True Pos

[[Da...]]

Hi guys. How would you measure this true position 0.25 ? I am doing best fit only with rotation. Is that correct ? By the way what is the tolerance for 44.00 and 22.00 ? Is 22+-0.125 and 44.00+-0.25 ?

[[Ra...]]

First, the 22 and 44 are basic dimensions. They "set up" the location of the True Position tolerance zone. Let's ignore that for now.

Second, because there are no datum references in the DRF, I would allow for rotation AND translation. However, I have never seen this on our parts, so I would suggest looking at the standard for clarity. The center lines and implied perpendicularity to the edges suggest you should control the location of the pattern to the edges as a practical matter.

Finally (most importantly?), because the 4 holes share the same tolerance and (implied?) datums, use a pattern in your True Position characteristic. In the tolerance box, use .250 mm. The 22 and 44 will automatically part of the characteristic. Those dimensions dictate where the center of your tolerance circle/cylinder should be. The tolerance circle/cylinder will be .250 mm in diameter.

I hope this helps! There are better GD&T/blueprint minds here that will surely get you squared away if this didn't.

[[Da...]]

If I will use rotation and translation the software will ignore diamension 22 from the center. It will onle check distances between 4 holes.

[[Br...]]

Yes that's correct. The drawing is incorrectly drawn. Either there needs to be datums in the datum reference frame, or the 22 basic needs to be removed. The FCF is stating the tolerance zone is only relative to the hole pattern. You might want to get further clarification from whoever drew this drawing or an engineer who knows how its supposed to function. More than likely there's supposed to be a datum reference frame. Typically you would only tolerance it this way in a composite FCF, whereas the lowest segment would omit any datums.

[[Aa...]]

Randy makes a good point about having to evaluate them as a pattern.

As far as going back to the edges, etc...if that was the intent, it was not correctly expressed.

If this is to the ASME Y14.5-2009 standard, then there's one more aspect that should be considered: simultaneous requirement (section 4.19) As I understand it, there is disagreement even among those on the Y14.5 subcommittee about whether or not simultaneous requirement applies when the DRF references no datum features. So, are there other position or Profile FCF's on the print with a DRF referencing no datum features?

[[Jo...]]

If this is an ASME drawing then the ONLY way this is okay is if the rest of the part is geometrically controlled to no datum reference frame (see how weird that sounds?) and the lack of a datum reference frame is the same as the same datum reference frame ( that sounds even more weird), and simultaneous requirements applies. Aaron is correct that there is disagreement about that, even among the committee members, but I'm not in this camp. To me, no DRF is no DRF so simultaneous requirements does not apply.