Steel frame for 8' * 4' cnc bed ?

[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.

[JAZZCNC]

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.

Oh god here we go again.!!! . . . Don't talk soft you can compensate for un-parallel rails in software.!! . . . How.???

Jonathan I'm not wanting to get into a fight with you again but you are miss-leading folks and plan wrong by promoting software can compensate for poor build quality. . . . please stop it because it curdles my noodle. (And I'll always challenge just for the sake of newcomers.!)

[Jonathan]

Oh god here we go again.!!! . . . Don't talk soft you can compensate for un-parallel rails in software.!! . . . How.???

In mach3 go to "Function Cfg's" then "Formulas". If you can measure the error (dial indicator etc) then it will be possible to work out a forumula to compensate, and eliminate it.

Of course it is better to get it right to start with, for obvious reasons, but if after doing all the things we suggest it turns out that there is a small error or, as is likely with a welded frame, it gradually moves, that's something you can do to compensate. Better than nothing.

Surprised Jazz didn't mention this as he has before ... but to damp resonance on a steel frame you can fill it with sand.

[JAZZCNC]

In mach3 go to "Function Cfg's" then "Formulas". If you can measure the error (dial indicator etc) then it will be possible to work out a forumula to compensate, and eliminate it.

So whats the formula to get Mach to stop loosen the rails eliminate binding and tighten back down.??? . . . Your talking tosh and again passing on missleading vaguely accurate impracticle information which just sends wrong signals to newcomers who only discover this far into the build when it's too late.!. . . . End result being they can become disspondent and give up.

Much more helpfull to give sound advice encouraging good accurate build quality than saying just build it and don't worrie because software will compensate for your shity work.!!
Software compensation and formulas etc are last ditch work arounds which never work correctly or has desired.!!. . . . .Effectively they are weak bodge's not to be encouraged IMO.

[Jonathan]

So whats the formula to get Mach to stop loosen the rails eliminate binding and tighten back down.??? . . . Your talking tosh and again

I think you're missing the point I'm making. I said in my last post that obviously it's better to get it right to start with and that software compensation helps if and only if it is a *small* error. I didn't think I needed to state that by small I don't mean binding or tight rails as you'd be foolish to try and run a machine like that for obvious reasons.

Software compensation and formulas etc are last ditch work around

Exactly ... which are better than nothing. If the machine is running smoothly then I see no reason not to use maths to improve it.

No matter how well you build the machine you will never get all axis perfectly true and perpendicular. The best you can get is the error being negligible compared to the tool deflection. If it's only just over negligible then the machine may well run smoothly and if so you can use the formulas to compensate.

Much more helpfull to give sound advice encouraging good accurate build quality than saying just build it and don't worrie because software will compensate

I thought I did say that in my previous posts ('it is better to get it right to start with') on this thread. If not I'll not bother.