[Beefy]

Like a few others without the space / cash, etc to build separate plasma and router tables, I'm building my new gantry for rigidity and strength to also handle some basic routing. Lets get silly and assume the gantry could end up as heavy as 200 pounds (I prefer to think worst case scenario and prepare for it).

I'll be using dual rack and pinion and if necessary I'd be happy to have dual 900 or even dual 1200 steppers. However, do larger steppers restrict you with their top speeds and direction change and acceleration abilities.

Has anyone built a heavy gantry for plasma cutting and are just using brute force to get the necessary acceleraton and speed for plasma cutting.

Would big steppers be OK or are there reasons why servos are better with a heavy gantry doing plasma.

Thanks,

Keith.

[will_eng]

As you have seen I am researching the same cause. I am finding if you look at the torque curves of the motor running in series vs parallel the slope will be shallower for parallel.
So a smaller motor running at the same amperage in parallel than a larger motor in series will have higher torque at running speed.

To maintain a constant acceleration requires a constant torque through the range of rpm because of the inertia of the gantry.

Since stepper motors lose torque with speed the actual obtainable acceleration is not constant and decays as speed increases.

Now my issue is that Mach3 will only allow a constant acceleration value. So you have to pick you worst case torque value which will be one at higher rpm.

I am working on a calculator to look at this more closely in my other thread.

[jimcolt]

All of the high end industrial (read high dollar) cnc plasma machines use servos. Properly matched servos (inertia matching to the size of the machine is key to best performance) provide the speed range necessary for plasma (10 inches per minute to about 450 inches per minute) and maintain a decent level of torque throughout that range....this speed/torque curve allows for decent acceleration throughout the range as well.

Many industrial machines have gantries that weigh in the 1000 lb (or more) range (multiple plasma's, bevellers, oxy-fuel torches all mounted) yet can still accelerate to over 400 inches per minute at up to 40 miligees. 40 miligees is relatively easy at 50 inches per minute, but hard to do at both 50 and over 400 inches per minute........this is the primary reason that steppers are not used on large, precision machines.

I know I will hear some arguments against servos in favor of steppers. Steppers are cheaper, easier (electrically) and accurate (assuming they do not miss steps)....Servos are more expensive, more complex, but have better speed/torque characteristics along with accuracy ensured by a closed loop encoder feedback system.


Jim Colt

Like a few others without the space / cash, etc to build separate plasma and router tables, I'm building my new gantry for rigidity and strength to also handle some basic routing. Lets get silly and assume the gantry could end up as heavy as 200 pounds (I prefer to think worst case scenario and prepare for it).

I'll be using dual rack and pinion and if necessary I'd be happy to have dual 900 or even dual 1200 steppers. However, do larger steppers restrict you with their top speeds and direction change and acceleration abilities.

Has anyone built a heavy gantry for plasma cutting and are just using brute force to get the necessary acceleraton and speed for plasma cutting.

Would big steppers be OK or are there reasons why servos are better with a heavy gantry doing plasma.

Thanks,

Keith.

[Beefy]

Thanks very much for the input guys.

Looks like the ideal is servos for heavier machines. Been looking at the extra cost to set myself up with these as opposed to just getting a couple of bigger steppers, and I'm looking at 4 or 5 times the price. Gearboxes, new drives, power supply, etc get added to the equation.

SO my question now is does anyone know if dual 1200 oz steppers would suffice for a gantry weighing up to 200 lb and give the required acceleration and speed for plasma cutting. Reduction ratio of 1 stepper revolution = 1 inch of gantry travel.

Thanks,

Keith.

[will_eng]

I posted a shot of the calculations I came up with in my “How fast do you accelerate your Gantry” thread.
I will run the numbers for your gantry.
What top speed, accel, do you want? Also any other limiting factors? Can you run series, parallel, max amps/phase, pinion rad…I will let the calculation select what ever you are not limited by.

[Torchhead]

The 960 oz-in with the 1" to ! per rev drive ratio will be enough to handle the 200 lb gantry and good acceleration (35 IPS/sec) up to about 200 to 250 IPM. If you can run the motors at higher voltage you get more RPM (but not more torque). It does afford you the opportunity to change the drive ratio some so you still have sufficient midrange torque for plasma and get more low end torque for contact cutting. A ratio of about .75 linear motion per motor rev buys you lower end torque.

servos do offer a flat torque curve and variable torque up to their peak that can get you out of some messes BUT servos and plasma need some good engineering to keep noise out of the controls and is not as easy to do on a home built design.

now if you start upping the weight to 300 or 400 lbs you are going to run out of motor in the stepper world. the answer with steppers is not always a bigger motor. In a lot of cases you will find that bigger motors have shorter torque curves so upper RPM characteristics are not much better that a smaller motor. At the low end ...yes but that may not be the optimum speed you want to cut plasma at.

For every pound you can reduce the gantry weight you will gain acceleration.

TOMcaudle
www.CandCNC.com

[Beefy]

Thanks Tom,

that answer is just perfect and explains what I need to know, much appreciated.

Not sure what you mean by home built, but if you mean the electronic system I should be OK (I'm using your MP3000-DTHC and all cabling is screened twisted pair, even the stepper power cables). Next time will be one of your complete kits thoughs, complete with motors, power supply, etc. If only I could go back in time.

Keith

[Beefy]

Hey Will-Eng,

much appreciate your offer of calculations but at present I'm just looking for others actual experience / knowledge of a setup like mine, and yes it will or no it won't work type of answers.

However, if it may be of any help, I've thought of something else you may want to factor into your calculations. If a gantry is using dual motors, one on each side, then the total weight of the gantry divided by 2 may not be very accurate for calculating the torque requirements per motor.

When the z-axis is fully to one side of the gantry then you could almost add the complete weight of the z-axis to the weight of half the gantry then use that figure as the weight per motor. Do you get what I'm saying. When the z-axis is right over at side A for instance then motor A is moving its half of the gantry weight + the z-axis weight (or at least a large portion of the z-axis weight). The z-axis weight will only be equally distributed when it's in the centre of the gantry.

Makes things a bit more complicated because you need to know the individual weights of the gantry and the z-axis.

Please tell me to shut up if you've already factored that in.

Cheers,

Keith.