Mounting part on setup tooling

it's not uncommon, but is also less than desirable, for the plate is not the actual datum. From a micron's perspective, that datum doesn't lay flat against the surface plate -- there are high points of contact that it is resting, due to it's deviation from perfect form. So no matter how you spin it -- it's not the same surface as the datum.

bad surface > poor alignment > poor dimensions > poor report.

While I THINK I understand what you're going through, you should really attach a picture, like I suggested before. Possibly somebody can recommend some sort of setup that will both rigidly fixture your part and allow you access to the features you care to align to or inspect. The first line of your last post does not do this.

Here's my assessment of your possibilities:

1) Bad idea. For the reasons I stated above, whereas the CAD and part will appear to lay perfectly on each other, in real life they don't adhere, due to manufacturing error. This will result in a poor alignment.
2) Not really possible to 'manipulate the alignment'. Don't forget that when you are taking hits on a surface, PCDMIS is using the CAD model's vectors to calculate probe compensation. IE: If you have a model of a 321 block, call Datum A the underside, fix it to a surface plate, program off the model, but then shoot the actual surface plate upon execution, what is going to happen?

PCDMIS is going to take your hit information off the surface plate (which faces up), and probecomp it according to the CAD vector information (which tells it that the surface is facing down). The result will be a surface that is off by the thickness of your probe radius, because we've inadvertently told PCDMIS to comp in the wrong direction.

The workaround that I've always used in this scenario when I have a Datum that (in this case) is flat against my surface block:

- create the plane off the model as expected
- edit the theoretical vectors to reverse them
- now that plane actually refers to the surface plate. it has the XYZ information that it (nominally) shares with the underside Datum, and the IJK information unique to the surface plate. So it will comp correctly.

however, you should make every effort to avoid cutting corners like this! I won't do it unless I've explored myriad setup options already. Good luck

it's not uncommon, but is also less than desirable, for the plate is not the actual datum. From a micron's perspective, that datum doesn't lay flat against the surface plate -- there are high points of contact that it is resting, due to it's deviation from perfect form. So no matter how you spin it -- it's not the same surface as the datum.

bad surface > poor alignment > poor dimensions > poor report.

While I THINK I understand what you're going through, you should really attach a picture, like I suggested before. Possibly somebody can recommend some sort of setup that will both rigidly fixture your part and allow you access to the features you care to align to or inspect. The first line of your last post does not do this.

Here's my assessment of your possibilities:

1) Bad idea. For the reasons I stated above, whereas the CAD and part will appear to lay perfectly on each other, in real life they don't adhere, due to manufacturing error. This will result in a poor alignment.
2) Not really possible to 'manipulate the alignment'. Don't forget that when you are taking hits on a surface, PCDMIS is using the CAD model's vectors to calculate probe compensation. IE: If you have a model of a 321 block, call Datum A the underside, fix it to a surface plate, program off the model, but then shoot the actual surface plate upon execution, what is going to happen?

PCDMIS is going to take your hit information off the surface plate (which faces up), and probecomp it according to the CAD vector information (which tells it that the surface is facing down). The result will be a surface that is off by the thickness of your probe radius, because we've inadvertently told PCDMIS to comp in the wrong direction.

The workaround that I've always used in this scenario when I have a Datum that (in this case) is flat against my surface block:

- create the plane off the model as expected
- edit the theoretical vectors to reverse them
- now that plane actually refers to the surface plate. it has the XYZ information that it (nominally) shares with the underside Datum, and the IJK information unique to the surface plate. So it will comp correctly.

however, you should make every effort to avoid cutting corners like this! I won't do it unless I've explored myriad setup options already. Good luck