Practical Screw Thread Information & Tolerances

[Wayne]

The main reason I opted for ISO 1502 is that there is an Amendment in BS 84 from 1966 that states, “... accordingly it has been agreed that BS 84 be rendered obsolete: it will be made obsolete in due course”. “Due course” seems to be a wording with quite a bit of elasticity
You also wrote, “ The class of fit for BS 84 if none is specified is: Medium.” Am I correct in writing that for a standard Whitworth nut the correct class is “NORMAL” and for the bolt “MEDIUM”?

I will admit to not owning a copy of BS 84. Can you confirm from your copy the intended classes-of-fit?

 

[stefanhg]

Hi Gordon,

Is the thread profile really Whitworth?
Flank angle 55 degrees is not enough!

For metric threads the basic standard is ISO 1502. Gauges for ISO 2901 trapezoidal threads, DIN 405 knuckle threads and other metric threads are based on the ISO 1502, the difference is in the increased with 45% gauge manufacturing tolerances because of the less flank angle - 30 degrees.

Please send us the thread specification (major, pitch and minor diameter, flank angle, TPI, external or internal thread). We can calculate for you the non standard thread gauges acc. to ISO 1502, ASME B1.2, ASME B1.16M or BS 919-2:2007 (if Whitworth).

You or your client can also calculate the gauges with the software QMSys Threads and Gauges 4.8 (we are the producer of this shareware product).

 

[Gordon Clarke]

If I had a tail I’d wag it

BS 84 (Whitworth) has three classes for standard threads.

1. NORMAL CLASS is standard for nuts and MEDIUM CLASS is standard for bolts
A finer class is 2. MEDIUM for nuts and CLOSE for bolts
The courser class is 3. NORMAL for nuts and FREE for bolts

If we set the pitch diameter tolerance to 100 for MEDIUM CLASS for bolts then the tolerance is 150 for NORMAL CLASS nuts (types 2. and 3.)
If we set the pitch diameter tolerance to 100 for MEDIUM CLASS nuts then the tolerance is 66 2/3 for CLOSE CLASS bolts and 150 for FREE CLASS bolts.

 

[stefanhg]

Hi Gordon,

Two classes of tolerance for nuts are recommended, a Medium Class
and Normal Class.
The Close Class nuts, are now regarded as non-preferred but, in view of their usage in the past and for special needs, their limits and tolerances are given in an Appendix to the standard BS 84.

 

[Wayne]

BS 84 (Whitworth) has three classes for standard threads. 1. NORMAL CLASS is standard for nuts and MEDIUM CLASS is standard for bolts. A finer class is 2. MEDIUM for nuts and CLOSE for bolts. The courser class is 3. NORMAL for nuts and FREE for bolts.

That is it! I knew it was something nice and simple like that.

That is just screwed up enough to keep people scratching their heads. Maybe it was an early form of technospeak. Keep others confused enough and they will keep coming to you for the answers.

Thanks for looking that up.

 

[Gordon Clarke]

Hi Gordon,

Two classes of tolerance for nuts are recommended, a Medium Class
and Normal Class.
The Close Class nuts, are now regarded as non-preferred but, in view of their usage in the past and for special needs, their limits and tolerances are given in an Appendix to the standard BS 84.

THANKS!!!

I've glanced through Appendix B of BS 84 and can see it deals with BASES OF TOLERANCES AND ALLLOWANCES FOR WHITWORTH FORM SCREW THREADS. please forgive me if I delay further responses until tomorrow as it is 22.15 (10.15 PM) here and I have a TV programme I want to catch

 

[Gordon Clarke]

Hi Gordon,

Is the thread profile really Whitworth?
Flank angle 55 degrees is not enough!

For metric threads the basic standard is ISO 1502. Gauges for ISO 2901 trapezoidal threads, DIN 405 knuckle threads and other metric threads are based on the ISO 1502, the difference is in the increased with 45% gauge manufacturing tolerances because of the less flank angle - 30 degrees.

Please send us the thread specification (major, pitch and minor diameter, flank angle, TPI, external or internal thread). We can calculate for you the non standard thread gauges acc. to ISO 1502, ASME B1.2, ASME B1.16M or BS 919-2:2007 (if Whitworth).

You or your client can also calculate the gauges with the software QMSys Threads and Gauges 4.8 (we are the producer of this shareware product).

The thread is definately Whitworth so the flank angle is 55 degrees. I've suggested that they add Whitworth (or at least a W) to the drawing to avoid any misunderstanding with subcontractors.
I'll hand on your offer to them and it's then up to them

 

[stefanhg]

Hi,

Designation of screw threads of Whitworth form

It is recommended that the following system should be adopted on drawings and related documents for designating the screw threads covered by the present standard.

The basic major diameter of the thread and the number of threads per inch should be added to the designation. If the thread is left-hand the symbol “LH” should follow the designation.

Examples:
1/4 in. — 20 B.S.W.
1/2 in. — 16 B.S.F., LH

For screw threads of the selected diameters and pitches, the basic major
diameter and number of threads per inch should be stated, followed by the symbol “Whit. S”.
Example:
1 in. — 20 Whit. S.

Screw threads of Whitworth form but not having the selected diameter-pitch combinations given in the standard should be designated as given below:
Example:
0.67 in. — 20 Whit.

Where it is desired to designate the class of thread tolerance, one of the four terms “close”, “medium”, “free” or “normal” should be added in brackets, together with the word ”bolt” or “nut” as appropriate.

Examples:
1/4 in. — 20 B.S.W. (close) bolt.
1 in. — 20 Whit. S. (free) bolt.
11/2 in. — 8 B.S.F. (normal) nut.

It is recommended that multiple-start Whitworth form threads should be designated as in the following
Example:
2 in. 2 start, 0.2 in. lead, 0.1 in. pitch, Whit.

 

[Gordon Clarke]

Hi Stefan,

I’m not 100% certain if your reply is (or replies are) to me, or to all in here in general. Perhaps both?

I have BS84 and know quite a bit about Whitworth threads. I thank you for your detailed response, but part of “my” problem is that the company that originally inspired my question apparently knows next to nothing about threads as their drawing information proves. If I “overload” them with too much information then I think they’d rather see my heel than my toe. Just getting them to add “Whitworth” to the thread denomination was a battle. Many designers (draftsmen) live in their own “holier than thou” world and hate to admit that there might just be something they don't know :mg:

I’m pretty stubborn, but I also know that the best way to eat an elephant is a bite at a time

As I’ve already mentioned, my interest and knowledge on threads is primarily focussed on component and measurement of thread pitch diameter. Thread inspection combined with solid gauging is something I try and leave to others.

Something you wrote intrigues me.
Is the thread profile really Whitworth?
Flank angle 55 degrees is not enough!
I agree 100% that writing Whitworth is much better than writing 55 degrees (and I did suggest either writing “Whitworth” or at the very least a “W”), but do you know of any thread type other than Whitworth that uses/has a flank angle of 55 degrees?

The “W” in BSW is for Whitworth but BSF, BSP and BSPT are all Whitworth profile threads. In fact the correct title of BS84 is “PARALLEL SCREW THREADS OF WHITWORTH FORM” but it doesn’t include pipe threads.

My family and I were in Bulgaria (Nesebar) for the first time last year and had a great time

 

[stefanhg]

Hi Gordon,

The Info about Designation of screw threads of Whitworth form is to all. I already know you are familiar with screw threads.