Thread: Cnc cutting the wrong size..
Hi guys i have my machine all set up with 3 nema 34 motors an drivers DM860A and all is working good execpt when Isend to cut a 100mm square it only cuts it 80mm can anyone help with this, thanks i will be very grateful.
Increase your present 'steps per unit' (motor) setting by 25% ....then a present 80mm cut will become 100mm.
Goto the motor settings of your CNC app & for example, if the steps per unit are presently 500, change it to 625 etc (625, equates to 500 + 25%)
(btw your motor setting almost certainly won't be presently set for 500 .... it was just an example!)
Last edited by HankMcSpank; 04-07-2013 at 09:54 PM.
Hi my motor setting are set to 200 steps they are nema 34 motors, ill give it a try thanks.
Well, the correct method is to tell us your leadscrew pitch & whether you are microstepping or not, we can then tell you what entry you should make for the 'steps per unit' setting in your CNC app, but in the absence of that info, if your 'steps per unit' setting is set to 200 at the moment & you are saying that a 100mm piece comes out at just 80mm then increase the steps per unit setting to 250 & that should see a 100mm piece come out at 100mm
Tom 200 is the full step count for your steppers but your drives will have micro stepping. Most drives don't allow full step and start at what's called Half (1/2)step or 400 Micro-steps per rev. Also shown as 2X on some drives IE: 2 x 200.
So the way to calculate steps Per Millimetre is to take the micro-step unit and divide by the pitch of the lead screw. IE: drives set at 800 MS on 2.5mm pitch 800/2.5 = 320 Steps per millimetre.
Regards setting or choosing the micro stepping then here's a general idea of the affect they have.
Choosing a lower MS number IE:400 will allow faster motor speeds in tuning due to less pulses required from the Parallel port but not give such smooth running of the motors. It can also cause motors suffer from resonance more has well.
Higher MS will give much smoother running of the motors but demands much more from the PP so less pulse's equal less speed in motor tuning. Doesn't suffer from resonance so much has well.
That said then there is a limit to how much MS is useful or usable and the rule of thumb is don't go much higher 10x or 2000 pulses has the motors can't resolve to much more than that and your just wasting and stressing the PP more.
Personally I prefer higher MS has rapid speed is of little concern to me and prefer smooth running motors so often use 2000.
Last edited by JAZZCNC; 05-07-2013 at 07:21 PM.
Hi Jazz thanks for that ill give that a go, does that mean i change the dip switchs on the drives to suit the higher steps or change it in mach 3. Thanks
Cheers ill let you know how it works out.
Hi guys i have been messing about with the cnc all day and have managed to get it to cut to scale ie: cutting a square 100mm when asked. Now to accomplish this i have the driver set at 2000 steps on the dip switch and i have it set at 240 steps in the motor tuning settings, it is cutting to scale but can you let me know if this sounds correct.
Thanks in advance.
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