How to Check Threaded Hole w/ RFS Requirement

[LargeNCharge86]

Hi all,
I am passing down a requirement from our customer to a supplier and it needs to match. The requirement is a threaded hole that has a true position at RFS back to a center datum hole. The supplier is unsure how to gauge this functionally via attribute check because of the RFS requirement, and my gauging knowledge is limited. Any suggestions on how they they should check this? Thanks!

 

[pylfrm]

LargeNCharge86,

In theory you could have some device with a plain cylindrical hole of known fixed size through the center and an external thread of variable size that expands within and aligns with the axis of the threaded hole in the workpiece. The hole in the device could then be gaged using ordinary techniques.

This will probably be deemed impractical, so approaches that stray further from theoretical correctness will probably need to be considered. It's hard to make useful suggestions without knowing the details of the requirement. Can you show us the relevant portion of the drawing? If not, can you at least show the thread specification and position tolerance?


pylfrm

 

[LargeNCharge86]

Hi pylfrm,
I have attached the portion of the drawing that should clarify the requirement. Thanks!

 

[dgallup]

I don't think there is any way to use an attribute gauge for thru position at RFS. You will probably need to use a CMM or possibly an optical gauge. You need an MMC modifier on both the tolerance and the datum to use an attribute gauge. Even then, 0.1 mm is perhaps too small for an attribute gauge, it will be rather subjective as the part looks like it's not very stiff.

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[Tmoose]

What feature is Datum -A- ?

 

[LargeNCharge86]

The three surfaces the holes that I'm asking about are in. It's in the drawing view I provided.

 

[pylfrm]

LargeNCharge86,

What is the drawing interpretation standard?

Why is the leftmost hole seemingly specified twice?

What is the intended meaning of "MINOR DIA" at the end of the hole specifications? Are the holes even supposed to be threaded?

This doesn't look like a good application for functional gaging. It doesn't look like a particularly good drawing either, but perhaps there's nothing you can do about that. CMM inspection may be your only hope.


pylfrm

 

[drawoh]

LargeNCharge86,

That is a cool looking part, whatever it is.

I do not see the point of specifying the position of threads at anything other than RFS. When you tighten your screw, it will centre in the hole. There is no play in an assembled thread.

If that were my part to inspect, I would measure to the minor diameter.

If I were desperate for a fixture, I would design something with pins at minor diameter minus the positional tolerance (0.2mm). How well controlled are pilot drills on threads?

--
JHG

 

[ewh]

I'm not very familiar with ISO, but IAW ASME Y14.5, you would be obligated to inspect to the pitch diameter unless MAJOR or MINOR were noted at the FCF.

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[drawoh]

ewh,

Ideally, you should inspect the pitch diameter, however, the minor diameter is way more accessible to tools. I believe that thread gauges inspect the pitch diameter. You could screw in the no-go gauge and measure to that. How badly can a pitch diameter not be centred on the tap drill?

--
JHG

 

[3DDave]

Thread gages typically only check for thread clearance. There are plenty of difficult to produce recesses that would behave identically to a perfectly formed thread when presented to a thread gage.

The advantage to using MMC for threads is that there is a definite boundary for the mating thread form simulator. The amount of clearance and centering ability are of little typical consequence in comparison to trying to locate the actual pitch diameter.

Even minor diameter is a fiction as only in multi-start threads are there even opposed surfaces to make a decent measurement from; instead the technique usually involves trying a series of plug gages for threaded holes.