Thread form error with GDT 3

[gabimo]

How to control the straight thread form variation/ form error with GD&T? (Unified UN, UNEF, straight thread--internal/female or external/male threads)

At work, people gave some creative suggestions: to apply total runout, to apply circularity/ roundness along with the positional requirement.

Is any of these a good idea?
Or maybe other suitable solutions, you can advise.

 

[3DDave]

Not every surface type can be easily controlled with default symbology. However an unequally disposed tolerance applied to a mathematically defined surface would be suitable. One can apply both a very small tolerance over a short distance and a larger one over longer distances.

What is wrong with the Unified Inch Screw Threads, (UN and UNR Thread Form) B1.1 - 2003 ?

 

[Belanger]

If you wish to control "form error with GD&T" then the most suitable GD&T symbol is probably profile of a surface. However, standard thread designations already have prescribed form (see above), so make sure you aren't getting too fancy for what's needed.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[greenimi]

Wouldn't cylindricity be a better callout to control the form error? I am just asking!

Why ? Because circularity should be verified (correct me if I am wrong) on each cross section on a plane perpendicular to the derived median line. You need to construct the necessary unrelated actual mating envelope, derive an axis a.k.a. calculate a derived median line, construct cross-sections normal to that derived median line ; Now, since the straight thread is a helix, how this said helix play nicely with the cross sections "normality" requirement (per the definition)

Cylindricity is a condition of a surface of revolution in which all points of the surface are equidistant from a
common axis. A cylindricity tolerance specifies a tolerance zone bounded by two concentric cylinders within which the surface must lie.

If the thread surface is anywhere within the two coaxial cylinders then the requirement is meet.
Please be gentle.

 

[CheckerHater]

Every thread is normally associated with some sort of gauging standard that prescribes how exactly to check the tread.

For example ASME B1.3 provides 3 gauging systems for inch and metric screw threads:

21 - normal

22 - precise

23 - what were they smoking?

So nobody is wasting their time trying to describe the thread in terms of "regular" GD&T.



"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[israelkk]

CheckerHater

"So nobody is wasting their time trying to describe the thread in terms of "regular" GD&T"

You will be surprised how many engineers are trying to save an accurate cylindrical portion adjacent to the thread for an accurate positioning and "invent" a "precise" thread as the positioning feature.

 

[CheckerHater]

There is nothing wrong about using GD&T on non-threaded cylindrical surface.

"Invent" implies non-standard custom thread, which makes it designers responsibility to dimension and tolerance it completely.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[israelkk]

By "invent" I meant that they are defining GDT with respect to the thread for very accurate parts such as for circularity of 0.01mm with respect to the thread instead of defining an short accurate cylindrical diameter adjacent to the thread where the thread stays relatively loose and is used for its main function for clamping and not for positioning. I have seen designs that manufacturing department had to reject parts all the time because it was almost impossible to achieve the requirements in the drawing. When you define a thread as a feature what is the feature? the outside diameter which is in the accuracy range of 0.1 to 0.2 mm or the inside diameter or the pitch diameter which is almost impossible to economically measure? That said for external thread. But, how you can do the same for internal thread where is usually manufactured by a standard tapping tool?

 

[CheckerHater]

The entire thread checked with appropriate gauge is a feature.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[JNieman]

There is no gauge I've come across that checks "the entire thread".

I'm open to new ideas.

 

[3DDave]

There are two outcomes from things that cannot be measured:

1) either don't create designs that depend on them
2) they don't affect anything because the affect is immeasurable.

 

[KENAT]

Functionally the pitch diameter may well be a functional datum.

I suspect most of us on here would consider it a last resort but I'm hesitant to say it would be fundamentally wrong to use it as such.

As alluded to, it's best to get away from the functional requirement for thread to act as datum but sometimes easier said than done.

If the issue is 'straightness' of a long thread then there are limits to relying on thread specs, as I recall they assume only a limited length of engagement so long enough screw & 'nut' combinations can interfere.

Usually not an issue for the typical 'nut' being no more than couple of diameters etc.

However, for things like threaded bushings on precision adjustment screws etc. can come into play.

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[drawoh]

gabimot,

Don't use GD&T to define thread quality unless you are inventing a new, Gabimot thread. The UN and the Metric standards control everything you are asking about.

--
JHG

 

[mkcski]

A side bar to the discussion on threads and thread-form using profile: I was working for a company in the 1980's that manufactured Navel Nuclear reactor hardware. Really fussy stuff - insanity comes to mind. Anyway...the thread form on many of the threads had to be confirmed. We used dental putty, took an impression, let it cure, sliced to get a narrow cross section, put in on an optical comparator (remember them) and compared the shadow to the form on 50x Mylar overlay - get your wallet out - but this worked. This was done at several places along the thread to prove thread-form consistency.

Certified Sr. GD&T Professional

 

[KENAT]

mkcski - did similar at a former employer.

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[mkcski]

KENAT - and I thought I was the only one exposed to this cruelty. hahaha

Certified Sr. GD&T Professional

 

[KENAT]

Well I didn't actually do it but that's how a lot of larger threads were inspected.

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[mkcski]

KENAT - I was the machinist thread-milling the threads (long before TM was commonplace), so I "participated" in the inspection by assisting the inspector. Geez I'm showing my 'advanced" age here. hahaha

Certified Sr. GD&T Professional

 

[JNieman]

We have a somewhat older optical comparator that is quite handy for some of the small parts we make. However, we also have a stash of metrology epoxy that has come out for odd situations. Maybe once or twice a year.

 

[mkcski]

Oooooooooooo... I like the fancy words "metrology epoxy". We called it "schmutz" if you are familiar with the PA Dutch in my area.

Certified Sr. GD&T Professional