Hello, I have downloaded CAD models of a rack and pinion, and I don't seem to be able to mount them up in PTC Creo without quite obvious interference. The pinion has a pitch diameter of 24 mm, and a modulus 2. The "pitch diameter" of the rack is exactly one modulus lower than the tip of the cogs on the rack. The modulus is, of course, 2 mm. Measurements on the CAD-models suggest that I haven't had a mixup of some kind. In Creo, if I mount the pinion exactly 12 mm above the "pitch diameter" of the rack, it's quickly obvious that something is wrong. If I put a cog from the pinion facing straight down between two cogs of the rack, I get interference on both sides. If I rotate the pinion, after a few turns, the timing between the rack and the pinion is off. Why is this? Creo says that it is moving the rack pi*24mm per turn of the pinion, and I can't believe that Creo could mess that up. Have I done something wrong? Increasing the mount height if the pinion to 12.5 mm, everything looks a lot better. The timing problem is still weird, though. A) Why is this such a mystery? B) Is it horrible to mount a pinion slightly off from a gear? This is for a feeding mechanism of a disposable item. The pinion stays in the machine, but the rack is supposed to be soft plastic, and it is discarded after it has passed the pinion.