There's been some debate here about what the CMM is capable of measuring to when it comes to the 10:1 rule. Obviously we can change the resolution on the dimensions we want reported way out. What number do you guys reference for this?
To Underspec :
An old standard here (NF E 02 204) said U/T <1/4. (U = uncertainy with k=2, T = total tolerance, so if you have ±t, T=2*t)
To Josh :
I desagree with 2 points of your post (sorry, it's unusual !!!) :
1- I believe that spec of the cmm (A+B*L) is not an uncertainty but a tolerance. You can't use it in a calculation of uncertainty, you just have to call the tech if you measure some gauge blocks with a deviation > spec... 2- About decimal places, I always use 4 (in mm), because sometimes the deviation in µm is null, and I don't want thhat the customer thinks the part is perfect. It also helps me to check that Pc-Dmis measures !!!!!
To DaSalo :
I measure usually a 1020mm Koba step, 26 steps (so 52 lengthes), 3 repetitions, 19 positions, so it gives 2964 deviations values for each test.
The standard dev of it gives a "picture" of uncertainty on length (about ±2µm with k=2). Some other tests (scan, rotation of the head, rotation table, probe changer) helps me to calculate CMM uncertainty on standards (from ± 3.1 µm A0B0 only for example). The uncertainty increase with the form of the measured part (for example, the distance between 2 planes depends on the flatness of each one, so you must take them into account in the uncertainty calculation). At the end, without taking into account the method, you are often close to 1/100 mm, but you can be better than it...
To Underspec :
An old standard here (NF E 02 204) said U/T <1/4. (U = uncertainy with k=2, T = total tolerance, so if you have ±t, T=2*t)
To Josh :
I desagree with 2 points of your post (sorry, it's unusual !!!) :
1- I believe that spec of the cmm (A+B*L) is not an uncertainty but a tolerance. You can't use it in a calculation of uncertainty, you just have to call the tech if you measure some gauge blocks with a deviation > spec... 2- About decimal places, I always use 4 (in mm), because sometimes the deviation in µm is null, and I don't want thhat the customer thinks the part is perfect. It also helps me to check that Pc-Dmis measures !!!!!
To DaSalo :
I measure usually a 1020mm Koba step, 26 steps (so 52 lengthes), 3 repetitions, 19 positions, so it gives 2964 deviations values for each test.
The standard dev of it gives a "picture" of uncertainty on length (about ±2µm with k=2). Some other tests (scan, rotation of the head, rotation table, probe changer) helps me to calculate CMM uncertainty on standards (from ± 3.1 µm A0B0 only for example). The uncertainty increase with the form of the measured part (for example, the distance between 2 planes depends on the flatness of each one, so you must take them into account in the uncertainty calculation). At the end, without taking into account the method, you are often close to 1/100 mm, but you can be better than it...
