Thread concentricity verification

[harleyrider151]

We both manufacture and purchase hydraulic manifolds which consist of SAE ports and cartridge valve cavities machined in aluminum, steel, brass, or iron. All of the ports and cavities involve a drilled or machined cavity with threads ranging from SAE #2 to #24. We are experiencing valve shifting problems with a purchased part and suspect an out of tolerance condition with the threads. The threads must be concentric to the bore within .002 and perpendicular to the surface within .001. We are looking for a reliable way to verify these tolerances. Can anyone help us out?

 

[MikeHalloran]

- Visit the vendor and make sure the threads, the bores, the seal cavities and the spotfaces are all machined in the same setup. It may be necessary to insist that their CNC program be arranged to change the tools and fully machine each port in sequence, instead of machining all bores, then all spotfaces, etc, which involves fewer toolchanges and saves time.

- Make up some functional gages to check the bore centering and spotface perpendicularity, and provide a set to the vendor.

- Review the tolerancing on your purchased part prints to make sure that your requirements are clear, and are specified in a way that is inspectable. This is probably an application where you absolutely should be using GD&T.



Mike Halloran
Pembroke Pines, FL, USA

 

[israelkk]

Relying on the thread as a guiding surface for concentric or perpendicular placement is a bad approach. There always should be an cylindrical surface following the thread and the concentric and perpendicular should be referenced to the cylindrical surface. Even when using one chuck setup operation you still have the issue of the accuracy of the thread. The tolerances on a standard thread even the 3A class are still too large for accurate placement.

 

[meintsi]

<Valve shifting problems> - is then problem with the cartridge cavities then?

For a cartridge to fuction - "All diameters concentric within .002" is all you need.

Control the perpendicularity of the cartridge cavity elsewhere, NOT by the thread.

Checking this is another issue. You could try testing the cartridge in a different cavity. Also after removing the cartridge from the suspect cavity, look for wear marks on the nose diameter (and others) for signs of rubbing. Creating an accurate gage for this is practically impossible due to the (I'm guessing here ......) 6H thread tolerances.

The SAE spec does use the thread as Datum -A-, but perpendicular to .008" at a predefined diameter on the spotface. (not .001")

Remember...
[navy]"If you don't use your head,[/navy]
[navy]your going to have to use your feet."[/navy]