scan of a spherical feature

[zbailey]

Question for those with more experience than I. I am looking for a way to take a scan of a spherical feature on a project I am working on. I know I have seen scans done on spherical IDs before but I cant for the life of me remember how. Sitting here when its slow trying to optimize programs and I hit a roadblock with this and it just wont leave my head trying to figure it out.

[AndersI]

There doesn't seem to be any scanning strategies on an autosphere, so I guess the only way is to

- measure the sphere with a suitable number of touch hits
- align (origin) to the sphere
- scan a number of arcs on the sphere (create one, paste with pattern [rotation] to get the others)
- construct sphere from all the scanned hits

[JEFMAN]

There doesn't seem to be any scanning strategies on an autosphere, so I guess the only way is to

- measure the sphere with a suitable number of touch hits
- align (origin) to the sphere
- scan a number of arcs on the sphere (create one, paste with pattern [rotation] to get the others)
- construct sphere from all the scanned hits

+1 !
You can also scan circles at different heights, then construct a bfre sphere, or use constructed filters / sphere.

If you use basic scans, you can select "conical scan" if you don't scan at the equator.

[neil.challinor]

If you're trying to scan "on section" - for example, around the equator to look at circularity - you might want to consider using the adjust filter
https://docs-dev.hexagonmi.com/pcdmi...ted_Filter.htm

[AndersI]

You can also scan circles at different heights, then construct a bfre sphere, or use constructed filters / sphere.

BFRE is imperative if you scan circles at different heights, as the circles themselves will be probe compensated at an angle to the surface..

If you use basic scans, you can select "conical scan" if you don't scan at the equator.

"conical scan"? How would PC-DMIS know which direction to probe compensate? I see the minimal text in the Help

The Conical scan check box allows for proper scan compensation on cones or spheres. This check box lets you scan more quickly when not perpendicular to the part surface. PC-DMIS continues to monitor the probe force as needed.

Does this mean that surface direction is taken from the probe force deviations? If this is a good way, why isn't it always used automatically? What about friction drag on the probe - that will give a partly wrong direction for compensation, the more the higher the friction (and let's not talk about bumps and scratches...).

[Matthew D. Hoedeman]

BFRE is imperative if you scan circles at different heights, as the circles themselves will be probe compensated at an angle to the surface..



"conical scan"? How would PC-DMIS know which direction to probe compensate? I see the minimal text in the Help



Does this mean that surface direction is taken from the probe force deviations? If this is a good way, why isn't it always used automatically? What about friction drag on the probe - that will give a partly wrong direction for compensation, the more the higher the friction (and let's not talk about bumps and scratches...).

'find noms' for the scan, if you have cad data, will eliminate that issue, if you are doing it from the SCAN window and not an autofeature window.... won't it?

[AndersI]

Probably, but my reaction was because the quoted Help didn't mention CAD at all, in the explanation of "proper scan compensation on cones or spheres"...

[JEFMAN]

Does this mean that surface direction is taken from the probe force deviations? If this is a good way, why isn't it always used automatically? What about friction drag on the probe - that will give a partly wrong direction for compensation, the more the higher the friction (and let's not talk about bumps and scratches...).

No, I think it's only a way to manage the "pumping" between lower and upper force, to avoid to the probe to go along the tangent of the sphere at the contact point. (wow, that's certainly the max that I can do in english )

[zbailey]

Thanks for the advice everyone, we are on 2014 and I DO have CAD, so I will experiment with it today. I wish that spherical scan would be something that could be added to make this possible. Just having it take the nominal vectors and apply it into rows of scans along the sphere would simplify this whole process. I know I have seen it done before but couldn't recall how.

I had tried a few different scans but the data never came out correct, so I am assuming it is the recomp that will correct the error I am seeing from the vectors being incorrect.

[SingularitY]

If you're using CAD, you could use linear open/closed scans and change the Cutvec to match the vector of what the scan would be if it was a circle. This will flatten the scan parallel to the workplane no matter the type of feature you're scanning.

[JEFMAN]

Just a thought about programming a circular scan on a sphere, if the cut plane doesn't contain the center of the sphere.
Then, the programed height and radius are different than the contact ones.
Depends if you know the height of the center ball ot the height of the scan that you want (contact height) :
scan on sph.jpg

So, in PC-DMIS, if you want to scan at 45° :
ASSIGN/HCONTACT=SPH1.R*SIN(DEG2RAD(45))
ASSIGN/RBALL=(PROBEDATA("DIAM")/2+PROBEDATA("PRBRDV)
ASSIGN/HPROG=(SPH1.R+RBALL)*SIN(DEG2RAD(45))
ASSIGN/RPROG=(SPH1.R+RBALL)*COS(DEG2RAD(45))-RBALL

[SingularitY]

Does using linear open scans while probe/cad comp are activated with the cut plane set to the workplane automatically compensate for this? I was told previously from Hexagon that it does. I do this often so if it doesn't compensate correctly, then I'll have to rethink my scanning strategies.