Cnc cutting the wrong size..

[Tomnewry]

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.

[HankMcSpank]

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!)

[Tomnewry]

Hi my motor setting are set to 200 steps they are nema 34 motors, ill give it a try thanks.

[HankMcSpank]

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

[JAZZCNC]

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.

[Tomnewry]

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

[JAZZCNC]

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

Yes you change the dip switches to the micro stepping you prefer then and only then do you change the steps per in Mach.
The steps per hisn't something you play with to tune the motors.? You set it to suit the MS/Pitch relationship then leave it alone.

[Tomnewry]

Cheers ill let you know how it works out.

[Tomnewry]

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.

[JAZZCNC]

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.

Strange figure that.? This suggests 8.3mm pitch screw.? Do you have any gearing ratio on it.?

[Tomnewry]

The belt is a 5mm pitch as for the ballscrew i dont know, any ideas how to find out also can this harm the machine or what problems will it cause.
Thanks

[HankMcSpank]

The belt is a 5mm pitch as for the ballscrew i dont know, any ideas how to find out also can this harm the machine or what problems will it cause.
Thanks

Sounds like you're now sorted (ie 100mm is cutting 100mm), if you're using Mach3 & have access to a dial indicator, here's how to get really granular with accuracy....


http://www.youtube.com/watch?v=XkO5tc-jSxw

[JAZZCNC]

Sounds like you're now sorted (ie 100mm is cutting 100mm), if you're using Mach3 & have access to a dial indicator, here's how to get really granular with accuracy....

Purposefully didn't suggest using the auto feature has it doesn't really help Tom understand whats going off with his machine. Much better if you know the inner workings of the machine, makes fault finding and smooth operation much easier and quicker.


The belt is a 5mm pitch as for the ballscrew i dont know, any ideas how to find out also can this harm the machine or what problems will it cause.
Thanks

Ok well there's a few ways to find screw pitch but easiest way is to simply accurately measure the distance travelled for one full turn of the screw. This will give you pitch then you need to count the teeth on each pulley to find any ratio. (NOTE: 1 Turn of SCREW not MOTOR)

The belt pitch doesn't have any bearing on setting the steps per unit or distance travelled it's just the spacing between teeth.

The other way to find pitch is measure distance between the ball track centres. This technique works ok to give quick eyeball idea but it can catch you out if the screw is multi start.? Multistart screws are common with lead type screws and some of the higher pitch ball screws.
Basicly multistart use 2 or more tracks added together to give affective pitch per rev. IE: 10mm pitch with 2 starts each track will measure 5m centres.
The measuring distance travelled technique is preferred has it's more accurate and less prone to error.

[HankMcSpank]

Purposefully didn't suggest using the auto feature has it doesn't really help Tom understand whats going off with his machine. Much better if you know the inner workings of the machine, makes fault finding and smooth operation much easier and quicker.

....when there are a few unknown variables involved (unknown pitch, pulleys etc), the auto calibrate may not be the most educational metho (from a getting deep down & dirty with steps per unit aspect), but it will likely be the most accurate/painless!

[Tomnewry]

Hi guys thanks for the info. Still not quite sure so im gonna measure between the screw that idea seems the easy one an then ill get back to you with the results, in saying that I cut something today and the size was exact i cut it 640mm by 400mm an it was good so as i say ill keep you informed. Thanks for all the comments.

[Tomnewry]

Oh and sooy about this one but where is the auto calibrate in mach?

[Tomnewry]

Hi guys on another note, I have basically ripped out all the electrics of the old machine and replaced everything but the transformer which runs the 120v porter cable motor but now for some reason the porter cable motor is not running at full speed, I have checked all the wiring and it a ll seems ok, any rough ideas.
Thanks

[Mansour1974]

Thanks Hank
I was looking for this solution since 2 days, mine was 320 steps per unit, I change it to 400
it is perfect and accurate now.

thanks again :)