Verifying "coxilality" to a thread

[prdave00]

One of my components has an external thread (see attached) that I'd like to have serve as the primary datum feature and then check the position of "coaxial" cylindrical features against. What's the best way to inspect position in this case? I thought of getting the opposite of a flexible hole location gage to thread into the part thread (such that the pitch diameter is referenced), but I'm not sure if something exists or would be cost prohibitive to have custom made. I'd hate to have to default to the Major Dia since it has nothing to do with the function. Any ideas?

The other thing I should mention is that the externally threaded part is a reinforced plastic.

I know this might not be the most appropriate forum, but I figured I'd have the best shot at a response.

http://files.engineering.com/getfil...6-42c0-b132-b4ed47750d8d&file=Ext_Image_2.jpg

http://files.engineering.com/getfil...4-4be1-9eab-81e5a2bca9cd&file=Ext_Image_1.jpg

 

[flash3780]

If you reference the datum at MMC, a functional gage may be threaded onto it. Otherwise, I think you need to grab it with thread measuring wires. Unless otherwise specified, the pitch diameter is the datum, which is likely what you're after.

To be clear, you're using nonstandard threads. Is this necessary? I'd crack open ASME B1.1 and use the same dimensioning scheme that they do for the threads. I'm guessing note 5 ties the major/minor diameters to the pitch diameter, but if not you'll need to add a control there.

I notice that you have a datum A and a datum C sharing the same axis. Why add in the extra stackup? It's fine if that's functional, I suppose.

 

[KENAT]

Sorry, not a direct help on the inspection issue but wrt calling out UNS threads correctly: faq404-1480

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[prdave00]

Flash3780:

If I understand you correctly, it would make sense to specify datum feature A at the maximum material boundary when it appears as the primary datum in the FCF for a related features (e.g. the .218 dia blind hole). This would allow me to use a functional gage built at MMC which I imagine would be similar to a GO ring gage for the thread but with a precision ground OD that's "coaxial" to the thread, etc.

I still can't grasp using measuring wires to grab the part since it wouldn't constrain the part 360 degrees around.

The feature Datum C is actually a "width" serving as wrench flats functionally, but also serves as a clocking feature for some profiled features not shown in the views I provided. I'll have to think this one over a bit.

By the way, Datum B is the flat perpendicularly constrained to Datum A (i.e. pitch cylinder of the thread). Functionally, Datum B controls how far the mating part can be screwed down.

The special threads are necessary given the design envelope and size constraints (including for the mating part) that I'm working within. All basic diameters and tolerances for the thread were determined per ASME B1.1 and the dimensioning scheme is equivalent to that in the standard. Note 5 states the following: UNS THREAD PER ASME B1.1-2003. MINOR DIAMETER ESTABLISHED BY CREST OF NEW TOOL. ROUNDED CONTOUR AT ROOT PREFERABLE, BUT SHALL NOT VIOLATE THE MAXIMUM MATERIAL BOUNDARY OF THE MINOR DIAMETER.

 

[DeanD3W]

prdav00,
While the default is that datum A would be an axis based upon the pitch cylinder diameter of the thread, this could be modified to greatly simplify measurement by adding the note (MAJOR DIA) beside the datum feature A label (see figure 7-35 of ASME Y14.5-2009 or figure 5-62 in ASME Y14.5M-1994)... Your tolerances are tight enough that it may be good to stick with the pitch cylinder diameter... That's not easy though since you need a non-standard, external thread version of something like the multi-ball gages shown at

http://www.trueprecisioncorp.com/

(no affiliation).

One major problem with the drawing you provided is that the threads are not long enough to serve as an adequate primary datum feature... I don't suppose there are similar coaxial threads or a coaxial feature at the other end of the part? If so that other feature could be datum feature B, with A-B as primary.

I hope this helps.

Dean

http://www.d3w-engineering.com

 

[flash3780]

Regarding the MMC reference, here is an example of an inspection fixture for internal threads:

http://www.tec-ease.com/gdt-tips-view.php?q=188

Your situation is obviously a bit different, but I think that the topic on the link is applicable. MMC allows for hard gaging to be used - the gage is made to represent the "worst mating part" so to speak.

If datum C is for clocking only, is a centralizing datum necessary? I think that if you use a single flat, it would be easier to inspect.

Also, I did a quick conversion and you're pretty close to an M7 metric thread. You might look into using a standard metric thread rather than a custom thread profile.

 

[prdave00]

Dean: The available engagement length has definitely been a problem. One gaging company no quoted me on a flexible hole location gage to check the mating part for this very reason. The thing is that the short engagement lengths have not caused me any discernible issues in terms of how my design functions. It's just a shame there is no easy (or relatively inexpensive) way to "grab" onto it to check location. I am looking into Tru-Pos Locators from LaVezzi Precision for the mating part and have queried Pennoyer-Dodge regarding their Twist'er gages.

flash3780: I'll look at the M7 thread form. It would be great if I could use a standard threads for my application. Any idea why there are so many gages to check the location of threaded holes but not threaded bosses? Also good thought about the datum C feature.