1.) Configure Motor Drive (Dip Switches or Software setting) and CNC12 Wizard Axis Configuration menu Steps Per revolution settings to match.
See Steps per revolution discussion below for details.
Note: For Open Loop Stepper motors the Drive, Motor and Power supply are required to be 'balanced and wired correctly. See GeckoDrive paper on how to do this. https://www.geckodrive.com/support/step ... asics.html
2.) Perform "rough" Overall Turns Ratio calculation and test commanded movement.
See Steps per revolution discussion below for details.
3.) Optional Perform "Fine" Overall Turns Ratio procedure.
Note: Usually Mill and Lathe users will take this extra step. Router and Plasma user may not need to take this step.
Steps per Revolution and Overall Turns Ratio Discussion
Steps per Revolution
- "steps per revolution" is the number of electrical pulses (steps) that will cause the motor to rotate exactly one revolution.
- Steps Per Revolution values are FIXED round numbers determined by the Axis Motor DRIVE step settings.
- If you have set the "Steps per Rev" in CNC12 software set to anything other than a round number like 1600, 2000, 3200, 4000, 6400 etc then it is WRONG!
- if you are using any steps per rev number below 1600 this is out of range, values below 1600 are not supported.
- Numbers like: 2003, 867, 1623 are all impossible Steps per Rev values.
- Steps Per Revolution is NOT USED to adjust Commanded Motion calibration! "steps per rev" is SOLELY determined by the Axis Motor DRIVE step settings!
- Set the Steps Per Revolution values for each axis to match the axis DRIVE step settings for each axis. DO NOT ADJUST Steps Per Revolution value to get the machine to move 1" when commanded to move 1"!!!
Overall Turns Ratio
Overall Turns Ratio IS used to calibrate commanded distance with actual distance moved. The Overall Turns Ratio is a absolute value..there is only one right answer for Overall Turns Ratio for each axis of every machine tool. The Overall Turns Ratio is a physical value determined by the ballscrew pitch and how the axis motor is belted/geared to that screw. For example a 5 turns per inch screw with a axis motor belted at 2:1 yeilds a Overall Turns Ratio of 10, said another way.. it would take 10 revolutions of the axis motor to make the axis move exactly 1" of travel. Not all screws or racks are perfect 5 turns per inch so a more accurate adjustment can be made to fine tune the commanded movement to the actual movement. The 'fine' Overall Turns Ratio is determined by measurement. There are several common measurement techniques that are typically employed by DIY CNC integrator. Marty has some videos that go over his techniques, see below for more info.
Steps per Revolution FAQ
“What steps per rev should I be using with open loop stepper motors?”
Answer: Typically 1600 to 4000 steps per revolution for stepper motors. Any values lower than this reduces machine performance and accuracy.
“How do I set up steps per rev?” On the axis motor drive, set the micro step dip switches on the drive (or use drive mfg software if provided) to yield 2000 steps per revolution of the motor or higher. (See drive manual for dip switch settings). 2000, 3200 or 4000 steps per revolution are good common values to use with most machine tools using open loop stepper motor/drives. For initial testing, set the drive dip switches up to produce 2000 steps per rev and then go into the Centroid Wizard and set the Steps per rev value to 2000 steps per rev.
Steps per revolution of a stepping motor as the name implies, is the number of distinct positions per revolution that the stepping motor can be commanded to move to.
When stepping motors were first invented the number of steps per revolution was small. 200 steps per revolution was a common resolution of the technology at the time. This is called a 1.8 degree motor and drive. As the stepper motor could be commanded to moved at its minimum resolution of 1.8 degrees per step. When connecting a 200 step per rev motor to a milling machine, it is not that great.. the resolution is not fine enough to produce smooth motion and accurate parts.
Then came along “microstepping” this is a technique was created and used by the drive manufactures to be able to hold the stepping motor in between any one of the 200 steps per rev on the 1.8 degree motor. The drives commonly use dip switches to set the microstepping multiplier..for example. A 2 time micro step = 400 steps per rev, a 4 time micro step = 800 steps per rev , an 8 time micro step = 1600...etc...etc..see the drive manual for more info on this. You want to use micro stepping to a limit. Most modern drives do a pretty good job of producing torque on an open loop stepper at 1600 or 2000 steps per rev..but the motor torque will fall off with higher and higher steps per rev. So just becasue the motor drive can run at 10,000 steps per rev doesn't mean you should do it. Use 1600-3200 steps for rev for most stepper drives.
