Template for plasma-cutting - shape-plot size-accurate on paper

The math for offestting a complex shape can get pretty complicated.

An easier way might be to jigsaw out the actual shape in a piece of thin plywood, then repeat on a second piece using the first piece as a guide for a router with a collar/bit that mimics the torch. You can use the "inside" or "outside" of your cutout to produce the "outside" or "insde" pattern.

I've plotted large patterns from Autocad on multiple printer pages and taped them together with good results - though not for plasma cutting.

them together

Jim, everyone

Thanks for response.

The shape & maths. Is it complex?

Chosing your shape is not that difficult. You play with a program like "gnuplot" where you enter a formula and it displays it as a graph. You play around until you get a pleasing shape which is right for the job.

The blank will "follow around the curve", and even living here in Cambridge, UK, where there are a lot of mathematicians (think of "Beautiful Mind" with Russell Crowe), didn't end up with an answer to that - how to calculate the blank shape. Basically you would need to do some calculus.

There is a much easier "approximate" method. You do a summation integral. The length of an "element" of length on the blank is given by Pythagoras. The equation you chose tells you exactly the delta-y for a small length delta-x. Pythagoras tell you that the length of the blank in this interval is Square-root(delta-x squared + delta-y squared). So you go along recording the y's against the sum of the Pyhagorean lengths to this point. A plot of these (x-blank, y-blank)'s gives you the blank shape. You do this for hundreds of points and the answer is very accurate indeed.

For the template, with these x,y points, you take the gradient between neighbouring points and record a point which is the cutting offset away from the line at 90degrees. Plot these derived points gives you your template.

This didn't prove that difficult. It's the plotting which is the problem to me.

I'll draw it if anyone wants.

I am afraid my code is in Lisp, which few people use.

Richard Smith

Sounds like a cad problem, something that is behind in the FOSS world. of course, if you can plot it out at the final, unoffsetted size, you can offset it manually with a pencil.

I have done this and related things numerous times. I can't imagine that a piece of M$ bloatware would give accurate drawings but ...

Let me first state my computer environment: I run OS/2 Warp 4 on an IBM ThinkPad. Running under this, and relevent to this discussion, I have Generic CADD, APL2 and PMView. Other potentially useful software would be Papyrus, Ghost Script and Ghost View.

Generic CADD is an oldy-but-goodie DOS drafting program but I have run it under W95 and heavily use it under OS/2. It is capable of producing very accurate drawings. I have printed layouts to my elderly Epson dot matrix printer and found them to be within a few thou. It has no driver for my recently aquired Canon i850 printer but the work around for that is to create a virtual Post Script printer to which it will print, read the drawing into the Ghost twins and print it from there. I recently did a degree scale using this method. See

Generic CADD has a useful ability for the kind of thing you wish to do. It will read or write an ASCII file of the instructions to produce a drawing and produce it. I can *very* quickly produce a wide variety of interesting functions and preview them while in APL on my graph plotter. Since I have written a 6-point Gauss-Legendre numerical integrator, functions involving integration are no problem. After creating something I wish to see outside APL, I use my little convert-to-CADD function which produces the above mentioned ASCII file. Generic CADD offers a variety of ways to deal with curves including Bezier curves and cubic splines.

If you can create a drawing with whatever that is proportioned correctly, you can re-size it very accurately in PMView and print from there.

These drawings can be imaginitive shapes such as you wish or layouts for drill, cutting and bending sheet metal. For plasma cutting, I just glue or clamp the drawing to the metal and cut. I don't care if I char the drawing - paper is cheap. I have never had one ignite.

Very acurate panel layouts can be done in colour this way. Print on good quality photo paper, coat with epoxy as I did for my degree scale and glue to the finished panel.

Once exposed to the concept, there is no limit but your imagination as to what can be done with some programming, a drafting package and some metal working gear.

If you wish to discuss this further, you can e-mail me direct or post here.

BTW, re software: If you can use them or translate them to something else, you are welcome to my relevent APL2 functions. I have been using Generic CADD for many years and it has served me well but it was bought up and killed by Autodesk - too much competition for Autocad bloatware. I recently aquired an unopened copy of AutoSketch for $2 at a used book sale but haven't yet time to learn it so I don't know if it will do all of the above.

Ted

I don't think so but others do. :-) Mind, I've enjoyed playing with math since before I used to do my babysitter's algebra homework for her when my parents went out.

This is certainly the hard way! My numerical integrator is a six-point Gauss-Legendre method which will do the job with a tiny fraction of the number of points. The algorithm is written up in Abramowitz and Stegun and I have it in Basic, Pascal and APL. The APl is the full blown one that handles pseudo indefinite integrals. Also very useful is the Runge-Kutta method for differential equations. I have had great fun playing with that one problems such as arrow trajectories (no closed form solution exists), coupled oscillators, orbits and many others.

If you have a fairly steady hand, you don't need offsets. I just stick the paper on the stock with glue or clamps and cut through it. The paper chars but I have yet to have one burst into flame.

See my post elsewhere in this thread.

Ted

What is "the FOSS world"?

Ted

Free and Open Source Software. Linux, BSD's and the like.

If you write C, I see Watcom has a cross-platform compiler. Haven't tried it yet (haven't done C in a number of years) but plan to look at it.

Ted