Quote Originally Posted by njhussey View Post
Surely the twist will be sorted when the rail is bolted tightly down, that's assuming that the epoxy is perfectly flat of course?
Quite likely yes - but how much force will bending the bearing to 'flat' impose upon it? The answer is a lot, since the stiffness of these bearings is still very high in the rolling direction. It also depends on how stiff the gantry is and how wide it is - something has to bend. A fairly high force wont make much difference to how smoothly the bearings roll, since as I mentioned in the last post their co-efficient of friction is very low. It will still shorten the life of the bearings though.

It's different with round rail bearings - clearly they have zero stiffness in the rolling direction, so angular misalignment doesn't affect the force of the bearings (within reason).

If you look in the datasheet for the rails and bearings you'll find specifications for the maximum linear and angular misalignment. These are based on getting the rated lifetime out of the bearings, so if you exceed it by a little, the bearings just wont last as long. Though on a home machine it might still outlive you!

Quote Originally Posted by njhussey View Post
I wanted to see if there was/how much twist there is in the rails. Placing the straight edge across the two rails on top of the carriages I tried to get a feeler gauge underneath the edge and couldn't. Shone a light from the other side and couldn't see a thing so it looks like the epoxy has done it's job in finding the level. Again I'm happy with that!
That's a good indication that they're close, but since you've got the straight edge you might as well measure it carefully. The issue with what you just did is it's hard to measure very small angles and you don't get a reading for what the angle actually is. Instead you need to do something like in my build log - stick the dial indicator on one of the bearings and mount the straight edge parallel to the rail, but some distance (say 200mm) horizontally away from it. Make the indicator touch the straight edge, with it's movement perpendicular to the surface. That will convert the angular reading into a linear reading and amplify it. You don't need to worry too much about getting the straight edge level, as if you plot the readings on a graph that will just make a linear error which you can subtract.

Just out of interest, how much did the straight edge cost?