3 Axis CNC router

[JoeHarris]

The more I think about how to brace this thing up I am moving towards a metal frame rather than timber. I would keep all of the aluminium profile and plate as is but bolt this down to a steel box frame. The question is, is it worth buying an arc welder and learning to weld for the gains in strength and hopefully accuracy??

[D.C.]

I came to pretty much the same conclusion about using steel, I can upgrade something like a spindle or motor at a later fairly easily to improve the machine but if the frame moves around like a jelly then the whole thing would have to go in the bin and get rebuilt from scratch.

At the moment I'm planning to hold thep pieces of steel together with brackets made from steel angle and bolts so that I can adjust it fairly easily, because of the problems welding seems to cause I'm also considering joining some bits with a metal epoxy. If you need to buy a welder just for this job (the really cheap mig welders won't do 3mm/4mm box properly) the epoxy route would work out cheaper and easier than learning how how to weld on a frame that needs precision work from the get go.

This guy has a lot of useful info for how to approach frame building, while spending £500 on a perfect straight edge to use for alignment may be out of my league there is some good stuff that can be applied on a lower budget.

Machine frame - MadVac CNC

[irving2008]

Learning to weld is a skill thats worthwhile in itself. The thing about welding up a steel frame is that unless you are careful it will distort with the heat. The mistake that most beginners make (and I include myself in that as I'm not an expert welder, I just get by) is to try and weld too much in one go. The trick is to tack things in place then go back and fill the gaps so to speak. You dont need to seam weld everything. Personally I'd go with a MIG welder rather than an arc/stick welder. Its easier to strike the arc and get a good finish. While a steel box won't necessarily improve accuracy it will improve rigidity and you can still use wood with steel; a basic 4-sided bed with 4 legs of steel and a carcass of mdf bolted over will give it rigidity (and turn the base into a useful cabinet)

[martin54]

Meant to say if you buy a gas less mig setup you don't have to worry about insurance as far as having gas bottles on the premises goes either. Not used gasless mig myself but know people that do & seems to be little difference.

[martin54]

Like irving has said welding is a useful skill to have but like anything does take a bit of practice to get right. Mig is certainly easier to learn than stick & probably more forgiving in that it is easier to keep a run going. Steel is pretty cheap to buy so the practice doesn't really cost a lot. If you picked up a decent second hand mig machine you would probably be able to sell it on if you decided not to keep it for much the same as you paid for it so would only really be borrowing it for your build.
Even with buying a welder a steel frame will probably work out cheaper than an aluminium frame I would have thought. Can't speak from a cnc point of view though as haven't built a machine yet myself lol.

[martin54]

That's an interesting read DC, haven't read it all yet but did read the bit about him using an epoxy to basically glue the steel sections together.
Have you done much research on this way of manufacturing a frame? The guy doesn't seem to go into to much detail & I couldn't see much on the link he provided but haven't read that properly either.
He mentioned using west systems 207 which is just a hardener so not sure what resin he used plus there is no mention of ratios for resin, hardener or fillers although there would have to be a low volume of fillers in it if you can pour it.

Determined to get some epoxy in your build somewhere aren't you lol.

[JoeHarris]

Wow that guy went to some extreme lengths on the accuracy of his frame then only used one screw on the long axis!!! Whoops.

[JoeHarris]

Thanks for the tips - i'll add 'learn to weld' to the to do list.

[D.C.]

Sorry for rambling post...

A few people on cnczone seem to swear by it, I've only ever used the stuff before for patching up rust holes in cars not for anything structural but watching this vid impressed me enough to seriously consider it:

DP-420 3M EPOXY overlap shear test by Doctorbass PART 1 - YouTube

(In part two he adds a load more weight and the join holds perfectly)

The thing with welding properly is it does take a lot of practice, if you don't prepare the joint carefully, don't make sure you have good penetration and don't make a consisitent pool in the weld than the strength of the join drops off dramatically, that is why people doing mission critical welding like nuclear power plants or submarines insist on x-ray inspection of every single weld. A weld can look and will look absolutely perfect but still be structurally weak, especially with arc welding where bits of slag can slip into the weld and be completely invisble to you when you have a welding goggle on.

If the people recommending repeated tack welds ever took an x-ray of the joints or sawed through them they would be shocked at how weak the joint they ended up with is in real life, fortunately the amount of steel that gets used for CNC machines is not there for strength, it is there to provided rigidity. (I know nothing about CNC but I can weld...)

Metal epoxy seems to provide more than enough strength for the job of holding a machine together but introduces another problem because the young's modulus of the various epoxy mixes seems to vary between 2-20 Gpa (compared to about 70 for ally or 200 for steel, higher means more rigid). I'm satisfied that any epoxy joint will structurally hold together on a CNC machine but I still haven't worked out what effect joining two pieces of steel together with what is by comparison a bit of rubber will have on the overall performance of the structure.

If I can't determine that my current plan is to build a frame from mild steel box section that is held together using brackets made from steel angle section welded to plate that is bolted to the box section. Enlarging the holes for the bolts mean I can perform accurate positioning before tightening the bolts and secure the joins using metal epoxy. That way I get the benefits of steel on steel rigidity and the epoxy makes sure that it will never move out position but if I welded a 'perfect aligned bolted structure' the thermal shock of the welding would distort the alignment as metal expands and then contracts. Bolting alone would mean loosening of the bolts over time.

My current thoughts anyway!
*Subject to radical change when I get a clue about what I'm doing. :)

[irving2008]

Have you considered Ally box section and ally welding like this: Durafix UK Ltd - Pages