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oversized holes for galvanising?
sdh2903 - 6/3/13 at 08:25 PM

I'm starting my trailer build this weekend and have a question for those in the know.

I have 10mm and 12mm holes to drill for M10 and M12 bolts respectively, the steel will be galvanised when i'm done butchering and wondered how much i should oversize the holes so the bolts still fit? Google brings varying results from 1mm to 2mm oversize.

Can anyone help?

Steve

tegwin - 6/3/13 at 09:09 PM

This website suggests that plating thickness is around 60 micrometers..... (about 0.06mm)....

http://www.galvanizing.org.uk/about_galvanizing/specifying_galvanizing/standards

So on a 10mm nominal diameter hole you are going to loose about 0.12mm from the diameter giving you a hole of 9.88mm

tomgregory2000 - 6/3/13 at 09:11 PM

Go 1mm oversize and you will be cleaning out the holes with the correct size drill bit when it comes back from the galv anyway

sdh2903 - 6/3/13 at 09:29 PM

quote:
Originally posted by tegwin
This website suggests that plating thickness is around 60 micrometers..... (about 0.06mm)....

http://www.galvanizing.org.uk/about_galvanizing/specifying_galvanizing/standards

So on a 10mm nominal diameter hole you are going to loose about 0.12mm from the diameter giving you a hole of 9.88mm


Cheers I had read through that but I believe the coating gathers a bit thicker around the holes, 1mm sounds a fair call, if I have clear holes out I suppose it will still leave a good chunk of coating behind.

tegwin - 6/3/13 at 09:35 PM

Yes I would imagine that to be true. Would be interesting to read some technical stuff on that kinda thing..... Perhaps a suitable topic for my dissertation....

sdh2903 - 6/3/13 at 09:36 PM

quote:
Originally posted by tegwin
Yes I would imagine that to be true. Would be interesting to read some technical stuff on that kinda thing..... Perhaps a suitable topic for my dissertation....


must be something more interesting out there than that!

tegwin - 6/3/13 at 09:38 PM

Hah, someone might find that REALLY interesting.....

Can't say I have given it much thought yet

Talon Motorsport - 6/3/13 at 09:47 PM

When we used to send stuff to the galvers it would come back 0.5-1mm thick it depends what time of the day your job is done. The tank is cold at the start of the day so the galv will be thick by the end of the day it will have got hot and you get a thinner coating. As for the holes I refer you to the hot-cold tank part of this post, they might be full of it not just lightly coated.

Confused but excited. - 6/3/13 at 10:21 PM

quote:
Originally posted by tegwin
This website suggests that plating thickness is around 60 micrometers..... (about 0.06mm)....

http://www.galvanizing.org.uk/about_galvanizing/specifying_galvanizing/standards

So on a 10mm nominal diameter hole you are going to loose about 0.12mm from the diameter giving you a hole of 9.88mm


I think you will find those figures refer to electo-galvanising, not hot-dip.

SteveWalker - 6/3/13 at 10:41 PM

quote:
Originally posted by sdh2903
quote:
Originally posted by tegwin
This website suggests that plating thickness is around 60 micrometers..... (about 0.06mm)....

http://www.galvanizing.org.uk/about_galvanizing/specifying_galvanizing/standards

So on a 10mm nominal diameter hole you are going to loose about 0.12mm from the diameter giving you a hole of 9.88mm


Cheers I had read through that but I believe the coating gathers a bit thicker around the holes, 1mm sounds a fair call, if I have clear holes out I suppose it will still leave a good chunk of coating behind.


It doesn't actually matter if cleaning them out leaves bare metal. Galvanising protects steel by its "galvanic" electrical effects and this effect works even where there are gaps in the coating.

Brian R - 6/3/13 at 11:09 PM

quote:
Originally posted by SteveWalker


It doesn't actually matter if cleaning them out leaves bare metal. Galvanising protects steel by its "galvanic" electrical effects and this effect works even where there are gaps in the coating.



This is indeed true. I normally overdrill by 0.5 to 1mm then clean out with the correct size drill when it comes back.

Grimsdale - 6/3/13 at 11:18 PM

quote:
Originally posted by Confused but excited.
quote:
Originally posted by tegwin
This website suggests that plating thickness is around 60 micrometers..... (about 0.06mm)....

http://www.galvanizing.org.uk/about_galvanizing/specifying_galvanizing/standards

So on a 10mm nominal diameter hole you are going to loose about 0.12mm from the diameter giving you a hole of 9.88mm


I think you will find those figures refer to electo-galvanising, not hot-dip.



most hot dip and spun parts are about 60 microns, but i guess you'ld have a job spinning a trailer, so would likely be thicker.
electroplated zinc is thinner still, say 10 microns (ballpark)

mcerd1 - 7/3/13 at 11:06 AM

at work we use 2mm oversize for hot dip galv 80 or 140 micron coatings (big lumps of structural steel with loads of tolerance) even then some of them might need cleaned out

85 microns average is the standard coating thickness for normal hot dip to BS EN ISO 1461 (the standard that they all follow - it also tells you what the min coating at any point should be and what imperfections are not allowable)



so for your trailer I'd go for about 1mm oversize holes and see how it goes

if you end up with bare spots you can spray them with this stuff (or similar) http://www.metaflux.com/products_singles_70_45_43.htm
its the same stuff the galvanisers use when they miss bits



[Edited on 7/3/2013 by mcerd1]