Re: building a 3or4 axis cnc

--- In CAD_CAM_EDM_DRO@y..., jim austin <bartles8971@y...> wrote:

>
> Tom,
> My plans are to be able to cut a 4x8 sheet of wood or steel. At

That gives us some more info to work with.

First lets address the basics.

There are three distinct pieces to the hobby CNC puzzle.

The first is CAD. Used to draw the item you are going to make, there
are dozens of solutions. If all of your work is 2D with the third
dimension (Z depth) then there are some really inexpensive
solutions. A torch table is basically a 2D process with Z just being
used to move the torch head down to the work and back up. 3D
modeling and construction opens up a whole new set of problems and
options (to be left to another response).

The Second is CAM. The raw CAD file (DRW, DXF, EPS, etc) is imported
into a CAM package. It can be as simple as one of the DXF to g-code
programs and some hand editing, all the way up to AlphaCam or one of
it's high cost commercial relatives. In the more sophisticated
programs you can use the graphical interface to build a cut file with
depths, and in some, even view the rendered finished product in 3D.
The final job of the CAM program is to spit out a g-code file that
will work with your controller software (discussed next).

The Controller Software takes the g-code and translates it into step
and direction pulses and basically generates the low level signals to
run your actual machine.


There is one more piece that could be considered part of
the "electronics" and that is the Motor controller(s). They take the
step and direction pulses from the software controller and turn them
into power switching to control the motors.

A true servo system uses encoders that give pulses back from the
moving parts of the machine so that the controller software
and "measure" where it is versus just trusting that the motors
actually went where they we told to. The terms "Closed Loop" is used
to describe a servo system where position feedback is used to adjust
to actual output to the motors. "Open Loop" woould be simple
steppers that are given pulses and we have to assume they actually
did what they were told. True servo systems are complex because of
the delay between the command to move and the actual movement. Since
the Controller Software has to be custom for the type and scope of
the feedback it all has to be designed together and is outside the
abilities of most hobbiests. (hang onto that thought)

Until recently controllers were proprietary "boxes" with their own
software to control a specific machine (Fanuc, GE, etc). You could
only use that controller with that machine and if it broke you had to
have that manufacturer fix it. One of the things that has made this
whole hobby CNC things explode is the availability of low cost
generic controller software that will run on a PC. There were
challenges. If you use the standard ports on a PC (parallel or
serial) there is a finite limitation as to the total number of pulses
per second (PPS) you can get out. The serial port can only pump one
bit at a time and must use some of the bits to identify which device
it is talking to. Some vendors have come up with solutions that let
the serial port "talk" to an external device and use it to actually
generate the pulses. A variation of that is a custom card that plugs
into the buss of the computer and outputs pulses. The limitation of
both of these approaches is that the conroller software must match up
to the external device/internal card so you are tied to the
combination. Several vendors decided to make things more flexiable
and use the parallel port to actually interface directly with the
Motor Controllers (step and direction signals) You can change the
parallel port controller software without changing or buying anything
else. There is a debate about whether DOS, LINUX or WINDOWS is the
better operating system on which to run the controller software. It
has been debated at great lengths on this forum.

Now back to the Motor Controller. Remember the servo issue? Well
several vendors have solved the complexity and proprietary issues.
Gecko and Rutex are two that come to mind. Either will take step and
direction commands in and with the encoders hooked back to them form
a closed loop system. The Controller Software dosen't have to make
decisions about pulses vs position, the Motor Controller does that.

Soooo. Now you can buy a low cost Motor Controller with built-in
Closed Loop functionality and use Servo Motors but control the whole
thing with flexiable software running on a low cost PC.

I apologize for the long response, but I sense there are a LOT of
guys just like you lurking in the wings (2900+ members) that are not
into asking open questions. All of these things (and much more) have
been discussed, re-discussed and debated to the point of boredom.
every once in a while we (the more prolific contributors) have to
take a deep breath and realize that the best thing we can do to
propagate our hobby and keep low cost solutions rolling is to bring
in to turn interested lookers into full blown builders and users!

Read the posts, check the achieves and ask questions. You can get
help here with just about anything regarding Hobby CNC. As you learn
more and your questions get more focused, so will the answers. One
thing is for sure: After you build your own machine and it makes its
first controlled cut, you will never be the same!! ;-)