True position of concentric holes. 3

[LittleTolerance]

I'm am trying to understand how to go about measuring the call out in the attached image with and AACMM. Is it possible with a cmm and does the call out make sense to begin with?

This is my first time posting hopefully it works.

 

[rsm7400]

Let me get a little more information from you. Just to be clear, is this an Articulated Arm CMM? I have never used one and deal primarily with simple x,y,z coordinate CMMs and optical CMMs but I still might be able to help you out.

Second question, according to the picture you attached it looks like the diameters are reference dimensions? (because they are in parenthesis) or do they have specs listed somewhere on the print with tolerances for them? This matters because of the Max Material bonus you might be able to take (M). Finally are there any other specs on your print that are in boxes other than true position is? For instance a length. These would be "basics" that help calculate the true position.

If the print doesnt have any basics and it is really just a true relation between the first hole and the "F" hole then it will be easy. There still needs to be something designating the sizes of the holes for the (M) bonuses.

Ryan M
Quality Engineer

 

[LittleTolerance]

Thanks for responding rsm7400

Yes it is an Articulating Arm CMM.

The diameters are reference back to the piece part for this weld assembly. The diameter call out is 1.008 ±.005.

There are no basics related to these holes.

I have no trouble understanding True Position when there are datums and basic dimensions involved. I just have never grasped how to set up a piece when the holes are coaxial. What defines the X,Y and Z? More or less learning on the job here no formal training.

Thanks ahead of time for your assistance.

Brad A
Quality Tech

 

[rsm7400]

Sorry I was not able to respond yesterday....

So it sounds like to me with no basics being called out and that your tolerance on the holes is ±.005, they are just looking to make sure they are aligned to eachother and on the same axis.

If you can position the entire part so that you are looking down thru the hole to the next that would be preferable. This true position has no regard for the 2.380 dimension so looking at it from the top down removes it from consideration

While looking at it from the top down thru the holes you can measure hole "F" and set it as your datum origin. Next you can measure the top diameter. If your machine is outputting the values in Cartesian (X,Y) or Polar (A,R) then the calculations are slightly different.


Cartesian:
Take the difference of your x values
Take the difference of your y values
Double the larger difference
Half the smaller difference
add them together for true position.
Note: if your difference is negative, just switch the order since you only want the absolute value (positive) value.

Polar:
Double the R value is true position.

Example; Datum set on "F" (0,0)
Top hole X,Y coordinates measure at (.005,.004)
Double the X value = .010
Half the Y value = .002
True position = .012


Do you know how to calculate the bonuses for Max material? Chances are if your true position is slightly more than .020 it will still be ok with the bonuses.

Let me know if this helped or if you need max material.

Ryan M
Quality Engineer

 

[LittleTolerance]

Sorry it took so long to respond rsm. This has helped some what thank you. I guess the only question I have is how I would set the part. Would I use a surface that the datum F hole is thru? I could set the centers of the holes up so that they are aligned but the material the holes go through may not be parallel to each other. So functionally if a pin was put through one hole it would be skewed and would not pass thru the second hole. Maybe I'm over thinking this or something is passing way over my head. Thanks again.

 

[dgallup]

There are no basics related to these holes.

There are two implied zero dimensions from the shown common axis.

This is not a valid call out. The datum F is too short to establish orientation, it can only control 2 degrees of freedom.

The above calculation for true position is not correct. Your CMM will do it for you.

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