Steel frame for 8' * 4' cnc bed ?

[Web Goblin]

80 x 80 x 6 box section can still distort. The size of the machine you are building will be rather heavy, 80 x 80 x 6 box section aint light. Unless you level it very well it will still move. If you decide to bolt it down then make sure it is perfectly level on its feet or you will pull it out of alignment with the bolts.

Ian

[Karl]

We would certainly be interested in hearing the personal experiences of anyone who's used epoxy et al to level the top rails of a larger frame. Our Mark-3 machine will be for 8' x 4' sheets. (That said, we're still assembling the bits for MK-2).

Karl

[flanagaj]

80 x 80 x 6 box section can still distort. The size of the machine you are building will be rather heavy, 80 x 80 x 6 box section aint light. Unless you level it very well it will still move. If you decide to bolt it down then make sure it is perfectly level on its feet or you will pull it out of alignment with the bolts.

Ian

Do you think that is over sized ?

[Web Goblin]

Do you think that is over sized ?

No I dont think the material is oversized.
If you want a really sturdy machine then you cant really oversize the frame material, within reason I might add. Box section steel itself isnt really that strong unless you start going into heavy wall thickness stuff and as Jazzcnc has said getting your construction right to start with will make all the difference and would save you £800 on machining.

Ian

[Jonathan]

80 x 80 x 6 box section can still distort.

Yep, anything will distort. It's a matter of how much and the effect it has.

Heavy frame = good, as long as you don't have to move it!

For a machine purely for woods it does seem a bit excessive, however you seem to want to use it for milling aluminium too in which case it pays to make it as strong as you can.

What sort of tolerances do you require as that it what determines if machining the rail surfaces perfectly parallel is necessary? You can compensate for it perfectly well in software.

[flanagaj]

Yep, anything will distort. It's a matter of how much and the effect it has.

Heavy frame = good, as long as you don't have to move it!

For a machine purely for woods it does seem a bit excessive, however you seem to want to use it for milling aluminium too in which case it pays to make it as strong as you can.

What sort of tolerances do you require as that it what determines if machining the rail surfaces perfectly parallel is necessary? You can compensate for it perfectly well in software.

Ideally, I would like to have an accuracy of at least 0.05mm. Worst case scenario I could stretch to 0.1mm, but no worse

[Colin Barron]

If you are not bothered about weight, why don't you buy an old machine and fit new drives and electronics. i would have thought you are more likely to achieve the level of accuracy you require.

[flanagaj]

If you are not bothered about weight, why don't you buy an old machine and fit new drives and electronics. i would have thought you are more likely to achieve the level of accuracy you require.

I have trawled in the interweb for hours on end looking for 2nd hand cnc machines, and not had a great deal of joy.

[Colin Barron]

what about this: http://www.ebay.co.uk/itm/Wadkin-UX-...item231485a663

[flanagaj]

what about this: http://www.ebay.co.uk/itm/Wadkin-UX-...item231485a663

That is slightly too large. I have another plan in progress which is to make the base out of concrete and the rails out of epoxy granite and all so that it can be dismantled easily.