Hello everyone,

I'd appreciate some feedback on this matter since I cannot come up with a smart answer ;

we are considering building a cnc plasma from scratch with a work envelope of approximately 50" x 100" to accommodate 4x8 sheets.

We have two machines on the floor and are relatively familiar with router and machining center mechanics since we retrofitted both of them (Thermwood 5x10 c50 router and biesse 20 ptp machining center).

The plasma will see occasional use but we'd like to build it as if it were to run often since this may be a whole new line in our business ....

After looking at many grooved bearings home made rails, we are of the opinion that commercially made linear bearings are the way to go,either thompson supported rod type or thk slide type (we currently have both on our machines....)

It seems that considering the rail lengths needed, Automation overstock has the best deals with 30 mm Hiwin heavy duty rails and blocks X axis master and slave and y axis for maybe $ 1,200.00 total.

Please do not hesitate to comment on choice .... We could not located fully supported rod sufficiently long ...

Here is were the question arises ;

I can see mounting fully supported rod type slides on a mechanical tube frame assuming that the slide geometry grants it extreme rigidity that would counteract the poor tolerances of tube steel and span the variations, but in the case of the Hiwin rails, I am concerned about mounting the rail onto standard mechanical tube ( we were thinking 3x4x .1875 hot rolled) with .03 tolerances and basically transferring this poor tolerance to the slide assembly ....

On our biesse, I believe the frame is made out of hot rolled tube but a "flat" was clearly machined into the tube where the rail mounts . I assume to maintain straightness ... having no practical way to machine the tubes, wondering if the frame members supporting those rails x axis and gantry should be made out of cold rolled bar (not too happy about cost) or if there is a simpler solution such as shimming ?

omitted to mention that the basic design at the moment (conceptual only) is a rigid mig/tig welded steel tube frame with linear bearings mounted on the outside face of top tube rails in the X with a gear rack underneath thus my concern of steel tube tolerance in order to maintain parallelism and straightness between both X slides (also stand for the gantry Y...)

thanks for any input ....

Pierre