GD&T Datums from two Holes

[Brokenengineer]

Hi all,

I was hoping if someone could review my use of Datums and FCFs in this example sketch. I am creating a production drawing for a part, and will be applying a GD&T scheme similar to this. But this is one of my first formal uses of GD&T,

To understand design intent: The 15mm hole aligns with the centre axis of a mating disc. The 6mm hole is used to align a detent to rotationally align the disc. The other features are items that interact at certain points on the disc.

My main questions are:
Is the use of datum A and B sufficient to position the Hole used for Datum C?

Can all other features then reference ABC as Datums?

Have I used the MMC correctly?

Is the outer profile FCF used correctly?

Please see attach image. Also I know the sketch is not drawn well, but I put this together in a rush.

Many thanks for your help! I am in in the process of booking a GD&T course with my company but this drawing needs to go out within a few weeks.

 

[Burunduk]

Adding the MMC to the specification validates the use of a flexiable thread gauge a bit more as it includes the bonus tolerance.

Not quite. Generally the idea of using MMC for providing additional tolerance utilized during gaging is that the gage element which mates with the feature is fixed in size, and simulates the worst case boundary generated by the smallest internal feature displaced from true position the maximum distance per the position tolerance. The usual MMC gage acts as a go-gage so you have more tolerance (bonus) for displacement of the feature as the size of the feature departs from MMC. The ASME and probably the ISO standards too default the PD as the feature of the thread to which the tolerance applied. Thread gages can be made to simulate the above-mentioned worst case boundary (called Virtual Condition in ASME, not sure about ISO) in terms of the thread pitch diameter*, but differently from the usual VC gages of internal features, they are never fixed in location relative to the datum simulators in the fixture, neither are the gages fixed relative to each other in a pattern. The additional "bonus" manufacturing gets is due to the very limited "play" available between the fixed size gage and the female thread, which happens despite the centering effect.
Also, thread gages for threaded holes do not act as go-gages as in the case of clearance holes, but are used to simulate the axis of the mating fastener. So what you actually get is some play between the simulated axis of the fastener and the thread feature. This is very unique to thread gaging.

Edit: the the pitch diameter of the MMC thread gage should be at the MMC of the internal thread, but it doesn't account for the allowed positional error - this is another difference from the typical Virtual Condition gages used for other internal features.

To make long story short, while the usual VC gages for internal features controled at MMC simulate an internal boundary inside the internal feature, the fixed size thread gages simulate the worst case fastener that is assembled into the holes in terms of the of smallest possible looseness, and can indicate the fastener axis behavior in assembly that may affect the risk for interference at the protruding fastener shank.