datum selection - stamped part

[AndrewTT]

My below questions will be about real world application of ASME Y14.5-2009.

Background: This part is stamped from sheet metal (.027” thick). This part is the trigger for a heat sensitive release mechanism. It is soldered to its mating part. The trigger is only soldered to the mating part on the surface that I have indicated as datum feature A. The pattern of 4 circular dimples that protrude from datum feature A towards the mating part are spacers to allow room for the solder between the two parts. These spacer dimples protrude .003-.005” beyond datum feature A surface. The other two dimples on datum A have a function that will not be discussed. These two dimples are conical, terminating in a spherical radius and protrude in the same direction as the 4 circular spacer dimples.

A functional gage is not planned on being built to check this part at this time due to a low EAU of this part.

Need: I am only concerned with the protrusion of all 6 dimples and not their indentation portion. Said another way, I don’t care what you push into the side opposite of datum feature A as long as what protrudes from datum feature A is the geometry I need. I need the inspector to verify the geometry of the dimples on datum feature A side only. Therefore, the FCF controlling these dimples will reference A, B, C.

Here is where my lack of real work experience in application gets me confused. The inspector will have to simulate datums A, B, C and the physical datum simulators will most likely block his access to these dimples for measurement. Scanning/CMM measurement of the part eliminates this issue but what if a scanner/CMM is not available?

Questions:

1) How is this type of situation handled in the real world? How was it handled before scanners and CMM were readily available?
2) Do I use datum targets in an attempt to free up space for my inspector (not sure how much room this would buy him, if any)?
3) Do I modify my datum scheme away from being based upon function and towards being based upon inspection to aid the inspector? Do these compromises in datum scheme happen often?
4) Should be the opposite surface of datum feature A be my primary datum feature regardless of any other issues, simply due to the presence of the protrusions on current datum feature A? (bad original choice of primary datum, opposite side is better choice?)
5) Who doesn't have a scanner or CMM, it is 2017? (We have a scanner)

Any feedback is appreciated.

Thank you.

 

[3DDave]

1) Height gages and dial indicators and swearing.
2) See 3
3) The scheme is not currently a functional one. I would flip it and make the tops of the spacer bumps datum targets, indicate the part is to be constrained such that those points are co-planar and then use profile to make sure the nominally flat surface is the correct distance from that datum to properly solder it. After all, it is the bumps that align and orient the part.
4) See 3
5) Lots of people don't.

 

[AndrewTT]

Thanks 3DDave. I really like your 3) answer. A little upset with myself for not putting that together myself while describing how the part works...

 

[TheTick]

Having a scanner isn't necessarily even close to having an accurate measuring device.

 

[AndrewTT]

3DDave -

Per the attached picture you can see that the part is not stable on the suggested datum targets. Is the correct application now to use the two lowest targets (nearest the edge that is lifted up off of my very accurate datum simulator...) and have a third datum target on the far end of the part to stabilize the part? Even if the part were stable on its own, can I use 4 datum targets to establish my primary datum? Am not I limited to three datum targets that are not in a straight line (3 points make a plane, the 4th part if not perfectly on that plane will allow for rocking)? Or can I use the 4 targets with a 5th stabilizing target and the part will still only sit on the 3 highest points?

 

[3DDave]

I put it before "the part is to be constrained such that those points are co-planar."

What will you use to hold the part in place when it is soldered?