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Composite GD&T Again

Composite GD&T Again

Composite GD&T Again

I've read several responses to a similar question. I'm asking again, for my specific issue so I know it's being answered for my specific issue.

Please see attached .PDF. This is referring to a bolt hole pattern setting on the |A| datum. Exactly what does this mean? How would you properly check this kind of composite call out? There seems to be as many theory's on how to check it as there are companies checking it. The top FCF is obvious, I'm referring to the bottom one.

Currently, I'm being asked to make a midpoint between two holes on that pattern. Then make that my XY zero, and finally translate back to zero from the midpoint that was created. Then to report the location of each hole in the pattern from there.

Some clear, concise, expert advice on this would be great to end the debate.

RE: Composite GD&T Again

What does it mean?
The Bottom(Feature)Frame
The Axis of all the holes must be with-in the True Position DIA 0.15 (M), I.E.; the center points of the hole @ top & bottom must be in the diametric zone, basically, perpendicular to -A- with-in the zone. And that hole axis (2 points) must also be with-in the Upper(Pattern)Frame zone.

How to check?
The following example is what we do on our CMM. Some CCM's have this type of composite tolerance check command built in, some do not.

1. Bottom (Feature)Frame. Check hole: take 2 points, 1 as close to top as possible, 1 as close to bottom as possible. Use the two points to create the hole axis (line). Use that hole axis line to intersect bottom plane and top plane of product. This will give you 2 new points @ hole top & bottom on hole axis. Obtain distance from point to point in XY (Datum -A- Z0). That distance is the actual zone, to be compared to tolerance zone.
2. Top (Pattern)Frame. After checking all holes per #1, check set of top points per Top Frame, then check set of bottom points per Top Frame.

Harold G. Morgan
CATIA, QA, CNC & CMM Programmer

RE: Composite GD&T Again

I understand the concept of what you're saying, but what if there is no plane at the bottom of the holes? What if it's cast, or just not accessible? Would you have to construct a cylinder in each hole, and then translate the main datum plane to the bottom of that cylinder and intersect to create a center point of that cylinder?

RE: Composite GD&T Again

Maybe we need to see your real world application.
There is always a ton of what-if's, a specific answer needs a specific situation.

However, the basics are:
Take 4 points in a hole at two locations as far apart and as near the hole edges as possible.
Use those points to create the hole axis line.
Intersect the Axis line with the end features of the hole, whatever they are.
Use those intersect points as the actuals to compare to the tolerance call-outs.

You can use the cylinder command if you want. By taking 8 points (4 points @ two depths) in a hole you are creating a cylinder.
However, you want to use the cylinder axis (line) for the intersect, so, we skip the cylinder and just create and use the Axis line.

Harold G. Morgan
CATIA, QA, CNC & CMM Programmer

RE: Composite GD&T Again


Assuming interpretation per ASME Y14.5-2009, and that datum A is a plane basically perpendicular to the holes, the lower segment of the feature control frame means the following:

The surface of the part must not violate a series of cylindrical boundaries, one for each hole, each located at true (theoretically exact) position and orientation relative to each other and to datum A, and each 0.15 smaller in diameter than the lower limit of the hole diameter tolerance.

As for how to check it, the simplest method to describe would certainly be to build a hard gage. However, it sounds like you're using a CMM or similar, so it will probably depend on your equipment and software mainly. Ideally you would measure some representative quantity of points in the holes and on datum feature A (or a physical datum feature simulator), and then use an appropriate algorithm to attempt to fit the boundary cylinders inside the measured points.


RE: Composite GD&T Again

Thank you guys for your advice, I appreciate it.

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