[lgalla]

No Answers? I will assume a 2510 ballscrew will multiply by 5 the torque to give 5625 oz-in.
Is high torque a good thing in a CNC machine?A dragster with 4.88"s will acceralate to top speed very fast in deed.Low gear ratios give rapid acceralition.Any use in CNC?
With 5620 ox-in would a crash shear off your end stops and remove the gantry from the table?
Larry

[Xerxes]

If you try different values in the link I posted, you get that 1125os/in (~8Nm) produces about 3.5kN force with 10 mm pitch. That's a lot more than you need.

With servos you can always limit the torque by reducing current limits.

[lgalla]

Xerxes thanks for the reply.I assume too much is ok with servos.Steppers are not very good above 640 oz-in.If you have tons of torque you can limit the current so in the event of a crash or what ever fault the drives will save from serious damage.Sorry for my lack of experience with servos.In a previous life I was a stepper guy.
Larry

[Mariss Freimanis]

You are correct. Servomotors are constant torque motors while step motors are constant power motors. How does this difference between motor types matter? If your application requires high torque at low feedrates mixed with rapid repositioning moves, it matters a lot.

Let's say you have a 5 TPI screw and you need 1,000 in-oz at a 20 IPM feedrare. That's 100 RPM on the screw so the required power is 74 Watts (in-oz times RPM divided by 1351). Let's also say your rapid is 200 IPM and 100 in-oz is enough to overcome mechanism losses. That's 1,000 RPM on the screw and it's still 74 Watts of power for the rapid.

1) A step motor (direct drive) will have 1,000 in-oz at 100 RPM and it will have 100 in-oz at 1,000 RPM because it is a constant power motor.

2) A servomotor (geared-down) will also have 1,000 in-oz at 100 RPM but it will still have 1,000 in-oz at 1,000 RPM because it is a constant torque motor. IMPORTANT: 1,000 in-oz at 1,000 RPM is 740 Watts! You must buy (and pay for) a 740 Watt motor and drive to do only 74 Watts worth of work. 90% of the motor's power cannot be applied to the load; there is nowhere to put it. Your rapid only requires 100 in-oz.

Servomotors are low torque, high RPM motors when compared to step motors. Excess RPM (4,000 to 6,000 RPM) must be traded in for badly needed torque. 740 Watts at 6,000 RPM is 167 in-oz of torque. It requires a 6:1 reduction to get the 1,000 in-oz torque you need. You will burn the motor up if you try to run it without gearing it.

A step motor can easily deliver 1,000 in-oz at the motor shaft at 100 RPM so it requires no reduction gearing. Throw away one expensive and/or complex reduction gearing unit if you use a step motor. It doesn't need it.

In this example a 74W step motor does the job of a 740W servomotor. This is because the example requires a mix of low-speed, high-torque and high-speed, low-torque cycles. If the application were different, one that runs over a narrow speed range and requires a lot of power, then a servomotor is the best fit.

Mariss

[lgalla]

Hay Mariss,thanks for dropping by.I thought this thread would just be a re-hash of previous subjects.
"Stepper missing steps"I this a general excuse for poor motor sizing?Does Mach3 say"Hay bud you missed some steps"?The software as I understand tells the stepper where to go but there is no feedback to confirm position.I would assume servos have the same problem.12'Oclock here must sleep on it
Larry

[ger21]

"Stepper missing steps"I this a general excuse for poor motor sizing?

Yes. It is possible that you could miss steps due to electrical noise or other electronic issues. But, if the motors are sized properly, it should never lose steps. If you want to cut at 400 ipm, and the machine can only do 390 reliably, then you'll lose steps trying to cut at 400ipm. Thousands of people use stepper machines and never lose steps. If you read through all the build logs here, I think you'll see that the only time people mention lost step issues, is when they are first setting up their machines.

Does Mach3 say"Hay bud you missed some steps"?The software as I understand tells the stepper where to go but there is no feedback to confirm position.

Yep, Mach3 will just keep telling the motors where to go, and has no idea of whether they are going there or not. You can but an add on encoder interface for Mach3 that will keep track of where your machine is and stop it if it starts getting out of position. But Imo, there's really no need for it. The money would be better spent on more powerful motors/ drives.

I would assume servos have the same problem.
Larry

Yep, same thing with servos, as Mach3 sends the same signals, and doesn't know if you're using steppers or servos. With servo's, though, The drives will fault if they lose position. You can wire the drives together so that if one drive faults, it'll stop them all.

[Andre' B]

You are correct. Servomotors are constant torque motors while step motors are constant power motors. How does this difference between motor types matter? If your application requires high torque at low feedrates mixed with rapid repositioning moves, it matters a lot.

