How do you you convert photo to 3d grayscale

There is no '3D greyscale'. 

It sounds like you may be referring to a 3D heightmap, which would have been found under bump map in older versions, but 3D functions are no longer reliable and are being removed from Photoshop. Even if you could produce a bump map, further software would still be required to convert to the instruction set for a CNC machine. That has never been a function of Photoshop.

Dave

What is »cnc«? 

First, you should be aware that 3D is being discontinued in Photoshop. Here's some info on how you may be able to access it:

• https://helpx.adobe.com/photoshop/kb/3d-faq.html (See What if I absolutely have to use the latest version of Photoshop along with the 3D features that are being removed?)

If you're able to access the 3D features, then it sounds like you'd want to create a new 3D mesh from a grayscale image:

• https://helpx.adobe.com/photoshop/using/creating-3d-objects-animations-photoshop.html#create_a_3d_mesh