Printable View
How are people dealing with out of straightness and twist tolerances on box section for router frames, parkers give a tolerance of 0.15% of length which is a lot, i cant see how people claim to hold 0.1 tolerance on a frame manufactured from tubing thats that far from straight?
I'm dealing with it by using epoxy to level the surface the rails will sit on. Doesn't matter about straightness or twist then.
I have a feeling that that is why there's been so much discussion about epoxy techniques recently! You might be able to shim out bend but I would guess that shimming twist is a bit more difficult.
My frame was 3mm lower at one corner after welding, I designed it to have bolt on beams to support the rails, I managed to shim those beams to within 0.5mm of each other but as Neale says there was also very slight outward twist. I therefore used epoxy to level the top surfaces prior to fitting the rails. As Neil says, if you take the epoxy route you don't have to worry excessively over the frame, as long as it's strong and stiff.
That's what I've found anyway but it's everyone's choice to do what's best for them.
The short answer, to the OP question is: They can't.
The steel supplier will likely be quoting from the British Standard for tolerances of form on hot formed hollow section. I don't have to hand, but they are not small numbers; quite high percentages of length and section size, etc. One can control the tolerances, to a point, when welding together. Other than that, epoxy or shim. The one thing I don't get about shimming is that if it isn't done over the whole length of the clamped joint, joint stiffness benefits are lost. In that regard, epoxy is better.
I designed my frame so that when I welded on the box section for the X axis rails I could get them as level as possible before welding them and I welded them by tacking and checking the level and then adding more tacks, once I was happy that the rails had not moved I welded over the tacks. One of the box sections does have a bit of twist in it but the epoxy will take care of that. I think if you're selective about the box section you use and are using supported round rail then you can get away without using epoxy. Jonathan's build and Kingcreaky's builds are testament to this.
You can see here that I welded 10mm thick flanges to the frame and also to the underside of the rail support beams, these were then bolted together with shims between where required.
It was the welding of the flanges to the underside of the rail support beams that spoiled it, it made the beam bend like a banana such that it was about 1 to 2 mm higher in the middle. Bolting it down made it like the Loch Ness Monster in that it developed humps between the flanges, obviously not very big but humps all the same.
http://www.mycncuk.com/forums/gantry...html#post54981
I think that if I had simply bolted the rail supporting beams to the frame flanges, i.e. without welding flanges to the beams, then I could have shimmed it successfully without using epoxy at the end.
Thanks for the answers, im glad i wasnt just missing something, makes the design a bit easier knowing im starting with the problems i thought i had!
My mill is a large Hardinge fanuc vmc and I had considered facing all the box off, but the envelope is only 710x400x500, so would take a long long time for 3000mm, plus i have to take the side door off and due to the stupidity of the designer the door is higher than the table so i would have to use couple of large angle plates and make brackets, and id have to move the receiver for my probe, and remove my tool height setter, all of which is a pain as all my tool offsets would need to be updated and i'm bound to forget and bugger something up, plus if the beam is out by 3mm i could be machining a lot of metal off, thinning the wall and stressing it into being more twisted, then flipping it and doing it all again until ive got a whole beam of chips in my hopper and no beam!
My other plan was to build a jig in the mill and drill the box section ream some dowel pin holes to align it and locally face it, then weld up some corner brackets from plate or GFS and machine them to be square, bolt it all together and then I could use some sort of resin as suggested to make the surface for mounting the rails, i wondered if anyone has used self levelling machine grout for this? Ive used it to make foot plates for my mill, its 3 tons so needs good footings and the grout leaves a very good surface, its good for 80N/mm2 so should have no problem with a reasonable light gantry(180kg) is my guess.
Cheers for confirming my fears.