Something to be aware of, I was using 6mm wall Al box section from Aluminiumwarehouse.co.uk. It was pretty straight, but slightly twisted. so that in 1m, on a 50mm box one side was about 0.5mm off the supported round rail. At the moment I have just built the thing, which I will probably regret later, though it is not the worst issue with the machine.
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Thinking about it if the box section is twisted then with a rail on either side they will still be the same distance apart so should run smoothly. In an extreme case I suppose it would mean the Z axis wouldn't remain perpendicular to the bed and would get slightly translated at the cutter tip - pretty negligible for 0.5mm though I think?
The number of times I change my mind on this because of all the advica I get/ ideas I have is getting ridiculous!
My impression is that the rails will resist torsion better than the box section anyway. I certainly don't have any issues with binding with supported round rail.
01-09-2010 #34To my mind, mounting a profiled rail to this slot slightly to the right or left of centre may result in the rail ending up rotated with respect to the plane or possibly twisting slightly along its length.
I don't pretend to understand much of this but precision aluminium profile (and box? it is extruded) apparently conforms to DIN 17615:
Outer Dimensions: Depending on size, 0.2 to 0.4 mm
Straightness Deviation: Max. 1.5mm/2m
Flatness Deviation: Max. 1.5mm/2m
also this is an interesting read, or would be if it wasn't so late
Some interesting reading there certainly...
Somewhere it says 'For one manufacturing set-up the variation within one profile is 0.0004 inches'. That's roughly 0.01mm which is negligible. So even if the 80mm dimension is +-0.45mm as it says in your first link, the actual dimension should be stable.
That's where I became totally confused Jonathan
if the 80mm dimension is +-0.45mm as it says in the first link and the actual dimension varies by about 0.01 then where do the straightness and flatness deviations per meter fit in? Also the 1.5mm/2m is not necessarily a gradual change along the full length, it could be bumps or waves or??? I don't know.
Brain is starting to fry, again!
This guy had problems with the concave surface of his ali profiles...
... his fix was a mix of long t-nuts and epoxy to flatten the face, but the area he was dealing with was quite small.
This got me thinking, could epoxy be used to to flatten all mounting surfaces on steel box and smooth over the dimension issues with ali profile? The above thread makes for good reading and at some point leads to this build...
This epoxy is amazing stuff, the two linked builds make heavy use of it to ensure the joins in the steel frames are true.
So, with enough epoxy, a precision level and a thick sheet of glass do you think it possible to true ali profile? The end result must be better than building with 1mm irregularities?
Two big problems:
1) The cost of ali epoxy, http://uk.farnell.com/araldite/arald...0ml/dp/1759545, need to add in the cost of a dispensing gun too, I think.
2) The fact that even with surface prep the special epoxy is not a guaranteed strong, long lasting join.
Since all I'd be using the epoxy for is to make the filling between two lumps of metal is it necessary to use ali epoxy? And indeed, am I obsessing over trivial detail ?
Not got time to look at those links now - about to go into school to make a few bits I can't do at home :)
Try this site for epoxy:
Maybe reinforce it with some of there glass/carbon fibre?
I've sortof assembled mine with just the box section as it is...and it seems ok. I'll have it assembled properly later on today, then we'll see!
I've a length of 3" square 1/4" wall aluminium that I'm intending making my router out of but I haven't had chance to do any measuring on it. I'm hoping it will be close enough that I can just use a thin layer of epoxy on the interfaces and by initially clamping the rails down very lightly until the epoxy sets ensure that they are not deformed by the box section. It'll be at least a couple of months before I get to that stage so I'm looking forward to reading how you get on
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