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In my last article on cmmquarterly (click here to see past article) briefly demonstrated how you can take advantage of the point cloud manipulation functionality in OpenDmis to do a very quick layout on a die cutter with minimum work on CMM machine.

But that was an example without any cad data. Since more and more people deal with cad most of the time in their daily work, I'd like to take one step further to show you how it can also work out to your cad perfectly.

Not too long ago, I got a roll-formed part (see image below)

from a client who required a full profile comparison to the original cad data. Again, I could have used conventional CMM programming routine, which is manually align the work piece, do DCC alignment then start to pick all features that need to be reported. But the problem with this standard method is: when you have a prototype part on which you might have large deviations even on features that you use for alignment, you'll probably get a wrong "image" simply because you didn't bestfit your data. Even with bestfiting you need time to decide on which features to use and these all have to be done on line occupying your CMM. And what's even worse is someone comes back later on asking for a different "what if" analysis, then you have to run your program again if you still have that part. So let's try my method on this project:

Step 1: Collecting data. That's really easy if you know how to scan, no matter with analog scanning or peck scanning. All you need is a few hundreds of point all around the whole profile. And in my case I scanned this work piece with a SP25M, which took me only 6 min. from set up to finish.

Step 2: Save you scanned data to iges file as points. This varies depending on your software but for sure there's option in any software that allows you to save them to points.

Step 3: Import the saved point file into your original cad file in OpenDmis. At this  point you'll only see the scanned point cloud totally in space, far away from the cad model because I didn't do any alignment on CMM when I was collecting data.

Step 4: Initial alignment. To bring your point cloud to cad co-ordinate system we need some "easy" features for alignment. And I believe most of you will think about tooling ball or tooling hole alignment. In any modern CMM software a "3 point bestfit alignment" usually takes just a few clicks to complish. So first I defined 3 circles using those bending radius

and then just simply pick those radius points from my point cloud with point cloud manipulator and DnD to its corresponding nominal circles. Next is use these 3 features to do a bestfit alignment, followed by "set model alignment". Now you can see that my point cloud is pretty much aligned to cad. But this not my final alignment yet.

Step 5: Bestfit alignment. This the best part about this method: Even though your part is not accurately aligned to cad but you can "accurately" get a big picture how the whole profile of your part will fit into cad, where is the area with least deviation and where is your problem area. From there you can easily find those reasonable areas for bestfiting otherwise if you do it on CMM then you really have to spend some time for analyzing or trial-n-error. In my case, it's quite obvious that the 3 inside walls making sense for bestfiting and I can increase number of points on any wall to add weight on it. After my bestfiting and set model alignment, it shows a very "true" comparison between my part to cad.


Step 6: Reporting. A picture worth thousands of words, I think if you just save this comparison as an image it will be a very good report because anyone can tell what the part looks like at his first sight. But of course it's even better with some actual readings and this how my report looks like.

With the point cloud capability in OpenDmis I can process more complicated cases, if you need such help (even you are not using OpenDmis) please feel free to contact me.

Ray Xing
GDTP S-0605
CGP Certified (ITAR equivalent)
CMM Application Specialist
RX Metrology Solutions


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