Tolerance requirements on tapped holes forZinc-plated Metric Fasteners

[Jieve]

Hello,

Background: I have been ordering parts for a machine design prototype used to demonstrate typical industry machine vibration phenomena in an academic environment. The machine is medium-sized (1m x 1m x 0.5m) and is mounted on spring isolators, which are designed so that resonance is induced at certain rotation speeds. It's made of all metal machine parts.

I have spec'd 8.8 Zinc plated hex cap screws for most of the loaded parts and some of the fasteners were only available in class 8.8 but without zinc-plating. I recently read that using zinc coated hex screws with non-zinc nuts or vice-versa results in either unacceptably tight or loose tolerances between the screw and nut, as the zinc-plated nuts are bored slightly oversize. For this reason I am trying to use only zinc screws and nuts together, and non-zinc screws/nuts together. Is this a reasonable general rule?

When the fastener hole is tapped into a blind hole, do I need to call out a larger clearance than standard if the fastener is zinc plated? i.e. 6G instead of 5H

Thanks!

 

[CoryPad]

By plating, do you mean electroplated or mechanical plated zinc? Hot dipped galvanizing? If you mean plating, then you can use them interchangeably with non-plated parts. Standard tolerances are 6g for external threads and 6H for internal threads. For hot dipped galvanized parts, it is common to use 6g for external threads and 6H or oversized 6AZ for internal threads.

 

[Jieve]

Corypad, thanks so much for the response.

The fasteners are being purchased from local fastener suppliers according to din/iso spec, specified as either 8.8 or 8.8 zinc plated. I'm assuming that this means that they are zinc electroplated and not hot dipped galvanized. Is it common to find hot dipped galvanized fasteners without specifying that process separately? Or are hot dipped more common for structural applications?

 

[CoryPad]

It is not common to find any parts with a coating unless it is specified specifically. Hot dipped galvanizing is common for fasteners for structures, not very common for general industrial applications.