Pitch Diameter as Primary Datum? 1

[Airfix]

I see lots of callouts where thread pitch diameter and gear pitch diameter are the primary datums.
I had an instance recently where an outlet hole in an oil jet had to be controlled back to a thread pitch diameter as its primary datum.

As a pitch diameter is a theortical diameter I'm having a hard time undertsanding how you would (in pratical terms) inspect back to that pitch diameter? How would the inspection fixture be set up for a primary datum on an external thread pitch diameter, a NO-GO threaded hole in the fixture to hold the threaded part?

As I encounter this situation a lot and I need to understand the inspection method more so I can really understand what the tolerances will be that are specified back to a pitch diameter primary datum.

Airfix

 

[CheckerHater]

Did you try Google search?

 

[Belanger]

The pitch diameter is found by simply threading two parts together; it's where they engage one another. So I would think that for a primary datum feature of a threaded hole, the pitch diameter's axis (which is the true datum) can be found by threading a standard gage pin in there. The subsequent measurements would be taken from the center of that pin.
Although I suppose that begs the question of material condition/boundary -- I presume there is no "M" after the datum reference?

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

 

[3DDave]

Pitch diameter is used because there's no better choice, neither the major or minor diameter control the orientation or location of the mating part and some diameter needs to be chosen - so that leaves pitch diameter.

You can make threaded simulators that are turned down to limit thread interfacing to just the pitch diameter, but mostly a device that has a precision location of thread to a more accessible feature is used. If you use and MMC reference on the thread, you can use a fixed size simulator; No clue about how to handle RFS threads; RFS is not a typical thread usage for location, though they are in pipe and other tapered threads. In straight threads I've never seen a sensible use of RFS.

 

[Airfix]

Did you try Google search?

That's quite helpful CheckHater I hadn't thought of that. Perhaps you could post a link of something relevant that helps clarify my understanding.

The pitch diameter is found by simply threading two parts together; it's where they engage one another. So I would think that for a primary datum feature of a threaded hole, the pitch diameter's axis (which is the true datum) can be found by threading a standard gage pin in there. The subsequent measurements would be taken from the center of that pin.
Although I suppose that begs the question of material condition/boundary -- I presume there is no "M" after the datum reference?

Belanger thanks for the response. There is no material condition associated and probably should be RFS. In my mind I'm think of an external thread where the pitch diameter controls the true position of a co-axial hole in the same part. The pitch diameter is theortical therefore when you thread two parts together and one of the parts is not the theortical perfect thread then the two axis may not be co-axial AND there may be some radial clearance. How do you account for that if you are making a fixture to check a true position back to a threaded pitch diameter?

Airfix

 

[Airfix]

You can make threaded simulators that are turned down to limit thread interfacing to just the pitch diameter, but mostly a device that has a precision location of thread to a more accessible feature is used. If you use and MMC reference on the thread, you can use a fixed size simulator; No clue about how to handle RFS threads; RFS is not a typical thread usage for location, though they are in pipe and other tapered threads. In straight threads I've never seen a sensible use of RFS.

Thanks Dave. That's kind of what I was thinking but I'd not be happy about using an MMC callout. The reason being that the hole position has to be controlled tightly for alignment (across a long oil jet)and I don't like the extra bonus tolerance you'd get with the MMC. Having said that using an MMC call out so a precise female part can be made might be the only way to inspect it.

I guess you could use a set of inspection balls (like doing a dimension over pins on a gear) over the thread in multiple locations and then use a collet to hold it all together to get a true (adjustable) pitch diameter in your inspection fixture.

I've just seen this called out many times and I was discussing it with a younger engineer and I realized I hadn't thought much about how it is inspected over the years. I'd just accepted it as the way it is done but I've always hated it when others tell me "well that's just the way it has always been done" without having a technical solution.

Airfix

 

[CheckerHater]

Perhaps you could post a link of something relevant

Links are usually underlined and highlighter in dark blue.

 

[greenimi]

Airfix,

How much "extra bonus tolerance you would get with MMC"? How do you calculate that?

 

[Airfix]

Links are usually underlined and highlighter in dark blue.

I appologise. I thought you were totally trolling but I wasn't familiar with the bold, large font, dark blue link. It's a little different from other BBs I've used. That's actually a useful link. Thank you.

How much "extra bonus tolerance you would get with MMC"? How do you calculate that?

