I’ve always assumed that e.g.an M4 bolt has a diameter across the threaded part of 4mm. Everything I’ve read on the internet backs this up, especially HERE which gives the ISO spec tolerances.
However, I’ve just measured three M4 bolts from different sources and they measure 3.80mm (±0.02mm). ISO spec min is 3.838mm for an M4 so these are out of spec.
My micrometer is one of the ‘better’ steel Chinese cheapie imports and measuring a 5p coin gives the correct diameter of 18.00mm so I don’t think it’s an equipment error.
Am I missing something here; Is it common for manufacturers to produce undersize bolts, have I got three identical rejects, or is there something else going on I’m not aware of???
for an M4 the bolt thread nominal size is 4mm and if its partially threaded it has to fit a 4mm hole. you can actually get M4 bolts with different pitch but if you think about it you need to manufacture the valleys and hills of the thread. the male and female has to match up and the spec allow machining tolerance so it wont lock.
Yep it’s a nominal 4mm thread size so your bolts are fine
Within the all encompassing term of “M4” or Mxx, the “xx” relates to the nominal bolt diameter, however the actual peak diameter (termed major diameter) will always be less than the nominal size due to tolerancing, ease of fitment etc etc.
For example, if you had a bang on 4mm bolt going into a bang on 4mm female thread, you would in effect be creating an interference fit and you wouldn’t be able to turn the bolt.
So in the case of an M4, a course pitch bolt would have a major diameter of between 3.978 and 3.838mm. The corresponding female thread max diameter would be between 4.000 and 4.219mm
For some reasonably detailed learning on matters bolted joints google boltscience - this is a company that specialises in joint assessment software and have a lot of good articles on their site.
Thanks all.
Strange, I do a fair bit of design for my 3D printer (hobbyist level stuff) and it never really clicked before, even though I’ve put in many M clearance holes. I usually measure the bolt diameter and then add (usually) 0.4mm for the hole, rather than working to the spec on a chart.
Everything has a tolerance and it would very much depend on the tolerance class of the fasteners. You will be measuring commercial grade, rolled thread screws with an upset head, and for their intended purpose they will be fine. You can specify the fit to very tight limits, but them you are looking at very specialist screws and very expense manufacture.
The true reason for the difference on your fasteners is likely just tool wear. It is within acceptable limits and so fine.
Metric screw threads just mean there are a 60 degree included angle thread. M3, 4, 24 etc. are just a classification of nominal sizes classed an coarse or fine. You can specify a metric (60 degree thread profile) to just about any size and tolerance you want.
