Base Alignment

[[Me...]]

We're struggling to hold the .002 True Position of the -D- bore...I've tried a few methods of Base Alignment. I've squared up the part in a V Block and double checked with the Micro Hite, it is close to what I'm getting on the Zeiss..

Just wondering how any of you would set up the Base Alignment for this part.

[[Me...]]

The potential problems I see with this part is the projected axis of -A- to the location of -D- and whether or not you're able to measure full circular sections of -A-. I would scan full cross sections of -A-, even if I had to stand the part up and go at it from both sides. As a cross check, I would create an axis out of the entire OD of the part using -A- and the .875 diameter to see how that differs from using just -A- as the axis for location.

[[Cl...]]

I'd use a step cylinder (-A- & .875 dia) for the planar rotation (X-)
Spatial (Z) using the plane around -D- & (-B- as X/0)

Then of course measure datum's A/B/C/D separately for any evaluations.

[[Br...]]

I'd do similar to what Clarke said only I would use the axis of my step cylinder as the spacial rotation and two of the origin locations, Then i would use one of the faces of datum feature C as my planar rotation. I think that should get you a really stable and repeatable Base Alignment.

Of course, like Clarke already mentioned, make sure you evaluate the position tolerance using the actual datum references and not the base alignment. The base alignment I chose here, does not reflect the actual position requirement for the Datum feature D bore. It is called out CBA, in that order. So "C" is your primary, B is secondary, and A is tertiary. The axis of A in relation to the rotational degrees of freedom it can constrain, has no effect on this DRF because the Datums preceding it take precedence. One problem I see on this drawing is the 7.572 distance to Datum Feature B. This needs to be basic in order to properly define the Datum D bore's location within the CBA Datum Reference Frame. Also, Datum C is not really of sufficient size in my opinion to be the Primary Datum feature in this case. It will be very hard to get reproducible inspections from this DRF considering the length of the axis of datum feature A in relation to the size of Datum feature C.

[[Me...]]

My alignment was correct. I used -A- as my spatial and -C- as my secondary...

The Operator's fixture was putting too much pressure on the part so we're going to finish the bore at another Op.

[[Cl...]]

OK, so you must have the part standing vertically?

[[Me...]]

I had the part sitting exactly as shown on the print, held on to a good portion of the Ø .875 shaft with a magnetic v block, indicating Datum C square.

[[Ka...]]

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The drawing alignment appears sound. As others have stated, a vertical set-up may be best. It gives full access around -A- (if you're scanning, be sure to filter results), and, more importantly, can minimize the effects of gravity, which could easily be a thousandth or more, depending on material characteristics.