Spline gages- how do they grind them?

[spigor]

Does anybody know how do they grind the internal teeth on small spline gauges? I'm looking at a DIN5480 gage with internal teeth, and the certificate says the errors are in 2 mikrometers, it looks like ground. But how can that be possible. Just out of curiosity...

 

[mfgenggear]

wire cut then lapped

also there are special custom made miniture internal arms with a grinding wheel if the spline is large enough fot it to fit.
driven by means of a belt.