Ot- some way to improve low contact pressure electrical connection?

I have a fixture which has a brass plate sitting on a graphite block with low contact pressure and it needs to carry some amperage.
The graphite block is energizing the plate for a electro-chemical process.
I am getting arcing and poor conductivity & either need to redesign the fixture or use some dodge to improve the contact area.

I had thought there was a paste that would work- just a dab will do ya?

Also- under the gun so I have to work this out with what I can find around town on a Sunday..

Contacts like that are really similar to commutator contacts, without the movement over segments, and with lower conductivity brass vs copper (of course the graphite conductivity is less than either). They have a limited amperage per square cm, and I don't know how much more pressure will increase it.

One thing that does, however, is getting the two "fitted" together well so there is maximum percentage contact.

How does the pressure get applied now?

If there is a graphite block, do I assume there is relative movement, so the block is a sliding contact?

This is a fixture for electrochemical marking.
The work piece is a brass plate held in the center of the fixture below just in contact with the carbon disk.
The contact pressure is only that applied by pressing the (felt covered) graphite marker onto the work piece.

The setup is shown below:



When marking the screen in its frame (on left) is placed over the work piece.

This is the setup ready to mark:



The power supply provides current ( ac & dc) from the graphite block and graphite marker through the work piece at up to 45 amps.


I am thinking maybe I need to toss the base assembly and remake out of aluminum as a collet type fixture which can securely grip the brass plate.

I used plastics and graphite contact due to the corrosive nature of the process & had thought I would loose the fixture if aluminum...

If I understand correctly, the arcing is occurring on the back side of the object to be marked. In that case the solution is to ensure that the contact area between the part and the graphite is complete and has sufficient pressure to ensure that complete contact is made. Presumably the part has the contact area machined truly flat, it not working that side on a sandpaper lap, say 120-180 grit, will make it flat. Likewise for the graphite. Then you may need to look at increasing the clamping pressure.

Yes- that is correct.
The fixture initially worked fine but rapidly degraded.
All surfaces are flat though the 18 ga brass disk is not ideal for this setup.

I've got a small sample of TechBond-NC, Basically RTV , filled with nickle coated graphite, for making conductive gaskets.
I believe I bought it from Mc Master Carr. Paper work , shows a p/n :72-00350 and a Tecknit phone # 908-272 5500.
My sample is terribly outdated 11-12-2008. It's listed as a conductive adhesive. dave [acme thread]

A not cheap long shot would be CoolAmp products. eg. Conducto-Lube

2¢

Note that the following will likely expose my ignorance of the process...

So, the pressure of the felt-covered graphite marker needs to be low, but the pressure (actually, contact) between the workpiece and backer needs to be higher to improve contact to prevent arcing. Can you pull a vacuum to hold the workpiece to the carbon backer, while still having light pressure from the marker on the top?

Basic shop vac may prove the concept, if marking duration is short. Longer times may burn up a standard shop vac due to lack of cooling air, but a real vac pump or a Fein-type vac may do the trick.

I assume the back of the work is very clean already.

Or, I'm completely out in left field. One of the two.

Chip

What kind of corrosive is the "electrolyte"? Acidic? or basic?

Aluminum is reasonably tolerant of acids, but not at all tolerant of bases. Dilute lye, for instance, causes it to instantly foam up with bubbles from the reaction.

JST said:

What kind of corrosive is the "electrolyte"? Acidic? or basic?

Aluminum is reasonably tolerant of acids, but not at all tolerant of bases. Dilute lye, for instance, causes it to instantly foam up with bubbles from the reaction.

Click to expand...

I believe it is a acid- not sure really though as these are commercially supplied electrolytes.
I will test some on aluminum.

Acid can make the carbon surface non conductive.

Maybe try some fine copper braid of flex spread out across the back of the part (maybe some arc welding cable stripped back?)

Most platers use racks out of titanium for this very reason, a block of that would unquestionably solve your issues. failing that stainless 316?

Would not waste time with the alu, its only conductive when clean, chances are the chemicals will cause it too to become a high resistance connection pretty quick and your back to the same issues!

Stabilant contact enhancer
Not sure how it would respond to your chemicals.

If you can, as you seemed to imply, clamp the workpiece somehow, it seems your troubles with contact would be over. But I get the impression that the workpiece may be thin and not adapted to clamping against the edge. As mentioned, that also might make a mark. But just the clamp might mark the surface as well, so I assume if you were able to clamp you could probably clean up that surface.

