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Losing steps
« on: June 17, 2009, 11:22:46 AM »
I've been running an old Dyna2800 that I converted to Mach 3 using the CANDCNC breakout board and keeping the original Dyna stepper motor drivers. Running in Sherline mode, it has performed well (a vast improvement on the original Dyna controller!).

This morning I ran a couple of parts and began losing steps on the Y axis. The Y axis motor "growls" and loses about 20% of the commanded travel, but only on the Y axis.

After messing around for while, I discovered that if I increased the feed rate from the original 2 in/min to 10 in/min (sorry for not using metric, but I'm an old guy whose brain thinks in what my first boss called "Christian Units") the machine ran the program perfectly.

It occurs to me that this is the first time I've run this machine this slowly (I'm usually making plastic parts), so that would explain why I never came across this before.

I suspect that I should just install modern drivers (I know, I should have done that in the first place), but I'm curious if there might be any other explanation for losing steps only a slow speed...


Re: Losing steps
« Reply #1 on: June 17, 2009, 01:26:02 PM »
Could be mid-band resonance, which is an inherent characteristic of stepper motors, and can be mitigated by using drivers that damp it out (i.e. - Geckos), or adding harmonic dampers to the axis.

Ray L.
Ray L.

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Re: Losing steps
« Reply #2 on: June 17, 2009, 02:37:13 PM »
If it is a simple job, swap the drivers - i.e. run the Y on the X driver card and vica versa.

If the trouble transfers to the X axis, this would indicate some problem om that card, which as Ray said, could be replaced (and I would not dispute using Gecko). If the fault does not transfer itself, then it might be a problem with the motor.

How many steps per inch are you using ? Is there any stiffness or wear on the axis that the slower speed has difficulty in overcoming.
Not me driving the engine - I'm better looking.
Re: Losing steps
« Reply #3 on: June 18, 2009, 01:54:05 PM »
Thanks for the suggestions.

I'm set at 5082 steps/inch, with a max speed of 40 in/min., Sherline Mode(required by the Dyna drivers).

After running the parts I needed by reprogramming at a higher speed and reducing the depth of cut, I played around with this a little more. All three axes have the same problem, but the onset is at a slightly different speeds on each axis. (The Y axis happens to be the highest speed at about 3 in/min).

I'm thinking that Ray's suggestion that it might be resonance in the motors is probably correct, especially given the sound of the motors when this occurs. At higher speeds, the motors"sing" with that pleasant sound I suspect we are all familiar with. The growling they make at low speeds is pretty awful to hear...

The motors and drivers are the original Dyna parts that have served well, but I think maybe its time to move into the modern era and replace them...

Thanks again,

Steve F.