Gantry design and FEA analysis

[eci22]

Well sort of.... The direct fixing of the rails thing was more aimed at how you're spacing the front and back Z plates apart. Currently you have the carriages fixed hard onto the back plate, and then some bits of square bar lifting the rails off the front plate presumably to clear the ballscrew bits. This will make the front plate wonderfully stiff front to back, but it would be worth doing your FEA to see what lateral/twisting rigidity is like. The other option would be to have the rails direct onto the front plate, and lift the carriages on a couple of bits of rectangular plate, if this leaves the front plate assy. too bendy, stick the stiffening bars onto the front plate instead of the back. My gut feeling tells me that getting the front plate stiff is more important as it's that that will be dangling down at maximum Z extension. It will be interesting to see what the FEA predicts for the different options.

PS do think about the locations of the fixings for both X and Z carriages - get them overlapping and you might end up with a wonderfully rigid assembly that it's impossible to assemble And also think about making sure there's a way of tramming the Z-axis (adjusting it so it's perpendicular to the bed)

Yes thank you for pointing the overlapping XZ carriages, this is something I tend to forget, I will try the FEA on your suggestion and post an update- thank you very much Voicecoil !

You then have to choose what measurements you need to make and instruments to buy in order to set those adjustments correctly. There are a lot of worms in the can but you learn a lot too.

Yes, just waiting for some of those instruments to arrive, got a surface plate, square, dial indicator/ test indicator and some 123 blocks I think that should have me covered

Can't beat experience and to get the experience you need to get on with it so put the PC down and get the welder out.!!

I completely understand where you are coming from, I just like doing it- it adds to the enjoyment of the build process to me personally. I'm not trying to get others to do it or recommend it as a technique to anyone else, it's just my personal preference.

Your concentrating too much on deflection and not thinking about resonant vibrations which are the real killers of tools and finish. That Z-axis shown will resonate because the plate is too thin with a large gap between the rails acting like a trampoline.

Going back to the 50mm box section then I can tell you from experience building machines that it's not the best choice unless you use at least 6mm wall thickness and good bracing. The rectangular tube is much better when used in the correct direction and braced correctly, however you still need a thick wall to help with vibrations.

The simple FEA you are doing won't tell you anything about how the machine will perform in real life. You would be better advised to drop the FEA and put the time into reading builds and asking questions about any designs you like. By that, I mean directly asking the builder and not posting a thread asking the questions about a particular design as you'll get lots of conflicting advice and often from people with little experience or worse armchair experience.

It's not a one stop solution to everything and I'm not going to treat it as such, but I do think comparing the deltas between different design choices is useful and interesting to do. It is also useful in pointing out the best candidates for bracing, this may be something that is second nature to someone like yourself and many others but for me, seeing this visualised is very helpful.

I know vibrations will something that needs to be compensated for, I appreciate 6mm would be better than 3mm and 8 mm would be than 6mm etc- but I have what I have and need to make the best use of it. I enjoy the flexibiity that CAD gives you in terms of experimentation and will try to combine it with the vast array of insight in the forum.

when used in the correct direction

Would you mind expanding on this please ? Thank you

Personal view here, but based on using my own machine for the last 2-3 years. It's described somewhere on this forum - search for "AVOR" and my username if you want to look it up. It's built mainly of 50x50x3 with some 100x50x3. Steel, all welded, various bits and pieces from 20mm Ecocast aluminium. I suspect that if you did a static FEA job on it, it would come out as being pretty stiff (for some definition of "stiff", anyway). It's not "Boyan bullet-proof" but it isn't bad. That 3mm box is strong enough - if well-braced. If I were doing it again, would I go the same route? No - it's strong enough but resonance is always a problem, and that's not something you can easily fix after the event. I should have gone for 5mm, I reckon, although there's not much else I would change. That would have meant that I could drill and tap directly into it for fixings instead of needing reinforcing strips or other workarounds.

So, what can t cut? Can it cut aluminium? Well, yes, probably, but it's horrible stuff to cut and with a plywood bed, I don't want to do "wet" cutting so I try to avoid it. Of course, it cuts wood/ply/mdf etc with no problem. But it also cuts - regularly - brass and steel. I use small (typically 2-4mm) carbide cutters which are happy to run at high speed and cut dry with no problem. No, I'm not carving engine blocks from large lumps of steel but I cut lots of fiddly little bits which would be difficult to do on the manual mill because of the detail needed - like a brass lost wax master for a lapel badge. One limiting factor is not machine strength or spindle capability but just work-holding - one area that I would revisit another time around is how to build a bed with clamping capability for the kinds of things I now do, as opposed to what I thought I would do when I first built it. Difficulty in just holding things firmly limits cutting forces you can apply.

Thank you Neal, I also believe your CNC machine is larger than mine ? my cutting area is 3040 with a 600x600 frame, so I'm thinking the relatively smaller size works in my favour. I believe the thicker material gives you a bigger operating window and the thinner stuff means your pocket is smaller and probably slower and I am OK with that.