Effect of Forces on Gantry Cross-Sections?

[WandrinAndy]

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

[wilfy]

Ok but mounting rails on 2 smaller pieces of box section is ok I presume because you can make some adjustment??

[Jonathan]

Ok but mounting rails on 2 smaller pieces of box section is ok I presume because you can make some adjustment??

Yes, unless the box section is really bad - for instance if it has been welded it would distort somewhat.

[irving2008]

Ok but mounting rails on 2 smaller pieces of box section is ok I presume because you can make some adjustment??

there are two separate issues - parallelism of the oppsite sides of the box and flatness of each side of the box; neither are guaranteed in the manufacturing process. The flatness issue means you might have to shim the rail to keep it straight when you bolt it down.. even if you have 2 boxes you still have this problem. The only sensible solution is to surface grind or face both sides of the box section to make them flat and parallel.

[D-man]

Sorry bit off topic but JAZZ trying to get hold of you but pm's just bouncing back as your hoarding you must be collecting them lol

[JAZZCNC]

Sorry bit off topic but JAZZ trying to get hold of you but pm's just bouncing back as your hoarding you must be collecting them lol

Sorted mate.!!

[Ross77]

Bit late to the topic and most of it has already been discussed but here's my take on it

Forces on a gantry are difficult to judge because they can change direction and magnitude and are mainly dynamic. To get an ideaof the problem of dynamic forces look in to why Marching Armys have to break step when going over bridges! At a basic level most beams only have to deal with a force in one direction and as such have a larger dimension in that axis, hence rectangular beams. however gantry's are also subjected to twisting so need to be able to resist torsion. circular sections are best for this and square the next best. In comparison rectangular beams are not so good especially if the are slender. Therefore a square beam would be the best option.

To get an ideaof the problem of dynamic forces look in to why armys have to break step when going over bridges

Box vs Profile

A made up beam using tee slot extrusions will have an new 2nd moment of area but the calculation would assume that all the faces are bonded together to make one solid section. any movement in the bolted joints will reduce its torsional stability.

Box sections are deemed the most efficient as they have all their mass at the extreme edges but can be affected by resonance. this can be damped or designed out.

Extruded sections can have the same parallelism issues as box and I have seen builds that have milled the top of the extrusion to get the accuracy they need.


So as Jazz and so many others have said its horses for courses but it does seem easy to get carried away and dismiss simple solutions. Just using linear rails an ballscrews wont result in a precision machine, they have to be correctly mounted and tweaked so they work correctly. so why not use an easy option and extend this tweaking to the mounting structure as well.

I had a go at designing a composite beam a few years ago but not had time to progress it any further. Planned to use extruded Ali box and channel to made a sort of closed I beam that had a recess for the ball screw. Top and bottom sections filled with epoxy granite to provide damping and compressive strength.