I notice that it “twitches” ever so often when taking a corner.
Yes, you need to tighten that.
Where you have your finger in that video, there are 2 bearings that are clamped in that way. If you carefully tighten those bolts, it will squeeze in the other 2 608 skate bearings. Tighten it just enough so that those bearings touch the rail. They are a little off now, which is what is allowing that motion.
Tighten it just enough so that they touch the rail and don’t allow that movement.
Edit: P.S. Congratulations on not overtightening things on initial assembly. This is much easier to fix than the other way around.
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Thank you. Will do 
it’s been cutting ok. But on pockets ever so often following a knot in the wood and going off track.
Can I ask you which tubing size you’ve got?
Sure
Okay, 2.54mm for me. Thanks I am still trying to figure out whether that only happens with the larger tubes, because we had it twice already and it was 25mm. But maybe only coincidence, because it should not matter.
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Aluminum won’t last long. The bearings will press flats into it pretty quick.
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Oops, didn’t catch that. 
Dammit. Ill have to rebuild 
https://www.metals4u.co.uk/materials/mild-steel/mild-steel-tube?outside_diameter=71856 Do any of these look ok?
Mild steel would be fine. I would probably go with the 16swg, but the 18 is probably OK for a LR3.
Aluminum would be OK with POM wheels, I think there are larger ones that are 22mm in diameter, but they still have a 5mm diameter hub, so it’s definitely easier to get the steel rails.
Stainless is nicer, particularly if you live in a humid climate. mild steel will require a little maintenance. You can wipe it down with a little light mineral oil or tool wax and that will help keep it rust-free. I actually like that tool wax, since I have several tools that need rust protection (Even though the climate here is very dry.)
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Thanks.
I always used wax. Sawdust won’t stick to it as much as oil.
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Is the wax just to stop rust or to help smooth movement as well?
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The primary purpose of the wax is to stop rust by preventing contact with the oxygen in the air, and keeping water from condensing in contact with the iron. Additionally, it should help maintain a smooth surface. It’s marketed for things like the cast iron surfaces of tablesaws, bandsaws, belt/disc sanders and the like, so it should also allow things to slide. This shouldn’t be important on the CNC where we are relying on the bearings for any motion. No moving parts should be sliding on the steel.
That said, it should reduce any abrasion to both the steel tubes and to any work pieces that might contact them while loading or unloading, in theory.
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Already been answered, and gone astray, so…
Do you need to tighten something?
Depends on if you are a Downocrat or Getuplican.
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