Hi everyone,

im new to cnc and i m building a 1.20 m by 1.80 m cnc using r&p system. i m running mach3 on an old toshiba laptop with windows xp and it seems to be working fine. My motors are nema 34 1600oz and i cant tune them to run smoothly (lots of vibration). Anyone knows how can i set the step per unit, vel and accel right? (Motor drivers are set to 1600 microstep setting)

Set the distance or steps per first - do this approximately by calculating the ratio between motor and axis, enter this in tuning, set speed to say 1000mm/min and accel to 100mm/s/s

Next get the distance accurate by using auto calibrate on Mach diagnostic screen (i think) - enter a distance, mach will move, then measure the actual distance it moved and accept the result - use a big distance if you can accurately measure this - a good steel rule or accurate measure is needed, the bigger the distance the better.

Then tune your motor speed, do this by increasing the speed in tuning in steps of say 500mm/min until the motors start skipping when doing a rapid jog - be careful if you have slaved motors or you can rack(twist) your gantry.

each time you increase speed, increase acceleration by 1/10 so if speed is 5000 then acc is 500.

Once it skips, reduce the number by say 20% and try it - if it works well and keeps position then all is well.

Next you try upping the acceleration until it screams at you or loses steps - then back off again.

The two are sort of inter-related but not directly

Eventually you will find a sweet-spot for your machine but once set you should never need to change steps-per unless you alter the drive system.

Thank for your reply Davek,

My motors gear is 1/3 ratio to the big gear which is attached to a small pinion which is 1/3 ratio to the big gear (as you can see at the pic i made with paint  ). So if i get this right and if my motors 1 turn is 200 steps and my pinions diameter is 25mm, i need 600 steps to turn the pinion 1 full turn, which is π*diameter=78.5 mm. Is that right?

I hope it wasn' t too long...

Thanks in advance

Yes you have 600 steps to turn big pulley 1 revolution if the ratio is 3:1

Now we need to know the pitch diameter of the small pinion - how far does it move for 1 revolution?

And also your micro-stepping settings on the drives.

My pinions pitch diameter is 25mm so i think it moves 78.5mm per rev. My microstepping settings are 6400. That is 600*6400 =3840000 steps for 78.5 mm so my steps per mm must be 3840000/78.5 = 48917,197.  Is that right?

Sounds like a good starting point, enter that then run the auto set feature in Mach3 to get the setting perfect.

thanks Davek,

all working good now

I just think that my motors are way too big. I cant get them to work smoothly at any speed.

Is this a plasma or router table??

My guess is router.

Nema 34's are generally seen as too big for many of our uses - they are heavy, the rotors are heavy and accelerate slowly unless you have very high voltage drives on them 0- this is rare as they cost £££

Do you have one motor driving two pinions on the gantry axis or two motors slaved??

Really, you have a bit of a problem to get that beast running smooth, you may help by adding torsional dampers on the shafts - large masses of hard rubber to help absorb the vibrations  but i dont know anyone that has done that.

Really, you need Nema 23 4Nm motors and DSP drives in there on something like 70v PSU I think, to get speed out of Nema 34's you need probably 150v PSU and serious drives - for that sort of cash I would have simply thrown in AC servo motors

Not sure what your best course of action is i'm afraid, maybe some one else can help here?

So generally speaking servos are more accurate than steppers?

Horses for courses

Both are accurate if set up correctly, the difference comes when things need to get heavy and/or move fast - steppers + heavy + speed = bad times, Stepper + Light + fast = OK to a point.

Servos are a different beast completely - most of stepper torque is down way at the bottom end of speed range and maybe only 5% left at full speed, servos have a linear torque and will give full power from 0 to max speed, they are also capable of stopping and starting heavy loads as they have encoder feedback so they just do not lose position - the servo will try to regain a loss in position or 'following error' almost until it self-destructs. The power from even a small servo is amazing.