Effect of Forces on Gantry Cross-Sections?

[Iwant1]

How come you didn't make a drawing of how JAZZCNC quoted in your build log.

The 45x90 will be better but I'd put it top & bottom and have the wide bit flat.
This way it ties into the sides better and gives more resistance to bending in lateral cutting directions.,

I've knocked up a drawing of this and also think this is a great way to go.

1. It gives a nice straight guide to fix the profiled rails onto.
2. Using the BK/BF blocks as spacers between the top and bottom extrusions, will help the profile rails being parallel with each other.
3. Makes a nice void for the ball screw to hide in and keeping the z axis close to the extrusion.
4. Better fixing to the gantry side plates by having 4 bolts in both extrusion ends, 8 for both ends.

Cheers

[WandrinAndy]

How come you didn't make a drawing of how JAZZCNC quoted in your build log. I've knocked up a drawing of this and also think this is a great way to go.

I don't think that's what Jazz was suggesting Adil... If anything I think having the two 45x90's flat like you've drawn would be lots weaker towards the middle of the gantry than the 45x45 which I was going to go for... similar to your original design... because the profile rails are so far removed from the back plate!

I think Jazz favours the GREEN option for an initial DIY build which I'm now thinking is a far better option that what I designed.... I was thinking of using the GREEN at one stage but somehow dropped it and went off on a tangent after seeing your design pics. lol

My thoughts are constantly evolving, and while I am looking at changing things, may as well look at the box-sections too, but I really need to understand things, hence the reason for this thread.

I suspect that the PURPLE is too thin, so have added another option which I've attached below.

ORANGE - 127x51x6.35mm

[JAZZCNC]

I don't think that's what Jazz was suggesting Adil...

Adil's correct that is what I meant in that post. It will be hardly any weaker in the middle over this width but will add far better strength and rigidity to the gantry than 45x45 would in the lateral directions.

Your also correct that I favor the green design but didn't want to get into that in your previous post because it's slight different design.

I don't like box section for gantry's has it can produce resonance and vibration unless thick walled.? The bolted together extrusions are far denser therefore absorb vibrations far better.!! . . .Any resonance will transfer thru to the finish.

[WandrinAndy]

Adil's correct that is what I meant in that post. It will be hardly any weaker in the middle over this width but will add far better strength and rigidity to the gantry than 45x45 would in the lateral directions.

Your also correct that I favor the green design but didn't want to get into that in your previous post because it's slight different design.

I don't like box section for gantry's has it can produce resonance and vibration unless thick walled.? The bolted together extrusions are far denser therefore absorb vibrations far better.!! . . .Any resonance will transfer thru to the finish.

Thanks Jazz and apologies for doubting your interpretation Adil. Afraid I still have nagging doubts though about the vertical strength of those sections lying flat like that, and reckon there must be extra sag in the middle when compared to the GREEN design.

Getting back to this thread, how would the design Adil listed compare to the GREEN design in terms of rigidity?

I wonder what size box section Jonathan's gantry is and particularly what wall thickness?

[Iwant1]

I wonder what size box section Jonathan's gantry is and particularly what wall thickness?

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.

[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.

[JAZZCNC]

Thanks Jazz and apologies for doubting your interpretation Adil. Afraid I still have nagging doubts though about the vertical strength of those sections lying flat like that, and reckon there must be extra sag in the middle when compared to the GREEN design.

Obviously there will be some tiny tiny difference but when you have some profile in your hands you'll realise why it's not a problem.!! . . . . The difference you would struggle to measure with a dial gauge and I'm 99.9% sure you have far more pressing problems ahead to overcome than if the profile sag's 0.001mm more than 45x45.??
The extra lateral strength and rigidity the extra bolting into gantry sides allows will far far outway the very very minute difference any sag will make.

[JAZZCNC]

I've built machines using the green design and please believe me you have nothing to fear the strength is easily over the top for anything but steel or harder.

If you use Bk/BF blocks then they line straight up with slots so easy fitting. It offers good strong attachment to gantry sides.
Yes it places the ball-screw slightly further back than what's considered ideal but again it's nothing and doesn't make any difference to how machine performs. The positive is the screw is away from the firing line and protected.

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

[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.