View Full Version : Cnc steps per setup in mach 3 help

21-05-2015, 04:02 PM
Hi im trying to setup my cnc im using mach3 software,
nema 23 stepper motors 200 steps per.
step angle is 1/8 and using trapezidac lead srews which are 10x3 i contacted the company i purchased them off and was told they do 3mm per turn (lead) just need to put it all together to find out what to enter in mach3 its confused the hell out of me lol.
would my table size in the display box in mach3 alter aswell as most gcode i seem to enter is off my table
any help much appreciated

21-05-2015, 08:41 PM
Ok well first make sure Mach3 units are set to Millimeters. If not then change and restart mach3.

Some explination may help first.! . . Your motors move in 1.8 degree steps so 360deg / 1.8=200 full steps per rev.
This is a very course step amount so your stepper drives will allow each step to be broken down into Micros steps. It's this Micro step amount you require to calculate the Steps Per setting along with the pitch or lead of the screw.

For the steps per it's a simple calculation. Divide the micro steps setting your drives are using by the pitch of your lead screw. ie 800ms / 3 = 266.666 steps per MM.

Now regards the micro stepping amount then you don't want to go too high other wise you will stress the system needlessly. People often believe setting the micro stepping high increases there resolution but it doesn't. All it does in most cases is stress the system causes lost steps and slows the machine.

The main advantage to micro stepping is it gives smoother motor operation but it does come at a cost in the form of pulses required. Depending on the motion control device or pulse provider ie: parallel port etc then you only have a certain amount of pulses available. Which if using the parallel port this isn't an awful lot and a high MS setting will drasticly slow the machine down and stress the parallel port. Which 99% of time leads to lost steps and stalled motors.

Like wise you don't want to go too low other wsie your motors will have a very rough action to them.
Your motors use 200 steps for one revolution. This would be known as FULL stepping. But if you run at this setting on your drives(if they will allow it at all) the action will be quite course and can lead to issues with resonance affecting motor/drive performance. So rarely is Full step used. Often stepper drives will only start at 400ms.!
Now for a machine using a 3mm lead screw and running with the parallel port then 400 or 800 is good amount as it won't stress the system and give decent amount of speed. Going higher will slow the machine and potentially stress the system. Never go above 3200 as theres very little gain and your just wasting pulses.

21-05-2015, 09:29 PM
thanks for the reply how can i find out the micro settings please sorry but all this is new to me and confessing here is a link to the stepper motors and driver i bought :-

also how does the accel and velocity work in mach3 motor tuning
thanks alot anthony

21-05-2015, 11:14 PM
Ok well just because your new and I'm feeling in a good mood I'll ignore the fact you have bought the worst kit in the world and skip to what you need to know.!!

On the board you'll see 3 banks of Dip switches. (blue with white toggle switches). The position of these switches set the parameters for the drives.
#1 #2 set the micros stepping amount. These drives have limited choices which are 1, 2,8,16. This means 1 x200=200ms or 2x200=400ms, 8x200=1600ms, 16x200=3200ms.
So your best bet will be 2x200=400ms. So set switch #1 =OFF #2=ON

While your there check the other settings to make sure the drives are set correctly or close for the motor current.
These drives are weak and fragile things and advert states these motors are rated 3A and the drives Max is 3.5A. But the current settings are limited to 25, 50, 75 or 100% of the drives rating. So we can't set them exactly to whats needed for the motors so your best going under rather than over and stressing the board. 75% of 3.5A =2.6 which is close enough.

So set #5=OFF #6=ON

Set the Decay to 50% so #3=OFF #4=ON

Now your Steps per will be 400ms / 3=133.333

The velocity and acceleration define the maximum the machine can travel at rapid feed. The exact setting is machine dependant and many factors play a part in the actual setting you'll be able to use.
The major players involved are Mass to move, Friction, Screw lead and efficiency, Voltage steppers running on, Motor current, Motor spec, Micro stepping, Resonance, and several others. So there is no way I can advise exact settings for this it's purely trial and error based on the machine setup and quality of the electronics etc. At best all i can offer is a guide and that may be miles off depending on the many variables.!

What I will say is you have THE WORST drives in the world. With poor spec Motors running on too low a voltage and too low current. Connected to high lead low efficiency screws so don't expect too much.!! . . . and thats without knowing anything about the rest of machine.!!

Working on the fact the steppers are only running 24V and the motors are high inductance motors which is the worst combination for speed then you'll be lucky if they reach 600-700rpm without stalling. 500 rpm might be usable depending on rest of machine, friction etc.
So this means with a lead screw of 3mm and Motor rpm of 500 a maximum velocity of 1500mm/min before motors will reach saturation point and stall.
In reality this will probably drop to 1000mm/min for a stable machine.
Accelleration will be in the 200-300s/s region. Just be aware that if you raise Acceleration then you'll proably need to lower Velocity or vise versa. You can't have both high other wise you risk missed steps or stalling motors.

Like I said before don't expect too much from this setup because you have some very poor Kit connected to some Low spec mechanics.!!

21-05-2015, 11:35 PM
thanks alot im starting to get it know, as it is the first cnc i built myself i just bought any old motors etc just to have a feel on how things work once i have the hang of the cnc world i would love to build a better and stronger unit, what motors and drivers would you recommend im so grateful for all the help you have given me allready
cheers anthony:friendly_wink:

21-05-2015, 11:44 PM
what motors and drivers would you recommend im so grateful for all the help you have given me allready
cheers anthony:friendly_wink:

Don't worry about this now. Ask again at the time your ready to upgrade has things change quickly. Also there's other factors that need to be considered based on the machine design and component spec to make the correct call.

Just have fun learning with what you have now and Don't buy anything else without asking or doing some research as it's very easy to buy wrong or poor spec kit.
When you have cut your teeth or let out the magic smoke on this lot then come back and we'll put you on the right track.