What thread does a Stanley Cap Iron Screw have? I wonder about getting a couple made up. I tried buying a couple of new one (Clifton and Quangsheng), but the screws are wider (larger dia) than the older Stanley ones.
Stanley Cap Iron Screw Thread
- Thread starter RobNichols
- Start date
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Sorry - should have googled first - plenty of information out there. Looks like 5/16-18 Whitworth was what I need. Looks like I should get myself a set of Whitworth taps and dies. If nothing else that will give me a good set of screws to test against.
Vann
Established Member
The cap screw thread is the ONLY standard Whitworth thread on the whole plane.
Stanley began making planes before there was such a thing as "standard" threads. Each manufacturer would select whatever thread count they felt appropriate for the application.
By sheer co-incidence, the 5/16" thread count Stanley adopted for the cap screw, was also the 5/16" thread count adopted by Whitworth for BSW.
Stanley never changed to standard threads (until metric). Possibly because they were all set up for those sizes and didn't see the need for change. Or possibly just to keep spare parts interchangeable.
In 1930 Record adopted the same - by then non-standard - threads for their new range of bench planes. Other companies did the same - such as Falcon-Pope in Australia, in 1945.
Probably again to keep parts interchangeable (maybe they hoped to capture the spare parts market).
Here endeth the lesson
Cheers, Vann.
Brilliant Vann. Thank you.
StephenArfer
Established Member
If it helps, you'll find that 5/16"-18 Whitworth and 5/16"-18 UNC are interchangeable for all practical purposes.
J-G
Established Member
Can't allow that to go unchallenged !
It would be possible to screw a 5/16 UNC screw into a 5/16 BSW Nut but only if the Nut was at the maximum allowed Dia. and the Screw was at the minimum. That is on the 'Effective' (Pitch) dia.
If those two are reversed then there would be close to a 2½thou interference which would make it impossible to mate the two.
It would not be possible to mate them by simply reducing the OD of the screw, you would need to run a split die down the screw to make it undersize on the effective dia.
StephenArfer
Established Member
The effective dia. of 5/16" UNC is 0.2764". Effective dia. of 5/16"W is 0.2769". Half a thou difference. The Whitworth thread angle is 55 deg. and UNC 60 deg., but I never found this to represent a significant difference in practice, nor was the slightly different root and crest form. Commercial bolts and nuts can differ widely from specification. The same camera threads are described as 1/4" and 3/8" UNC in N. America exclusively but for some quaint reason they are often described as BSW this side of the pond. Either will fit either perfectly in practice so it really doesn't matter.
In my early days of aircraft maintenance on a mixed British/American fleet mixed hardware threads and specifications were a nightmare. The practice of just picking from hanger storage racks disappeared when a BEA Captain was half sucked out of a BAC 111 after the just replaced screen attachment bolts failed. The bolts were the wrong specification but appeared to fit the nuts. The aircraft landed safely under control of the 1st officer, but with the captain still more than half out and only prevented from going further by the cabin crew hanging on to him. In those days flight deck security wasn't an issue and the cockpit door was often left open. Just as well eh?
In my early days of aircraft maintenance on a mixed British/American fleet mixed hardware threads and specifications were a nightmare. The practice of just picking from hanger storage racks disappeared when a BEA Captain was half sucked out of a BAC 111 after the just replaced screen attachment bolts failed. The bolts were the wrong specification but appeared to fit the nuts. The aircraft landed safely under control of the 1st officer, but with the captain still more than half out and only prevented from going further by the cabin crew hanging on to him. In those days flight deck security wasn't an issue and the cockpit door was often left open. Just as well eh?
J-G
Established Member
You are neglecting to consider the whole story - you quote the Effective Dia for 5/16 UNC only for a Class 3A fit - and even that has a tollerence :-
0.2764 - 0.2692. -
The equivalent figures are for
Class 2A - 0.2745 - 0.2716
Class 1A - 0.2764 - 0.2692.
The BSW standard has similar (though less complex) tollerences
Close Fit = 0.2769 - 0.274,
Medium Fit = 0.2769 - 0.2725
Free Fit = 0.2769 - 0.2703
Here is a drawing of the two threads showing the potential clearance/interference in extremis depending upon which 'fit' the two threads have been manufactureed to.
The Green sliver in the top image is the clearance that would be available with UNC on Min & BSW on MAX. In the lower image the Yellow sliver shows the equivalent interference.
The coloured lines to the right show the position of the Effective Diameters.
I don't doubt that there could be a 5/16 UNC Bolt that would fit a 5/16 BSW Nut but it isn't a 'given' as your original statement claimed.
