Hi,

Firstly im only in the planning stages of my first cnc mill, and so please excuse any stupid questions which may follow.

I am planning to use thk linear rails for the moving table of my mill, and would like some ideas as to what material to use to make the frame. From what i have read such rails require a very flat surface to be mounted to, and i dont have the facility to skim cut 80/20 extrusions, so would i be better off just working with mic-6 tool and jig plate throughout, or is there a way to mount the plate to the extrusions without either warping the plate, or having to mill the extrusions?

I wouldn't worry too much about the flatness of aluminum extrusions. They are pretty flat. In fact, I see many manufacturers mounting rails directly on to extrusions. I also imagine that the rails are flatter than the extrusions, and that when they are bolted toghether, the rails may slightly pull the extrusions flatter (though this may be wishfull thinking). Regardless, I'd use the extrusions as they are.

I guess it boils down to how precise you want your machine to be.

Carlo

Oooh - this is not one of those quick answers....

Basically - I agree with cbass - the extrusions are pretty straight and flat - but they are going to conform to whatever structure they are mounted to, if any. The rails are not going to straighten them out. The addition of the rails will stiffen the extrusion structure, but the rails will also conform to the mounting surface. They may not be perfect, but the straightness of the extrusions are probably close enough. If not, they can easily be manipulated to make corrections.

Straightness, flatness and twist are the concerns of the individual rails. For a plural arrangement add parallelism to the considerations. The design of the base structure is key to making a structure that can be adjusted to perfection. All you need is a baseline to compare to for level, straightness, flatness and parallelism, the means to make the adjustments and patience. Considerations in the design can easily provide for the adjustments without sacrificing rigidity.

Scott

On a small machine, mounting the rails to the extrusions works just fine. There are many examples on this forum.

Some people have used bedding compound to set the rails.

Here's a thread with a good discussion on mounting rails and associated problems. You may have to read through it aways to get to the rail discussion.
http://www.cnczone.com/forums/showthread.php?t=6840