Another Sand Blaster Grounding Problem

I have read several threads about media blasting and grounding and I can't seem to stop getting shocked. We have a good Empire blast cabinet we use for glass beading aluminum. I have a grounding wire going from the gun to the cabinet. I have a wire grounding the work piece to the cabinet. I have a wire grounding me to the cabinet. That same wire goes from the cabinet to an 8 ft. grounding rod. The cabinet is also grounded to the steel building.

A severe part of the problem probably lies in the set up inside the cabinet. The cabinet walls are lined with adhesive backed plastic sheet and the floor has a piece of plastic mesh because the part being blasted cannot be scratched and it is pretty heavy. The plastic is supposed to be some sort of static dissapative sheet from McMaster. My thought was initially that this plastic is keeping the part from being grounded and building up a good charge itself, which is why I added the wire from the piece to the cabinet. I also have bare wires that lay across the face of the plastic sheet and then to screws in the cabinet. I did this thinking the plastic sheet was building up a charge that was not being grounded through the adhesive. I just finished blasting a piece and when I reached for it got a massive shock that I could feel leave me through the grounding wire I wear connecting me to the cabinet. I think the charge entered me through the glove which is a regular heavy rubber glove like most cabinets have.

So I think my question is, am I grounding things correctly? Also, is there a better solution for the inside of the cabinet to prevent the work piece from being scratched but also maintain a good ground?

I hope this makes sense. Thanks for any help you can offer.

The cabinet I occasionally use has an expanded metal shelf to keep the parts and whatnot from falling down into the media. A piece of 1/4" plywood holds smaller parts and prevents scratching.

Some conductive plastic sheets are conductive only on their surfaces, ie., the core is still an insulator. Plywood is an insulator but will conduct a lot better than most plastics. When it's sitting on the expanded metal it'll conduct enough of the charge to the enclosure to minimize any static buildup.

Cheers

Is there a vacuum system running while you are removing your part?

First off, what abrasive ?

What does Empire have to say about it (I'm sure they have seen this).

Maybe the gloves were replaced ? (and not static dis-appating rubber).

Ussualy the wire in the glove works well.

Grant2k said:

I just finished blasting a piece and when I reached for it got a massive shock that I could feel leave me through the grounding wire I wear connecting me to the cabinet. I think the charge entered me through the glove which is a regular heavy rubber glove like most cabinets have.

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Where do you have the ground connected to you? From the statement above, it sounds like somewhere other than the gloves, and that is just asking to have the current travel through your body. Ground the gloves and leave yourself out of the current path.

Dennis

With the gun, work, and cabinet grounded the static charge just about has to be building up on the plastic. There is a lot of surface area there and thus the potential to hold a large charge. I'd look at replacing all of the plastic with something conductive. For the walls sacrificial AL sheet or heavy foil would work. I don't know how it would hold up, but the carbon filled foam used to protect IC's should work to support the part and not mar it.

Why not follow normal practice and line it with rubber? Rubber oftern has enough carbon black to be conductive enough to remove any static charge generated. Equally its way more blast - wear resistant + gental on the media + parts.

Grant: If at the end of the blasting operation, and you touch some part of the cabinet and you feel no shock, then the part that was blasted holds the charge. If so, try using a grounded wire to touch the part and discharge the buildup in the part. If none of this works, then you are holding the charge. A common trick in avoiding shock from seatcovers, is to hold a metal key and touch it to metal (car) and discharge yourself before touching the door handle . Something like this, might work with the blasting operation. Good luck.

JH

From a safety point of view, grounding yourself is normally done through a reasonably high value resistor, like at least 1 megohm (1 million ohms)*. That keeps you from getting seriously zapped if there is a "real" shock due to a fault, and actually does drain off your charge.

Most likely, it would have also kept you from getting a big static shock such as you report, it cuts the current surge.

If everything really IS connected together, it's hard to get a shock. Since you did, something was NOT connected.

The sand, or abrasives, pick up a charge on their surface as they exit the gun. They then transfer some of it to the part.

But they are not conductive, usually, and so only part of the charge goes into the item being blasted, even if it is grounded. The abrasive particles may be still partly charged, and you can get shocked later from charges they induce on things, or from charge you yourself pick up from them.

If a decent amount of metal particles get mixed into the grit, that probably will cut back a bit.

*
If you make one yourself, don't use one single resistor, the high voltage static charge will probably just arc over it if you touch something charged. Use several smaller value resistors, like 10 100k ohm ones in series. Less likely to arc over at high voltage.

Make sure you are using a sandblast hose that has the wire coil inside, when trimming the hose, leave the wire long at both ends, allowing it to make contact with the media and ground it to the pot, also making sure the pot is grounded. The worst I have seen is steel shot media for building static, but glass bead and garnet build up a little too, anytime you convey material through a hose, your gonna build some static.