Quote Originally Posted by njhussey View Post
Another question. I was planning on using 3" x 2" x 1/4" Aluminium box for mounting the rails on (X & Y axis) but noticed that others have used steel in their builds. I was always under the impression that steel was not straight enough? I'd prefer to use steel as it's cheaper and I can weld the frame up and I need 3 lengths of it for the existing frame so wouldn't have as much waste.
Man wish I'd got £ for every time get asked this.. .Lol

What makes you think Ali box is any straighter or flatter than steel.?. .. It's not.!! Also contrary to popular belief often neither is Alu profile.! . . Steel is far stronger and cheaper and with careful selection can be very straight and flat. It's also very easy to work with if it's not perfectly flat.

There's very few off the shelf materials that you can use without some form of correction for inaccuracies and that includes Alu profile. People think profile is straight and flat but in reality it's often not, yes it can be better than extruded profiles like Alu box but it will still need some form of correction to the surface at some point. It's also 5x the price.

The main area of concern is for the rail surfaces being straight and flat. Extruded Alu like Box is rarely straight or flat enough to use without work so will need some form of correction, either filling or sanding high spots.
This also the same for steel but the difference being steel is quite easy to scrape,file,grind,sand than Ali has being harder it doesn't clog tools etc.
In my experience steel box has just the same inaccuracies that Alu box does but steel is much easier to correct. Often all it needs is a quick run over with grinder knocking off any high spots, Low spots can be filled. Then just shim the rails to get perfect.

If your using the Chinese round type rail then I can tell you with 99% certainty the base will not be flat or straight and will have more error than the steel your fastening it to.!!. . And thats without correcting the steel.!!

Have no fear of steel, it's cheaper,stronger and very easy to work with and manipulate to your needs. Other than being lighter Alu has just the same issues steel does and unless you have special welding equipment is actually harder to work with.
If you bolt together while bit easier to drill alu suffers from crush and hole deformation much more than steel does. Unless it's thick material doesn't hold tapped threads very well like steel does. Steel will easily hold fine threads in 3mm thickness for fastening rails etc and allow decent torque on bolts, try that in Alu.!