Fast corrosion of aluminium while milling.

I've had several times this week aluminium 6082 plate (two different sources) which has pitted while on the mill. The first time it happened over night, today it happened in a couple of hours on a fresh piece of aluminium.

The plate is clamped to the table with a piece of normal A4 paper underneath to raise it slightly off the table so the sides can be milled. Lubricant is Multi-Spec G at 25:1.

It happens where the paper touches the aluminium. Even where I've used paper under the clamps it has pitted, but not as much. By chance I've used two different types of paper!

Any ideas?

I think it is electrolytic corrosion where the paper is wet. Perhaps you can put an additional layer of tinfoil in so that corrodes instead, or clingfilm to break the circuit. I haven't looked up the spec of the aluminium to see if it contains copper that is a bit of a corrosion magnet.

I wonder if caustic soda is an element of the bleaching/whitening agents in your paper?

Paul

Most paper is acidic, thats why when mounting artwork you should use "acid free" or "neutral ph" paper and board

Interesting, we mill a lot of aluminium for our label application machines and never have this problem. So we can assume that it is the paper?

Our engineers never use paper to raise, or clamp surface protect and I never have a problem with their finishes, as I am the BOSS who has to be pleased before a machine leaves our factory. All our aluminium is anodised in either natural or red finish and again this always looks great.

If your milling the edges as you say and raising the piece to clear the bed the paper your using must be very thick to achieve this, so why not use another piece of plate to raise it?

Just a thought

Bri

I've been using ordinary A4 copier paper under clamps for years with 6082 and never seen this problem. Given that the 6082 came from two sources and the paper was of two different types, the only common factor is the coolant?

Regards,

Andrew

Just a thought that the aluminium, paper, cast iron (presumably) bed and acidic coolant would make a reasonable battery so the effect could be electrolytic.

Try a multi meter across the aluminium, and the mill table, you might just find .5 volt approx.

Maybe better to use kerosene/paraffin.

Ian S C

Hi Chris

I wonder if the clear semi rigid plastic A4 sheet as supplied by office supplies for ring binding would work as well as paper. Being plastic it would creep a little under compression but I doubt it would creep very much. It is not particularly slippery.

With luck you will have an old manual or similar to use for a trial.

Being an insulator electrolysis would be stopped. And no problem with the PH.

Regards
John

Some interesting replies, all of my multimeters are broken except one which I have left at work. I will try and measure voltage this week.

I've changed to using gauge plate under parts while they are on the milling machine now with masking tape on top. It's not happened since. Although I left last night the gauge plate with masking tape on top of the rotary table. The gauge plate was against the table and the masking tape on top of the gauge plate. The gauge plate had corroded under the masking tape.

There is a couple of reasons, 1st to look at is the Ph of the coolant. If new coolant still does the same result, it could well be an electrical issue with you mill . A bad earth or insulation breaking down and creating electrolyses.

Seen 1st hand what happens when a motor windings have the insulation breaking down. Even though you do not get an electrical shock, there is current running through it causing electrolyses.

Neil

Is your machine earthed, it may appear so, but worth checking. Ian S C

I will dig out the Megger and do some tests!