[imitheo]

Hi guys, have anyone tried to run a CNC machine using high RPM on steppers. I wish to run using 1200 RPM, that seems like a lot to me.

But the deal is, it needs only the 1200 RPM for a really brief period, and only one time! It's a really specific application.

My concern is, i need to mill a part after this "sprint", BUT i can rehome the CNC machine, so i ask you guys, can i rain hell on the stepper for a brief period and then rehome to regain my home position?

Will some NEMA 34 hold agaisnt this 1200 RPM?
Will it lose much steps?
Can it be done in any software? This "routine"


A friend of mine told me of some "StepServo", like this: (I don't know how to hide the link)

https://pt.aliexpress.com/item/Free-...0-05b0c5764ebb

He says that can be operated at higher speeds without losing too much torque, is this true ? What is this step servo?

[Biggs427]

A stepper is a stepper; it will lose torque at higher speed.

I've seen high rpm on steppers, but only on nema 23, with 40+ volts PS and low inductance steppers. (Less than 2.0mH)

Why do you need to "over drive" your machine so much?

What are the specs of your, motor, drive, PS&

[imitheo]

It's a competition in my university envolving cnc mills. One of the tests is this, reach 12m/min on each axis.

But the cost counts on the overall game, so i don't want to go servo, i want to try using a cheaper option, like a step servo or stepper motor.

I was considering these options: Leadshine ES-M Series - Easy Servo Motors

We still don't have a motor of our choice.

[ger21]

Do you need to move a lot of weight?

A smaller, low inductance Nema 34 motor (±3Nm) running at higher voltage should easily achieve 1200rpm. Leadshine AM882's from Ebay are inexpensive, and will work.
But, it really depends on how much weight you need to move.

[imitheo]

I think the gantry will weight max 35kg, or 30kg.

[awerby]

All servo motors aren't super expensive. You can get a regular dual-shaft brushed DC motor and mount an optical encoder on the back shaft, transforming it into a servo. You can drive it with a G320 drive from Geckodrives using the same step-direction pulses as a stepper. If you really need those short bursts of speed, that might be the way to go.

[imitheo]

I would prefer to go with an "off-the-shelf" solution

[A_Camera]

It's a competition in my university envolving cnc mills. One of the tests is this, reach 12m/min on each axis.

Is that 12m/min on each or at the same time? Because that's quite different. Also, how fast the axis moves also depends on if it is direct drive, or using some sort of gearing or belt, ball screws or trapezoidal, and of course the screw pitch. I am using direct driven 1605 ball screws and NEMA 23 steppers. The drivers are driven with 43VDC and the maximum speed I can get is about 12m/min but that's the absolute maximum, so I run slower. Speed also depends on the rigidity of your machine. Mine is a moving table type, fixed gantry is easier to make and is considerably more rigid and can get the speed up then a moving gantry type, which is often limited because of the mass which is needed to be moved around and also the flexing and vibrations you get from the moving gantry type. So, if speed is important then a fixed gantry, moving table wins.

But also, speed is just one factor and maximum speed is not that important unless you can also get high enough acceleration AND milling speed and of course, accuracy and repeatability comes before any speed requirement. So if it is a university competition, they should also look at all those factors. It is totally pointless to drive fast if you are missing steps or can't repeat the moves.

[imitheo]

I am aware of this "pointless" issue, but the score goes to the team that reaches 12m/min. Everybody is probably going to servo it, but i'm using a Ø6 endmill, i'll not be able to use the 12m/min to mill of course, then my machine will actually mill it much slower.
That's the catch i'm trying to do, get the score by raining hell on a stepper, get a reduced cost and then feeding my rapid and G1 much slower(Because using it slower will garantee my work piece precision and repeatability).

My design is a moving gantry, that weights 30kg.

One axis per test, not the three all together.

[Jim Dawson]

Just to put things in perspective, I routinely move my 300KG table at 15M/min, at 0.13G acceleration, about 570 RPM, with a 1280 oz/in NEMA 34 stepper. Geared 1.5:1 with a 40mm lead ball screw. Feeding the drive with a 80V power supply.

