[TheForge] Re:3 axis plasma cutting

Sat Nov 8 00:22:26 2003

I have a CNC plasma that was originally programed via a tracer eye that 
followed a line or edge of a line. The machine can cut a 72 x 128 plate 
and was built by ESAB / Heath with a Compupath controller. There were 
several short comings with this type of setup. First the speed needs to 
be slow for the tracer to stay on the line when it came to a sharp 
corner or it would get an off line error. The eye would sometimes get 
confused on inside corners and just stop dead. Even worse a microscopic 
break in the like would cause the eye to reverse direction. This would 
be very frustrating on a complex shape when the eye would jump the line 
after it spent a half hour creeping around the edge of the pattern. 
Fortunately we were able to record the trace before actually cutting the 
part. The trace could then be scaled up or down and played back at a 
faster or slower speed for cutting.

Optical tracers are somewhat complex. They have their own light source 
and use a small oscillating convex mirror to read the  black on white 
line or edge of a silhouette. Dirt is always a problem. A dirty lens or 
dirt on the pattern can cause problems. I once had a fly land on a 
pattern and the eye followed it around for a while. Luckily it wasn't 
cutting an expensive plate at the time. The pattern must also lay flat 
and stay put. If part of the pattern curls up the line will be out of 
focus and can not be read. If the pattern moves during programing the 
part will be distorted.

After the controller on the machine was fried in a lighting hit we 
eliminated the eye and went strictly CNC. Now if I want to cut a part 
from a free hand drawing I scan it and use Corel Trace to create a dxf 
file. Then I use a CAD-CAM program to generate the G-code that the CNC 
plasma uses to cut the part. For the most part this works much better. I 
can actually cut more intricate parts and simple parts are a snap. If 
the drawing was done on a napkin it might take a while to clean it up to 
generate good clean code. But a good clean drawing can go from scan to 
code in about the same time as the old tracer.

The down side is the new setup can not retrace in reverse. Reverse trace 
is especially helpful when cutting a part and the torch stops cutting. 
The torch could be backed up along the cutting path by pressing the 
Back-up button and the arc could be reestablished where it was lost. 
When the button is released the cut proceeded as normal. With the new 
setup we have to return the torch to the home position or start of the 
cut. Then do a dry cut until the torch approaches the point where the 
cut was lost. The machine is than paused until the cut is reestablished. 
Corner slow down also has to be programed in.

Some systems can do both but they are not cheap. With most systems the 
operator must tell program which part of a part needs to be cut first. 
The holes in a part should be cut before the outside of the part is cut 
or the part may drop before the holes are cut.

Jay Hayes

shannell Sugrue wrote:

>Yup, CAD or however you use a puter to do a .dxf or whatever type of file
>your cnc software uses can be a NIGHMARE!
>Is this a table you built or bought? The optical trace is intersting and I
>havent really seen one up close. Its got a computer in the loop somewhere??
>how does it scan and find cuts within the main outline and how does it
>determine, what to cut out first? or has that never been a problem, its
>really only matters with small pieces that will drop to the floor through
>the grate. What is the eye? do you need to illiminate the drawing? Does it
>work on a scale up method, you draw at x and the machine cuts at 3x?
>----- Original Message -----
>From: "Ries Niemi" <[email protected]>
>To: <[email protected]>
>Sent: Saturday, November 08, 2003 5:19 AM
>Subject: Re: [TheForge] Re:3 axis plasma cutting
>
>
>  
>
>>My plasma table, which is not cnc, but optical trace, has 3 axis
>>motors, and I wouldnt think of having one without it. The third, z
>>axis, is controlled by  a voltage sensing circuit that samples the
>>voltage from the plasma power supply. It then adjusts the height
>>accordingly, so the arc gap is always consistent. This means the torch
>>is always cutting at its most efficient, but more importantly it will
>>follow the warped surface of a piece of plate. Even though I built my
>>water table flat, then levelled it in the shop, not every piece of
>>sheet metal is flat. And then, when you start cutting it, it warps like
>>crazy. Especially thinner stuff like 16ga- It can warp an inch or more
>>in height as you cut parts out. If the plasma cutter doesnt have z axis
>>adjustment, the torch either runs into the metal, and you lose
>>registration, or it gets so high you lose contact and the torch goes
>>out. Either way, bad news. So 3 axis is needed unless you want to
>>manually adjust the torch height every inch of the way.
>>Personally I prefer my optical trace table to a full on cnc one. I can
>>do a full size drawing of the part up to 4' x 8', and it goes on the
>>right side of the machine, then I lay a 4x8 sheet of metal on the water
>>table on the left. It is a little slower than full cnc when you are
>>cutting more than 20 or 30 parts, but I dont do that kind of quantity
>>anyway very often. More often I will be cutting a full sheet in a very
>>complicated pattern, once. I just finished a job where we had to make a
>>4 foot by 16 foot artwork panel out of 3/16" plate, and to troubleshoot
>>the design in software on the computer would have taken twice as long
>>as cutting it would have. Plus, I am an artist, and it is easy enough
>>for me to draw what I want.
>>
>>ries
>>
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