Rule #1 and form thread

[greenimi]

Is rule#1 applicable to the form thread?

I have a situation where an internal thread must be verified with a Go gage (plug gage). The question is: does the go gage --plug gage-- need to be (must be) as long as the female thread that is checking or can be shorter? If the go gage-the plug--is shorter do we run the risk that the female thread (internal thread) being bend and the plug won't catch that form error?

Any clarification on my dilemma will be greatly appreciated.
(manufacturing is using a go gage with only 3-4 spirals to check the size of the female thread which is 8-9 spirals long).

Any standard (ASME, AISI, etc.) I can use to get more info?

 

[MQSCI651]

Why not use such kind of gauge which handle cross section is smaller than thread diameters?

 

[Belanger]

Rule #1 states that it only applies to a "regular feature of size." A regular FOS is one that is a cylindrical surface (or sphere or parallel opposing surfaces, etc.). So it looks like that doesn't apply.

Unless you're using a weird, customized thread size, wouldn't the form error be covered in the thread specs given by Machinery's Handbook or similar guide? (I don't have MH here at home so I'm just tossing it out there.)

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

 

[greenimi]

Okay. So if rule#1 is not applicable then how the form is controlled?

And another question is: if the minor diameter is verified (with a GO gage pin) is it acceptable that the GO gage pin length’s to be only the thread pitch size?
½-20 UNF (pitch size .05). Minor diameter is to be verified with a Go gage pin. The length of the pin that must pass thru the thread could be only .050? Or must be at least as long as the thread is?

 

[dgallup]

Threads are usually inspected with specialized thread gauges, not just pins. There is a lot to inspect beyond the minor diameter.

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[CheckerHater]

greenimi,

You may want to look into something like ASME B1.3 for more information.

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

 

[greenimi]

B1.3 does not say anything about the gage length.

4.10.8 Runout.
The pitch and major cylinders of the threaded portion of the GO segments or rolls shall not exceed the runout as determined by mea­surements of runout (full-indicator movement) on each gaging member, with respect to pitch cylinder. Runout shall not exceed one-half X gage major di­ameter tolerance.

Isn't it more stringent than rule#1?

 

[CheckerHater]

B3.1 refers you to individual standards.

Individual standards say something like this:

"5.6 Length of Thread Engagement
5.6.1 Normal Length of Thread Engagement. The pitch diameter tolerances specified herein are applicable to the limits of the normal length of thread engagement LE in Table 7. The limits were calculated by formulas given in para. 6.4."

So, normally your thread GO gauge has to correspond to "normal engagement length" specified in standard for your particular kind of thread. NO GO gage may be shorter, just like on this picture:

http://files.engineering.com/getfil...4c-b73b-f1e4dcd910c5&file=Wkg_Thrd_Plug_0.jpg

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

 

[greenimi]

Ch,

Where is your statement from? (5.6 length......) I cannot find it in B1.3.

Is it from ASME B1.1 (Unified Inch Screw Threads)? Or from ASME B1.2 (Gages and Gaging for Unified Inch Screw Threads)
or from other standards?

Please kindly let me know.

 

[CheckerHater]

This is from B1.13M - 2005 Metric Screw Threads: M Profile. What's your thread?

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

 

[greenimi]

5/8-14 ACME-4G
ASME/AISI-B 1.5

 

[CheckerHater]

According to the standard you referenced, the tolerances apply to length of engagement not exceeding 2 times basic major diameter.

(For whatever it's worth)

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

 

[BiPolarMoment]

It seems to me that regardless of what the 'rule' is I'd want to be checking more than a dictated gage length if the length of engagement is considerably longer than that. Of course this is all dependent on function--maybe a joint with the threads galled together because of pitch mismatch is perfectly functional, it wouldn't be the first time I've seen that concept being used.

 

[CheckerHater]

@moment: If you define your own non-standard thread you take responsibilities for the consequences like galling, don't you?

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

 

[BiPolarMoment]

@CheckerHater: Of course; fortunately most of my non-standard threads interface with much softer materials that may not even have a female thread form to mate with (i.e. self tapping) so it's less of a critical issue.

However, it was my understanding that even most standard thread classes have an 'upper limit' of engagement length where you need to start worrying (e.g. >2 major diameters as you mentioned) and there might be a (possibly poor) reason someone chooses to exceed that. That's more of what I was referring to. I'm not sure if that's necessarily the case here.

 

[CheckerHater]

All I'm saying, that it's not the standard's fault.

OP was asking if the length of gauge is defined somewhere, so there it was.

I agree that there might be reason(s) for using special threads, that's why they call them "special"