Plasma cutter for ribs?

[wingnut13]

Hi guys,

I haven't cut one rib yet but I was wondering if anyone has used a plasma cutter for the job. The newer machines seem to have minimal issues with cutting aluminum. It seems like you could really knock them out quick. Anyone here done it before?

~Todd

[alaskabearhawk]

This was discussed a year or so ago. The action of the plasma arc leaves the edge of the cut brittle, making them susceptible to cracking. At least that's what I remember the answer to be.

[X'N]

I agree. If you were to do it and than aneeal and temper them I would think they would be fine.

[Enewton57]

Can a person control temperatures and cooling sufficiently to get the correct "T3" temper in thier home shop with regular equipment?

[wingnut13]

I was thinking about trying it out after seeing a guy cut out a body panel dent and the paint right next to the cut wasn't hardly affected. I know there is heat from a pass with the router, but that's not nearly what the plasma arc puts out. I've got work in the shop to do tomorrow and I might just play with some aluminum scraps to see how it cuts. But I guess I'll keep all my "flying Aluminum" to router shaping.

[JohnF]

What about rough cutting prior to router shaping? I too have seen body panel cuts where the paint right next to the cut wasn't affected. Some differences to note are that steel oxidizes in this process but aluminum does not and I guess is cut more from the heat (I don't know this part for a fact - anybody?). If I leave 1/16 to be trimmed via the router will the HAZ be trimmed away? It just seems like it would be a zillion times faster than a hand held shear.

[Gerhard Rieger]

The question is, do you really want to risk a crack or even worse on a wing that will take you a year or two to built, and you want to fly the thing. Not worth it, stick to what works, and you can expect to have a good wing. Its going to be a lot of work, don't go for short cuts.

[N3UW]

What about rough cutting prior to router shaping? I too have seen body panel cuts where the paint right next to the cut wasn't affected. Some differences to note are that steel oxidizes in this process but aluminum does not and I guess is cut more from the heat (I don't know this part for a fact - anybody?). If I leave 1/16 to be trimmed via the router will the HAZ be trimmed away? It just seems like it would be a zillion times faster than a hand held shear.

John, it just does not take that long to cut out all the pieces before routing. I think I spent less than 2 hours cutting all the pieces out. I used a combination of tin snips, air shear and band saw. I see you are close to me. I live in a Westminster near 104th and Sheridan.

[JohnF]

I'm sitting in an office at 120th and Huron.

Tin snips and air shear is where I'm probably heading. But it would be neat to know if the Powermax 30 would do the job. It would be just about as quick as drawing a template with a sharpie.

[JohnF]

Of course I want a wing that is as strong and reliable as any that Bob would build. Process shortcuts do not necessarily increase one's risk; I understand that the kit factory hydroforms the ribs, a shortcut method I can't afford so pay instead with time. From what I can tell the bee's knees approach to making ribs in the scratchbuilder's garage is to hack out a rough shape from sheet with a air/electric powered hand held shear, clamp into forms, and precision shape with a router. (Note: a shortcut method to the old precision draw and hand cut with hand shears.)

I fully understand why one would not want to plasma or laser cut precision 2024 parts and risk exactly what you are talking about. But I'm talking about the rough cut prior to router forming. I would be totally comfortable doing just what I am talking about if I left a 2 inch margin (the HAZ will for sure be removed) except it would be a waste of material and take longer. I accept that it might not be possible; where the time you save rough cutting is eaten by the time it takes to trim away the HAZ, I was just curious if anyone knew. If I don't find a good answer I'll be making noodles like most.

Maybe I'll see if I can get an expert to weigh in...

Okay, I sent a request to Hypertherm because I know they know the lowdown and because a Powermax 30 is my tool in question.

[X'N]

A jig saw or small band saw is much cheaper and will get that job done just as fast. Or just have your sheets sheared to rectangles with in a 1/2" of each blank. Or you could buy a cutting torch being you would need an OA setup for the fuselage build...

[JohnF]

Oxy-fuel cutting doesn't work well with aluminum and would put too much heat and distortion into the metal. Seems strange to think that plasma at something like 20,000 degrees puts less heat into the metal but you zip along so fast that when you are done you can touch it. At least for sheet steel, I don't know about aluminum.

[jimcolt]

While I am not sure exactly the type and thickness of aluminum being discussed here, generally thin aluminum parts cut very efficiently (fast and with nice quality) when done on a cnc machine that can maintain correct cut speed and height. Expect no warpage, a heat affected edge of less than .010" thickness, and the parts are slightly warm after cutting. Air plasma systems will creat a relatively rough, porous finish on the edge that likely would need to machined, sanded etc., for best structural integrity. You can see the edge roughness on the picture of the .080" sample.

High heat input and warpage is only caused by plasma if the cut speed, cut height, and or process power level is incorrect for the cutting application.

