Welding bed for CNC machine ?

[flanajb]

Hi,

I am in the process of researching what is involved in building my own 8' * 4' cnc machine. I am currently looking at the frame design and wanted to avoid using aluminium profile.

I came across a chap who made his frame out of mild steel, but upon reading his article he has sort of said that welding is not suitable ?

He epoxy joined the key joints. I just wanted to ask any experienced members whether he is correct in what he says or whether it is over kill ?

The snippet below is from his web site. If you want to read the full blurb the link right at the bottom will take you to the web page.

Appreciate any comments you have.

"Welding is excellent when you want strength and simplicity in joining of awkward pieces together but it is not good enough for precise alignment since the weld shrinks and warps the steel even with thoughtful pre-stitch welding (and a few of us have stress relieving oven for steel handy or a precision grinder for later). Also, plain structural steel is not rolled perfectly square and flat, so plain bolted steel to steel joints are not workable because the contact area is only about 15-20% if you are lucky and torquing things tight together is very bad since it leaves the steel in strained condition and the structure will keep twisting and creeping and you will never align the machine. It may not be visible by a naked eye but in the 0.01" scales and smaller the movement is very noticeable, not to mention the fact that by bolting together unground steel will NOT make an accurate machine."

http://www.oneoceankayaks.com/madvac/machine_frame.htm

 

[Hitch]

Depends on what machine you are going to build id say....?

Something like a router could be important, depth of cut on vgrooves especially will notice as a wider groove. Where as on a plasma, its not that much of an issue.

Even for a router though, id say a decent fabricator (with a suitable flat bench should be able to get it pretty good, then, if theres goingto be a secondary bed on top, it could be shimmed to suit once the carriage etc is all on.

 

[flanajb]

Depends on what machine you are going to build id say....?

Something like a router could be important, depth of cut on vgrooves especially will notice as a wider groove. Where as on a plasma, its not that much of an issue.

Even for a router though, id say a decent fabricator (with a suitable flat bench should be able to get it pretty good, then, if theres goingto be a secondary bed on top, it could be shimmed to suit once the carriage etc is all on.

It is a cnc router I am going to build so it does need to be accurate. I think you are correct that there are ways to work around any inaccuracies

 

[Drains]

Are you concerned about the bed being level for stock, or level for the cnc head? The best way is to make an accurately flat steel frame, but to use separate rails from aluminium or whatever which you can level with adjusters. The head can then run perfectly accurately, while the work sits well on a flat surface. If you can sit the work on a jig if it needs turning over, all the better.

Anyway, the essential thing is to make the main runners separate to the frame so they can be aligned separately, if you can't make the frame accurate enough.

But to weld up a frame accurately isn't so hard if you square everything up, minimise the amount of weld used and be careful to weld in places, areas and directions which will minimise distortion.

 

[flanajb]

Are you concerned about the bed being level for stock, or level for the cnc head? The best way is to make an accurately flat steel frame, but to use separate rails from aluminium or whatever which you can level with adjusters. The head can then run perfectly accurately, while the work sits well on a flat surface. If you can sit the work on a jig if it needs turning over, all the better.

Anyway, the essential thing is to make the main runners separate to the frame so they can be aligned separately, if you can't make the frame accurate enough.

But to weld up a frame accurately isn't so hard if you square everything up, minimise the amount of weld used and be careful to weld in places, areas and directions which will minimise distortion.

Both aspects really. The main frame will have 2 parallel linear guide rails (x axis), on top of this will slide the gantry which has the y and z axis.

The 2 rails have to be spot on as you dont want any twist. Maybe the best way is to not join the 2 rails and just have each of them fixed to a bed. They can then be levelled independently of each other until they are parallel and perfectly level to each other. The bed itself can then be welded and the mdf that sits on the bed can be cnc flat.

That might make it easier.

My intial plan if I was going to weld a rectangular frame for the rails was to level 2 pieces of steel on my concrete garage floor and then rest the t x axis rails on these. If I clamp them down onto the rails on the concrete floor it would at least make sure they are level. That is as long as I can find a very accurate level to level the 2 steels on the floor.

That is probably as clear as mud, but you might get what I am saying. Maybe a drawing would be a better idea ?

Thanks

 

[Drains]

Maybe the best way is to not join the 2 rails and just have each of them fixed to a bed. They can then be levelled independently of each other until they are parallel and perfectly level to each other.

Yep, that's what I meant. You can get the rails bang on that way.

My intial plan if I was going to weld a rectangular frame for the rails was to level 2 pieces of steel on my concrete garage floor and then rest the t x axis rails on these. If I clamp them down onto the rails on the concrete floor it would at least make sure they are level. That is as long as I can find a very accurate level to level the 2 steels on the floor.

I'd be more likely to make two H frames (each side of the table) and join them together, tacked, squared, checked on the diagonals and visually checked for winding (which you can do very accurately by eye, better than with a bubble), then welded up with minimal welding in a sensible sequence. You should be able to make it to within a millimetre or two if you take your time, then adjust the x axis rails to suit.

Don't forget that a steel table frame that's not flat can be "adjusted" by dropping, appropriately.

 

[flanajb]

Yep, that's what I meant. You can get the rails bang on that way.



I'd be more likely to make two H frames (each side of the table) and join them together, tacked, squared, checked on the diagonals and visually checked for winding (which you can do very accurately by eye, better than with a bubble), then welded up with minimal welding in a sensible sequence. You should be able to make it to within a millimetre or two if you take your time, then adjust the x axis rails to suit.

Don't forget that a steel table frame that's not flat can be "adjusted" by dropping, appropriately.

I think we are agreed. I have posted a picture.

 

[GeorgiePorgie]

This guy is doing summat right with his CNC,he has used Mild Steel box section but has also used Aluminium shims to correct the distortion
http://www.cooperman.talktalk.net/files/2.htm

 

[flanajb]

This guy is doing summat right with his CNC,he has used Mild Steel box section but has also used Aluminium shims to correct the distortion
http://www.cooperman.talktalk.net/files/2.htm

Thanks. That looks like a really informative web site. I have trawled google a lot on my travels, but never came across that one.

 

[Pete.]

You're never going to bring a weldment to cnc-machine standards of alignment. The only way is to build it as close as possible, get it stress-relieved then fit the rails and shim into alignment.