Curve Best Fit Alignments/Alignments from Several Curves

[[Jo...]]

I recently completed my Curve training and the one thing that I left confused by, was the alignment options for individual curves. And maybe this just stems back to some sort of misunderstanding about best fit alignments to begin with. My vaguest memory of covering any best fit alignments in all my training is that it's mostly only when stuff is not fully constrained. Someone please correct me if I am wrong on that account.

Going back to alignment of curves; are these options only intended for curves that are not fully constrained by a datum structure? An example would be a curve(s) dropped onto a spline, on some outside geometry that lies perpendicular to a plane. In the example the plane would be a datum, and the splines would be used in a profile callout only called back to that one plane, which would obviously not be fully constrained.

The real world problem I have is we have several new parts we are making that I am programming, with splines that define the outside geometry on. The parts are mostly "circular-shaped" but have several pieces of different geometry (planes, huge radii with small radii and tiny arc segments that transition onto other geometry). In one example, I have a .002 profile back to A-B-C, which this datum structure does fully constrains the part. Does this render a curve best fit redundant since it's fully constrained? Just out of curiosity I did create curve best fit alignments, then alignment from several curves, and they did improve the results which makes me feel.... uneasy.

To put my method simply: I created curve best fit alignments, used those to create a multiple curve best fit, created a secondary alignment using A-B-C and referenced the multiple curve best fit. It improved the results... but I feel like I'm doing the equivalent of forcing a gage pin through a hole that barely fits just so I can say "the gage pin goes".

[[Ro...]]

Hi Jon,

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For the most part, yes. But they can also be used to create secondary alignments and used in other ways to analyze geometry.

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That's one use for them. There are others. It's just another tool in your toolbox. I use BF alignments for all sorts of things. For example, usually I'll scan a curve and apply no filtering, outlier removal, etc. to get just raw data. Then I'll copy that curve, recall it into another curve or curves, and apply whatever best fits, filters, outliers, etc it needs for my needs. I also use Curve Form plots a lot. Even though you may have geometry that is fully constrained, providing a Best Fit curve plot will show the machinist the offsets needed to bring a bad part into tolerance and/or improve the process.

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If you're Best Fitting geometry that is constrained you're not getting accurate results, as a general rule. If you have a specific callout and/or print you can share I can give you my 2 cents on how I would measure it and why.

Cheers,
Robert

[[Jo...]]

Hi Robert,
Thanks for the insight. Unfortunately I cannot share the print on this forum due to export control regulations on the drawing, but I think you've answered my question. I am basically in the situation you've described; trying to provide the machinists' data so they can make adjustments accordingly and make improvements on their process, while also trying to get parts to pass final inspection. I feel like I don't have enough experience with best fit alignments yet, in general, and that's where my deficit is as far as my understanding of it goes. I am definitely in the learning stage of best fits, and I am mostly concerned with making sure I'm not making the parts look better than they actually are.

Thanks!