Cylinder incorrect Output after scanning

[[AG...]]

I am curious to what i should be doing here about this cylinder i am picking up and it is showing oversized. I am using a 90mm length probe at a 55;60 angle to get to the feature to measure. When the machine scans the feature it shows way larger than what it is. Is it because of the angle that it is taking it as or is there something that i am missing as far as turning on a part of the strategy. I have had some crazy things happen lately and i am not sure the reasonings why. If there is any help i can get or direction of something i am missing here that would be great. Thank you and i have to say that this is an awesome forum and a lot knowledgeable people for answers.

[[Ja...]]

In general small arc segments are going to be tricky given some of the calculations used, in both accuracy and repeatability. I am not sure what amount you have there but it looks like it is maybe ~20% judging by your angle range. Some things to consider;

Personally, I would recommend starting by adding a lot more circle paths along the entire feature length, especially if you are trying to evaluate as a cylinder. Change the "Number of Sections" to ~8, and then use the start height and target height to encompass the full geometry of the feature. Also, I would try increasing the amount of points per section/decreasing the step width. Based on the form I am seeing you might not see too much change given how this part that is presumably a metal is machined but I always am on the side of more data... (I work with plastics so I am always a bit data heavy given how they can warp and etc..) If this is a production inspection program you can later optimize retention of accuracy and repeatability vs. time.  

The real tip or trick though with small radii/arcs though is to utilize some constraint(s) and this is often recommended by the Zeiss application engineers as well. If you go to the evaluation tab of the feature you will see the evaluation constraints section, based on the coordinate system I see if you are interested in reporting the diameter you might want to constrain the Y and Z. What this does is lock in the center point of the cylinder/circle and then asks for the distance between all of the points for your diameter, rather than trying to also calculate the center point of the cylinder. Some will also recommend at this point to also get out of scanning entirely and utilize single probing points along the available geometry. Use this with careful discretion though.. constraints can really influence your results but sometimes it can feel like there is no other option honestly.

I see you are using your base alignment as well. Perhaps a more localized (secondary) alignment in that region would be helpful. 

Last piece of advice that we can all tend to miss is to go double check on another piece of equipment/with a different method if possible.

Hopefully something of what I said can put you in the right direction.  🙂 

Edited February 10

[[Ma...]]

I think Jace meant to check X and Y since your cylinder is Z axis.

We had part with R95 with angle of 22° - because of ISO i ended with 7 points per circle to have repeatability - but it's forged and blasted part.

You can came with own strategies after you know how it's milled and it's form.
But constraining nominals is not always good ( depending how it's milled and so on - you can have another alignment origin than on machine - so you can bring errors ).

If you have curves then start with them and alignment from many curves - this will bring you closer.

[[Ja...]]

Thanks for the correction Martin, you are right, I didn't look at the space axis. I was looking at circle paths relative to the coordinate system graphic next to it.

[[AG...]]

Thank you for the response', much appreciated. My other question would be when i am in 0,0 with my XXT compared. When i use this position compared to the 55,60 the positions and diameter are closer to nominal. 

[[Je...]]

20° of arc segment will prevent you from having repeatable measurement results.  Your data acquired and evaluation length compared to diameter will further prevent you from getting repeatable results. 

Change evaluation type to CIRCLE as your axis vector is so short its irrelevant in this application. 

Constrain the center point for size reporting. 

Constrain the radius for location reporting.

 

Better yet, report as Profile of a Line to ensure the surface falls where it should relative to a nominal center (what probably matters anyway)

 

Also, your step width and speed are excessive if you are looking for good, repeatable reporting data.

Stylus angle (RDS XXT rotation) shouldn't matter if your speed is set properly

[[Ja...]]

Hey AGH1977, I have been browsing the forum while I run some GRR... I recognized your part from another post and two more things came to mind.

First it looks like you might have more portions of the geometry that you can measure as independent features and then use 'Recall Feature Points' in constructing a new cylinder/feature which would give you access to a lot more geometry for the feature. Of course it really depends on your print callout but just throwing that out there.

Second.. and it might be entirely my perception, but are you actually looking at an ellipse? 

Not sure why you are seeing differences with the 55,60.

 

[[Da...]]

On Short arc sections of cylinders and circles, I have found that single points can reach further around the circle than a scan. Scanning will eliminate starting and ending points. 

[[Ma...]]

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Thats why i did points instead of scan. 20° of R95 with tolerance of +/- 0.8mm was impossible to explain to customer - i was even posting that minuscule deviation at center with min and max radius to show cuustomer how riduculus is that tolerance on forged and blasted part 😄

[[AG...]]

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I do like the recall feature points that i will try. This particular fixture is solid steel. It looks like an Ellipse but is a Diameter. It might be the way i snapped the pic. 

[[AG...]]

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I will try individual points for this and see what results i get. Thank you.

[[AG...]]

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This particular feature is under the another feature. The feature you see where the small dia's are is calling for the same profile. I didnt have an issue with that one. its a big block of steel that was machined at the same time. You were saying using single points and call it a circle instead of cylinder and report it as a profile of a line. I will slow down the speed and the step width.