CNC Plasma Motor Selection Design Guide

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03 Jan 2020 16:09 - 07 May 2020 00:32 #153900 by thefabricator03
Hi Guys,

Thought I would put this together to help anyone looking for help on selecting motors for their CNC plasma.

So over the last couple of months I have been looking for cost effective motors & drives for my large CNC plasma machine. I originally brought a complete package from CandCNC and got some DC servos that were suppose to be good for heavy gantrys. Had alot of issues with the CandCNC setup so I decided to ditch it and go with Linuxcnc to control my machine. I decided to use the Gecko 320X drive from my CandCNC controller and I could not get the fault circuit to work.I looked at other DC servo drives but could not find one that had the features I needed.

So I started looking for motors and drives as a set. I came across these - oceancontrols.com.au/MOT-184.html - They are Nema 34 closed loop steppers. I paired them with these drives - oceancontrols.com.au/SMC-184.html

My main concern with Steppers vs Servos is the exponentially decaying torque curve. So to prove to myself that these were going to work I used the following calculations:

First the fundamental basic calculation,
F = MA
Force ( in Newtons) = Mass( in KG) * Acceleration ( in m/sec^2)

I know my gantry weights 140kg and the minimum recommended acceleration by Hypertherm is 40 milliG
1G = 9.81 m/sec^2
9.81 m/sec^2 / 1000 = 0.00981 (1 milliG in m/sec^2)
40 milliG  = 0.3924 m/sec^2
Therefore
F = 140*0.3924
F = 54.963 N

So the force needed to move the gantry is 54.963 Newtons - this is ignoring friction.

But we need to know the force needed in Newton Meters to get a basic idea for sizing motors so we need to know how the force will be transmitted.

In my case I am using a planetary gearbox with a 10:1 reduction ratio and a rack and pinion transmission.
My pinion gear according to the manufacturer has a Diametral pitch of 47.4mm

So to calcuate force in Nm we need to times the force in Newtons by the radius of the pinion gear in meters.
Force in Newtons * (Diameter of pinion / 2 / 1000) 
54.963 * (47.4/2/1000)
54.963*0.0237
1.30 Nm

So we now know that to move the gantry at the minimum recommended acceleration speed we need 1.30 Nm of torque. For my machines Y axis, it is duel drive so that number gets divided by 2. So that number now is 0.65 Nm of torque for one motor on the Y axis to move the gantry.

But there is more to the story than simply working out the torque needed. We need to consider the type of motor to be used and any gearing.

So with my machine I am using 10:1 planetary gear box on the duel drive Y axis and the X axis. With gearing it will multiply the torque coming out of the motor by the gear ratio. So my motors 8 Nm rating will become 80 Nm of torque. Sounds like over kill - not so fast. Steppers have a exponentially decaying torque curve.



So the stepper motors that are suppose to be rated at 8 Nm (really 7 Nm if you go by the chart) but they only have that at very low RPM. As RPM increases the torque decreases ( Note: servo motors dont have this problem - my DC servos were rated at 1.6Nm but the torque curve was flat)

At 2400 RPM the motors are rated at 0.5 Nm * Gear ratio = 5 Nm which is still above the 0.65 Nm needed to move the gantry.

But what if we want to know the torque at a given RPM?

Refering to my Hypertherm cut charts in my user manual - for me to cut 0.5 mm thick mild steel at production setting I need to be able to move the machine at 12510 mm/m.

So to work out what RPM the motors will be at for those speeds, you need to work out the circumference of the pinion gear.
47.4*Pi
47.4*3.1415
148.9071 mm

For one revolution of the pinion gear the gantry will move 149 mm, but we need to divide this number by the gear reduction ratio to get the gantry movement per one motor revolution.
149/10
14.9 mm
Now to calculate the RPM for a given speed, we need the required speed divided by the gantry movement per motor revolution.
12510/14.9
839.597

So for my machine to run at 12510 mm/m the motors need to spin at 840 RPM. Looking at the torque curve chart that puts them at around 2.0 Nm - with gearing this comes out at around 20 Nm - Still way more than required.

One other point for those trying to size the rack needed or who want to calculate theoretical acceleration - the force at the rack is proportional to the torque at the pinion divided by the pinion radius in meters, in my case
Torque at 840 RPM = 20Nm 
Pinion Radius in M = 0.0237
20/0.0237
843.881 Newtons

Now once we have force at the rack in Newtons we can then calculate theoretical acceleration using basic algebra,
F = MA
Rearrange to make A the subject
F/M = MA/M
A = F/M
A = 843.881/140
A = 6.0277 m/sec^2


Now these calculations are very basic - we are omitting friction and mechanical losses. But for 95% of people who will read this you will not need to worry about those things unless you are cutting your motor sizing very fine. Looking at these numbers I could of used smaller motors but I like to over do things.


After reading all of that I have attached the easy way to do the calculations to this post - A memeber of CNC Zone forums named "thegolfer" has done the hard work and created a excel spread sheet that is far more accurate than the above calculations. But when in doubt the above will get you 90% of the way there. Also attached is a list of calculations by the same author which is a bit more technical. Also the excel spread sheet has allowances for those using ball screws.

In the end the closed loop stepper motors are working very well on my machine. I have cut some small artwork with lots of small sharp cuts and details and the motors ran with no issues. I am very happy with the outcome.
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Last edit: 07 May 2020 00:32 by thefabricator03.
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03 Jan 2020 17:22 - 03 Jan 2020 17:23 #153907 by thefabricator03
Last edit: 03 Jan 2020 17:23 by thefabricator03.

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03 Jan 2020 17:32 #153908 by tommylight
Nice, thank you.
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03 Jan 2020 17:50 - 04 Jan 2020 02:39 #153913 by thefabricator03
No worries Tom, I am happy to give back to the community that has given me so much.
Last edit: 04 Jan 2020 02:39 by thefabricator03.
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03 Jan 2020 20:10 #153922 by rodw
Very interesting Stefan, so it looks like there needs to be yet another chapter in the plasma primer!
It would be cool if AgentWD40 made another online calculator

It would be interesting if you ran your own numbers based on a gantry weight of say 30 kg which is where you can get the weight down to to see how much of a penalty your heavy gantry design is impacting.

Acceleration is king for plasma. The interesting thing I found was to do a few what ifs in the spreadsheet and reduce the acceleration time from your 0.5 sec but stay inside the motors torque curve. If you convert m/sec^2 to mm/sec^2, you should be able to use this result in your INI file. It sure beats all the trial and error I did!

Also, when I dug up my torque curve on the motors I used on the gantry, what really stood out is how much torque I have left on the table by running these motors at 48 volts, not 80 volts. Look at the difference in the 40v and 80v curves. I'm running these motors at about 950 rpm top speed.

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04 Jan 2020 02:09 #153941 by thefabricator03
Thanks Rod,

I am open to putting this information anywhere it will help people the most - including it in the plasma primer makes the most logical sense. The spreadsheet by thegolfer is really a good resource for anyone building a plasma or a router for that matter.

I agree that acceleration is king with plasma machines - the main thing I was worried about when I brought these steppers to replace the DC servos. I have to say though that I am very happy with the results I am getting with these motors. But I would try to make the gantry lighter if I was to re-design the machine.

And yeah I did not mention that but my toroidal power supply is rated at 80VDC so I am using the top end of the curve. I do find it interesting that with steppers you can get more performance by applying more voltage.
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