How thick is galvanizing?

[BOOF]

Hi all
I'm having some parts laser cut from 8mm plate steel, and once ive got the thing im building welded im going to get it hot dip galvanised.

How thick is galvanising likely to be inside holes? for example I have a lot of M12 bolts, I'd normally do the holes at 12.25mm if not galvanising... should I allow more?
I want to keep tolerances fairly tight, but dont want to have to ream / re-drill after galvanising incase i go through to the steel.

Thanks!

 

[rtbcomp]

Drill the size hole you require then remove the galvanising afterwards, you can't rely on a consistent thickness.

 

[Hitch]

Standard practice would be about +2mm. So a 14mm hole for M12 bolts.

However, small holes still skin over a little sometimes, but no hardship to run a cordless drill through usually, 13mm bit. if its a 14mm hole)
problems occur if they have used a hole to wire it up for dipping. Now and then they will just cut it off and leave a bit of wire stuck through the hole, making t a pain to drill out.

Do the holes really need to be that tight on tolerance- no idea what you are making, but dont forget that the process can also cause other problems which may give you problems on such tightly toleranced bolt holes.


The galv is 'self healing' to a point, a tiny scrape from a drill wouldn't cause a problem.

 

[BOOF]

hold on, +2mm???? galvanising isnt that thick is it??? or does it get thicker inside a hole due to surface tension?

 

[BOOF]

I suppose as long as the bolts are all greased it should hold off any corrosion for a good while.

yes tolerance is quite important on this build - hence ordering the holes lasered at +.25mm

 

[J.B. Welding]

Like Hitch says, tension caused by the welding will relax in the heat of the bath and if there are a lot of bolts to line up over several components it WILL move, and this will depend on how you weld it. If you want to allow the bare minimum I normally recon the galv. to be 0.5mm thick.

 

[Hitch]

I fitted a job only last week, 12mm holes and M10 bolts had to be knocked through the majority of them.
Although the coating is theoretically about 80microns minimum (i think), you get a lot of drips/lump/unevenness that effectively make the coating a lot thicker.


Are you using galvanised bolts through the holes?
Galv nuts are tapped after galv'ing.... a little scrape from you running a drill through after won't result in a massive rust scab all around the hole.

 

[BOOF]

damn! ok...
maybe I should go +1mm, then drill/ream to +.5mm, leaving plenty of zink.

No they will be stainless shoulder bolts, the most important bit is a pair of hinges that allow a chassis to fold in half, but are also VERY load bearing.

 

[tlovatt]

You are aware that stainless should not be left in contact with galv as it will tend to strip the surrounding area of zinc and cause corrosion due to the effect of dissimilar metals. If they are going to be in an outdoor environment it is normally better to use galv bolts, or have the stainless steel bolts galvanised

 

[Hitch]

Stainless you say....
I'd be more concerned about the reaction between the stainless and the galv....than not having galv in the holes....

 

[BOOF]

oh! I didnt know that!, odd but ok, so cheaper zink coated bolts it is!

Thanks, I know steel / aluminium will react but never considered SS would...

 

[Gareth J]

Don't fret too much about leaving a coating inside the holes, if you're drilling them out after, unless you're very careful, you'll break through the galv somewhere. If tolerance is that tight, I'd be to tempted to use the 12.25 bit to remove all galv so at least the holes are where you put them.

When clamped up, what's inside the hole will be protected.

 

[rtbcomp]

Also bear in mind that standard SS bolts aren't as strong as 8.8 HT bolts, so you'd have the extra expense of buying HT SS bolts. BZP (Bright Zinc Plated) 8.8 HT bolts are the way to go and use Coppaslip on them.

 

[BOOF]

Yes true, thanks :-)

 

[clift_d]

If they're small parts you could get them spin galvanised - similar to what is done with galvanised bolts. It gives a tighter tolerance of finish without the drips and runs that you get with dip galvanising although you will still have to allow a bit for buildup. If however tolerance is really critical e.g. machined recesses for bearings, then zinc plating would probably be best - it doesn't the same level of protection as galv but it means that you don't have to fettle afterwards.