Tolerance on Thread Angle - Drawing calls for M4 external thread to BS3643

[barbter]

Hi All,
1st time newbie here, and hopefully posting this in the correct place...


This could be a silly question but....

Drawing calls for M4 external thread to BS3643 (by memory).
What angular tolerance is there on the 60 degree form?

I looked through an old(ish) copy of part 1 and 2, and can't see it for the life of me.
Our gauge supplier's said there was (by memory) +/-16 minutes on the gauge, but they couldn't find what tolerance should apply to the component.

Anyone got any ideas?
Thanks,
Terry

 

[Stijloor]

Hi All,
1st time newbie here, and hopefully posting this in the correct place...

This could be a silly question but....

Drawing calls for M4 external thread to BS3643 (by memory).
What angular tolerance is there on the 60 degree form?

I looked through an old(ish) copy of part 1 and 2, and can't see it for the life of me.
Our gauge supplier's said there was (by memory) +/-16 minutes on the gauge, but they couldn't find what tolerance should apply to the component.

Anyone got any ideas?
Thanks,
Terry

Terry,

Welcome to The Cove Forums!

I noticed that you posted the same question on other Forums...

The standard you are referring to is BS 3643:2007.

We have a few thread/gauge expert Members here. It is weekend, somewhat quiet here, but I am sure that you will get a response.

Stijloor.

 

[barbter]

Thanks for the welcome.
Yes, I posted it over at Practical Machinist forum, but to no avail.

Anyone here can help please?
Thanks

 

[Wayne]

This could be a silly question but...

Terry; it is said that there is no such thing as a silly question, but then you have: What color is 5 x 6?
Now that we have settled that, your question has merit.

Drawing calls for M4 external thread to BS3643.

For clarification, when the thread specification is given without a pitch, the standard does give a default pitch. For accuracy, I personally like specifying the pitch in all cases. So in this case I ASSUME that we are talking: M4 x 0.7-6g external thread.

Our gauge supplier's said there was (by memory) +/-16 minutes on the gauge,…

Your gage supplier hedged his answer, and for good reason. The answer is not straightforward. We are talking gages here for this part. For gages the flank angle tolerance is variable depending on the thread pitch and is thus a moving target. For the 0.7pitch the gage flank angle tolerance is +/-18 minutes. The tolerance becomes less as the pitch becomes coarser. This is a practical thing related to the length of the flank and the capability of measurement machines to report accurately the actual value.

…external thread to BS3643.

To lay some groundwork; BS3643 is technically equivalent to ISO 68 which is technically equivalent to ANSI/ASME B1.13M. The standards are all created to produce fully interchangeable final products. The standards are nearly copies of each other in the data presented, not the lay-out or wording, but the finished goods all have the same (with some extremely minor variances) size and tolerance requirements.

Drawing calls for M4 external thread to BS3643. What angular tolerance is there on the 60 degree form? I looked through … part 1 and 2, and can't see it for the life of me.

You did well. It is not there. This feature is considered to be well enough controlled by other features and thus not needed to be defined with rigid tolerances. It is also so complicated to define that the experts who wrote the British Standard may have decided not to worry the average quality engineer with the details of the calculation; however; the authors of the ANSI/ASME B1.13M were kind enough to provide the information.

Are you ready for this?

ANSI/ASME B1.13M paragraph 10.1.5b: The allowable flank half-angle variation in minutes of arc is equal to 125 divided by the square root of pitch for pitch diameter tolerance grade 6 and 80 divided by the square root of pitch for pitch diameter tolerances grade 4. For other standard tolerance grades, multiply allowable angle variations for tolerance grade by the multipliers in para. 6.6.1.1. Round calculated angle variations to the nearest 5 min.

And if you think I am kidding, I have attached an excerpt from the standard.

Also read paragraph 10.1.4 where it states that if the flank angle is critical to the function of the product, the flank angle tolerance then is required to be specified on the product drawing.

I hope that this answers your question.

 

[Stijloor]

Wayne!

Super job as always!!

Thank you so much!

Stijloor.

 

[barbter]

Wayne,
Top job. Many thanks for the reply.

I summised that for the BS, it didn't matter what the flank tolerance is, providing all other tolerances (OD, ID, Effective and root radius) are met.
But I really couldn't believe that this was the case. It seems so open, especially compared to standard engineering drawings where drafty's put +/- 1/2 degree tolerance on debur edges, which don't matter at all!
But thanks again.

 

[stefanhg]

Table of allowable variations in 30 deg. basic half-angle of metric screw threads acc. to ASME 1.13M:2005.

Multipliers for other tolerance grades:
T(3) = 0.5*T(6)
T(4) = 0.63*T(6)
T(5) = 0.8*T(6)
T(7) = 1.25*T(6)
T(8) = 1.6*T(6)
T(9) = 2.0*T(6)