[andy1989]

I'm planning my first cnc build. Current plan is to use cnc router parts style carriages on bright steel flat.
It looks like most people use aluminium t slot extrusion for the frame.
Is there a reason why people don't use steel box section? Is it just an ease of use thing?
To me steel is the better option. It's stiffer and cheaper.

What are other people thoughts on this?

[awerby]

Steel is harder to deal with for most people. Welding is not a skill many have, and it tends to warp when it gets hot. Even cutting steel is harder than cutting aluminum.

Aluminum extrusions are generally flatter than steel as it comes, and having all those slots makes it easier to assemble, especially with the special hardware the 80-20 system provides.

That said, those extrusions, and that hardware, are really expensive, and as you point out, they aren't as stiff. If you're able to weld things without undue warping, and can deal with cutting, drilling and tapping steel, it makes sense to use it at least for the parts of your design that don't need to move. Since aluminum is lighter, there's a good case for using it for the gantry and Z axis, although you can find it in other shapes besides those popular t-slot extrusions.

[atomarc]

Whatever you build from welded components that will later be machined in any fashion, and you want that component to stay true, flat and parallel, you are going to have to stress relieve it in some fashion. This is typically done in a furnace. Unless you stress relieve your "weldment", every time you touch its surface with a cutting tool it will change shape..it'll move. The foundation for any accurate machine, CNC or otherwise, is a accurate frame.

Stuart

[andy1989]

I wasn't planning on welding the frame, just lots of nuts bolts and brackets.
Straightness is a concern, and like atomarc says I agree an accurate frame is essential!

The extra work cutting and drilling isn't a problem. I'm more than happy to put in a little more time building the machine if I can save a few pounds.

I guess aluminum box section could be a sensible compromise. Cheaper than equivalent stiffness t slot, and straighter than steel box (please correct me if this is wrong!).
I will have a 6mm thick steel flat bolted to anything I choose, so that will add a lot of stiffness.

[ger21]

Steel is definitely the better option, if you have the tools and ability to work with it. There's a reason that any router over $20,000 is made of steel, rather than aluminum.

[joel0407]

I can weld TIG and Stick but the reason I would choose Extrusion over Steel is the flat and trueness of the aluminium over steel RHS. I had considered it but I figured both where the rails mount and where the bearings mount both need to be machined flat. That's why I believe most use steel then an epoxy spacer between the steel and the rail. The epoxy is poured quite liquid and gravity does it's job to make it dead flat.

Having said that, I have read that software can now compensate for uneven rails. I believe the way it's done is as a tool is moved through it's travel, a measurement is taken to a known true base, the offset is measures at set intervals. The software will then compensate in steps to deal with an uneven rail.

Not having finished my first machine yet, I think if I build another, it will be a CNC Plasma. The accuracy in that will be less than my current machine and a steel frame will work fine.

Happy Days.