ground through the spindle bearing even if it were bronze. I think you’ll find things will go better with brushes bearing on the spindle itself or contacting a steel or iron disk clamped over the shaft. Spindle bearing will wear quickly with bronze & if you pass current through ball bearings they’ll fail very-very quickly.

    Brass or carbon/graphite? Both will have some arcing & graphite goes better for not chewing up a steel or iron surface by fretting. Both will have slight voltage drop through the rotating connection when in use.

    I’ll attach some brushes, 5/8x2” and 3/4x2” T900 GE type (traction motor use). When you check resistance through a good static connection they’ll be around 2 ohms. The pigtails on both sizes can handle a lot of power in open air.

    As an aside, I’ve also built what you want to do with a rotating torch. Then you can shove what ever you want in front of it (with some tube contactor changes).

    Good luck
    Matt
    Attached Thumbnails Attached Thumbnails t900brushb.jpg   t900brusha.jpg  

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    Rotostar welding positioners are made right down the road from you in Clackamas. Go talk to Rob about what you are doing. I'm sure he can fix you up.

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    ATI torque converter welders use one of these held stationary clamped around a rotating spindle. As long as you aren't turning it fast it should work good.TWECO 300 amp Copper Ground Clamp #92051130 For Sale | Tweco | Ground Clamp | Welding Accessories | Welding Equipment | Welding | Welders Supply Company Beloit-Big Bend-Burlington Wisconsin and Rockford, Illinois

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    Some years ago I worked in a fab shop that made air duct systems. They had a positioner made from an old lathe headstock. A flat copper brush, spring loaded, rode on the back of the chuck. Worked fine for them. While you are at it, why not make a tailstock to hold the bolt and eliminate the tacking step?

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    We ended up using the "engine block ground" method (ground strap wrapped around the rotating shaft) and, thus far, it has worked well although it's still in try-out mode. This was about $15 for the ground and a clamp nut so I would consider it to be the easiest and cheapest of all the methods suggested.

    Quote Originally Posted by PocoLoco View Post
    While you are at it, why not make a tailstock to hold the bolt and eliminate the tacking step?
    Actually we are looking into that, although we will likely go with a V-block so that the finished assembly can easily "tip out" when unclamped.

    Thanks all!
    The Dude

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    Glad to be of service.
    Good luck with it.


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