Author Topic: comparing stepper motors  (Read 10420 times)

gocnc

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comparing stepper motors
« on: October 17, 2009, 11:08:44 AM »
comparing stepper motors

This is from Gariels project
If you search 3PCS new NEMA 23 Dual Shaft Stepper Motor 425oz-in you should find them


I found this from the internet
 HT23-260-4 Stepper Motor

I am having a hard time understanding how the strenght of stepper motors work
Can someone tell me which is  a better stepper motor
Thanks


Jon

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Re: comparing stepper motors
« Reply #1 on: October 17, 2009, 07:28:45 PM »
I'll take a guess at it before someone who actually knows the whole answer replies.  

There must be some standarized test to see how much weight a stepper motor can lift and/or hold in position.  Maby there is a standard shaft diameter, maby the existing shaft on the stepper motor, or maby 1 cm, or maby 1 inch sized shaft or wheel.  Maby for example a 30 oz rated stepper motor can hold a 30 ounce weight and as the motor steps through its rotaton movement.  Thats my take, I hope its close.  Also, external gears to the stepper motor can alter the speed and torque of the power available for use.

From Wikipedia, but it does not say how the torque rating or weight test is made:  http://en.wikipedia.org/wiki/Stepper_motor

Theory:  A step motor can be viewed as a synchronous AC motor with the number of poles (on both rotor and stator) increased, taking care that they have no common denominator. Additionally, soft magnetic material with many teeth on the rotor and stator cheaply multiplies the number of poles (reluctance motor). Modern steppers are of hybrid design, having both permanent magnets and soft iron cores.
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To achieve full rated torque, the coils in a stepper motor must reach their full rated current during each step. Winding inductance and reverse EMF generated by a moving rotor tend to resist changes in drive current, so that as the motor speeds up, less and less time is spent at full current -- thus reducing motor torque. As speeds further increase, the current will not reach the rated value, and eventually the motor will cease to produce torque.

Stepper motor ratings and specifications

Stepper motors nameplates typically give only the winding current and occasionally the voltage and winding resistance. The rated voltage will produce the rated winding current at DC: but this is mostly a meaningless rating, as all modern drivers are current limiting and the drive voltages greatly exceed the motor rated voltage.

A stepper's low speed torque will vary directly with current. How quickly the torque falls off at faster speeds depends on the winding inductance and the drive circuitry it is attached to, especially the driving voltage.

Steppers should be sized according to published torque curve, which is specified by the manufacturer at particular drive voltages and/or using their own drive circuitry. It is not guaranteed that you will achieve the same performance given different drive circuitry, so the pair should be chosen with great care.

« Last Edit: October 17, 2009, 07:57:31 PM by Jon »


Jon

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Re: comparing stepper motors
« Reply #2 on: October 18, 2009, 02:15:53 AM »
I recently found this article on motors.  It shows a drawing and short discussion of how torque is measured with a motor, and probably the same process is done with a stepper motor to give it a rating:

https://csel.cs.colorado.edu/~bauerk/legorobots/motors.html

gocnc

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Re: comparing stepper motors
« Reply #3 on: October 18, 2009, 11:58:55 AM »
Hi Jon!
Thanks for all that info.
There are so many choices and it is very confusing
Before I make my purchase there are a few details I need to go over
If I want to make a machine something like this one
http://www.dumpstercnc.com/
or
http://buildyourcnc.com/blacktoe.aspx
What are the steps I should take to complete the process.

I want the cutting area to be about 4 feet x 3 feet.
I have a few design ideas I have seen on few web sites
They have details here is one of them
http://buildyourcnc.com/default.aspx
 
I could follow his steps but I want the frame to be from metal preferably steel.
What I want to know is should I purchase a kit
http://www.probotix.com/3_axis_stepper_motor_driver_kits/

Which accesories would I need
for example the rail, the bearing lead screw
Should I ask this company for rails system.
http://www.lm76.com/speed_rail_osg.htm
Also I dont want a weak machine.
I am thinking about milling Aluminum
What do you think about that.
Thanks

travis77

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Re: comparing stepper motors
« Reply #4 on: October 18, 2009, 08:55:19 PM »
To start, are these the motors you were talking about? http://www.probotix.com/specs/HT23-260-4_specs.jpg

Those are 260 oz-in where the ones Gabriel uses are 425 oz-in motors. 425 oz-in are what most everyone building a large CNC use. But it also depends on what voltage you are diving them at. I would like to see some comparison data with 260 oz running on 32 volts versus 425 oz on say 12-24 volts..

