Re: CNC PLASMA

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "billchambers64"
<bchambers@a...> wrote:

> <SNIP> I DONT KNOW IF THESE MOTORS ARE STRONG ENOUGH NOR DO I KNOW

IF THIS

> PACKAGE WILL DO WHAT I WANT.
> I WOULD LIKE TO BE ABLE TO SCAN IN IMAGES AND TURN IT INTO A
> CUTTING PATH FOR MY PLASMA TORCH. I WOULD ALSO APPRECIATE ANY KIND

OF

> ADVICE ON LOW BUCK TIPS THTA ANYONE MAY HAVE FOR THIS PROJECT. I

HAVE

> SEEN THE TORCHMATE AND OTHER SITES LIKE IT. WAY TOO MUCH MONEY FOR

ME.

> I HAVE THE TABLE AND LINEAR MOTION STUFF FIGURED OUT BUT I AM

NOT

> MUCH IN THE WAY OF COMPUTORS.
> THANKS FOR ANY HELP
> BILL C

(Long deep breath...) Well Bill there are a lot of things to
consider about plasma cutting with CNC. Depending on the material
you plan on cutting, you may need feeds of 200 or even 300 IPM. When
you start runing your numbers for feeds and speeds try and design for
that range. If you are going to use the moving gantry design you
will have to have enough power to accelerate and move it at higher
rates. The problem with steppers is that they can be REAL tricky to
get high RPM's out of. The type of motors and the drive design along
with the max supply voltage have a lot to do with how fast you can
move things.

You will find that simply scanning in a picture and having it in a
form that the torch can cut is not a piece of cake. Here are some of
the issues:

1. The torch has to move down slightly above the material and do
an "Arc Start" (piercing of the metal with the arc). The amount of
time this takes varies with the thickness and type of material. The
pierce point makes a divit about 2X the normal diameter of the cut
kerf. You don't want the motion to start until the piercing is
complete but the longer you delay after the metal is pierced to
bigger the divit hole gets! It helps to have a CAM package that will
let you define the toolpaths and automatically calculate a "lead-in"
cut to statrt to torch in the waste part.

2. You can't cut a bunch of lines close together. A plasma torch is
not a precision tool. The flame is not "solid" like a milling cutter
and is sort of like dragging a piece of rope. Changing cut directions
will cause shifting of the flame. Cutting lines too close causes the
center piece to melt out. The torch will spew molten metal, "hiccup"
and try to restart as it goes across a void. You need to plan the
order of your cuts for best results. Most packages that take a
picture (bitmap) and turn it into a vector file that the conversion
software that generates g-code do a marginal job of converting and
for decent results you will spend many hours cleaning up the
resulting files. You want nice smooth lines and arcs on longer cuts
to keep the machine from dancing across the floor.

3. As stated earlier the torch tip cannot touch the material (unless
its a drag style tip) so you have to either keep the material real
flat since the optimum cutting height is between .125 and .187 above
the material or you have to have a way to automatically adjust the
torch height based on the gap voltage (which is directly purportinal
to the gap distance. Commercial Torch Height Controls (THC's) are
expensive.

4. The cost of consumables is a real concern. I have found several
on-line sources for the electrodes and tips that are MUCH cheaper
than buying from your local supplier. Drop me a line and I will
share them with you.


5. Your controller software needs to support continuous contouring
(Constant Velocity = CV) so the moves are smooth and constant. Jerky
moves cause the floppy flame to "color outside the lines" The
controller software is one of the pieces you need in the CNC
equation. It takes g-code and puts out signals to the motor control
modules. Hobby versions of controller software are available that
support CV.


There is a group at MACH1MACH2CNC yahoo that is for a controller
software package that works well with plasma. We have developed a
low cost THC add-on for the software that will make your cutting
experience much more fun. I just finished doing final testing on a
version that is going to be available assembled and tested. If you
are electronically inclined a schematic is available for free on the
above site.

All of the issues I listed need to be considered and planned for but
don't let them stop you from building your machine. There are
solutions and sources for artwork that will help..We (the cnc
Torchheads) will be glad to help you along and tell you about our
experiences.

Tom C