Hi there,
My mill has been converted for quite a while now, but I am having trouble with accuracy. Today I was machining a small model engine part in an area of about 3 x 3cm. I set the zero point to the centre of a pre-drilled hole and then machined the part. The part included some holes that were meant to line up with holes on the engine's crankcase. After the part had been machined, the wall thickness of one side of the part was thicker than the other side, when they were meant to be the same. When I put the part onto the engine, all 4 of the mounting holes were offset to the right and down a bit. This is a bit irritating, as the 2mm holes are out by more than 0.5mm. I don't think it is inaccuracy in the leadscrew; 0.5mm over 3cm is ridiculous -I know my leadscrews are more accurate than that! I think it may somehow be accumulating an error. The reason why I say this, is because the features on the part that were machined first are close to the correct dimensions/positions, while the features that were machined later on are further out. You can almost tell what order everything was machined in just by looking at how inaccurate it's position is

. It is offsetting everything in the same direction.
All the code was hand-written, and I am confident that it has nothing to do with this, as the problem has occured on other parts too. I was using mostly G02 commands in the code. My machine is a Sieg X1 using the original leadscrews and TurboCNC for control.
The leadscrews are meant to have a pitch of 2mm and that is what I have set it up to be on the software. If for whatever reason my leadscrews are not quite 2mm pitch, but the software thinks they are, can this cause an accumulative error?
Does anyone have any idea what might be going on here? I need to sort this problem out, as having these slightly offset parts is worse than not having the part at all. It also gets irritating when you have to remake a part 3 times, just to find the third one is no better than the first...
Thanks
Warren