I could follow his steps but I want the frame to be from metal preferably steel.
What I want to know is should I purchase a kit
http://www.probotix.com/3_axis_stepper_motor_driver_kits/


Personally I wouldnt buy that kit. I would buy 250-425 oz-in stepper motors and a 24-32 volt power supply from http://www.kelinginc.net/ and a 3 axis driver board from http://www.hobbycnc.com/

You would also need wire to connect the motors to the driver board, a DB25 parallel cable to connect the driver board to your PC and should get a fan for the Hobby CNC board. But this is assuming you have minimal soldering skills since you have to build the Hobby CNC board yourself.

Which accesories would I need
for example the rail, the bearing lead screw
Should I ask this company for rails system.
http://www.lm76.com/speed_rail_osg.htm
Also I dont want a weak machine.
I am thinking about milling Aluminum
What do you think about that?


What do you mean by accessories? The type of rail and bearings depends on the design you have in mind... For lead screws the cheapest way to go would be 1/2 x 10 acme threaded rod. The way I would design something would require different parts than the way you or someone else would choose to make their CNC.

Should you buy rails from that company? Depends on the price and quality. I've never heard of them, but i havent researched profile block and rail stuff. I cant give you a fair answer. I've only seen THK rail but I'd assume that the other brands are similar in quality and design.

About milling aluminum, I dont think that your first CNC machine will be able to mill aluminum. The first machine is a huge learning curve. You would have to spend a lot of time thinking and building (and money) to mill aluminum on your first garage built CNC. You'd have to put a lot of thought into it beforehand.


gocnc

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Re: comparing stepper motors
« Reply #5 on: October 19, 2009, 09:29:20 AM »
Hi travis

You are giving me lots of good ideas
I think I would have to build 2 machines
1)  larger machine like I am talking about here in this thread
2)  A machine for cutting aluminum to make dies will have to be smaller and stronger.

Someone said I should buy plans
If anyone has  links to plans that would be nice

You are right to say that my first machine will be experimental and with lots of faults.
But I got to start somewhere
Thanks

gocnc

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Re: comparing stepper motors
« Reply #6 on: October 19, 2009, 06:46:12 PM »
For now I want to make a machine like this one
http://www.cerebralmeltdown.com/cncstuff/mycnc/index.htm

I would like to make modifications to that
I would like to have a frame like this one
Steel / 8020 Hybrid Version
http://www.pdjinc.com/pilot%20rails.html

Also I would like to  have stepper motors that is more powerful.

The idea of milling molds is on hold for now
I think I would need a different structure for that
I will make this later on
For now I want to concentrate on this.

I was told that befor buying the motors and the drivers
I should first make the body

If anyone can give me give me an idea for the motor size that would be nice
Also some ideas about the body
Thanks

travis77

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Re: comparing stepper motors
« Reply #7 on: October 21, 2009, 03:04:31 PM »
Motor size depends on the size of your machine for the most part. Also have to take into consideration the leadscrews you will be using, voltage you will be driving them at, as well as wiring them in a bipolar or unipolar setup. For a large machine, 425 oz-in is pretty standard. For small to medium machines i've seen people use between 180 - 260 oz-in.

My interpretation is this: The larger the holding torque, the faster you can cut in IPM through a material without losing steps or stalling.

So you could run a machine with smaller motors, but you wont be able to cut as fast. But it also depends on other variables such as TPI of your leadscrews compared to the Torque vs RPM curve of your motors. Stepper motors lose torque at higher RPM's so having leadscrews with less threads per inch, means the motors will spin slower to achieve the same result, therefore putting the motors higher in the torque curve.

gocnc

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Re: comparing stepper motors
« Reply #8 on: October 21, 2009, 05:11:26 PM »
Hi travis77
Thanks for your help.

I got a reply from here
http://www.k2cnc.com/shop/proddetail.asp?prod=KT-2514-09&cat=26

This is their price
Frame only $1595

If I was to buy all the parts myself and make something like this
Steel / 8020 Hybrid Version
http://www.pdjinc.com/pilot%20rails.html
This doesnt look like a difficult design
wouldnt it cost me less.

travis77

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Re: comparing stepper motors
« Reply #9 on: October 22, 2009, 09:51:41 PM »
Ya the design is simple, but the parts are going to cost more. But you get what you pay for, linear bearings are stronger and are more compact than DIY bearing systems.