Pitch Diamter Rule - How is it controlled? 1

[johnchrc]

I would like to use the thread as a datum or position reference. The pitch dia. seems to be the only way the control a threaded connection relative to other surfaces. HOW IS THIS INSPECTED?

Thread bore or od appear to be a useless callout unless you care about thread strength. Does anyone disagree?

 

[Belanger]

True, if you are concerned about the thread as it actually engages with another threaded feature, then pitch diameter is the way to go. There are special pitch diameter gages (unfortunately, they are pricey!) that can be obtained for standard size threads. They lock onto the pitch dia.

Other options might be an optical method such as a comparator that can magnify the silhouette of the thread profile, or perhaps using gage wire wrapped around the threads to then check position in a more traditional manner. (The wire sits snugly in the "V" of the threads.)

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

 

[CheckerHater]

The short answer would be "don't", and there are multiple discussions here explaining why.

But if you really have no other choice you use product like this:

http://www.lavezzitrupos.com/home.aspx

to represent your hole during the inspection.

 

[johnchrc]

Great points and good feedback. Thanks.

Bottom line is most shops won't have the ability to check, unless they use the wire method, which would probably be time consuming and costly. I like the Tru Position Indicators. However, we farm out our manufacturing and most shops probably don't have anything like this. I am going to forward the website to some.

I design downhole tools which are generally made from tubular products with straight threads and seals for connections between parts. It is critical that the threads stay concentric to both ends of the part to some degree. If your thread OD or Bore are spot-on with your pitch diameter, as I would assume they are, then perhaps they can be used as a positional reference. I don't know.

The attached is a very short and simple downhole release tool used to latch and pull equipment from the hole and release during circulation of brine or drilling mud. Most parts are much longer and have more tubular layers nested inside.

We have not used GD&T in the past and have always just specified, "Ends concentric within .003 TIR" or " Ends A and B concentric...." The GD&T is applied different within all our R&D groups.

 

[CheckerHater]

Now I see where you are coming from.

As much as I personaly dislike this kind of advise, but maybe you should reverce your scheme:
It appears your parts have fairly large cylindrical surfaces, that can serve as datums, and you may try to control your pitch diameter in relation to OD of the part.
Just a thought.

 

[drawoh]

johnchrc,

You can make a precise male thread, and inspect it with wires. If the male thread screws up through a clearance hole in your fixture, then that clearance affects your accuracy. A male thread solidly attached to your fixture gives you no control over rotation.

The minor diameter is not as accurate or significant as the pitch diameter, but it may be accurate enough for your purposes.

Could you re-design your part so that there is an accurate boss concentric with your thread? This way you have an accessible datum. There must be some way to test the run-out of your thread with respect to the boss.

JHG

 

[johnchrc]

I could add a boss for those purposes, where particularly critical.

I have resorted to using the OD's as datums and calling out a cylindricity tolerance (same as flatness for datum A on a rectangular/square feature) so I have a true cylinder as my base feature.