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Motor Tuning Issue
« on: March 16, 2009, 07:23:04 PM »
I am new to CNC milling. I haven't even cut anything with my machine yet. So far, I have just done simulated cuts. By "simulated" I mean that I load simple G-code and run the program without the router attached to the machine. The machine seems to go through all the right motions. The problem is, the X-axis and the Y-axis are extremely jerky while jogging or while running a program. However, and this is the strange part, both axis move fine while in the motor tuning menu (when I press the up/down arrows on the keyboard to preview the settings). The stepper motors sound good and their movement is smooth. When I exit the motor tuning menu and jog the X/Y or run a program, the movement is back to being jerky. I don't have any trouble with the Z-axis though. What could the problem be?

I'm not sure what information would be relevant here. I am using Mach 3 version R 3.042.020. My machine is a 3-axis, stepper motor with rack and pinion setup.

Offline jimpinder

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Re: Motor Tuning Issue
« Reply #1 on: March 17, 2009, 05:25:26 AM »
The thing is, without a machine attached, you can't have a proper go at motor tuning - which depends on your motors on your machine, not on Mach 3.

I have Mach installed on two computers - one in my "office" - front room - where I do all my programming and GCode work, and the other in the workshop.

The one in the workshop attached to my lathe/mill is tuned to that - i.e. rapids 40 ipm acceleration at 3. This is the fastest my machine will move reliably - because it is reliability that you must aim for when machining.

The one in my office, I have the rapids set at 160 ins per minute (my lathe bed is only 20 inches long) - four times faster, so that when I am running a test program, it speeds things up a bit, when I am watching. A G1 move does not matter, because the speed for this is specified in the GCode anyway.

The only way to reliably set the motor tuning is to attach the machine. If you ony have the one computer, keep the machine attached, but do not put any tools in so nothing gets damaged.
Not me driving the engine - I'm better looking.

Offline Tweakie.CNC

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Re: Motor Tuning Issue
« Reply #2 on: March 17, 2009, 12:46:03 PM »
Hi,
Two things to check:-
1) Are you saving the changes you make under the motor tuning menu ?.
2) Do you have 'constant velocity' checked in the general logic config page ?.
Answers to both the above should be yes.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: Motor Tuning Issue
« Reply #3 on: March 17, 2009, 04:46:27 PM »
I appreciate the replies. I am saving the changes that I make and constant velocity is checked. It seems that the best thing to do is what jimpinder hinted at, and that is to disregard the performance that I get in the motor tuning menu. I have found a steps/velocity/acceleration combo that seems to work well outside the motor tuning menu, so I'll stick with that.

Offline jimpinder

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Re: Motor Tuning Issue
« Reply #4 on: March 19, 2009, 02:47:24 AM »
Now you have got this far -

The steps per unit is a fixed number - it is nothing to do with tuning. It is the number of "pulses" the computer has to put out to move the axis by one unit - eg mm or inch. This is usually quite a large figure - eg.
In inches - my motor require 200 steps per rev, my drives also do 10 micro steps per step - so that is 2000. I have step down gearing to run my leadscrew - 3 to 1 - that is 6000 and my leadscrew requires 10 turns per inch. = 60,000. So my steps per unit is 60,000. That is a fixed number of pulses and is not negotiable, otherwise you machine will not be accurate. You must do your own calculation for your machine.

Modern motors seem to be 200 steps per rev, drives give 4,8,10 or sometimes 16 microsteps and this you can set. Gearing - if you have a direct drive ignore it. The only thing is the leadscrew - and this should be clearly set out in the lathe/mill  documentation.Beware someone saying you have inch screws, when infact they are metric - i.e. 25mm.

Once yu have the steps per accurately set, you can start tuning. Set the speed of the axis about 10 per minute,acceleration at 1 and jog. If it takes that, increase the speed in chunks of 5 until the steppers clearly baulk and miss steps. Back off the speed until you get reliable movement again.

Repeat this with the acceleration and you are set.



 
Not me driving the engine - I'm better looking.