Comments sought on new build: A CNC Router for RC Gliders and Planes

[Clive S]

Dean, how do I make sure one rail is straight without buying a proper striaght edge? My frame is not straight enough to align the first rail, or at least I don't think so as it's just plain 100 x 50 x 4mm steel, with any sort of accuracy?

I was thinking of getting some of my 10mm thick Ali and using the milling machine machine as much as I can down one edge and then turn it round and then using my edge finder set it parallel again and machine the rest to give a straight edge. Would this do?

How about using some fishing line stretched very taut along the length. ..Clive

[njhussey]

How about using some fishing line stretched very taut along the length. ..Clive

Wouldn't that be a little flexible? Plus I've got an end stop on one end so can't wrap it round the rail, would have to accurately drill a hole through the steel.

Just looking at a Veritas straight edge http://www.axminster.co.uk/veritas-steel-straight-edge which I could clamp to the rail to keep straight?

[Clive S]

Wouldn't that be a little flexible? Plus I've got an end stop on one end so can't wrap it round the rail, would have to accurately drill a hole through the steel.

Just looking at a Veritas straight edge http://www.axminster.co.uk/veritas-steel-straight-edge which I could clamp to the rail to keep straight?

You could clamp the line to the rail with a spacer say 1mm between the rail and line then do the same at the other end with spacer. With the line tight you can then measure with a feeler along the length after fixing one end first. ..Clive

[Neale]

As Jazz said, sliding a carriage along is a pretty sensitive test of whether or not the rail is fairly straight - you quickly feel the graunching sensation if it isn't! My design uses an aluminium block with two carriages bolted to it (gantry then bolts to these later) and having two carriages fixed in line is even more sensitive. No way am I going to buy a 1.8m straightedge for alignment purposes.

Just bolting it down by eye (I loosely bolted one end of mine, then wiggled it a bit to let it find its own line) is probably good enough for a first pass, with a sanity check of running the carriage up and down. It's not going to be many thou' out, I reckon. I then set the second rail to this, knowing that it's probably not absolutely straight but good enough to work, and will then fine-tune it by cutting test pieces once the machine is in a state to actually cut something. It's another example of the "you can't build it accurately enough but you can tweak it afterwards" home-workshop approach...