How to calculate the tolerance band on the major diameter of an internal metric thread?

[Euan Foster]

Hello all,

I am trying to calculate the tolerance band on the major diameter of an internal thread?

This is for a component I am designing at work that has two bores that run perpendicular to one and other and require a bezel ring to lock some optics in place. The issue I am facing is that I need to control the wall thickness between the two bores and the major diameter of the threaded bore is the dimension driving this.

I am uncertain whether or not I will have to employ my own custom thread in this application so I am seeking to verify how the various thread tables you can find online calculate this out. I have had a look at the following tables you can find online and they state max and min diameters on the major diameter.

I have been referring to iso 965 and this gives formulas and precalculated tables to work out the tolerance band on the pitch diameter and minor diameter of an internal thread but not the major diameter.

I have attached a spreadsheet in which for M9x1-6H internal thread I am trying to verify the tolerances stated in the table I linked above ^. As stated above I have used the iso standard to verify the tolerance on the pitch and minor diameter of this thread but have been unable to verify the major diameter tolerance. I have tried using basic geometry to extrapolate the major diameter tolerance based of the minor and pitch diameter tolerance but with no luck. I have also confirmed that these numbers appear on multiple websites so are most likely right.

In summary, I am a bit lost in all of this and would very much appreciate if someone could advise on the tolerance band of the major diameter of the internal thread?

Regards,

Euan Foster

 

[drawoh]

Euan Foster,

I am not sure what you mean by tolerance band. Send both pieces to the same machine shop, and the custom threads will be made to fit.

Are you worried about thread minor diameters? Check out Parametric Threads in SolidWorks. This should work on any 3D[ ]CAD.

--
JHG

 

[racookpe1978]

I too don't understand your concern: The tolerance on threads are minute fractions of the thread pitch and height, or the bolt thread and nut thread can't match together , much less tighten after they turn.

If your threads are so close together you are worried about them overlapping because of thread tolerances, you cannot machine them in the real world to provide strength in the few thousandths of an inch remaining between the major diameters of the two tapped holes. So, accept that the threads may overlap. Further, even if the female tapped threads do overlap after drilling, tapping and assembly by an amount determined somehow by the thread diameter tolerances, the male (bolted) threads of the two male threads will not touch each other.

Or move the tapped holes. Or use a smaller tapped hole: There will be no loss of joint strength, because there is no metal around the two tapped holes now at the crossing point.

 

[pylfrm]

Euan Foster,

ISO 965-1:1998 does not give a maximum major diameter for internal threads. I don't know whether ISO 965-1:2013 has changed this.

ASME B1.13M-2005 gives the following requirement:

[pre] D_max = D_2_max + 0.7938566 * P

Where:
D_max is the internal thread major diameter maximum.
D_2_max is the internal thread pitch diameter maximum.
P is the pitch.[/pre]

ASME B1.1-2003 gives the same requirement for unified inch screw threads. The constant 0.7938566 comes from 11/12 * cos(30°).

In the real world, I wouldn't want to count on that requirement being met. Thread cutting tools with sharp points are common, and these will result in an oversize major diameter unless the pitch diameter is held toward the low limit.

pylfrm