[imitheo]

Jim, do you have any pictures of your machine ?

[Jim Dawson]

Jim, do you have any pictures of your machine ?

Here is the best one I have. This is a moving table machine. I could supply more pictures if it would be helpful to you.
http://www.cnczone.com/forums/attach...d=364926&stc=1

[Trell]

Lord. What is that 50 mm rails on that thing. Yes more pictures please. Would love to see more of that design


Sent from my iPhone using Tapatalk

[109jb]

I would prefer to go with an "off-the-shelf" solution

What he quoted IS an off the shelf solution.

DC Servo NEMA23 Kits Archives - Stepper Motor | Stepper Motor Driver | CNC Router | Laser Machine | 3D Printers For Sale Stepper Motor | Stepper Motor Driver | CNC Router | Laser Machine | 3D Printers For Sale

Everything he quoted for the DC servo is included in the kits on the page linked above and the price is probably about the same as the steppers/drivers you linked.

[A_Camera]

Just to put things in perspective, I routinely move my 300KG table at 15M/min, at 0.13G acceleration, about 570 RPM, with a 1280 oz/in NEMA 34 stepper. Geared 1.5:1 with a 40mm lead ball screw. Feeding the drive with a 80V power supply.

Yes, table... but not the gantry. That's what I am saying. Fixed gantry is better if speed is important, but it is not as popular and sexy as a moving gantry type.

[A_Camera]

I am aware of this "pointless" issue, but the score goes to the team that reaches 12m/min. Everybody is probably going to servo it, but i'm using a Ø6 endmill, i'll not be able to use the 12m/min to mill of course, then my machine will actually mill it much slower.
That's the catch i'm trying to do, get the score by raining hell on a stepper, get a reduced cost and then feeding my rapid and G1 much slower(Because using it slower will garantee my work piece precision and repeatability).

My design is a moving gantry, that weights 30kg.

One axis per test, not the three all together.

I think moving gantry will be difficult to move that fast. Anyway, if your maximum speed is 12m/min then that is the rapid move speed. You can't feed the rapid (G0) moves slower since that is always at maximum speed. Also, if you think you get accuracy and repeatability if you have too high rapids which you can only keep for a few seconds, not repeatedly, then you are mistaken. A machine which has too high rapids will NOT be usable for milling because as soon as you issue a G0 move you will risk stalling or losing steps. You might think that "...oh, well, I'll never issue a G0 command in my G-code..." but that won't work since there are G-codes which internally use rapids as well, like the peck drilling commands and probably others as well, and if you use those then again you will get problems.

Also, in a competition the rules should be clear, even if it is a pointless competition. In this case, if I was writing the rules I'd describe how far a fast move must be minimum to be qualified as a fast enough move, and also how many times it must be repeated. Are you sure there is no such definition? How does the judges measure that you ACTUALLY moved as fast as you claim you did?

Another thing I forgot to ask you is the acceleration you are aiming at. No point of having high speed unless the acceleration is high enough to reach that speed. Actually, the problem is almost always with the acceleration. Set it too high and your motor will stall. Set it too low and you never reach the speed unless you have a long travel.

[imitheo]

I think moving gantry will be difficult to move that fast. Anyway, if your maximum speed is 12m/min then that is the rapid move speed. You can't feed the rapid (G0) moves slower since that is always at maximum speed. Also, if you think you get accuracy and repeatability if you have too high rapids which you can only keep for a few seconds, not repeatedly, then you are mistaken. A machine which has too high rapids will NOT be usable for milling because as soon as you issue a G0 move you will risk stalling or losing steps. You might think that "...oh, well, I'll never issue a G0 command in my G-code..." but that won't work since there are G-codes which internally use rapids as well, like the peck drilling commands and probably others as well, and if you use those then again you will get problems.

