Quote Originally Posted by Neale View Post
- (and this one is probably a bit more contentious) - consider very carefully if you really need a bridge. I'm not sure if it causes problems due to shrinkage or not - I don't think it does but others believe passionately that it does - but it is a pain to set up, block leaks, etc. What does a bridge buy you anyway? It cannot improve the "horizontalness" of an individual epoxy strip (but might make it worse). What it does, theoretically, do is make both rails the same height. Can you build a gantry that is so accurate that both "feet" are exactly flat, in the same plane, etc? Commercially, of course you can, but then you would probably be able to machine the tops of the rails anyway. I know that I am going to be shimming between the feet of my gantry and the X carriages, so I am happy to take up the odd mm or two height difference in the shimming. My latest epoxy layer hasn't used a bridge, and so far the epoxy surface is looking good when I look along it at reflections of the wall. I doubt if I'll be taking up more than the odd millimetre of height difference. Why make things difficult for no benefit?
In which case don't bother with epoxy at all and buy straight edge to straighten and level the top rails. If the surface isn't pefectly flat it takes nothing but a file to knock off high spots and bit of epoxy filler to raise low spots. this can be down in the time it takes to setup and build good dams.

The whole point of the bridge is that it allows both rails to level on the same plane and to be honest if anyone can't setup a decent bridge or dam that doesn't leak I'd be quetioning there abilty to build the machine in first place.!

To me epoxy is ok but to be honest I only use it in certain cases like recently when I'm correcting someone else frame work. Rest of time I find it more work than it's worth and can reach the same results or better quicker using straight edge and shimming. But for those without precision straight edges then realise it's a quick and easy method if done right. (thou you'll still need a straight edge at some point so don't bother and buy straight edge is my thinking. .Lol)