Hi all,

Please excuse my naivety. We do mostly lathe work with 2nd, 3rd, and 4th op mill work with a 3 axis vmc. Most of our work is not mill work, mostly lathe work. Most of the drawings we work with have GD&T that is incorrectly applied. We use a haff and schneider 3D probe. We do not have a renishaw probing system.

I understand that a lot of the aerospace/defense work uses GD&T pretty extensively. We have done occasional defense/aerospace work. I know GD&T fairly well and how to interpret it. What I am not familiar with is renishaw probing.

My question is about hitting datums on a 5 axis mill. See below:

Say you have a hole. The surface at the top of the hole is datum A (primary datum), the side of the part is datum B (secondary datum), and the other side is C (tertiary datum). According to the GD&T 3, 2, 1 rule - the primary datum is 3 points (forms a plane), the secondary is two points, and the tertiary is 1 point.

So, do you probe the datum A 3 times to form a plane, datum B twice, and datum C once? Or do you just probe the part like this during inspection on the CMM and not on the mill during machining? When adhering to datums in the feature control frame for the hole, what is the process for probing your parts before machining features (such as a hole) to hold the true position tolerances?

Thanks,

Chris

Please excuse my naivety. We do mostly lathe work with 2nd, 3rd, and 4th op mill work with a 3 axis vmc. Most of our work is not mill work, mostly lathe work. Most of the drawings we work with have GD&T that is incorrectly applied. We use a haff and schneider 3D probe. We do not have a renishaw probing system.

I understand that a lot of the aerospace/defense work uses GD&T pretty extensively. We have done occasional defense/aerospace work. I know GD&T fairly well and how to interpret it. What I am not familiar with is renishaw probing.

My question is about hitting datums on a 5 axis mill. See below:

Say you have a hole. The surface at the top of the hole is datum A (primary datum), the side of the part is datum B (secondary datum), and the other side is C (tertiary datum). According to the GD&T 3, 2, 1 rule - the primary datum is 3 points (forms a plane), the secondary is two points, and the tertiary is 1 point.

So, do you probe the datum A 3 times to form a plane, datum B twice, and datum C once? Or do you just probe the part like this during inspection on the CMM and not on the mill during machining? When adhering to datums in the feature control frame for the hole, what is the process for probing your parts before machining features (such as a hole) to hold the true position tolerances?

Thanks,

Chris

Last edited: Jan 27, 2022