Cylindricity on threaded part?

[Madhu454]

Hi All,
I need your help; I was referring to one the old drawing. Please see the attached drawing where threaded feature as called as datum feature “A” and it is qualified with cylindricity callout. Is it appropriate to use cylindricity on the threaded feature? I felt which is wrong. Also I felt it is not possible to measure the cylindity of threaed feature?? Can anybody suggest the best way to dimension this?

This part is made of tungsten and is fastened to anchor blocks. Used for receiving the lightning strokes.

 

[leeekim]

Madhu454,

The cylindrical callout tolerances the pitch diameter of the threads. I've seen them measure the threads by the 3 wire method then reverse calculate to find the pitch diameter.

Regards

 

[DanStro]

It looks like this drawing is calling out the the cylindricity on the pitch diameter but the datum is the major (or minor) diameter.

The choice of MD as the abbreviation is a little unfortunate but I think since the datum leader is attached to the outer edge it probably means that the datum is the major diameter (this also makes sense from a practical standpoint also).

I am not sure any of this affects the end result but I thought it would be worth pointing out.



Han primo incensus

 

[Madhu454]

Hi,
Thanks for the reply.
The MD refers to major diameter. So that it is easy to catch the threaed part using a collect and establish the datum axis.
I guess the cylindricity is given to the major diameter in this case, to qualify the datum feature (surface of all peaks of thread). But how do they measure the cylindricty of major diameter?

By considering the function of the part, all I need is the head and the threaded portion can be offset by say 0.25mm.

By Functionality; The primary datum feature is the threaded feature (Major dia is okay by considering measurement), but in case of planar datum’s the practice is to use flatness to qualify the datum’s, I hope the designer has used the same approach here and used cylindricity in this case.

Is it logical to use cylindricity for the threaed feature?
How Do we qualify the datum feature in case of threaded feature?

 

[CheckerHater]

In my opinion you can drop cylindricity requirement.
Most of the time cylindricity is only relevant for fits.
Unless you press your threaded part into round hole, there is nothing to worry about.
Your MD is already completely defined by metric thread standard. When you add extra requirements, your thread becomes “special” and not just regular M10. Is it worth the trouble for your part? From your description I’d say not.
Still, you are the only one who can make final decision.

 

[CoryPad]

Cylindricity is not used widely for threads. The more common options to limit variation on the threaded shank is either straightness or runout or position. The MD modifier is in accordance with ISO 4759-1. In this ISO document, the straightness requirement for a part of your dimensions would be 0.14 mm, quite a bit higher than the 0.05 mm cylindricity requirement on your drawing.