What equation is used to calculate a true position callout?

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eknytych
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What equation is used to calculate a true position callout?

Post by eknytych » Fri Mar 27, 2015 4:19 pm

Greetings CMM Manager Forum Folks!

I am using a Mitutoyo Bright-A707 CMM with a Renishaw PH10 probe to validate a true position callout, but something does not make sense. If the basic dimensions for the point in question deviate less than .005" for a feature with a tolerance zone of .020", how can the true position deviate more than .030" from nominal?

I am confident that I have entered the data correctly in my program, but I have been asked to "math it out" using the basic dimensions provided by CMM Manager and then compare my results to those computed by CMM Manager; I have also been asked to figure out how CMM Manager calculates true position.

So what equation does CMM Manager use to calculate true position?

Thanks!

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US_Helpdesk
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Re: What equation is used to calculate a true position callo

Post by US_Helpdesk » Sat Mar 28, 2015 10:49 am

This is not so simple... For a point, circle or other point reducible feature we just use the following...

2 x SQRT(X² + Y²)

This doe not account for Material Modifiers that could inflate the allowable tolerance. Also line reducible features such as cylinders have some extra calculation that identifies the worst deviation of the c-line.

There is a common mistake when performing True Position in CMM-M. Where exactly are you entering the Basic Dimensions when reporting? You should be entering basic into the area circled in green below... The Feature Report data is not related to True Position calculation in any way.
tp_report.jpg
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eknytych
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Re: What equation is used to calculate a true position callo

Post by eknytych » Mon Mar 30, 2015 10:08 am

I have been entering the data in the feature report section. I had no idea that the basic dimensions were to be entered elsewhere. I'll try your suggestion.

Thanks!

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Re: What equation is used to calculate a true position callo

Post by US_Helpdesk » Mon Mar 30, 2015 10:21 am

It's the fields highlighted in Green above... They're shown grayed out above because no feature has been selected yet in the Feature field.
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eknytych
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Re: What equation is used to calculate a true position callo

Post by eknytych » Mon Mar 30, 2015 10:51 am

I entered in the X and Y values in the green area and got good results, but there is more. I think what I was looking for initially is a spherical true position that will include deviation in Z as well, but the option to enter that value is still grayed out. What am I missing?

eknytych
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Re: What equation is used to calculate a true position callo

Post by eknytych » Mon Mar 30, 2015 11:09 am

I got it. I created the program to check the true position of a circle, and only x and y are available for this type of feature. When I edited the program to look for the point in the center of the circle, I was able to check all three values.

Yay me!

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Re: What equation is used to calculate a true position callo

Post by andycmm » Mon Aug 28, 2017 5:45 pm

I found this post looking for an issue we were having reporting a true position. I had recently been to Brighton for the training and there we were told that in order to properly report a true position we would have to report the position of a point. The trouble I am having/the question I have is: How exactly does CMM manager the true position of a point in 3D space.

This thread is helpful as I now know that the second page "Feature Report" has nothing to do with the calculating of true position. This was confusing because every time I printed the report those were the values that were reported but none of the math worked out and when I told my superiors and machinists that those numbers have nothing to do with the final calculation, they didn't feel I knew what I was talking about. because there it is in black and white on the report.

So here is the real problem: We have a lot of true position call out issues. I need to be able to tell the machinist what direction he needs to go to get a feature in tolerance. The problem is when I look at the X Y Z values and base my recommendation off of those values reported I should in theory see a change similar.

My method is to make a point and then when I report TP on the" Position Report" tab I set the X Y Z values to the true position values on the the CAD/blueprint in relation to the origin I have set in CMM manager. At one point we had X, Y, and Z all within .002 but the true position .010 was still out. That just does not make sense to me. If everything is within .002 to nominal how can the true position .010 be out? How is it being calculated? I see the yellow triangle, that makes sense, but then how does the leader from the tip of triangle to the constructed point play into it? I have read through many threads on this topic but the question is never fully answered. What makes it more difficult is that when we have these true position issues and then we check it manually with trig it the part is in tolerance.

If you take a look at this print, is there any reason CMM Manager would have a hard time calculating these true positions? Specifically the 3 X .112-40 equally spaced.

Thanks in advance, Andy
Last edited by andycmm on Wed Dec 27, 2017 10:25 am, edited 1 time in total.

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Re: What equation is used to calculate a true position callo

Post by CrashN8 » Tue Aug 29, 2017 9:38 am

True Position for 3D point is calculated as follows:
true_position.jpg
Position = (dX² + dY² + dZ²)^0.5 * 2

Notice the Nominal Position cells in the Position dialog... These are nominal (Basic) values that will be used to calculate True Position (not what's shown in Feature Report) You should see that unchecking any component (X, Y or Z) will reduce the True Position value since that component is longer contributing to overall position calculation.
true_position_2.jpg
true_position_3.jpg
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