Re: Still a little unclear

Don Hughes wrote:

> From: Don Hughes <pencad@...>
>
> Members:
>
> Could you please give me a little more understanding of what all
> hardware is required for the CNC systems. I have been watching posts
> from Jon and Dan and a few others about the controllers and the cards
> etc., but I am still unclear as to what all is involved.
>
> Am I right in assuming that there are many various ways in getting a DXF
> file or whatever format to the machine for processing. From what I
> believe thus far, I can produce with my CAD a file that can be
> translated into code by a CAM program and then at this point I become
> unsure of the "various" means of getting that information to the
> machine. I have downloaded many demo programs from the CAM side of the
> house, and these have helped me to get somewhat comfortable with the g
> and m codings.

Yes, there are many programs that will turn some information into the
G-code (RS-274D) common to most machine controls. I use Bobcad/CAM,
which will take in a DXF, or allow you to create a drawing in Bobcad
itself. But, for most simple operations, I have my own, text-based
programs, which ask a few questions, and then create the G-code to
do simple operations like boring holes with an end mill, milling out
rectangular pockets, drilling a circular array of holes, etc. So, for those,
ther is no cad drawing.

> I do realize that I have to have a controller that will send the info to
> the motors for processing of the g-code, but it is all this interface
> from the PC to the machine that has me unsure of the sequence that this
> hardware must be in so that all the parts work efficiently.

Well, for steppers, many systems use the parallel port, and these signals
are connected to stepper drivers. The stepper drivers are then connected
to the stepper motors.

For DC servos, generally, a plug-in card generates analog voltages that are
proportional to the velocity desired. These analog voltages are fed to
servo amps, which drive the motors. My servo amps use DC tachometers
so that the servo amp can make the motor run at exactly the velocity
commanded. Often, the PC plug-in card also has quadrature encoder
counters, so that it can monitor exact machine position. There are many
variations on servo architecture, so not all of them work this way.
Some have the computer calculate actual velocity from the encoders,
so no tach is needed. Some systems drive brushless DC servo motors,
some drive AC (Induction) servo motors, etc.

The above description with DC servo motors, etc. is what I'm using.

Jon