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?
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...
Well as I said earlier I've a low power laser (that i was going to use on the mill for alignment) that I've played with. Taped it to a DTI magnetic stand and aligned it so it shone along the rail. You can tell any high spots as the line gets brighter. The spots are dust/dirt. Will tweak this tomorrow but the rail looks straight enough. The carriages certainly slide easily with no binding, I'll try 2 bolted on a piece of Ali tomorrow after machining a reference edge to line the carriages up to.
Neil, fix a nice plate to both bearings on your choice of master rail, then pop a clock dial on via a mag base featuring a long reach arm/boom to get to the other rail with the clock, then traverse the master rail via the plate and bearings dialing in your second rail as you go.
Ideally you would want to do this with out the bearings and plate and so just moving the mag base along the master.
That's how I always do it for the machines I build from wood .
I've decided to cut the 20mm piece of Ali I currently have to the correct width for the bed which will leave me a 100mm (ish) wide strip 1000mm long (ish) which I can use for mounting blocks. I'll cut a piece 200mm long and machine a carriage block...
Then after attaching the carriages on the master rail I'll move it up and down and clamp the rail ready to drill.
I've made a small drill guide to go in the rail so the drill bit is kept at 90° (or as near as damn)
I've put a piece of 10mm Ali plate on the bed and moved my Pillar Drill on to it to drill the holes, a bit like below...
Should make drilling easier (but slower) and I hope not to snap any more drill bits as I snapped one in an end hole on the rail
Neil, It's obviously best to drill the holes spot on but if they are tapped at an angle it's wasted effort, I like your drill guide so maybe a similar thing is reguired for the tap or maybe you could just open out the drill guide hole slightly.Spelling mistakes are not intentional, I only seem to see them some time after I've posted
Sliding carriages along the rail gives some indication of the straightness, but very little indication of the twist. Fortunately for us these rails have very low co-efficients of friction, but when it comes to aligning them this makes small errors in straightness harder to detect as they don't produce much force.
Using an indicator mounted on one rail carriage to measure and fix the distance between the rails is good once you've got one rail straight. If you do if before then, you'll just copy the error - so if one rail is banna shaped the other one will be the same banna shape. To be fair that's still better than having a banna and some other fruit...
From a practical point of view, if you've got the rails aligned well enough that the bearings run smoothly then the bearings should last a long time, so it's arguably OK to use but you will need to have realistic expectations of the accuracy.
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08-01-2015 #390Spelling mistakes are not intentional, I only seem to see them some time after I've posted
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