True position for only one feature has to be done by Legacy.

[Jignesh]

I am having some drawings in which base of the part is primary Datum-A. Datum-A is having 4 same diameter holes. I find center of patern of 4 holes by constructing circle passing through origin of holes. True position of circle is o.3 mm. with reference to Datum-A . Whenever, I tried to find True position by Datum feature frame, it does not give me measured value. It always shows measured value 0, and dot in the picture of True position stays in center. However, Legacy dimension gives me right measured value for this True position. It looks like, when I have to find True position of only one feature at once with reference to only primary Datum-A, Datum feature frame is not helpful, but it can be determined by Legacy dimension. Datum feature frame is helpful and finds the measured value of True position with reference to only Datum-A when I had to find True position of more than one hole of same size at once. Any feed back? How could I determine True position of circle with reference to only Datum-A by Datum feature frame (non Legacy).

[Jignesh]

I can attach drawing later tomorrow if it is necessary for review.

[Darrollh]

In using TP in Xact, you have to define the datum in the program. Once you've defined the datum, you can pick the features you want to reference back to that datum. Once you've done that, you can pick what parameters you want to measure out. If you're picking the bolt circle as a feature of reference, then you need to construct the bolt circle and pick the construction as a feature to reference against the datum. Or you can pick each individual hole to get the true position. The numbers will be the same in both legacy or xact. If not, you have a bad alignment.

[vpt.se]

Post a snapshot of the drawing with callout and datums visible.

[NinjaBadger]

If FCF (Feature Control Frame, not Datum Feature Frame by the way) is for single holes (i.e. each hole has it's own FCF) and these are on top of the part (with datum A being the base) and you've measured them as circles then they will all report zero.

Each hole is being best-fit in X&Y as there are no other constraints. In legacy I imagine you're simply reporting to the coordinate system - if you select Fit to Datums they will also report zero.


Let us see the drawing and we'll advise you on how to approach it.

[JacobCheverie]

Just to elaborate a bit here - If you are asking for the position of a circle to a plane where the circle is in a plane parallel to the datum plane, the result will always be 0. The intention of a position control with the single primary datum reference is to control perpendicularity. To determine, you need to measure cylinders.

It sounds like you have a pattern of four holes. Measuring as circles will yield the relative positional error if evaluated appropriately (simultaneously) but will ignore any and all perpendicularity error.

Measure the four holes as cylinders and select all four in a single XactMeasure command.

[Jignesh]

Yes. If I measure all 4 holes at once in Feature control frame, it gives True position with correct measured values of all 4 holes at once. Let me have some time. I will update with drawing.

[Jignesh]

Drawing attached. Will add few more.

[vpt.se]

Define datum A (probably the plane the holes are drilled in).
Create a TP in Xact (non-legacy).
Setup the FCF using datum A.
Select all your holes (4x).
Hit create.
Done.

[neil.challinor]

Drawing attached. Will add few more.

Your drawing shows all four holes needing to be reported as a pattern with reference to datum A. The correct way to do this is how vpt.se described in the previous post. Measure datum A as a plane, measure the holes as cylinders and then report all four together in one XacMeasure position command (FCF). The pattern of holes is also used as datum B, presumably for other call-outs elsewhere on your drawing. To do that you need to construct a feature set from the four cylinders, you can then use the set as datum B.

[NinjaBadger]

If you want to use legacy you can do a best fit alignment to the 4 holes first. (2d Rotate and translate) - assuming you're leveled and origined on Dat A first.

Then report using Legacy Position (Don't use datums, select all 4 holes at once).

[Jignesh]

When I use Feature control frame and try to find T.P. of only one hole. It shows True position value 0. I had to select all 4 holes at once in one Feature control frame to find True position so that FCF was able to give me measured value of True position. Do you know why it is happening in FCF?

[vpt.se]

With one hole and "fit to datums=on" PC-DMIS will likely rotate and translate so the TP is at its best = 0. With two or more holes, PC-DMIS is trying to rotate and translate so the sum of all errors ends up as small as possible (I think).

[neil.challinor]

As vpt.se says, it behaves differently when reporting a single hole as opposed to a group of holes because of the different rules in play. Your drawing calls for the holes to be reported as a pattern so you should select all four in a FCF position command. Dimensioning them individually would not control the relationship between the holes and would be wrong in this case - you could end up passing parts as acceptable when in reality they have failed and would not assemble correctly.

[Jignesh]

Thank you very much.