Thread form error with GDT
Thread form error with GDT
(OP)
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.
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.





RE: Thread form error with GDT
What is wrong with the Unified Inch Screw Threads, (UN and UNR Thread Form) B1.1 - 2003 ?
RE: Thread form error with GDT
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Thread form error with GDT
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.
RE: Thread form error with GDT
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
RE: Thread form error with GDT
"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.
RE: Thread form error with GDT
"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
RE: Thread form error with GDT
RE: Thread form error with GDT
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Thread form error with GDT
I'm open to new ideas.
RE: Thread form error with GDT
1) either don't create designs that depend on them
2) they don't affect anything because the affect is immeasurable.
RE: Thread form error with GDT
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.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Thread form error with GDT
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
RE: Thread form error with GDT
Certified Sr. GD&T Professional
RE: Thread form error with GDT
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Thread form error with GDT
Certified Sr. GD&T Professional
RE: Thread form error with GDT
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Thread form error with GDT
Certified Sr. GD&T Professional
RE: Thread form error with GDT
RE: Thread form error with GDT
Certified Sr. GD&T Professional
RE: Thread form error with GDT
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Thread form error with GDT
Not passe per say (pun intended). But since a lot of mfg moved to "other parts', they are not as prolific as before. I teach for "die-shops" that make "2D" parts and they still use them.
Certified Sr. GD&T Professional
RE: Thread form error with GDT
This thread was the first time I heard the sentiment as well. This is the first company I worked at which has any, and I think it's a pretty useful tool. I've used it for many quick checks and doing some investigation. We have more expensive and elaborate vision systems but those are in a building ~100 yards away, opposite our parking lot and, well, I guess I'm lazy :)
We maintain calibration and get it verified by a 3rd party on an annual basis - if it ain't broke, why stop using it?
RE: Thread form error with GDT
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Thread form error with GDT
Would you trust a gage (thread ring) or a comparator (with mylar thread form) when checking an external thread?
What about if these inspection methods provide conflicting/opposing results (good/bad parts)?
RE: Thread form error with GDT
1) either don't create designs that depend on them
2) they don't affect anything because the affect is immeasurable.
RE: Thread form error with GDT
I guess the OP's question is a more general than specific to his case.
ASME B1.3M states: "“Within each gaging system, a choice of gages is specified for each characteristic. Acceptance by any one
gage specified for a characteristic shall be the criterion for acceptance of the characteristic.” "
Now, how the comparator came to play, I 'll let more experienced gentlemen to chime in.