Datum interpretation

[Disal]

Hi all, Happy New Year!

i have a cylindrical part that has 6 equally spaced holes (exact size, exact location), and they're calling this a datum. Can't post print sorry. How would i do this? Measure all 6 holes and construct a best fit line or something?
It's a primary datum for a couple other features and i'm not sure how to do this

[Rich P]

Center point of the constructed bolt circle of 6 ID's is your origin
As a primary this is troubling but if you had a datum simulator with a plate and 6 pins or rods located perfect square and perfect location made to mating MMC size there you go...visually I can see it. PC_DMIS try a constructed feature set and see if the software can handle that as a primary and let exact-measure do it's magic

[Disal]

Center point of the constructed bolt circle of 6 ID's is your origin
As a primary this is troubling but if you had a datum simulator with a plate and 6 pins or rods located perfect square and perfect location made to mating MMC size there you go...visually I can see it. PC_DMIS try a constructed feature set and see if the software can handle that as a primary and let exact-measure do it's magic

Thank you for helping. PCDMIS doesn't let me define a constructed feature set as a datum

[Schlag]

If it is a BOLT HOLE pattern, make a constructed circle from each of those features and make that your DATUM.

[Disal]

Schlag My secondary datum is the OD (for most features). I just wasn't sure how to create this datum. I hadn't seen a datum defined like this before

[RanDawgg]

Realistically, I think that pattern would constrain all DOF. I'm just not sure how PC-DMIS would handle it. I also don't know how I'd define it within PC-DMIS.

[zbailey]

I just posted this same thing haha, I wasn't sure if I was missing something or if there was an issue with using them as such in DMIS. I swear I've done a set like this as a datum at my last employer and it worked fine.

[zbailey]

Pardon the ignorance, newbie here but I understand a little. If you were to use all 6 of the cylinders as a datum would it not constrain all 6 DOF? If you use them as 1 feature set you would have Y and Z controlled by the holes that lay in the X axis, X and Z controlled by the holes in the Y axis, and rotation given that they were measured as cylinders. Maybe I am wrong but to my mind that seems feasible unless I am missing something. I am fairly certain I have in the past done datum structures like this but Ive dealt with such a volume of programs between my last employer and the new one that I don't recall anymore.

[Douglas]

yes it would you are correct, I detailed how in a comment below

[UKCMM]

From the picture I would say datum 'A' is a cylinder and should be used to Level Z and Translate to XY. Datum 'B' ( side bore ?) is used to Rotate to x or Y about Z. Datum 'C' the 6 bores I would measure as cylinders then construct 6 points at the intersection of diameter 'A' and the side cylinders and from these 6 points construct a plane and use that as datum 'Z'.

Now this is pure guess work without seeing a proper sketch or the print but that is my take on the information supplied.

[Douglas]

construct a plane from the center points of each circle, level to that. Rotate to one of them, and put origin at the center of the pattern.

[Disal]

Thank you everyone, sorry i haven't been here much. They told me to not work on this yet-so when i do go back to it, i'll try all suggesstions. Thank you all