How heavy of a gantry can I build for a 425oz stepper?

I was wondering about this I don't want to build it too heavy but I want it to be sturdy and rigid? Would appreciate some input thanks guys!

There's no real answer for this until you specify the screws you plan to use (assuming you're using screws to drive this machine). That will determine the mechanical advantage you get, so that you can figure out what it takes to reverse the direction of your gantry without losing any steps. More threads gives you more torque, but less speed. Ball-screws take less torque to spin than conventional screws. Also, you can use two motors to drive the gantry, which avoids the "stuck drawer" effect and doubles the available torque. If you want to build a really heavy gantry, you can make it a fixed "bridge" design, and move the table instead. Of course, then the weight will vary, depending on what you load onto the table.

Andrew Werby
www.computersculpture.com

There is software available on some of the stepper motor and servo motor manufacturers websites which will help you with this question.

I'm guessing you don't have much experience building and operating a CNC machine yet, but you will by the time you are done. Things that factor into your question are how fast do you expect to be able to travel (velocity), and how fast you want to be able to change velocity at (acceleration). Slow velocities and accelerations mean you will be able to build your gantry heavier but your cut times will increase. Faster accelerations and velocities reduce cut times but increase stress to frames and components.

One thing to keep in mind is it is beneficial to keep all axis' fairly close to the same accel ad velocity values as one weak link will limit the others during moves that involve that axis.

Your stepper motor should have a chart available for toque over RPM. Once you figure out things like your desired acceleration, your gearing and how much mass you plan on moving it's fairly simple to figure out the how fast your motor can push your gantry. It's been awhile since I did the calculations for my machine so I can't help you but a little bit of googling will fill in the blanks.

Conversely you could work backwards by taking your desired speed, acceleration, and gearing to figure out how heavy you can build your gantry.


If calculations aren't your strong suite it wouldn't hurt to check out other peoples builds who used similar sized steppers and see how massive their gantries were. That would at least give you a feel for how to design your gantry. I don't advocate this, but it is an option.

Thanks man, I really appreciate it. I have everything I need so far to build a complete machine, I'm gonna use 12mm all thread to start out and see how that works a while before I start spending money on acme or ball screws. I have rexroth linear bearings and rails for the build now that I bought cheap off a buddy of mine so that part should be very stable. I was thinking the first time around building the gantry outta angle iron kinda like a bridge or heavy overall beam that way it would be sturdy and lightweight but we will see. I'm gonna start the build on the gantry this week. Thanks for your guys input. I got plenty of steel and stuff laying around as we'll as some 80/20. Maybe ill build on out if both and see what I like the best

425oz motors tend to have very high inductance, and have less torque at higher rpm's than smaller motors. That makes them a very poor choice for driving allthread, which requires high rpm's to get good speed.

You will be able to move a lot of weight, but you probably won't be able to do it very fast.

Ok thanks ill see what happens

Ger is absolutely right about allthread and larger steppers being generally a poor combo.

I highly suggest you look at either ball screws or rack and pinion. Ball screws can be had fairly cheap on ebay, and rack and pinion can be ordered from industrial supply stores for surprisingly cheap. Driving a machine built with Rexroth linear rails would be similar to putting a Pinto motor in a Corvette. I should know, my first machine was built from sawmill wheels, I-Beam I found in the woods and half inch pitch chain and sprocket. It worked, but in the end it was a big waste of money. Sometimes it's better to shell out for the right stuff the first time! Just my two cents though, hope it helps. If you have any questions feel free to ask!

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