new table design question

[KTP]

Hi guys,

I want to make a light duty but accurate gantry style router. I scored a pair of almost new 60 inch LWH25 rails with 4 carriages for $175 (including shipping!...fastest I have ever hit the buyitnow button) and would like to use those for the long axis. I would really like to use 8020 for practically everything if possible.

So the question. If I go with 1530 8020, is it straight enough out-of-the-box to bolt the rails directly to it, or should I plan on shimming or possibly surfacing the extrusion prior to mounting?

[GaryCorlew]

I made mine from 8020 also, it is 1020 and it is great it is very stiff even at 1x2. Mine is 60" long so far I have only posted one picture of it but it is in my gallery

[ger21]

So the question. If I go with 1530 8020, is it straight enough out-of-the-box to bolt the rails directly to it, or should I plan on shimming or possibly surfacing the extrusion prior to mounting?

Depends how straight is straight enough. If you look in their catalog, they list the straightness tolerance, but I'm sure it can vary quite a bit from piece to piece.

[KTP]

Ok, I found the specs on the 8020 site...except they don't seem to list the specs by type, just overall. In other words, the twist and straightness per foot are the same for 1010 or even 3030 extrusion. Personally, I would have guessed 3030 would be a bit "straighter".

"Straightness 0.0125" per foot of length"

I realize this is a worst case spec, but I would like to bolt my nice thk rails to a surface that is at least within .002" per foot.

I see the options as

1) Shim the rails bolted to the 8020 extrusion

2) Surface the extrusion on my mill (not real sure if this would stress the material and cause it to bow even more?)

3) Hope the pieces I get are pretty straight

[PsyKotyk]

I'm not positive, but I think the thick 25mm hardend steel rails would bend the aluminum to tru rather than the other way around.

I realize this is a worst case spec, but I would like to bolt my nice thk rails to a surface that is at least within .002" per foot.

I see the options as

1) Shim the rails bolted to the 8020 extrusion

2) Surface the extrusion on my mill (not real sure if this would stress the material and cause it to bow even more?)

3) Hope the pieces I get are pretty straight

[chuckknigh]

What about laying your nice steel rails onto a thin bed of epoxy, and use screws to tension it to perfectly flat?

BTW, the aluminum and steel would bend each other. The steel is stiffer, but the aluminum would be bigger -- they'd both affect each other. So, I can understand why he wants to start with a surface as flat as is possible.

-- Chuck Knight