[TheForge] Re: 3 axis plasma cutting

Sat Nov 8 05:49:01 2003

My "rope wrench tools" were cut on an optical cnc using an ESAB plasma 
torch.   I had originally designed them on that wonderful CNC program called 
MS Paint <g> but ran the resulting image print thru my Brother Fax Machine 
which used a roll of carbon type paper, which the CNC optical then followed 
without a glitch. 

Another neat thing about an optical CNC is that an artist can make sketches 
using a special ink, and these can translate directly into steel.  Following 
this thread, it should be possible to marry a sheet of thin "scrap" steel to 
wood like from an old 55 gallon drum...  to wood, and get it to burn a copy 
of the design on other media. 

Dann Johnson 

***********
Jay Hayes writes:
********* 

> 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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>  
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Dann Johnson 