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Ian B
09-03-2005, 03:48 AM
I'm about to get various bits of steel hot dip galvanised (farm machinery, and other stuff).

There are some captive threads that I don't want to get clogged with zinc - what's the best way to protect them? Screw plugs in and hope they'll come out afterwards, or leave them and retap afterwards? Any other suggestions?

Grateful for any advice,

Ian

SGW
09-03-2005, 07:12 AM
The place you're getting the galvanizing done ought to be able to advise on that, I would think. But IMO your second idea (re-tap afterwards) is probably the way to go.

Another thought is to tap oversize (you can get oversize taps) and not do anything after galvanizing. That's done with plated parts, but I have an idea galvanizing may be too thick and/or too unpredictable in thickness for that technique to be reliable.

Timleech
09-03-2005, 07:42 AM
I used to get bolts spun galvanised, the galvanisers tapped the nuts oversize. I can't remember for sure whether this was before or after plating. I think for ordinary hot dipping you'll have to re-tap afterwards anyway as there's no real control over plating thickness & you may end up with globs of zinc in the thread. Anything you put in to protect the thread will have to withstand the hot cleaning bath (caustic?) as well as the molten zinc, and the galvanisers will want to be absolutely certain there are no hidden pockets which could hold moisure & cause an 'explosion' in the hot baths.

Safest way is to ask the galvanisers.

Tim

JS
09-03-2005, 12:15 PM
Paint in the screw holes prevents the acid from preparing the material for the zinc

The acid cannot eat the paint so I have been told when I was at the TL piviot manufature to get product gavanised. We had to sand blast everything for the process.

Larger tapped hole works too.


galvanise is not just zinc it is an alloy of several metals it just depends on the manufactor/ platers preferance

Ries
09-03-2005, 04:18 PM
We do a big galvanized project once or twice a year, and we always plan on having to retap any threads, and often ream holes out as well. Hot dip is just not very reliable about the thickness of the coating- so in the places where you need a precise fit, its somehow always thicker than the allowance you planned on.
I buy a can of spray galv, for touching up the piece after I file, grind, sand, tap, and ream out all the spots where there are big boogers.

Forrest Addy
09-03-2005, 10:39 PM
I used to locally heat the tapped area to the zinc's softening stage and spin in a round wire brush. ZIP! In and out. Do not dally. It leaves some zinc but removes the excess so the threads will fit. Use an oversized tap if you can otherwise you might have to retap a few holes.

[This message has been edited by Forrest Addy (edited 09-03-2005).]

wbleeker
09-04-2005, 03:37 AM
we got some irrigation plant hot dipped a few years ago and i'm pretty sure we filled the threads with grease and sealed the holes with silastic (silicone)
Will