Gantry design and FEA analysis

[routercnc]

Glad you found the CNC videos useful. It was a good way to pass on the ideas I had seen, and add some of my own thoughts.

In the analysis:
Yes, vertical direction forces are mostly due to gravity but they will be much higher than 100 N as this only represents a Y and Z assembly of about 10 kg. More like 25-50 kg depending on the design, possibly more. This is to see how much the gantry will sag if you are surfacing a plate as in the extreme you would cut a dished shape. There is also a pull down force when using spiral fluted cutters, and there will be forces when drilling or plunging.

The lateral direction analysis is correct for the load direction but because of the grounded sides in your simple analysis it won't mean much on the results you have there.

The fore/aft analysis is in the wrong direction - it would be in the 'Z' direction using the coordinate system you have in the top right corner and would cause the part of the Z axis plate which is hanging down to all bend forward or rearward, and cause the gantry to twist about the 'X' axis as per your coordinates in the picture. This is usually the worst case of all the load conditions.

There are lots of square and rectangular gantries out there but the L shapes gantries were 'invented' by JazzCNC:

[Boyan Silyavski]

Combine both and that's what you will have if you do not design properly the machine.

If you design your Z properly , use proper bearings and mount your spindle correctly, variant 2 will be non existent, so variant 1 is the real deal.
As if you clamp your spindl properly its body will strengthen the Z and only a couple of braces will make the Z equivalent to like a solid chiung of 100x100mm metal


Here is some design wisdom to you: it does not matter what profile you use. A well designed machine will have such design as to simulate at least 10cm wide x 3cm thick steel plate against all cutting directions. Against the 3cm no the 10cm. Or the equivalent. in whatever material/s you do it.

So go and make it 50x50mm but remember what i said if you want your machine to cut vibration free aluminum or even steel.

Check my build at page 7 for the gantry and page 16 and 17 for the Z, to grasp the idea of how serios a gantry and a Z have to be to machine fully extended at 200mm.

Its not only my machine i am bragging about. There at least a couple machines on forum that are seriously heavy duty and one way or another they are build like that.

[Voicecoil]

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)

[JAZZCNC]

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.

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.!!

[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.

[JAZZCNC]

Would you mind expanding on this, please? Thank you

Well, it's strongest when used on its edge, so for instance if fastening rails onto it you wouldn't use it flat unless it's supported from below. When properly used and supported it's very strong. Hence why my "L" shape gantry is works so well because together 2 pieces of rectangle form a very strong gantry in both directions but still allow reasonably lightweight Gantry that is stiff and dampens vibrations.

Now don't get me wrong I'm not saying don't play with FEA or design in CAD.!!... ALL... my machines are designed in Solid works down to the very last detail and done FEA on some of them in some key areas just for my own interests to see if it matches the real thing. And even with the detail, I've model them to they rarely match with anything I can see or measure.

Just pick a design that suits your needs and get on with it.!! . . . . If needs to be stronger get the welder back out.! . . . Or start Mk2 because I can tell you know no matter how well you model the design and try to think things through there will always be things you'd do differently the next time around.!

[Neale]

Or start Mk2 because I can tell you know no matter how well you model the design and try to think things through there will always be things you'd do differently the next time around.!

Now, ain't that just the truth!

[Boyan Silyavski]

You don't like to hear it and i would not say it again, but one last time for the benefit of all people who will read that in the future:

-50x50 is not right for a machine that will mill aluminum, 80x80 or 100x100 3mm is the right profile to form the gantry and the sides or the whole frame

-saving on box section size=5 times more cutting and welding diagonal braces, where if 80x80 or 100x100 there is no need for diagonals and simpler construction

-there is no such thing as 0.1- 0.3mm accuracy, especially cutting aluminum. If the machine is made right it will be repeatable 0.05mm and accuracy will be ~0.01- 0.02mm
0.3mm accuracy is for MDF machines or crappy cheap machines, in other words trash that will not cut aluminum at all.

-Modeling wil get you to nowhere, only wasting your time. Check what people build on forum, choose a build, see the results and imitate it. Then you will be successful. Make a build log, ask questions and listen to them answers..

-use common sense, if you lack it, there is no software to help that. Go to a local welder and find a 2m piece of that 50x50 profile, 80x80, 100x100, touch it , try to bend it then for 1 min everything will become clear to you. 3m 100x100 is flexible and 50x50 is like a macaroni

-every crappy machine can cut aluminum, scratching or slow, but the better the machine the most perfect the finish will be.

-if on a build of 60x90x20 for example machine you think that 10cm more or less is a great thing, better not start it. 10cm of rail and steel more or less in abuild are no more than 100-150e, and if you dont have 3k for parts and 500-800 for tooling at least, you are in the wrong place.

[JAZZCNC]

-use common sense, if you lack it, there is no software to help that.

Boyan that's a Class statement and 100% spot on.!! . . . . . That's going to be on my Tag line..

[Neale]

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.

I'm also in the "just put something plausible together, listen to comments based on experience, then fire up the angle grinder/welder" camp. The time you spend analysing you could cut and weld a couple more braces...