Re: From the Begining

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Daniel T. Medlin"
<dmedlin1963@y...> wrote:

> I WANT TO CONVERT A BRIDGEPORT TO A LAPTOP CONTROLLED
> 4-AXIS SERVO CNC SYSTEM. PRECISION IS NEEDED MORE THAN
> FEED RATE.
>
> I HAVE BEEN A CNC OPERATOR FOR YEARS, BUT I AM NOT
> COMFORTABLE WITH THE SELECTION OF ELECTRONICS
> COMPONENTS.
>
> FROM MY RESEARCH ON THE WEB LEADS ME TO BELIEVE THAT I
> WANT TO USE MACH 2 AND GECKO SERVO DRIVES, BUT THIS IS
> WHERE THINGS GET CONFUSING.
>
> WHAT MOTION CONTROL BOARD, SERVO POWER SUPPLY, SERVOS,
> ENCODERS, AND BREAK-OUT BOARDS SHOULD I USE?
>
> CAN I USE THE EXISTING DRO AS THE SERVO ENCODER FEED
> BACK?
>
> THE MILL ALREADY HAS HIGH PRECISION BALL SCREWS.
>
> I KNOW SOMEBODY OUT THERE MUST HAVE ASSEMBLED THEIR
> OWN, RATHER THAN BUYING A MUCH MORE EXPENSIVE
> PRE-ENGINEERED KIT.
>
> PLEASE ADVISE ME ON COMPONENTS.
>
> DANIEL T. MEDLIN
> danieltmedlin@y...
>
>

Daniel. First turn off the Caps Lock key on your keyboard. In web
communications caps are like yelling. (not trying to be nasty).

There have been many of the group that have done exactly what you
want plus there has been an article in HSM (Home Shop Machinist) for
doing a Bridgeport conversion.

The Gecko (and Rutex) servo drives a quasi-closed loop. What that
means is that the control software (MACH 2) "thinks" it is talking to
a step and direction motor controller (which it is). The Gecko's
readed the encoder position, read the requested steps and direction
and try to adjust the motor speed to make the two signals match up.
The controller software is unaware of any error and only if the
Gecko's fault from running past the error count can the software be
told there is a problem and e-stop the system. All that being said
there are hundreds of mills out there running with the exact
combination you listed.

As for using the existing DRO outputs the answer is "maybe" (Don't
you love the consiseness of that answer!). Any backlash in the
system between the motor and the encoder could cause the Gecko to
over correct and possibly break into a violent oscillation. If the
ballscrews are zero backlash and everything is really tight you might
get away with it, but if it's that tight you would get equal
performance from motor mounted encoders without the risk of "The
Dance" as has been described by others.

There is one system out there from Servo's To Go (segway for Jon!!)
that works with EMC and closes the loop in the controller software
and you can adjust the deadzone to cut down on the oscillation. EMC
as you probably know, runs under Linux so you need to be willing to
deal with that as well.

Read through the archieves (groan) and slog through the posts on
Bridgeport conversions and you will gain a lot of insight.

We will help where we can and I am sure others will add to this
thread. Welcome to the world of DIY CNC. You might want to join the
group DIY-CNC. It's a smaller subset of the membership here (same
twisted group of guys) but harbors more of the experienced cnc
machinists.