Re: Using CNC as DRO to save $$$

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "camfambmw540"
<jcameron@f...> wrote:

> In the name of conserving $$$ to be used on needed tooling, I am
> trying to save the expense of a DRO. While I now realize how
> valuable they are (just tried milling a slot in a 2" thick CRS

plate

> without one...yikes, keeping track of rotations to tell me how far

I

> have moved drove me nuts), I was thinking that since I am planning

on

> converting my Bridgeport Series I to CNC(I have the Servos, Amp's,
> encoders and Turbocadcam, need power supply and controller...still
> working on electronics, plan on talking with Jon Elson again when I
> get up to speed..just trying to understand what I am buying before

I

> buy it as well as understand how to hook everything up), can't I

use

> the CNC display on my computer screen as a DRO??? The GUI I have
> seen (MACH I comes to mind, but I could be wrong) shows what looks
> just like a DRO display.
>
> Immediate questions would be 1) Can I still operate it manually and
> get a display 2) If yes, can I still compensate for backlash (still
> have ACME screw until I can afford the G for ball screws. 3) Any
> other issues that my newbeeness has overlooked?
>
> Thanks very much for any help.
>
> 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.