Machine build – columns

[Dave SParticipant @daves59043]

I’m building a CNC mill. I though I had it planned, but now I’m having a bit of a which one sort of dilemma.
The basic structure is cast iron base with linear rails for the Y, then twin columns with rails for the Z, bridged with 2 pieces of 50mm equal angle (about 6mm thick) and some more linear rails for the X.

The base and bridge I’m pretty happy with. The structure is stiff enough – more massive – than the rails, so I’m confident that the rails will conform to the structure when bolted down. The rails are, as expected in the free state not straight in either plane.
The columns I’m a bit less sure of…
They are 80mm square hot rolled welded box, 3mm nominal thickness. Being none precision steel by the time I’ve made then flat I’m assuming the flat that the rails will bolt to will be about 2.5mm thick.
Seems like that might not be enough to pull the 20 mm rails flat.

Given I haven’t yet done the make precision work on the columns now is a good time to switch if required.
I could brace the inside with some more 50mm angle, which would give me the box section benefit in that it’s 80mm box, with added stiffness where the rails are.
I also have a length of 200mm channel which I could split and use as the columns instead. That would give unequal angle of about 60×100, which is less than 80 in one direction and more in the other. It is however 10mm thick, so probably stiff enough to hold the rails flat. Downside is I would have to cut it in half and that’s a lot of effort compared with backing the box with a length of 50mm.

I was planning on filling the box with sand to dampen any “ringing” which of course is not possible with an open angle.

Thoughts please, I’m probably overthinking this hugely, but there you go…

Dave

[Dave SParticipant @daves59043]

[JasonBModerator @jasonb]

Why not mix the sand with resin and fill the box solid – epoxy concrete with a steel shell. Have your inserts for fixing the rails go well into the epoxy and that should pull them flat

Alternative could be to weld say 10mm flat bar to the face of the box, machine it true and tap for fixings

Edited By JasonB on 07/04/2022 20:14:23

[Dave SParticipant @daves59043]

I was hoping to not need to fill at all – more of a backup plan in case of vibration.. So I don’t have sand (or epoxy) currently.
How would you go about putting the threaded holes in if that was a solution? Seems like you’d want to fill it first, but then the epoxy / filler fill is in the way and not a simple machining job to tap.

Dave

[Phil H1Participant @philh196021]

Dave,

I think the threaded inserts would be added before you pour the epoxy. Bolts or short pieces of stud could be used to keep the epoxy out of the threads.

[JasonBModerator @jasonb]

Or just drill and deeply CSK some holes and weld in some short lengths of bar. Then machine the face flat which will also flush the weld beads and drill and tap blind holes. By tapping after welding you can ensure the holes are where you want them should there be any distortion during welding

This thread may give you some ideas, also note how the rails are adjusted sideways with the "cam" washers

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