[kadirilkimen]

If you don't limit the way of machining, you can count microchip manufacturing as machining. So, we can easily talk about nanometers.

If you consider machining only physically removing the material by friction of two different materials, then I think it is practically about 0.001mm. Even this dimension is not always possible. because in this level of precision requires to consider even ambient temperature. If you machine the material with a cold coolant and wait for 10 mins to measure the result. You would end up a bigger part. If you machine it under a warm coolant, then you would end up a smaller part.