Also, in a competition the rules should be clear, even if it is a pointless competition. In this case, if I was writing the rules I'd describe how far a fast move must be minimum to be qualified as a fast enough move, and also how many times it must be repeated. Are you sure there is no such definition? How does the judges measure that you ACTUALLY moved as fast as you claim you did?

Another thing I forgot to ask you is the acceleration you are aiming at. No point of having high speed unless the acceleration is high enough to reach that speed. Actually, the problem is almost always with the acceleration. Set it too high and your motor will stall. Set it too low and you never reach the speed unless you have a long travel.

Indeed, the judges will use optical sensors to check the speed of the tool when it travel along the X axis in 12m/min, then Y and then Z. The only thing they state it, is that it needs to pass through the sensors with 12m/min and the machine will have a 75mm to accelerate it to that speed. I didn't gave a thought for the acceleration, but yes, it's good to remember that the trick is in the acceleration rate of the machine. I intend to change the rapid to a lower value to mill it, just use the 12m/min to get the scores. It's not my fault that the judges didn't put something in the rules saying: "This rapid move needs to be used while milling", they did not, so i'll be using this agaisnt them.

Also it's the first competition of CNC machines we promote, so it's normal to start soft, to see where the engineers can go and what they can actually build. There are a hundred other rules about safety, milling capabilities, operations that the machine can realize, so it's not pointless.

Nevertheless, the point here is i should choose a stepper motor with low inductance? Also check the RPM x Torque curve of the stepper to see if it has the torque that the gantry requires to be moved at this speed ? One other issue is, this RPM x Torque curve does not count for acceleration, so i should be careful to accelerate my stepper too fast ? What are the issues this could cause?

[A_Camera]

Indeed, the judges will use optical sensors to check the speed of the tool when it travel along the X axis in 12m/min, then Y and then Z. The only thing they state it, is that it needs to pass through the sensors with 12m/min and the machine will have a 75mm to accelerate it to that speed. I didn't gave a thought for the acceleration, but yes, it's good to remember that the trick is in the acceleration rate of the machine. I intend to change the rapid to a lower value to mill it, just use the 12m/min to get the scores. It's not my fault that the judges didn't put something in the rules saying: "This rapid move needs to be used while milling", they did not, so i'll be using this agaisnt them.

Also it's the first competition of CNC machines we promote, so it's normal to start soft, to see where the engineers can go and what they can actually build. There are a hundred other rules about safety, milling capabilities, operations that the machine can realize, so it's not pointless.

Nevertheless, the point here is i should choose a stepper motor with low inductance? Also check the RPM x Torque curve of the stepper to see if it has the torque that the gantry requires to be moved at this speed ? One other issue is, this RPM x Torque curve does not count for acceleration, so i should be careful to accelerate my stepper too fast ? What are the issues this could cause?

75mm to accelerate is HUGE...

Personally I would not participate in this challenge because it is totally pointless nonsense. Waste of time and proves nothing.

[Jim Dawson]

Also check the RPM x Torque curve of the stepper to see if it has the torque that the gantry requires to be moved at this speed ? One other issue is, this RPM x Torque curve does not count for acceleration, so i should be careful to accelerate my stepper too fast ? What are the issues this could cause?

Once the gantry has accelerated to speed, the torque requirement drops off rapidly. At that point you are only looking at friction losses, and if cutting, the cutting forces.

If you accelerate too fast, the stepper will decouple internally and growl at you. In other words, the field will be rotating faster than the rotor can keep up with. No damage will occur, but it won't turn again until you reduce the pulse speed so it can re-couple.

[Jim Dawson]

Yes, table... but not the gantry. That's what I am saying. Fixed gantry is better if speed is important, but it is not as popular and sexy as a moving gantry type.

Moving table or gantry makes no difference, mass is mass. Force=Mass * Acceleration It requires the same force to accelerate a 300 Kg table as it does a 300 Kg gantry, or in the case of imitheo's machine, a 30 Kg gantry. From an machine design perspective, it's a little easier to apply the force to a moving table rather than to a moving gantry.