True Robin, I think Cam is confusing resolution with accuracy. To be accurate to 0.1mm you need to resolve to something better, 0.05mm being an obvious one, ideally better than that. For a router cutting wood, 0.2mm is probably accuracy enough, needing 0.1 or better resolution. You'll never be as accurate as the lowest resolution you can maintain. So if you want 0.05mm accuracy then you need a 1/2step of 0.025mm or better. However that is just the accuracy of the drive train. Once you allow for backlash, flex of the supporting environment (even without cutting forces the mechanics will flex as they move), etc. your accuracy will suffer...

For a model wing rib, 0.25mm accuracy should be good enough, so 1/2 step resolution to 0.05mm plus some flex allowance should get you close...