Gantry Balancing

[Tautog13]

Hey folks,
So after lots of reading on this great forum, I have begun designing my own.....less than "original" 5'x10' plasma table. (plasma only)

I have not come across any discussion(s) about "balancing" the gantry. (that is not to say they do not exist)

Seems to me it would be best practice to try and "center" the weight(s) placed on/about the gantry. (realizing it is dynamic x/y/z)
So if trying to keep the over all gantry weight to absolute minimum, centering the weight as best one can on the gantry would result in less "top heavy" rotational force exerted (if weight too high), or if too low the same rotational "force" exerted. (make sense?)

I also realize setting a desired clearance above the slats will force the placement of the Z axis module/assembly.

So in the attached draft copy example, using CandCNC Super ZX module, by "centering" the Super ZX based on its travel of 5", the resultant gantry tube ends up at 12" above the table slats.
Although at present the motor driving the z axis is "higher" than the motor driving the x axis, it may be better balanced against the opposing Super ZX module.

Comments/Crits?

Gantry_Qs_01_26_2018.png

[smalltownpaul]

@ Tautog13
I agree with you on the lack of discussion in this matter. I also agree with your foresight into the forces at play when the gantry is in use. Many people forget how much force there is when changing direction as fast as possible, especially when the trolley is mid gantry. When I built my table I went out of my way to keep the motors lower than most and weight somewhat balanced... I still don't think I went far enough though. When I rebuild mine I'll put the Z motor horizontal just above the gantry tube. Another thing to keep in mind is that by making you gantry tube larger (not necessarily heavier) you can further reduce deflection. That is another change I'll be making when I rebuild.

Paul

[Tautog13]

@ Tautog13
When I rebuild mine I'll put the Z motor horizontal just above the gantry tube. Another thing to keep in mind is that by making you gantry tube larger (not necessarily heavier) you can further reduce deflection.

Paul

Thanks Paul.....all good info and thanks for sharing ideas.....I placed my X motor underneath the gantry tube today, centered and perpendicular to Z Axis Assembly, with a very light heat shield to protect the motor. (I think you meant X in above statement)
It all looks pretty well balanced now.
My software has a plug-in for center of gravity.....but I have only used it with same “mass” and haven’t gone to extent of sorting out densities etc. (& probably wont bother to that degree)

If anyone reading this knows the actual weight of the Hypertherm Powermax Machine Torch w/25’ cable (PN# 059477) I would greatly apreciate it. (I emailed Hypertherm Tech. & they were very quick to reply w/answer but only had shipping weight info @ 7.9 lbs)
Even better would be weight of torch & cable separately. (as entire cable doesn’t ride in gantry)

[Bobkovacs]

While I'm sure it has some effect, I'm not sure it's enough to spend a huge amount of time hyper-analyzing it. Unless you're cutting 26-gauge sheet at 400ipm all day, most cutting still takes place at a relatively slow pace so as long as your gantry and your linear motion systems are stiff enough that they won't deflect when direction changes, I don't know that I'd be overly worried about it. That's not to say that I'd mount the Y-axis gantry 4 feet above my water table, but an inch or two one way or the other isn't likely to hurt anything.

[Tautog13]

While I'm sure it has some effect, I'm not sure it's enough to spend a huge amount of time hyper-analyzing it................That's not to say that I'd mount the Y-axis gantry 4 feet above my water table, but an inch or two one way or the other isn't likely to hurt anything.

Thanks Bob.......I’m sure thee is a sweet spot in there somewhere.....and I have read enough to know plasma cutting has limitations not worthy of too much hyper analyzing to use your phrase. (Though I am very proficient w/cad software so its easier for me to anaylise)

My main goal is to shave as much weight from the gantry assembly as possible
(& maintain rigidity), hopefully end up with total Gantry weight at around 100lbs. even.
(The few designs I have researched come in around 150 lbs. (pretty much all steel construction/4x8 table)

Total Gantry Wt. = Side supports w/carriages & (2)motors + Horizontal X tube/beam w/motors/cables/torch & lead/X linear guide/carriage & gear rack/Z THC/cable track/(2)motors brackets/associated hardware/etc.

My current total as of writing is approx 110 lbs. (hence the Q about actual torch weight....my guess is the packaging is probably 1 lb. or so and I could probably deduct another 2lbs off for torch lead not on gantry)
I may still change the X linear guide and carriage to a slightly smaller one to save another few lbs.

Anyway, thanks for your reply, I greatly apreciate it.