Oppositional readings via incorrect methodology.

[[Ri...]]

I have a part with two adjacent rails.
We are seeking the distance and parallelism between the two rails.
This is one of those nightmare parts that has a tendency to spring when all the material is hogged away so when the part came in, it was immediately scrutinized and failed at assembly.




I followed the cookbook recipe for taking points off the two rails

3mm Tip
10mm/s scan speed
0.31mm step width
Outlier ±3s
No prefilter (short polylines)
Filter: Wavelength 2.5mm gauss
Took 4 linear polylines along the length of each rail.

What I came up with was a failed part for both for distance and parallelism, as anticipated.

But, being the curious creature that I am, I created a series of 5 smaller planes on each rail along their lengths and created caliper distances between each opposing plane so I could see if it was springing open or closed.

Problem is that each of these 5 distances show within tolerance! ❓

Clearly, Im leaning towards my evaluation methodology being flat out WRONG with the cookbook eval being correct

What's so curious to me is that I show a differential of almost 200 microns between methods. Can someone explain to me how and why checking the distance across the span in this way is incorrect and yields such erroneous results?

What would be the better way to achieve the goal of getting incremental distances and ascertaining if a part is springing, bowing or curving?

Again and always, thank you all for answering my pedantic questions!

[[Br...]]

One thing you could do to immediately recognize the direction of the taper is to look at the plot of the Parallelism tolerance. What evaluation method are you using for the planes, and what types of characteristics are you using to verify the width?(e.g. Caliper distance, Cartesian distance, etc.)

[[Lo...]]

Richard,

Here is a presentation I copied off here some time ago. It may help. I didn't make this presentation, I think Ryan Stauffer deserves all the credit for this. I miss the very informative help he use to provide on the older forum. Enjoy.

The Forum wont let me post a powerpoint. Send me your email and I will email you the power point.

[[Da...]]

Lonnie, zip can be uploaded to the forum.

[[Ri...]]

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This is the thousand dollar question for me.

==============
Current eval method for those datum planes:

Z400L
"Parallelism for function check" for all three planes.

Datum planes: "Outer Tangential" (as per R400LR) (pg 95 in the cookbook)
10mm/s speed, .31mm step width.
Clpr distance.
========

I realize there is almost never a "correct" evaluation method, but what would be the "prudent" method in a scenario such as this?
Is caliper distance applicable or proper in this scenario? Would Cartesian distance the better method?



I have since re-orientated the part on the surface plate and in the program to match the print and its datum structure/
Datums A, B and C comprise my base alignment.
Now that I have the part setup correctly it shows as being in tolerance!! ❓
I need assistance so I know how to proceed properly in the future.





Again and always thank you guys for holding my damn hand through all this stuff. 🙄

[[Br...]]

Good to hear. I can't say as to why you were seeing the differences. Could've just been in the setup for all I know. But as for that parallelism tolerance, the Datum Feature should use Outer Tangential, and the tolerances feature can just be LSQ, but I don't think it matters because Calypso just uses the actual individual points to verify if they land within the tolerance zone. Outer Tangential for a plane feature will always evaluate a plane that contacts the high points of the surface. Inner Tangential will contact the low points. Outer Tangential best represents the "Datum Simulator" and "Actual Mating Envelope" described in ASME Y14.5.

Regarding which characteristic to use for a size tolerance of a width like you have, I tend to favor Cartesian distance for the sake of simplicity. The only problem is that it favors one plane or the other. It levels/aligns with the second plane you enter and measures up to the first plane. A more optimized width can be done with caliper distance where you create a mid-plane from the two planes, create an alignment from those two planes, then use that alignment to evaluate the width of the planes. To go even further you can constrain the normal vector of the planes while using Outer Tangential. This will output the width similar to the way Maximum Inscribed works on a Circle feature for a diameter measurement. This of course is a lot of work. I hardly ever do this. There are some other ways people optimize this measurement as well to perfectly satisfy the Perfect Form at MMC requirement (Rule #1) from ASME Y14.5. But none of them are easy. The cookbook says to recall two planes into a symmetry plane feature. That might be easier but I've never really tested that myself.

[[Ri...]]

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If only the documentation were as concise and to the point as you!!
Thank you MASSIVELY, yet again. 🙂

[[Lo...]]

here is the power point I was referring to as a .zip