Aluminum would be "OK" for a clamp, IF the electrochemical reaction does not attack it, and clamping is possible. Buy while aluminum is generally resistant to acids, adding the electrical current/voltage may change the situation in a bad way, changing the equilibrium. I doubt it would work well if the part just rests on it. Probably you are thinking of it because it is soft and non-damaging.

That's a problem anyway. I assume the face has a resist on it that allows the artwork to be etched as desired. Resist elsewhere would obviously prevent marking, but also prevents electrical contact. Inevitably, the electrolyte is going to get on other places, under or around clamps, etc, and cause marking.

You may be "presented with" these parts already sized and finished, with "just the etching to do". In that case you are stuck, because you can't afford any marking other than what is intended.

The ideal situation would be a sacrificial contact "tag" that can be removed after etching, from some surface that is not seen, or that can be cleaned up. You could clamp a wire to that and have a good contact that would not cause trouble.

Possibly a low temp solder attaching a tab to the backside? If the parts come to you with the resist already in place, that's obviously a problem, because heat will almost surely mess it up, but otherwise that might be OK.

maybe a thick salt/water solution will help? (until you find a better way.)

Thanks for input everyone.

I am not going to waste time with a aluminum fixture re- the comments above.
I will make a brass collet fixture to hold the plates from this:



JST- the process uses a screen mask and is equivalent to electrical silk screening.
The disks are finished with only some post process polishing.

The typical solution for things like slip rings is to silver plate the surfaces. I can make you a beryllium copper spring to replace the carbon brush, formed into the desired shape and silver plated or if you want to do a lot of them, I would silver braze or rivet a silver piece on the end. The strip I have is about 3/8" wide. You could use several of them. See my website

Typical Projects click the 4th thumbnail.

I can make ones with the bifurcated contacts for more reliable contact. They are punched out in a blanking die and bent afterwards, then heat treated, so I can bend them any way you want. They can be formed a certain amount after heat treat but they don't like it.

Bill

Silver makes a lot of electrical stuff work that otherwise wouldn't. Brass and silver work well together.

Nice site Bill.

Thanks again all.
I had initially thought a leaf contact spring of copper would be the best approach.
I saw the weakness in the fixture before I started but was hopping I could get away with this mess....

When in use initially the fine screen of the art template is the only thing in positioning contact with the disk.
The disk is held in position for half its thickness in a recess milled in the fixture and would displace vertically if the contact had spring pressure against bottom.

Design problem more than anything else.
Note to self- when you see a obvious weakness in a setup- fix the damn thing on the front side instead of screwing around...

I tossed the whole works and am making up a collet from 316/brass.
I will use a bit of Di-electric grease in the seat for the disks and am confident this time that the good clamping pressure will provide the required contact.

Trboatworks said:

I will use a bit of Di-electric grease in the seat for the disks and am confident this time that the good clamping pressure will provide the required contact.

Click to expand...

Di-electric grease is an INSULATOR. PM me and I will send you a small vial of conductive grease.

OK- done.

Right to the wire but got them finished and delivered on time.
I ended up building a whole new fixture which is a securely clamping collet made from stainless and brass:





The work was a bit hard to get a handle on the process for me.
I was doing very deep etching on a relatively large workpiece so had to balance ac and dc volts, dwell time, cycles, and electrolyte choice.
In the end as I discovered the sweet spot for these parts, the etching was fast, deep and of very high resolution.

The subject has come up for steel marking and I will post a thread of more depth on process as I found the procedure to be flexible and and inexpensive to set up for custom graphic marking on metals.

This is tough to photo well but here is one of the medallions before buffing:

View attachment 117448

It doesn't look it but the fill is a deep nice blue/black.

One aspect of the process I could not get a handle on was the ac side plating which deposits a dark base in the etched mark.
I could only get a red mark to produce and will have to send a test piece off the the supplier of the electrolyte for testing.

Running out of time on these- I gun blued the entire piece and buffed off the face to get contrast.
A nice result but many more steps to produce the work.

Phil- thanks for the generous offer.
I used di-electric grease to shield the contact areas from acid and not as a conductive grease per se.
The collet worked quite well but should have been made entirely from stainless as these were the only portions which withstood all effects from the acid.