Projected tolerance

[prdave00]

I have a part with complex surfaces (I.e. machining requires the use of a 6-axis CNC and is almost 70% surface milling). This part also has 2 threaded holes with trajectories at compound angles. The location of the holes is critical to prevent thin walled sections that would compromise other features and the trajectory is equally important to make sure the fasteners don't collide. We've tackled this with a positional tolerance and the cylindrical tolerance zone was calculated to make sure fasteners don't collide. Now inspection has become an issue and hard gauges were designed to thread into the holes for inspection either with an overlay or CMM. Part of the gauge protrudes about an inch above each hole and part of the gauge extends 1" below each hole. Both ends are domes. The protruding lengths will be used to simulate the axis by probing the domed ends to estimate their centers and then using these points to simulate an axis. I'd much rather prefer stating a projected tolerance zone from the bottom surface of the part and using the length and cylindrical surface of the gauge protruding from the under side of the part to simulate the projected axis of the screw. My colleague has raised concerns about this method since it will force us to tighten the positional tolerance or run the risk that the hole location will not be controlled tight enough. Is there a way to have both and still use a hard gauge to simulate the axis of the hole? Maybe a composite tolerance with the top FCF having a loose projected tolerance zone and the bottom having a tighter tolerance zone to control location?

 

[restedup]

It is not readily apparent why you want the gage to extend above and also below the holes in question. Also I would question the use of 'domed gages' and then trying to determine the axis. A sketch would certainly be of benefit arriving at any possible solution.

What Standard is applied towards the Dimensioning and Tolerancing of this part?

 

[prdave00]

We are currently standardized to ASME Y14.5-1994 but moving to 2009.

The domed ends were more for use with an overlay since the holes axises are at compound angles with respect to the datums. When you backlight the part, the ends of the gauge are round no matter what angle you hole the part. Hope that makes since.

I'll post a graphic illustration later once I remove all the proprietary info.