UniPolar vs BiPolar?

[kolias]

It’s kind of confusing to a newcomer into the CNC world the UniPolar / BiPolar motors choice.

I can understand that the UniPolar does not have good low speed torque but holds the torque at high speed. I can also understand the BiPolar Series / Parallel benefits & drawbacks.

But since I’m building my 1st CNC machine how do I know which one to choose for a hobby CNC router machine? Is it important to have good torque at high speeds or low speeds?

So which one is more popular?

[rlrhett]

There is a good article about that here http://www.probotix.com/stepper_moto...polar_bipolar/

[Sharion]

What is the best software program to use for wood products on a Holzher or onsrude machine?

[kolias]

Thank you rlrhett

I know about this article but it does not answer my question.

My question again: on a hobby CNC router machine, is it important to have a good torque at high speeds or at low speds?

[AVolta]

I think it's hard to answer that question with a general answer, since the amount of torque that you will need depends on a number of factors, and is governed by the coupling ratio of step motor shaft rotation to linear motion of your stage. Put another way, for a lead screw driven stage, a finer pitch lead screw would behave better with lower torque and greater velocity, but a coarser pitch requires greater torque at lower speeds. However, for lead screws, greater shaft velocity requires more torque for changes in direction to overcome shaft inertia (the lead screw behaves like a flywheel).

This has less to do with bipolar vs unipolar, and more to do with the selection of motors and drives. In stepper motor world it is relatively easy to get something to work, but the issue is whether it will work to your expectations. Running some quick numbers might help.

Here is a formula relating torque to pushing force using lead screws

Force = (2 x pi x T) / (distance from thread to thread)

Force is in ounces
T = torque in ounce-inches
(distance from thread to thread) is in inches

So for example, directly driving a ten pitch lead screw with 120 oz-inches of torque generates the following pushing force:

F = 2 x 3.1417 x 120/0.1 = 7540 oz = 471 lbs

Here is a formula relating linear velocity to step rate:

Speed (in/min) = 60 x (steps per sec) x (distance from thread to thread) / (steps per revolution)

So for example, if your step motor turns the lead screw one revolution in 400 steps, and you are using a ten pitch lead screw, the cut speed using a controlling signal of 1000 steps per second would be as follows

S = 60 x 1000 x 0.1 / 400 = 15 inches/min

So using these ratios you can work back and forth between the speeds that you expect from your stage and the published torque vs step rate curve for the motor and drive that you plan to purchase.

Now none of this accounts for force the necessary to overcome inertia for speed changes, which is more complicated to calculate and often estimated. But getting a feel for it is easier when you first have control over the steady state portion of your system (working from the known to the unknown), and I am hoping that a little ratio math will help you in the process. :-) www.abilitysystems.com

[cdndave]

... so it depends?

I'm going to reread this post after I get my first machine up and running and I'll bet it'll make more sense to me.

D

Originally Posted by AVolta I think it's hard to answer that question with a general answer, since the amount of torque that you will need depends on a number of factors, and is governed by the coupling ratio of step motor shaft rotation to linear motion of your stage. Put another way, for a lead screw driven stage, a finer pitch lead screw would behave better with lower torque and greater velocity, but a coarser pitch requires greater torque at lower speeds. However, for lead screws, greater shaft velocity requires more torque for changes in direction to overcome shaft inertia (the lead screw behaves like a flywheel). This has less to do with bipolar vs unipolar, and more to do with the selection of motors and drives. In stepper motor world it is relatively easy to get something to work, but the issue is whether it will work to your expectations. Running some quick numbers might help. Here is a formula relating torque to pushing force using lead screws Force = (2 x pi x T) / (distance from thread to thread) Force is in ounces T = torque in ounce-inches (distance from thread to thread) is in inches So for example, directly driving a ten pitch lead screw with 120 oz-inches of torque generates the following pushing force: F = 2 x 3.1417 x 120/0.1 = 7540 oz = 471 lbs Here is a formula relating linear velocity to step rate: Speed (in/min) = 60 x (steps per sec) x (distance from thread to thread) / (steps per revolution) So for example, if your step motor turns the lead screw one revolution in 400 steps, and you are using a ten pitch lead screw, the cut speed using a controlling signal of 1000 steps per second would be as follows S = 60 x 1000 x 0.1 / 400 = 15 inches/min So using these ratios you can work back and forth between the speeds that you expect from your stage and the published torque vs step rate curve for the motor and drive that you plan to purchase. Now none of this accounts for force the necessary to overcome inertia for speed changes, which is more complicated to calculate and often estimated. But getting a feel for it is easier when you first have control over the steady state portion of your system (working from the known to the unknown), and I am hoping that a little ratio math will help you in the process. :-) www.abilitysystems.com

[AVolta]

Yes, every factor is a dependency, including your expectations. I think the best way to experiment, however, is to make a rough cut estimation and then purchase one motor and drive. Try it out on the X stage, which is generally the most demanding. If it doesn't meet your expectations, then move it over to the Z stage, which is generally the least demanding. www.abilitysystems.com

[JB Laser Tech]

Originally Posted by kolias It’s kind of confusing to a newcomer into the CNC world the UniPolar / BiPolar motors choice. I can understand that the UniPolar does not have good low speed torque but holds the torque at high speed. I can also understand the BiPolar Series / Parallel benefits & drawbacks. But since I’m building my 1st CNC machine how do I know which one to choose for a hobby CNC router machine? Is it important to have good torque at high speeds or low speeds? So which one is more popular?

Hi kolias

The answer is easy. BiPolar
If it's torque you're worried about, then buy a larger stepping motor. eg. 400 oz/in or 750 oz/in
I cut aluminum on mine and I just have 275 oz/in
Speed is really not a problem, because every material has its own cutting speed.
Here's what I build. http://www.SolarScore.com

[BobF]

Originally Posted by kolias It’s kind of confusing to a newcomer into the CNC world the UniPolar / BiPolar motors choice. I can understand that the UniPolar does not have good low speed torque but holds the torque at high speed. I can also understand the BiPolar Series / Parallel benefits & drawbacks. But since I’m building my 1st CNC machine how do I know which one to choose for a hobby CNC router machine? Is it important to have good torque at high speeds or low speeds? So which one is more popular?

Most 6 and 8 wire motors can be driven as both bipolar and unipolar. Several sellers now publish ratings for either drive configuration. bipolar gives the most choices and can be configured as serial or parallel (with 8 wire motors).
As for popularity, it depends :-) again. Many start with a simple unipolar driver because it is usually cheaper. You can then choose to upgrade to bipolar later if it suits.
My machine uses a hobby CNC kit that comes with motors (8 wire) and a simple and inexpensive unipolar driver. Some have used this kit and upgraded later to a Gecko G540 driver. The hCNC motors I have are rated at 305 oz/in in unipolar and 425 oz/in in bipolar. If I changed the driver I would gains some torque. I would probably want to change to higher pitch lead screws to take advantage of the torque.

[kolias]

My question has been answered

Thank you

[pminmo]

This video might give you some perspective: http://pminmo.com/which-stepper-motor

[kolias]

Originally Posted by pminmo This video might give you some perspective: http://pminmo.com/which-stepper-motor

Thant is an excellent video and info.

Much appreciated