New Day - new problems :(

[kickn]

After some carefull testing and trials I think it was down to the anodising layer.

Which brings a question. I'm experiencing very high voltages - any ideas on what that could be? I'm using the 720 rule strictly by the book and when I start out I get a voltage that about 15 or so. My surface area calculations are exact as I'm using a cad program to model the parts first and pull the values from there.

Another thing I'm a bit perplexed byis that when I initially power the tank up (gradually increasing the current) it seems to hold at a very low value (say .5A or somethign) and then jumps up - is this due to capacitance of the parts?

I also have to find more power since I can't anodise a complete set of parts at once which I would like to do to keep the finish as close as possible. Is it possible to add power supplies in series or anything? I have a good 10A one here but need more. My parts are about 5sq ft (4.96 to be exact) so I'm estimateing that I'll need 15A @ ASF3 or which I would prefer - 22A @ ASF4.5. Any ideas bar a single BIG psu?

Proper rinsing has also helped with the streaks and water marks. I think the water here is pretty heavy.



(I'll try to do the video later to day and put it up somewhere)

[Fibergeek]

Another thing I'm a bit perplexed byis that when I initially power the tank up (gradually increasing the current) it seems to hold at a very low value (say .5A or somethign) and then jumps up - is this due to capacitance of the parts?

That's not right; in CC mode the voltage starts at a very low value and increases after some minutes to something close to the peak voltage, the current stays the same (at the value you set it for) throught the entire anodization, regardless of the voltage. If this isn't happening you aren't running in CC mode.

In CC mode there is no point in increasing the current slowly; that isn't doing anything, good or bad.

Causes of this in no particular order:

1. Your PS isn't setup correctly, or something is broken.
2. Marginal cathode connection; this connection doesn't anodize like the anode connection, but since its close the acid it does corrode quickly. This is the most likely based on your symptoms.
3. Marginal anode connection; since I think you are welding this connection, if this is what's happening you are the very first to experience it (AFIK) the cause would be a lousy weld.

The dielectric constant of the electrolyte is very low (its a conductor not an insulator) the resulting capacitance would be a few pF at most, totally irrelevant.

Large power supplies often have the capability to be paralleled for more power directly; one acts as the master and it controls the others as slaves. If your's can do this it will say so in their manuals. This isn't common in small power supplies (10A is small).
The other way is to connect them in parallel through isolation diodes, one per supply. The diodes are wired to the PS + terminal with the cathode facing the work. The diodes must be sized to handle the current and power dissipation they will experience. Both PS must be capable of CC operation, and each has it's current set for the portion of the load current it has to handle, easier to do if the two PS are identical units. If you don't follow this explanation don't try it. One big PS is obviously the best way.

Since we have capable assistance from other members in this thread now, I will restrict my input to electrical issues.

[kickn]

I think its either the power supply or the cathode. The connection to the cathode is outside the acid and its bolted tightly on multiple plates. I have about 8 cathodes around the tank.

When I said that I turned the current up slowly I wasn't very clear. I turn the dials slowly but only take say 15 secs. to get up to where I want it. I just do it fairly slowly so that I compensate for the initial fluxuations. The delay that is sometimes there with the amp/voltage only generally lasts for say 30 seconds or so and then it clicks in and not all the time. My guess is the PSU but I didn't know if I was experiencing anything else.

I'll keep a look out for a 20A psu with current control.

Thanks again

[Fibergeek]

and then it clicks in and not all the time

A ha, a voltage compliance issue.
This means that your PS is having difficulty sourcing current when the required voltage is real low, not uncommon with cheaper power supplies.

You should be able to fix this by putting a power resistor in series with the + terminal, anything between 1 and 4 Ohms and rated for at least 15 Watts should do. This will drop some voltage and allow the PS to turn on normally. After maybe 5 minutes you can short out the resistor as it isn't doing anything useful anymore, but it won't hurt anything if you leave it in.

Tightly bolting the cathode leads doesn't guarantee the connection won't corrode.

[Sid03]

kickn- I have noticed the same PS issue you stated with my 20A CC PS. Although I dont doubt at all what Fibergeek says.....im not 100% sure as I sometimes have the same issue when the required voltage is rather high and my gains are up fairly high.

I think I was having luck by making sure I turn the current dials completely back to zero when starting up the PS, then turning the fine dial up halfway then adjusting the main dial. I cant remember offhand, but thats just something that you may want to play around with. I know exactly what your talking about......thought my PS was broken when I first got it......esp when it makes the strange sound when it actually starts outputting the current.

Just curious....where did ya get your .01 micron filter and hows that working for you?

[kickn]

My psu doesn't always do this - just sometimes. I'm not really bothered if its not process related as it only takes no more then 10 secs to start up

I got my filter from farnell here in the UK. I thought it would work a treat but I'm still having problems with the bead blasting changing the surface of the ali. I'm not sure what it is but it doesn't look like oil now (you would think that a 0.01 filter would take care of that). If I hold the gun 6" away its fine - any closer and I get marking.

The only way I've found around that is to then etch it for say 10mins fairly aggresively and then lightly bead blast it afterwards. THe only problem with that is the the etchant leaves a film on the ali sometimes. Probably not enought suffocant or something in it but if I scrub it its fine. A long process but its working.

Anyone have any other suggestions?

[Sid03]

did you try running your blaster at a lower pressure?

I ended up running mine much lower then the recommended psi due to similiar issues.

[kickn]

I did - right down to 60psi but I figure there is a balance - if you run it so low that you have to spend ages and work really close to the part - then you might as well up the psi a bit and then work slightly farther away - no?

Anyways - i'll keep messing about.

What psi did you end up using? What sort of nozzle do you use? I'm using a no7 which is the biggest I have right now.