Metric screw thread measurements?

wrote:

The 0.2mm is purely related to the pitch of the thread. The overall diameter of a metric coarse or fine pitch screw thread is always the same regardless of the nominal diameter.
With regards to ISO metric threads every dimension is derived from the nominal diameter and the pitch. The angle is always the same, 60 degrees.
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Thanks to everyone for all the explanations.
I'd never previously bothered with any of this, but maybe like the OP I bought a couple of digital calipers from Aldi. Using vernier calipers, although good enough for identifying drill bits, there's always a possibility of user error or misreading such as you wouldn't want to swear blind as to any particular reading. Without using a magnifying glass anyway .But now for just £8.50 everybody is an expert. The outside diameter of the thread of the 6mm (Toolstation) bolts I just measured is 5.86 mm. And yes I pressed zero first.
michael adams
...

Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
michael adams wrote:

That's within the defined tolerances. The diameter of a bolt is the diameter of the unthreaded portion, which is a close fit in the hole. The threaded portion is specified to be smaller to allow for easier assembly and to allow for manufacturing tolerances. If you were to continue the shape of the thread to a sharp edge, then it would be the same diameter as the shank.
Read this page:-
http://www.boltscience.com/pages/screw2.htm
and the following ones for a reasonably complete explanation of the history and theory of bolts.
In fact, the whole site is a mine of information about threaded fasteners.
--
Tciao for Now!

John.

Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Indeed so. But without my £8.50 Aldi digital calipers I would never have thought to measure the diameter of any screw. Metric or otherwise. Similarly I doubt that the OP would have been quite so confident in his measurements in order to ask his question.
I now seem to remember having come unstuck discussing the finer points of screw dimensions on here before.
> The diameter of a bolt is the

I have actually read a history of screws and screw threads and their role as an essential element in the evolution of precision tools - Ramsden Maudslay and some French engineer and their original screw cutting lathes - which could then be used to produce lead screws for lathes which could then be used to duplicate the originals
With the real precison laying in arranging the screw threads perpendicular to one another. i.e one compete revolution in one threaded rod only rotating another threaded rod by a 1/60 th of a turn. Which in turn etc. etc.
Without checking ISTR the screw is a form of inclined plane.

michael adams
...

Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

It would be very unlikely to come across fine and coarse threads in the smaller sizes.
--
*Cleaned by Stevie Wonder, checked by David Blunkett*

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
  Click to see the full signature.
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Dave Plowman (News) wrote:

Looks at two computer screws, both metric, both about 3.5mm, one coarse theaded, one fine. CBA to get out the micrometer to check if they're both metric, but as they both came out of the same case....
--
Tciao for Now!

John.

Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The coarse one is very likely an american thread. Most often found for HD fixings.
--
*Forget the Joneses, I keep us up with the Simpsons.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
  Click to see the full signature.
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tuesday, October 23, 2012 3:43:42 PM UTC+1, Dave Plowman (News) wrote:

+1 Hard disk fixing screws are normally UNC, CD/DVD drive fixings are normally metric, so you'd expect to find both in any computer...
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
John Williamson wrote:

Nope. One may or may not be metric but one is almost certainly UNC -a US thread used principally to hold disk drives in place.
--
Ineptocracy

(in-ep-toc’-ra-cy) – a system of government where the least capable to
  Click to see the full signature.
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Please tell me you are joking.
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Partly correct for some thread series

No
No
Total bollocks
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

As you're clearly very knowledgeable on the subject, as the OP was trying to discover why the diameters of various metric scews apppear to differ by small amounts, perhaps you would be kind enough to explain why this is.
michael adams

Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
michael adams wrote:

They almost certainly weren't all metric screws.
At or around 3.5mm is M3.5, 4BA, and something like 5/32" UNF. I domnt think Ive seen UNC or whitworth done that size, but there you go,
In addition there is BSP, and another standrad thread whose name escapes me.
All in all there are probably 10-20 thread forms in use worldwide, or which have been in use in the last 60 years. None of them fit each other.

--
Ineptocracy

(in-ep-toc’-ra-cy) – a system of government where the least capable to
  Click to see the full signature.
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Some do, depending on your definition of 'fit'. Some UNC and some metric sizes being the obvious one.
--
*I will always cherish the initial misconceptions I had about you

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
  Click to see the full signature.
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Dave Plowman (News) wrote:

Fit as in not the way wide legged trousers 'fit' chavs.
--
Ineptocracy

(in-ep-toc’-ra-cy) – a system of government where the least capable to
  Click to see the full signature.
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

0BA and M6 x 1 being a case in point
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 23 Oct 2012 14:33:09 +0100, The Other Mike

Camera threads of nominal (historic) 1/4" Whitworth are commonly bodged with UNC bolts. Since they usually only need three or four turns, it's got away with.
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 23/10/2012 13:39, michael adams wrote:
<>

If I were so very knowledgeable about threads, I would have answered on that basis. But I am sufficiently with it to try to find an answer. Further, I might well post bollocks on occasion, but most often I have actually tried to look something up, if appropriate.
The Wiki link identifies quite a lot about threads.
--
Rod

Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 23/10/2012 13:39, michael adams wrote: ...

The OP is fairly expert at posting apparently reasonable requests that end up spiralling into long and involved threads, if you are tempted into answering them. I have a book that devotes 18 pages exclusively to measuring threads. It is not as simple a subject as it might appear.
Colin Bignell
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.