Basic CNC

[jumps4]

ok i'll give this a basic shot
a stepper motor is controlled by pulses (steps) the common type has 200 steps in 1 revolution or 1 step every 1.8 degrees. microstepping is dividing 1 revolution into smaller steps with electronics. 1/2 stepping the motor would double the number of pulses it takes to complete 1 revolution 0r 400 pulses or .9 degrees per pulse. the greater number of pulses the motor uses the smoother it runs but because it does not reach full current in the time allowed per pulse the torque reduces. because the pc is limited in its speed it can send the pulses through a parallel port the controller handles the higher speeds. the higher the pulse the less torque the motor has but the accuracy of the motion increases.
if you were to use a stepper motor at the 200 steps per revolution mode connected to a 5 thread per inch shaft to move a milling table for example then one inch is only capable of being divided into 1000 steps ( 200 x 5tpi )or .001 per step. this sounds ok for most uses but the downside is that at slow feed speeds the steps are a series of hard thumps and vibration can be severe at some speed due to harmonics in the motor and drive parts. this all comes back to surface finish loss. now if you microstep that same 5tpi shaft at 2000 steps per revolution then it would require 10000 steps to move the table 1 inch. the series of thumps now is a steady buzzing. the downside is to get the torque at higher speeds you may need a larger motor or higher voltage. the pulses (steps) are too fast for the motor to reach full current in the time allowed per step.
most stepper motors are marked with voltage and amps, the amps are what is required to reach the advertised torque. the voltage is missleading and can be as much as 10 times what is marked. voltage is electrical pressure and because the step is so fast the motor does not have time to reach full current we raise the voltage ( pressure ) to make the motor reach the required amperage faster.
so a motor marked at 8v may be powered with a power supply putting out 80v.
the other advantage of microstepping is accuracy the smaller we divide an inch the more accurate we can move the axis.
steve

 

[jumps4]

i'll add a little more
servo motor are controlled by a slotted wheel or pickup the senses the motors movement. the sensor is located on the motors shaft or the object being moved. it is a set number of divisions per revolution or distance and not changeable. for this reason stepper motors are more desireable for very fine movements. take todays microscopes to move an object at millions of an inch would be impossible with a servo motor because of the mechanical pickup. your not going to make a pickup with millions of slots for the sensor to detect. this has to be done with electronics. thats microstepping in the smallest of uses and may not be a motor that turns at all it may be a linear motor.
steve

 

[8ntsane]

the more torgue you need the bigger the winding needs to be nema is the standard for interchange if the requirement excedes the frame size 17 you move up to the next size 23 and so on hey you forgot my nema42 4200 oz/in that motor has a 3/4" output shaft and is rated at over 1.5hp thats a heck of a stepper motor .
steve

Steve
Just want to verify that Nema 42 motor. You wrote 4200 oz/in, Is that accurate? Ive never seen one rated that hi.
Straighten me out if I have it wrong.


Edit:
Never mind, got my anwer on google :nuts:

 

[DMS]

i'll add a little more
servo motor are controlled by a slotted wheel or pickup the senses the motors movement. the sensor is located on the motors shaft or the object being moved. it is a set number of divisions per revolution or distance and not changeable. for this reason stepper motors are more desireable for very fine movements. take todays microscopes to move an object at millions of an inch would be impossible with a servo motor because of the mechanical pickup. your not going to make a pickup with millions of slots for the sensor to detect. this has to be done with electronics. thats microstepping in the smallest of uses and may not be a motor that turns at all it may be a linear motor.
steve

The sensor type you are talking about is called an "incremental encoder". It can be a slotted wheel, a photo-etched glass wheel, or any of a number of other configurations. The key here is that incremental encoders don't know where they are, they only know how far they have gone. So, in order to get accurate position out of an incremental encoder, you need "home switches" (a topic we should cover in more depth later). Incremental encoders give you a pulse when you move forward, and a pulse when you move backwards (it's a little more complicated than that, but we'll leave that for later). Incremental encoders tend to be cheap, and operate at at very high speeds, in harsh environments. They are also very repeatable.

Buuut....

That doesn't mean they are the only game in town. In the scenario you talked about with the microscope, you could do it with a stepper motor, yes. You could also do it with a servomotor, and an analog position sensor (like an LVDT or resolver). Neither of which you are likely to see in a home shop scenario, so this is kind of an aside.

To get back on topic (and Steve touched on this). The PC that is running your software is limited on how fast it can send pulses to your motors. And this is going to effect the encoder that you get for a servosystem or whether you can use microstepping on you drivers (assuming they are capable).

 

[jumps4]

to overcome the limits of the pc and windows and the fact that the cnc controller is not directly tied to the pcs internal clock there are controllers that are themselves a cpu they have their own internal clock that is dedicated to just moving the axis and reading the encoders or scales. these controllers take only the locations or parts desired and do the steps and pulses internally they are much faster, many times faster allowing for greater microstepping and less likely to make a mistake or miss steps. examples are: smooth stepper, dynomotion and uc100. each of these are very different in the way you use them but all remove the load off the pc to make the steps required. with them you can listen to music on the pc if you wanted, it wont delay or interupt your motors working normal.
steve

 

[Bill Gruby]

It will take a little time to absorb what is being discussed at this time. So I will not post any question from me till we are all on the same page.

When everyone is comfortable with this we will move on. If I don't hear anything in a day or so I will poet the last question givin to me. PM me or post here if you are ready to move on. Thanx.

"Billy G" :thinking:

 

[Bill Gruby]

I can only hope the the precedeing info was understood by all watching this thread. I see nothing posted or have received no PMs to the contrary. So it's time to move forward. There are three main components involved is a CNC set up. The Motor, power supply and encoder. Yes there is also the PC. I will let the experts pick the next component to be discussed.

"Billy G" :thinking:

 

[jumps4]

it's not encoder it driver
not all cnc has an encoder
are you looking for ballscrews next?
steve

 

[Bill Gruby]

Steve;

Pick it up where you think it should go next. I really don't know.

"Billy G"

 

[7HC]

Ballscrews, Acme Screws (can't help but think of Wile-E-Coyote and Roadrunner :lmao, couplers, Delrin nuts, and the individual effects that each of these have on backlash when deciding on the components needed for a conversion, might be worth explaining by those who know.

I'll pick the easiest, couplers.

Two main types, the three piece 'LoveJoy' style and the single piece slit aluminum type.
Both allow for some small misalignment between the stepper motor and the driven shaft, and for a little expansion too, but the LoveJoy type has a far greater potential to introduce backlash over time as the center insert compresses.

Here are the coupler pics, and I'll leave the screw thread issues to those that know more than I do.

First the LoveJoy then the Split Aluminum:




M