Plasma tube notcher?

[ty1295]

I am slowly collecting parts for a CNC plasma table.

Cutter is on order, servo motors are here.

Had a thought. Could I add a 4th axis to rotate tubing and notch it. (also called fishmouth)?

Right now I use my mill with a roughing end mill and set the tube in the vise at the angle needed.

Thinking if you could map out the appropriate geometry, you could rotate the tube and plasma cut it to acheive same result but quicker.

Also short of trial and error for the profile, is there a way to calculate it without a ton of calculus?

[Hobbiest]

Just set a chop saw to 30 deg, cut one side, turn over and cut the other side, sharpen the edges with a grinder. If I were better at CAD, I'd draw up some diagrams for you to see. Easiest way to notch tubing, if you plan to weld it together anyway.

[HuFlungDung]

If the tubing has a significant thickness, such that you would be concerned about the actual angle of inclination of the cut, the rotary axis working underneath a fixed plasma head would give you a cut that was normal to the tube axis throughout the rotation. This might not be what you want, as it would not be equivalent to what your endmill cuts exactly.

On the other hand, you could incline the plasma stream a bit (if they will cut that way), to cut a sort of cone-shaped end on the tube. This might fit up a little bit better, but still not as good as milling it.

So for best results, you might also need one degree of freedom on the plasma head, so that it can precess properly as the tube rotates beneath it.

As for calulating the path, even for a 4th axis wrap, it beats me how you'd do it. It might not be too bad if you only cut 90 degrees at a time, then indexed it. For this, you could use solid modelling to create the profile edge you would need.

[ty1295]

Hobbiest: I think your term notching and mine are slightly different.

I need to notch them so they intersect another tube midstream.

Probably most known case would be a car roll cage.

HuFlung: Yes, thought about the angle deal. Being I am welding them, and tubing is .125 max thickness I don't think it will be a big deal.

Here is a picture of what I am building. You can see tubing intersects with the bushings.

[Hobbiest]

No...that is exactly what I mean by notching. This is how we build handrails out of pipe. It is the easiest way to cope tubing to fit it together. Takes a little trial and error, but works great for welded joints.

[M_D]

I have thought about the same thing, I think it would work well with the right software to quickly create the code.

I designed a tubing vise to use on a mill-drill to fish-mouth 4130 tubing for racecar chassis'. It is nice because you can set the angle quickly and accurately, plus cope off center. The typical fish-mouthing tools sold use hole saws and don’t have that feature. This tubing vise I designed has a bushing that fits counter bores on the center of the rotational axis on the V block and vertical mounting plate. An angle plate makes it easy to mount to a machine table. There are marks engraved every 1º, the range of adjustment is 95º. It locks in position with a 1/2" bolt through the slot, with the bolt threading into the V block.

With stops and or some more structure, it could actually work fairly well for repetitive cuts in a production environment. Most race chassis are hand built to fit a certain body, so the need to make multiple identical parts isn't great.

The cope on a good tube joint has to fit very well so the weld joint is tight and even. For one thing, you don't want to fill big gaps with a TIG weld on a 4130 chassis; it isn't of good enough quality. I use hole saws with custom made arbors, you want something like a 3/4" shank for rigidity, and you can chuck it in an R8 or similar collet. The hole saws actually work quite well, since they are only cutting a thin kerf compared to an end mill it is easier over all. That’s a piece of 1-1/2” 4130 .080” wall tube in the picture for size reference.

I have used CNC's and a ball nose to cut fish-mouths for acute angles, where the joint was about 10" long. In that case I programmed it with 3D software. It is slow and I wouldn't want to do production work that way.

There is one heavy-duty commercial tubing notcher made that uses large end mills and a sliding feed table, much the same a dedicated mill. I forgot the name, but it might be a good alternative.

[Hobbiest]

TIG welding would definately be difficult with a gap. I have mig welded pipe coped in this fashion with incredible strength results however. On the machine you designed, what actually did the cutting?

[M_D]

I use the fixture mounted to a mill-drill table. Most hole saw arbors have a hex to chuck in a drill, so I made a heavy bi-metal hole saw arbor and chuck it in a 3/4" R8 collet. I use the "old" style hole saw, without the built in hex shank. Then its just a matter of adjusting the holesaw to tube relationship to where you want it. It's fairly fast on the cuts, and the fishmouth only needs a light deburring to finish. It's nice to have good tight joints, because it make the fixturing and welding simpler.

[timewarp]

http://www.taomc.com/studio_machines/plasma_cutting.htm

http://www.metalgeek.com/static/cope.pcgi

[Ries]

I just mentioned this on another thread, but it applies here as well- with auto torch height on your z axis, there is no need for 4th axis indexing, at least on small pipe. Maybe if you were cutting 12" diameter pipe, but it gets expensive and complicated to rotate the 4th axis- industry standard is a Haas rotary indexer, and they get about $5500 bucks for those.
My store bought C&G plasma system has a voltage sensor on the torch that runs the auto height on the z axis, and it has about 4" of vertical travel. This is plenty to birdsmouth, or notch, pipe or tubing up to 2" or so. Yes, the orientation of the plasma is vertical, but so is the end mill you are talking about- so it should give about the same result. A little less precise than an end mill, but a lot quicker.

[CNC Pro]

If your looking for a hole-saw type notcher, I can recommend this one from Pro-Tools.

http://pro-tools.com/hsn500.htm

It's a well built piece, and simple to use.

[braidmeister]

TimeWarp is on the right track with this links. You can use the calculator to generate the necessary unwrapped profile of the intersecting tube, make a DXF from it and toolpath it. There is no need for a torch height controller if you don't have one. Set torch at .2-.3" above crown of tubing.

Notching tube with a plasma CNC is gorgeous! You can do most notches (or fishmouths...or mitering) in a matter of seconds compared to several long minutes of setting up each one on a mill, drill or lathe. For instance, it would take just less than 6 seconds to fishmouth a 3" diameter tube with a .125" wall thickness on my machine with 4th axis.

The finish cut is absolutely dead on, burr-free/smooth and ready to weld. There are no gaps between intersecting tubes provided that your torch is dialed in and you are not showing excessive bevel on the inside wall. They TIG together beautifully.

If you are doing less than 10 notched tubes, then use a mill-based tubing notcher. However, if you are in production mode and are making bicycles, motorcycles or roll-cages, then the plasma with 4th axis makes the most sense.

-Brady