In the modern age, ideally for a milling machine or lathe using open loop stepper motors the bare minimum recommended steps per rev resolution is 1600 steps per rev. Modern inexpensive stepper drives such as Gecko and Leadshine are capable of much higher resolutions so take advantage of better performance in the 1600 to 3200 range. Don't run your machine/motor/drive setup lower than 1600 steps per rev. (If your drives don’t have microstepping feature they must be 20 years old or older so, time to buy some new drives and gain some much needed performance.) For open loop stepping motors, 2000, 3200 or 4000 steps rep rev is an ideal steps per revolution value for a Milling Machine, Lathes and Routers
From one extreme to the other...
“So if my drive is capable of very high motor steps per rev...then why not just crank up the drive to the highest micro step resolution possible? That way I’ll have all the accuracy I need!” Well that may be true.. you will gain accuracy but most likely you will loose maximum feed rate in which the machine can move…. So, this is a simple balancing act… a trade off between torque, accuracy and top speed of the machine. If top speed is not important to you, then go for the more accurate set up but be mindful of reduced torque at higher steps per rev.
So what micro steps per rev dip switch setting should I use?
1600 to 4000 typically is the range that is good for open loop stepper drives. 2000 to 8000 works good for closed loop AC and DC stepper hybrid drive systems, exact values depend on drive system settings and the mechanical configuration of the machine, its important to MATCH the Centroid Wizard “steps per rev” to the motor drive system settings of steps per revolution.
Definitions:
1.) Micro steps per revolution dip switch setting. (explained above)…
2.) Maximum step frequency of the drive. This is the maximum rate in which the stepper motor drive can receive step signals from the Acorn board. A lot of modern drives can except 200 Khz or higher, see the drive manual for the maximum step frequency specification of the drive but be wary.. some drive manufactures will claim a high step frequency but in the fine print list a specification for minimum pulse width or 5 or 10 micro seconds. So rule of thumb..run the Acorn step frequency at the lowest setting that will result in motor RPMs just a little faster than the machine rapid rats. The Acorn cnc controller can output step signal up to 800 khz frequency. The higher the step frequency allows both Accuracy and Speed to be obtained at the same time. Typically only high tech drives such as Yaskawa, Delta can actually perform well at those frequencies.
A high micro steps per rev value at a low step frequency yields accurate movement but limits top speed (rapid rates).
A high micro steps per rev value at a high step frequency allows both accuracy and good top speed but run the risk of lost steps due to wiring and or noise.
So, again..why not just crank it up to the max? Step frequencies above 200 Khz need special attention paid to the cabling of the step and direction signal in regards to wire type, shielding, drain and grounding. If you are not familiar with this type of wiring choose 200 Khz or lower to reduce the possibility of lost steps due to poor wiring practices. The Lower frequency (100 to 200 Khz) is much more forgiving and is reliable. Only use the higher frequencies when the signal cable is proper.
3.) Overall turns ratio of an axis. This is the mechanical constant of any axis, it is easy to determine. To put it another way ask yourself “How many turns of the stepper motor does it take to make the machine tool table move ONE inch? Some common examples are:
a.) The stepper motor is direct coupled to a ball screw, the pitch of the ball screw is 5 turns of the ballscrew yields one inch. Therefore the overall turns ratio of that axis is 5 turns per inch. It takes 5 full revolutions of the stepper motor to move the table one inch.
b.) The stepper motor is belted 2:1 to a ball screw, the pitch of the ball screw is 5 turns of the ballscrew yields one inch. Therefore the overall turns ratio of that axis is 10 turns per inch. It takes 10 full revolutions of the stepper motor to move the table one inch.
Here is a video by Marty explaining how to measure and set the Overall turns ratio.
So lets bring all this together……..
I set my dip switches for 2000 steps per rev. My machine X axis has an overall turns ratio of 10 and my drive can accept 200khz step frequency from the Acorn..
What is the commanded motion accuracy of the axes?
(Steps per rev) X (Overall Turns Ratio) = Steps per Inch
2000 x 10 = 20,000 steps per Inch
1 inch / 20,000 = .00005” of machine movement per step of the motor.
Some might say...“I don’t need that much resolution! My machine is lucky to hold .002” so why bother?” even if your machine tool mechanical resolution is poor you still want to run the axis motors at a decent resolution, 2000 steps per rev may be much finer resolution than the machine tool but it makes the motor run smoother and react better.
DON’T do it like Mach3 did it, and don’t do it like a 3D printer either!
Some DIY CNC software mixes "Steps per Rev" and "Overall turns ratio" into one number this is a terrible way to do things.
Ok..So, now that you decided on a microstepping value setting on the axis motor drive,
Set the steps per rev value in the Centroid wizard to MATCH this value.
