Datum Targets...

[[Ca...]]

I have a smaller part with datum targets on cast surfaces. Just for conversation, lets say the targets are Ø 0.200. When I use single points, according to the basics, the parts measure fine. I've been advised I should be scanning 0.200 diameters. When I scan circles the size specified by the print I get very different numbers. This is my real first brush with datum targets. I understand what they do, and the function of giving the target size, like 0.200.

My question is, what are people doing?

Are you taking points, or scanning surfaces?

Anyone care to share specific strategies?

[[Me...]]

We recently have been getting a lot of castings, I try to avoid scanning whenever possible, as I have a feeling, depending on stylus material and surface finish/material of the casting, that it probably causes a lot of abrasion to the stylus, but we have had both really rough cast, and some smooth, so the smooth I'm sure is not as big a problem. Most of the targets I have seen have been planes created by single points anyways, or single distance points, for machine loading orientation. I'm just guessing maybe scanning on cast is giving you different numbers due to form change? Maybe if you have to use target diameters, you could try creating circles from single points instead.

Sorry keep updating this as I get time here, so if you do need target diameters, you could create 3 planes, using circle on plane to size of target diameter, and then recall features points to one plane perhaps?

[[Me...]]

We also have cast parts with target datums. I scan a Ø.250 target @ .250 circular path and also a .100 circular path to try to pick up more of the potential deviation of the cast, then set it to outer tangential to find only the high points. It seems to mimic the fixture pretty close.

[[Ma...]]

For me it depends on the part and machine fixture. If the machine fixture has a single contact on the part, then a single point, but if the machine fixture has a round button type contact then I scan the surface. If I don't have access to this information then I generally use a single point. With castings though I will always loop the alignment and output on the report the delta of the alignment deviation.

Mark

[[Ra...]]

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I would only add one thing to this excellent post: I use a larger ruby when scanning datum targets.

[[To...]]

I feel typically if there is a diameter call-out on the datum target, it is implied that a fixture designed to hold the casting should use stops/pins of the same diameter. On the other hand, if the surface is not normal to the primary axis, which would be common due to a draft angle, then it is only going to make contact on the highest surface which negates the effectiveness of the fixture and probably is making contact away from the desired nominal values.

Mark is right that it doesn't mean the fixture was made to match the drawing so communication with customer should be considered.

[[Ca...]]

Update:

I originally was scanning the surface with 2 diameters, one the same size as the datum target, and another diameter about half that size inside it., and using OTE. It just seemed like it satisfied the intent of the drawing, and would mimic any results hard gaging would find. My numbers were bogus, because Tom is correct, my surface wasn't "normal" to the BA, which bumped everything out a few thou. After I switched to single points, everything came in to focus.

The customer is reviewing my process for acceptability.

[[To...]]

Has anyone considered creating a Point Set? Or, maybe that is what you guys are already referring to.

[[Ca...]]

...and I'm back on this....

Any advice on recalling groups of points, on individual pads, with each pad at different heights, all inside a small section of a 14" diameter? I have points set up, per the basics on print, with point sets inside them, probing points covering the datum target zone.

When using my points to create planes, should I use Recall Feature Points, plain Recall, or something else?

FYI, plain Recall gives me a message advising use of Point Recall.

I really hope that all makes sense.

I can't really post pics, this is the best I can do.

Capture.JPG

[[Ca...]]

New (or next) question!

I need to probe 3 points. A1, A2 and A3. I need to translate point A3 backward 0.200, so it's in line with points A1 and A2, then creates a plane.

How do I translate Point A3, at the top, backward to be in line with points A1 and A2, at the bottom?

Capture.JPG

[[Ma...]]

Open an Offset Plane. Features/ Special Geometries/ Offset Plane

Click on Evaluation and select Edit under Point Modification

Enter the values, if just moving Point 3 then leave the others at zero

Now go back to the front page by clicking OK. Now Recall the 3 points in the order you want them.

Mark

[[Ca...]]

Thanks Mark!