Effect of Forces on Gantry Cross-Sections?

[WandrinAndy]

How about using components from my yellow and your green, tada. I added a 60x45mm extrusion in between the top and bottom extrusions. These can all be bolted through from the top and also screwed into the 10mm back plate. Now your worry about the weaker wider section is dealt with, plus that extra piece for 800mm length is only £12, sweet.

Thanks Adil, that looks incredibly strong in all directions... my gutfeel is it's even stronger than GREEN!

Jazz's GREEN looks by far the simplest to build and has better accessability for changing belts and pulleys... certainly if this is to be done within the gantry ends... as my current limited space dictates a compact design. And it moves the ballscrew away from the mess of the cutting chips, but also away from the profile-rails.

Your YELLOW looks the strongest, and brings the ballscrew between the profile-rails, but also closer to the mess of the cutting chips.

Anyway, my beddy beckons and tomorrow is but another day to decide....

[WandrinAndy]

Not trying to sell it to you but I know it works with no detrimental side affects and plenty of positives.!!

Well, you have sold GREEN to me Jazz... even without trying! I have updated my design and am moving on, although I'm still curious about how the thicker-walled box sections compare, and the effect this may have on the machine.

For instance if Jonathan were to build a new machine and dosh wasn't a limitation, I wonder if he would still use box section, what size it would be, and why?

Thanks for the ideas guys.

[JAZZCNC]

For instance if Jonathan were to build a new machine and dosh wasn't a limitation, I wonder if he would still use box section, what size it would be, and why?

If NO DOSH limit then I'm 99.9% confident he wouldn't and it would be a hefty lump of ground cast Iron or some fancy composite affair.!!

Can't hide from the fact box section sings like a canary and resonance and vibration are to be avoided so density wins every time, that's why cast iron's used so much.

This is the kind of thing you'd buy(New) if Dosh no limit.!!! . . . .
Here's the Y Axis carriage(pic1) which carries the Z axis(pic2) of my future 5 axis toy(Doubt I'll touch it this year thou.!).!! . . . It's one big cast iron lump with a counter balanced Z axis which is very long lump of cast and ground aluminium that runs along the roof of the main machine frame which is one big structure of steel and ground cast Iron.. . . Very very very strong and heavy 3.8ton the frame weighs.!!

[WandrinAndy]

Very very very strong and heavy 3.8ton the frame weighs.!!

Nah... doesn't match my needs... my touring caravan frame wouldn't handle it... ;- )

[JAZZCNC]

Nah... doesn't match my needs... my touring caravan frame wouldn't handle it... ;- )

You could live in the control cabinet put wheels on it and it's touring CNC machine. .

[Jonathan]

For instance if Jonathan were to build a new machine and dosh wasn't a limitation, I wonder if he would still use box section, what size it would be, and why?

The simple answer is wait and see ;)

That is an extremely difficult question to answer since it depends primarily on the intended purpose and size of the machine. If the machine was required to do everything I could ever want, i.e. 5-axis able to cut most metals, then I would consider a steel and epoxy granite construction since it offers great versatility and strength in the design. Next step down perhaps would be a gantry made from aluminium plate. Neither of these would follow a style remotely similar to what has been posted. If the machine was only ever intended to cut woods and plastics with a short Z-axis then I would use multiple pieces of steel box section for the gantry, after appropriate FEA to ascertain and suppress potential resonance issues. None of this really helps you though since your budget is regrettably finite, so I'll leave it at that.

Jonathan's gantry uses 80x80 box section probably 3-4mm wall thickness. But as he says in other posts, the ball screw is not in an ideal location, the distance from the ball screw to the cutter is too far.

Mine is 80x80x3mm since that was cheap at the time. It is adequate since the SBR20 round rails introduce a far greater error than deflection of the box section. With profile rails then something stronger is warranted.

I would discount purple and blue due to the potential problems with accurately mounting the rails on material that may not be made to sufficient tolerances to maintain an accurate rail spacing. Additionally the ballscrew position is sub-optimal on these, since there is a horizontal distance between the ballscrew an the rails which allows the bearing blocks to move (slightly since they are mounted on something that is not perfectly stiff) without the ballnut moving. This is a significant limitation on my machine, made more significant by presently not having profile rails on the Y-axis.

It's harder to say between green and red, in my opinion they are both fit for purpose. I'll post more about them later.

Edit: In fact, if you send me or post the drawings (ideally just the cross section as a .dxf) I will run a FE stress analysis simulation since as irving has pointed out it would be good to get some idea of the torsional stiffness and calculate the second moment of area.

[WandrinAndy]

Edit: In fact, if you send me or post the drawings (ideally just the cross section as a .dxf) I will run a FE stress analysis simulation since as irving has pointed out it would be good to get some idea of the torsional stiffness and calculate the second moment of area.

Thanks Jonathan.... Interesting as always.

Second-moment-of-area is beyond me, but thankfully I probably only need to understand that it's different to having a senior moment.

Afraid I currently only have SketchUp, where one can do a section plane if this would help, but haven't used it before so cannot promise... I've just recovered the files as is from the recycle-bin, let me know if it would be worthwhile me sending or posting them... they're 300-600KB each (Green and Red?). Would like to see the results for Adil's most recent Yellow too if that's possible.

[Jonathan]

let me know if it would be worthwhile me sending or posting them... they're 300-600KB each (Green and Red?). Would like to see the results for Adil's most recent Yellow too if that's possible.

Yes I should be able to work from those.

[WandrinAndy]

They are invalid file types for this forum so have uploaded them to Models by MacTavish - 3D Warehouse Search

[Iwant1]

Would like to see the results for Adil's most recent Yellow too if that's possible.

If Jonathan doesn't mind I will send him a section of my yellow design.

I'll send two, my first yellow one which I designed how Jazz advised in your build where I have the z axis to the right.


And a newer one I've just made which is very similar to your green 'L' but a 'C' shape. This allows us to fix the Y axis motor mount easily and also have a surface to run energy chain. Have a look remember the z axis is now towards the left, and the ali plate is removed. I wonder if having this plate will increase its strength?