For instance If the drive setting is for 3200 micro steps per revolution then set the Centroid Wizard to 3200 steps per rev. Its that easy. Don’t adjust this number, don’t try to add or subtract steps for backlash, don’t scale this number to get the axis to move a different amount, or any other crazy idea.
3200 steps on the axis drive = 3200 steps per rev in the Centroid software nothing else!:-))
Overall Turns Ratio
Overall Turns Ratio and Commanded move calibration to Actual measured movement.
“OK I got my steps per rev sets the way you describe above but my machine moves more (or less) than the commanded value.”
For instance if you command a 10” move on X axis and you measured that it really moved 12.375”…. then guess what? DON’T adjust the steps per rev!! Once you have steps per rev set to match the drive setting, Steps per rev has NOTHING to do with commanded motion vs actual motion.
Machine movement calibration is done with the “overall turns ratio” value. ( This is the mechanical constant of any axis) If you command 10 inches and you don’t get 10 inches of movement the “overall turns ratio” value is incorrect. Use this TB36 (http://www.centroidcnc.com/dealersuppor ... ads/36.pdf) and /or view Marty's video (http://centroidcncforum.com/viewtopic.p ... t=10#p8852 on how to adjust the “overall turns ratio” to fine tune axis movement.
Using the Wizard and Tuning the Bench Test Rig
Attached is the Overall ratio calculator for doing the math when you are ready to fine tune.
It is an .xls file. Google sheets will open it..
Turns Ratio Notes:
- when setting up turns ratios on a Lathe X axis, be sure to use the actual distance moved by the X axis (don’t cut in half or multiply by 2 trying to make up for Diameter/Radius! follow Marty's video)
Max Rate (G0 ) example calculation.
100,000 steps per second step frequency
- 4000 Steps per revolution of the axis motor
- 1.25 axis motor revolution per inch "overall turns ratio" (1.25 is a typical router rack and pinion overall turns ratio, milling machine are 5 or 10 axis motor turns per inch)
so, lets calculate how fast the machine can go...
4000 steps per rev x 1.25 turns per inch = 5000 steps per inch of linear travel
at 100,000 steps per second step frequency
100,000 steps per second x 60 seconds = 6,000,000 steps per minute
6,000,000 steps per minute divided by 5000 steps per inch = 1200 inches per minute.
if you want to go faster than 1200 ipm reduce the steps per rev or increase the step frequency and set the drive hardware settings to match.
AC and DC hybrid drives
Since Hybrid drives use Encoders.
Hybrid drives have a “Step per rev INPUT Multipler” or sometimes also called “Transmission” or “Ratio” or “Input Resolution”
Steps per rev INPUT on a hybrid drive = the steps per rev coming from the Acorn.
For example:
Delta AC drives like to be run at 4096 steps per rev INPUT from the Acorn.
Set Wizard Steps per rev to 4096 and set Delta drives to 4096 for the input steps.
Clearpath like to be run at 6400 steps per rev input for milling machines and lathes (even with the 800 count encoder model) so set the Steps per rev to 6400 in the clearpath software and in the Wizard steps per rev at 6400. Routers and Plasma can use 3200 or 1600 to gain maximum high Rapid rates.
Gecko G320 at a 1:1 Multiplier with a 2500 line encoder step per rev input = 10,000
Set Wizard to 10,000 steps per rev when using 1:1 multiplier with 2500 line quadrature encoder (2500x4 =10,000 count per revolution of the motor)
Notes on Machine Design for nice smooth motion and cutting action.
- Motors large and powerful enough to move each axis without approaching the limit of the motor.
- Very important to have the overall turns ratio (a mechanical constant defined by machine design) and the steps per rev (a electronic constant defined by axis motor drive setup) be in the CNC Happy Zone.
Milling and Lathe Machine Happy Zone.
- Happy Zone Overall Turns Ratio for milling machines and lathes is 5 to 10 turns of the AXIS MOTOR yields 1" of table/gantry movement.
- Steps Per Revolution of 3200 or higher.
- Axis motor Accell/Decell .375 - .5 seconds
- CNCPC CPU with single thread benchmark of 1500 or higher
Router and Plasma Sweet Spot
- Happy Zone Overall Turns Ratio for routers is 1.5 to 2.5 turns of the AXIS MOTOR yields 1" of table/gantry movement.
- Steps per Revolution of at least 1600
- Axis motor Accell/Decell .3 - .5 seconds
- CNCPC CPU with single thread benchmark of 1500 or higher
- Low lash as possible.
- Rigid machine design so machine movement does not flex or shake the machine itself.
Plasma CNC Machine Stepper Motor Sweet Spot gearing