Re: [CAD_CAM_EDM_DRO] Skipping steps, avoiding resonance

At 10:35 PM 12/1/02 -0800, Randy Gordon-Gilmore wrote:

>I have a perplexing problem. Recently I've put together a gantry milling
>machine from mostly surplus components--a Techno-Isel stepper-driven

gantry

>table (500x500x100mm theoretical travel, with zero-backlash leadscrews),
>three General Controls "Dragon Driver" 2.5A bipolar chopping drives (fed

by

>a 28V transformer) and a Sherline headstock, for which I made an adapter
>plate to bolt it onto the Z-axis carriage.
>
>I finally got the thing running this weekend, and it is not running
>reliably. I have the drivers in half-step mode, which with the 4mm-pitch
>leadscrews gives 2540 steps per inch. I can jog the X and Y axes at
>insanely fast speeds (200 ipm,) but I have fast jog set at 60 ipm. I'm
>using CNCPro running under DR-DOS 7.03.
>
>Running 3-axis Gcode generated by Visual Mill Free, all 3 axes "drift"
>during machining. I'm cutting acrylic with a .50 roughing endmill, with
>cutting speed set at 10ipm. The motion is actually more like 2.5D--it

goes

>to a depth and then pockets that level before going to another depth and
>repeating. It spirals down into the material.
>
>Cuts at each depth should line up, but I end up with a "terraced" effect
>because the X and Y will shift by .005-.050 inches between depths. I have
>also measured the depths (each "level" is set at .050") and they are not
>consistent either.
>
>I'm baffled, but suspecting that the spiraling between levels is what is
>throwing things off, along with maybe arcs in XY. When I got the drives,
>they were set for full stepping, and the table lost steps very easily. I
>think that the coarse leadscrews contribute to that, because the motors

are

>stepping fairly slowly. Now that I have the drives set for half stepping,
>and have a decent acceleration programmed into CNCPro to get to running
>speed quickly, and keep all my motions to 5 ipm or more, I seem to be
>avoiding the low-speed resonances OK for straight moves.
>
>But what can I do about arcs? By their nature they will be moving the

axes

>slowly through the suspected resonance range, and spiral leadins will do
>that for all 3 axes. How do I avoid that?
>
>I truthfully don't know for sure that the arcs are the problem. But
>straight-line motion with handwritten Gcode works fine. I have the

stepper

>motor cables away from AC cords as much as possible. CNCPro works pretty
>reliably on my Sherline mill, so I don't think that is the problem. I

have

>run the Gcode through editNC to backplot it, and it looks fine so I don't
>suspect that. So that pretty much leaves arcs and spirals, doesn't it?
>
>Newbie again,
>
>Randy
>
>Randy Gordon-Gilmore ,----.___________ ______________ _________________
>ProtoTrains // = = === == || == == == = || == == == = ==

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Randy Gordon-Gilmore ,----.___________ ______________ _________________
ProtoTrains // = = === == || == == == = || == == == = == =|
Rio Vista, CA, USA /-O==O------------o==o------------o==o-----------o==o-'
zephyrus@... http://www.prototrains.com