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

[mitchejc]

Neil your build is looking great. I bet you can smell the wood chips already :-)

Please take what I’m about to say with a huge pinch of salt as I just fitted my first rails ever on my first build, so no real experience here but I’m putting it out there hopefully to get corrected or affermed. Look, I don’t know, maybe my expectations of final accuracy is just low or I got very lucky with the batch of 15mm Hiwin rails I received but those reference edges are as straight as Charles Bronson. Keep in mind my build is relatively small so I guess on longer rails it might be a totally different story. Few of us has the right equipment but I believe Jazz’s way with machining the dowels is the best approach assuming the mill you cut it on is accurate over that distance. I’ve seen builds where straight edges are clamped to the rail before drilling but I don’t think a straight edge is still straight after clamping it to something that’s potentially not straight?

I did not use a clamped straight edge for my reference rail. To ensure I didn’t bend it out of shape I clamped it to the epoxy base, drilled, tapped and fastened the ends and then repeated the process by moving the clamps for the rest of the holes always making sure I only drilled next to a clamp if that makes sense. On my rails the holes are 4.5mm and the bolts are 4mm which leaves a huge amount of adjustment as far as these things go assuming one does not stuff up drilling and tapping the holes.

For drilling I used a method proposed on here, I think Silyavski : Using the rail holes as reference, I took a 4.5 mm drill and drilled though the epoxy to the point where it made a good tapper dimple on the steel below and then drilled the rest of the hole with a 3.3mm drill for the tapping. That 3.3mm drill has no choice but to run to the centre of the dimple. Touch wood, I’ve done 40 something holes so far and not one out of centre so it appears this method is working really well. The important thing is that there is no binding whatsoever when you put the bolt in otherwise it will push the rail out of straight or best case you won’t any room for adjustment later. The steel on the inside of these rails where the holes are, are softer than I imagined so I guess the outsides where the bearing run are only case hardened. Not ideal but I’m totally prepared to enlarge the hole in the rail a little to ensure the bolt is not touching sides due to bad tapping or drilling the tap hole at a slight angle. I then used my straight edge and a feeler gauge (ok, it’s not really a straight edge but it’s a good quality stainless steel ruler and I know the one edge is relatively good) to check that the rail. I believe I have enough adjustment in the rail holes to fine tune it somewhere in the future if I get a real straight edge but I’m pretty confident it’s already better than my expectations of accuracy.
I then used my gantry after aligning it perpendicular to my first rail etc to align the second rail and then repeated the clamping drilling, tapping and fastening exercise on that side. As pointed out by Jonathan, I realise that these rails are quite slippery and it would require a huge force to actually get them to bind, but I did the best I could to move it around while aligning the second rail to feel for any sort of friction and I’m very happy with how smooth it’s running.
Even with this potentially dodge method I followed it still took me more a full day to get my x rails on so no quick wins here

Best of luck with fitting your rails.