Re: [CAD_CAM_EDM_DRO] Re: Using CNC as DRO to save $$$

It will eliminate the backlash to the DRO, but it will NOT eliminate the
backlash associated with the cnc control system and the system will just
sit there and "hunt", sometimes violently. I tried that and unfortunately,
it didn't work.

At 08:34 PM 3/24/2004, you wrote:

>Hi, don't know your name..but thanks.
>
>This hole thing begs the question. Can an accurate DRO (one that
>tells you where the table is rather than how many times the screw
>has turned (converted to travel of course) be used to power the CNC
>controller instead of the rotary encoders that I have? My thinking
>(now anyway) is that this would be a way to eliminate backlash as
>well as provide for a DRO when working in manual mode.
>
>Thoughts...comments???
>
>Thanks again
>
>John
> > >
> > > John
> >
> > In order to use the Software Controller as a DRO you need two
> > things. You need a encoder mounted on your equipment (it could
>take
> > the form of the encoders on your servos if they can be installed)
>or
> > even linear scales on the table. The encoder output needs to be
>dual
> > channel.
> >
> > The other thing you need is controller software that will read and
> > display the outputs from the encoders with the proper scaling
> > factor. MACH2 does this and I think EMC does as well. While the
> > DRO's are quite usefull for machining they will read where THEY
>are.
> > That means that if they are mounted on a part of the machine that
>has
> > backlash (i.e. current leadscrew) they will faithfully record the
> > backlash as part of the count. While backlash compensation is
>part
> > of the controller program I don't think it is implemented in the
> > direct readout of the encoders. EMC may have that capability.
> >
> > If you do not have your motors mounted and still want just a DRO
> > function check out the yahoo group over at ShumaTech. They take
>low
> > cost Chinese Scales and use them to build a 3 axis DIY DRO. If
>you
> > can make your own PC boards you can throw one together real
>cheap.
> > Even if you buy the complete kit you will pay half of what you can
> > get the cheapest commercial unit for.
> >
> > Once you have your DRO installed you will walk over to the wood
>bin
> > and use a 2 X 4 to pound yourself on the head for being so slow!
>An
> > accurate DRO (one that really reads where your TABLE is) makes
> > cutting accurate slots, drilling precisely positioned holes and a
>lot
> > of other general milling tasks easy and quick. Want to build a
> > mounting plate that has to match up with another? Draw them in a
>CAD
> > package and fetch the XY cooridinates of each hole. Now just dial
> > the number into the DRO with your manual handles and put the holes
> > exactly where you want them. It would have taken me much longer
>to
> > build my CNC machines had I not first outfitted my mill with a
>DRO.
> > I have my big mill partially converted to CNC. It has DC motors
>on X
> > & Y and a lowcost variable speed controller. I also left the
>handles
> > intact. I can use the motors to rapid to a spot then fine tune
>with
> > the handles while watching the DRO. Since it is mounted to the
>table
> > it automatically takes care of backlash. When I finish the
> > conversion and install my zero backlash ballscrews and the Z axis
> > motor the existing DRO will become redundant. I will probably
>move
> > it over to may lathe which will be the final machine I will CNC.
>

Bubba

OLDER THAN DIRT
Country Bubba
(Actually the inventor of Country and Bubba)
In God We Trust

axtein@...
LaGrange, GA

http://ns1.dicomm.net/~axtein/dro


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