True Position Of Datum Features

[TAC45]

A couple of questions about true position call outs for datums.

1) I have a 4 Way "B" Datum hole. This hole resides on the primary Datum "A" surface. I have a diametrical true position call out of .1 of the hole to Datum "A" on the drawing. Is this feasible? If so how would it be possible to calculate TP of the hole to Datum "A"?

2) The same drawing has a call out for the 2 Way slot Datum "C" which also resides on the Datum "A" plane. The call out is .4 to "A" and "B" at MMC. I am confused as to why you would ask for TP when the feature itself has been used to rotate the alignment and set to nominal? What are you checking it to A/B since the alignment has not been established until rotating to "C"? Is the intent that you are verifying alignment by calling this out?

Sorry if confusing, I really appreciate any help here!

[Matthew D. Hoedeman]

1) PERP to -A-

2) They are controlling the spread between the holes.

[VinniUSMC]

I nearly universally align to ABC first, and then report the location/position of the ABC features. Since TP|A is just perp, and TP A|B is just spread, there's no harm in doing it this way, and there's nothing gained trying to do it any other way.

Also, if your C is a slot, the length of the slot is really the spread, but probably is to ABC and not AB, so the true Datum C (the opposed surfaces of the slot) doesn't actually have a "spread".

So, your TP for B should be 0, unless you can measure a full cylinder and that cylinder is skewed (sheet metal doesn't really have that concern), and your TP for C should be 0 (this changes is C is a hole instead of a slot, then there is truly a "spread".)

[Francisco Rebelo da Cruz]

Good morning, can someone tell me what the DA is? Thank you

[dph51]

Maybe “Deviation Angle”

[Schrocknroll]

Good morning, can someone tell me what the DA is? Thank you

As dph51 commented it's the angle of the position deviation.

[TAC45]

Thank you for the replies! I should have specified this originally but this part is .8 MM thick sheet metal and the "B" datum is obviously measured as a circle and not a cylinder thus perpendicularity cannot be calculated. It seems as though this call out of diametrical true position of hole "B" to plane "A" is invalid based on the feedback here, correct?

[Matthew D. Hoedeman]

Thank you for the replies! I should have specified this originally but this part is .8 MM thick sheet metal and the "B" datum is obviously measured as a circle and not a cylinder thus perpendicularity cannot be calculated. It seems as though this call out of diametrical true position of hole "B" to plane "A" is invalid based on the feedback here, correct?

Not invalid, but not checkable. Basically, it means that your stamping die needs to pierce that hole normal to surface. I see it a lot. I have fixtures for everything, I align the fixture (it's own program), save to external alignment, then check the part in a 'check' program (that recalls the external alignment). I will then measure the 4-way (B) and the 2-way (C), then do an alignment in the check program using JUST those 2 features, construct a line, rotate to the line, origin to the B, then report B (has ZERO deviations since it is origin) then report the C, it has ZERO deviation in the locating direction and a deviation in the non-locating direction.

[TAC45]

Thanks for the help!