That's a really good point but that's exactly what MMC does is to provide bonus tolerance. While calculating bonus tolerance on a pitch diameter may not be possible (I don't have easy access to the thread specs or a machinist handbook at the moment) it can certainly be measured. See the link CheckerHater provided. Which is why 3DDave said that if you use an MMC modifier you can create a "fixed size simulator" which is essentially what CheckerHater's article is discussing.

It is interesting that the article rejects the use of adjustable inspection devices that locate on the pitch diameter. I like the philosophy of creating an inspection fixture to simulate how the part will actually be used.

What I'm taking from this is that if I create designs that call out a pitch diameter as a datum I need to always include an MMC condition in order to be able to inspect it. It makes me wonder what I actually did in previous designs.

Is it impossible to inspect back to a thread pitch diameter that is RFS or LMC?


Airfix

 

[CheckerHater]

I owe you apology as well. I am posting from work and not always have time to create long extended explanation when it's due.

The issue of using thread as a datum was discussed several times here. You can use "Google custom search" (sorry) window on the very top of the page to get listing like this:

http://www.eng-tips.com/gsearch.cfm...-8859-1&q=pitch+diameter+datum&sa=Go&siteurl=

http://www.eng-tips.com/viewthread.cfm?qid=374050&ref=www.eng-tips.com/viewthread.cfm?qid=374050&

I myself support the opinion that threads have ability to self-center, so the measurement is always RFS.

But for those who disagree and believe that solid thread guages still create MMC/MMD condition someone invented flexible gauges.

If those are not self-centering, I don't know what is.

 

[Belanger]

Threads do have tolerance, though. So there is the mathematical potential for bonus. But granted, you won't feel it, because the thread self-centers and tightens until you hit the desired torque.

Maybe it's better not to say that MMC is for the purpose of creating bonus tolerance. Rather, MMC simply looks to an envelope, and if you come it different than the envelope, there is the potential for bonus.

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

 

[greenimi]

From Mark Foster (AGI) one of the best GD and T experts,
His opinion is (copy from linkedin)

"Threaded features are deceptively complicated little creatures, and the standard somewhat "glosses over" how they are to be treated. The standard does say that, for screw threads, the default "feature" to which a geometric tolerance applies, or to which the datum feature symbol is referring, is the Pitch Diameter. That information is somewhat helpful in that we now know that the default is not the major or minor diameter, but none of these diameters, major, minor or pitch, is truly a diameter at all. They are really helixes. The "diameter" is just the imaginary cylinder that would capture said helix. So it is next to impossible (not impossible, just close) to actually measure the actual size of the "actual" pitch diameter cylinder.

I do NOT believe that we should be attempting to measure the actual mating sizes of PD's and making use of the "bonus" tolerance in the same fashion as if they were clearance holes. In other words, the "bonus" tolerance for threads cannot be readily measured (and therefore calculated). The bonus tolerance for screw threads really manifests itself in gages."

• Regarding MMC use on threaded features, let me put it this way: If you are only measuring, and never gaging, your orientation/location of your threaded features, then you could always treat all threads as RFS and you will probably not have any issues (except for perhaps your ability to measure consistently -- since that is another issue altogether). The reason is that the amount of total potential bonus tolerance is generally pretty small compared to the orientation/location tolerances, and any physical bonus tolerance that might exist is not measureable (only gage-able) and is not a 1:1 ratio anyway.

So if you went the rest of your life ignoring any potential bonus tolerance on any threaded features (and just a reminder that I am only talking about Pitch Diameters, here), then you will be conservative and probably not have any problems.

As I have mentioned in this thread and previously,there are two camps of people, those who believe that ALL threads should only be specified RFS (like you), and those who believe that only tapered threads should be specified RFS (like me). I think that we can coexist. As I have also mentioned previously, the bonus tolerance that physically exists between straight-threaded features' Pitch Diamters is not a really measure-able thing, and so really only can be assessed through gaging and actual parts, not through CMM measurement/assessment.


Mark Foster (AGI)

 

[Airfix]

This is all good stuff. Nice post there greenimi. My thought process on how to measure bonus tolerance was in reference to a gauge. You have have an ideal MMC female thread gauge and you thread a male part that is something less than MMC then any radial play would be legal "bonus tolerance".

The reality is typically you'd thread the part to be inspected to a given torque thus all radial play will be gone and therefore so will all bonus tolerance so I can understand why it's not smart to think of threads having bonus tolerance.

Next time a make a callout to a thread pitch diameter I'll have to think much more carefully about what I'm doing!

Thanks

Airfix