If someone can fill me in on the part details....I'd be happy to comment regarding plasma capabilities. If plasma can do the job it would be very inexpensive to farm this work to a small shop with a cnc plasma. If better edge quality is necessary...a fiber laser could do it, and the best edge quality with no heat affected zone would be with an abrasive water jet. The laser and water jet processes would be more costly.

Jim Colt Hypertherm

[JohnF]

Hi Jim!

Thanks for stopping in. Welcome to the crazy world of homebuilt aircraft. It contains a spectrum of individuals from those of us that like to turn sheets into airplanes to those that like to turn airframes into custom airplanes. We do have the option to buy ready made parts and take advantage of efficient processes; for example, the ribs that we are speaking of in this thread are available as finished parts from the kit manufacturer. (They probably use a water jet)

Take a quick look at the page at http://www.mybearhawk.com/wings/noseribs.html (I added a link but it looks like it doesn't go inline). Eric, one of our Gurus, cut rough nose ribs with tin snips and then went back and finish cut them (again with snips - and they look good!). Follow to the second page and you see how he went sheet to part.

The process many of us are using is to:
1. Using a rough part template, layout parts on the sheet with a sharpie.
2. Cut between the rough parts using the methods described in the thread. (snips, hand held air shear, various shears, score and snap,...)
3. Clamp a few parts between form blocks and trim to final dimension with a router/trim bit.

It just seemed to me that if we trade the plasma cutter for the sharpie we could skip step 2 with no consequence. That is, air cutting and then machining to final dimension.

You mention a HAZ of .010 using optimal cnc. What would be the HAZ in .025 2024 aluminum from a guy using a Powermax 30 dragging around a mdf template? (Assume proper average user)

Also, if a person were to perform this process wrong and extend the HAZ into the part *** could they tell? In Eric's case it is very easy to see that you've strayed with the snips and need to cut a new one. *** - I used a rough margin of 1/16 and it occurred to me at that point of the question that maybe it would be pretty tough for a person to get a HAZ of > 1/16 at all, so I guess that is another question, Eric used 1/4 margin and I suggested 1/16 margin so is there a margin for a sloppy slow user?

Thanks again Jim!

[jimcolt]

The heat affected zone will get wider as your cut speed slows, narrower as your cut speed increases with a plasma cutter. Although on thin aluminum the heat is disipated rather quickly (as compared to steel) so I doubt that there would ever be more than 1/16" HAZ on aluminum thinner than 1/8" if cut with a 30 amp Powermax30 plasma cutter. So....guiding the torch by a template will produce a nice cut with a steady hand, and a bit of a jagged cut with rough, unsteady motion.

The learning curve with a plasma torch is pretty short, so you likely would be cutting excellent parts and saving a lot of time in short order. If it would help I could cut a few sample parts....by hand and by machine and drop them in the mail to you. Aagin, I would need to know the thickness and type of aluminum.I was looking at the parts being cut and the slow process.....plasma could surely speed things up!

I am somewhat familiar with homebuilt aircraft as I watched two neighbors build homebuilts in my youth (60's, 70's) and another rebuilt an Aeronca Chief. My Dad was an FAA technician in W. Lebanon NH for his career, and I finally made it to the Oshkosh show (this year) for business purposes. Hypertherm was demoing plasma cutting systems, both hand held and cnc controlled in the Praxair booth at the show...I was the demo guy.

Jim Colt jim.colt@hypertherm.com

[Jflyer]

I have the Thermal Dynamics Pak-Master 25 Plasma Cutter. Runs on 110v. I've cut thin metal roof panels to size for the roof and some thin aluminum practicing with scroll work. I've also cut 1/8" steel angle.
I can dial up the current for thick pieces and dial it down for thin. It does leave a burned edge (HAZ), but a little filing removes that. I agree with the idea of rough cutting maybe an 1/8" above and then finish with the router. If you're using by hand, I doubt you'd get a true and even cut. Even if running along a template. A little wobble cuts a long way.

[S Lathrop]

I used a CNC machine to make wood templates of the various ribs. I then use a stomp shear to rough blanks for each rib. The final rib cutting was done with a router. I used dowel pins to align the blanks and the templates.

I found that I could do 3 ribs at a time. It took just a minute or 2 to cut a stack of 3 ribs. The only down side is there are chips every where. You might have to pass the router over the cuts a second time to get a very acceptable finish.

To keep from scratching the ribs as I passed the router over the ribs, I made a rib from thin plywood to cover the actual ribs. This also was a test of the template.

Once I made the tooling, it took just a few hours to do all the ribs. The templates were a day's work.

[wingnut13]

Yea I have picked up a Hypertherm Powermax 30 recently and that's what had me pose the question. Thanks for stopping in Jim. Nice equipment, and I'm only about 2 hrs from the Hypertherm plant. . . ..