Effect of Forces on Gantry Cross-Sections?

[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.

[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.

[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?

[wilfy]

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.

am i reading this right that mounting on steel box section is a no no?? if so how big of a problem are we talking here?

[Jonathan]

I will have a go at the analysis tomorrow evening and report back...approximately what distance do you expect between the tool tip and the bottom of the gantry?

am i reading this right that mounting on steel box section is a no no?? if so how big of a problem are we talking here?

Yes, mounting on opposite sides of steel box section is a no-no since the distance between those sides when the steel is manufactured has a much greater tolerance than for aluminium. This means that there is a high probability of the dimension being sufficiently even for the bearings to run smoothly upon the rails. Profile rails especially tolerate very little misalignment, so it is important to ensure they are mounted accurately to avoid premature failure due high stress.

[WandrinAndy]

...approximately what distance do you expect between the tool tip and the bottom of the gantry?

Could you use some hypothetical distance at this stage... I'm more interested in a general comparison and haven't got to the Z-axis of Mk1 design yet and things may well change if the recent past is a good yardstick... lol

Are you able to volunteer a distance you would like to be used Adil?

[JAZZCNC]

am i reading this right that mounting on steel box section is a no no?? if so how big of a problem are we talking here?

Depends which world you live in.? The theoretical or the real.!! . . . . . FFS wish folks would stop over complicating things.!!

Here's the real world deal.!! . . . Irving is absolutly correct that unless you can surface the two sides parallel and flat then you can't be 100% sure every things correct.! . . . BUT . . . This is true in one way or another for ALL the surfaces the rails mount onto whether they be 1 rail on top & 1 on front, or both on front etc and unless you can put these surfaces complete and assembled onto a milling machine and accurately machine all surfaces flat and parallel or perpendicular to each other then you are going to have to accept they will need some adjustment or tweaking.!!

How you do this will depend on the design But having the rails on 2 separate sections, no matter if that be profile, boxsection, ali plate will make very little difference and It's FACT that some shimming or filing or grinding will be needed to bring the rails into perfect alignment and parallelism either way.!

Now I'm not going to blow smoke up your arse's saying this is easy and quick because it's not and doing this takes patience and a lot of measuring and masses of time if you want good accuracy.!. . . BUT . . . If your just cutting wood,foam and bit of plastics then even box section will be close and with slight bit of clean up and shimming you'll be there in no time.!

I know it's hard if you haven't hands on experience of the forces cnc impart on frames and accuracy required but really for the application of mainly wood use then any of the designs shown here would be plenty strong enough.!! . . . For real meaningful Aluminium and steel work then all of them are not really up to it.!!

Don't fry your noddles trying to create the perfect flex free gantry for cutting woods etc because your just wasting braincells and time when it's not needed.!

[Iwant1]

. . . If your just cutting wood,foam and bit of plastics then any of the designs shown here would be plenty strong enough.!! . . . For real meaningful Aluminium and steel work then all of them are not really up to it.!!

Don't fry your noddles trying to create the perfect flex free gantry for cutting woods etc because your just wasting braincells and time when it's not needed.!

Thanks for the comforting post Jazz. Since this thread started, I've been thinking I got my machine drawings all wrong, and thinking about it day and night. I'm building my machine for general use and for a multitude of materials. I definitely won't be cutting steel, but would like to know it could do the odd bit of aluminium such as the 'Aztec calender', lol.

Is there any chance we can see your machine's gantry or get a description, as I know you mostly work with aluminium.

Thanks