I am guessing you are simplifying this based on the typical home hobby builder.
And you know that the servo motor, driver system switches from constant torque to constant power when the driver runs out of voltage head room just like a stepper system.

And that a servo can be high torque, low speed or high speed, low torque depending on what you pick for voltage and current constants when you spec out the system.

Just that hobby builders tend to take what they can get for as little money as possible. :cheers:

[Al_The_Man]

I must ask Al the man.When using ball screws is the pitch similar to gear reduction?

Larry, I must have missed this post somehow, smoke in my eye's.:devious:
A ball screw can be thought of as a class one lever where the load and effort is either side of the fulcrum (B.S. centre).
The load is pitch and the diameter effort.
So just like a lever, you can decrease the effort by lengthening the input lever (increase dia.), or shorten the load side length (decrease pitch).
Al.

[Xerxes]

It's very true that continuous torque of servo is relatively low compared to same size steppers. However, one should notice that servo motor has about equivalent PEAK torque than similar size & price stepper.

In CNC one can size servo motors using peak torques because the load is highly varying and most of the time low torque is enough. Peak torques may be used during very fast accelerations and when milling tool enters in material.

Some day closed loop steppers may become the winning choice (low cost drives needed).

[lgalla]

Found some useful formulae.
For RPM of a screw.IPMxTPI.Say you want a feed rate of 200IPM and have a screw with 5TPI.200x5=1000RPM.Then go to Nook critical speed calculator and select the correct diameter screw.
BTW is threads per inch the same as turns per inch?
On motor size required.
Watts=IPMxweight you want to move divided by 531.If you want to move say a gantry of 500Lbs at 200IPM....500x200/531=188Watts.Really?Does not look like a big motor to move that mass.Is my math correct?Perhaps I mis placed a decimal.
Motor watts =VoltsXconstant current.I will chose a Keling DC servo.model KL-34-180-90
90Vx7.8Amps=702Watts
Are these calculations correct?
Larry

[Al_The_Man]

BTW is threads per inch the same as turns per inch?

Same.

On motor size required.
Watts=IPMxweight you want to move divided by 531.If you want to move say a gantry of 500Lbs at 200IPM....500x200/531=188Watts.Really?Does not look like a big motor to move that mass.Is my math correct?Perhaps I mis placed a decimal.

It requires little effort to maintain a load at rest or at constant speed.
There is inertia to consider, the industry recommendation is not to exceed motor to load inertia ratio of 10:1.
The inertia ratio decreases by the square of any reduction.
If you download the Kollmorgen s/w from the Electromate site, it is a graphical sizing program that will give you an idea as to how inertia (acceleration/deceleration) affects motor sizing.
Al.

[JLSD]

Hi all,
The math looks OK, and that motor would do even more work than you think, since the calc. is for power to LIFT the weight you're talking about! !! BUT WITHOUT LOSSES!!
Also, and this is the big BUT, it takes no account of acceleration, which needs to allow for rotating bits as well as carriages etc.
My builds aren't even started yet, but I picked up some sweet ballscrews, thinking "what'll do a lot'll do a little!" so 32x5 looked great, (and cheap!)
My own education as to inertia of rotating masses is way past sell-by, so I tried dusting it off, and got seriously scary answers!!
If memory serves, the radius of gyration of a cylinder, for estimating purposes, is approx 2/3 R. This means the mass of the screw (as above) is rotating at over 12 times the linear speed of the driven piece!! No problem once you've got there, but not to be ignored. Now, like I said, this is from a very distant education, and I'd welcome correction if it's wrong.
Any comments about an unnatural fondness for hefty screws discreetly please!

Regards,

John

[lgalla]

John I only have 2 THK ballscrews for a dual Z.One is 2005 and the outher 1605 C-3.Never the less they are fine pices of machineing.I am kind of partial to the BK-15's
larry

[lgalla]

here is a stupid question:withstupi
Can a servo drive be used as a stepper drive?Or vise versa.
The reason I ask is many beginners are shy of servos and some posters say to begin with steppers and up grade later.If a drive was dual purpose only the motors would be changed.Sorry for the stupid questions.(chair)
Larry

[lgalla]

Thanks posters.I would have used the wrong ballscrews motors exc.My cutting speed would have been 200ipm before whip.Any ideas for a device to reduce whip on long screws?I attached a poor sketchup drawing.
The idea is 2 spring loaded nylon pulleys.The nut would push the pulleys down while passing.This would reduce whip an allow higher RPM.Is it a good idea?
Larry

[Jason3]

Hi Larry,

Have you considered the rotating nut type ball screws? Might be a nice way to avoid the inertia of an enormous ballscrew, while keeping the zero backlash advantages. You need a timing belt drive or something similar to drive the nut, which would also allow you to integrate the reduction you'll probably need from the servo.

Best regards,

Jason