Home-Made Circle-Jig for my Plasma Cutter

[tomrscott]

Well, I've got all 42 disks cut out now. The plasma cutter worked just dandy. With a fresh tip that cuts real clean, when I moved the torch steadily, I could cut at about 1 inch to 1.5 inch per second with about a 1/10" kerf, and an edge wiggle that was +/- 0.050" or so. When a tip had cut half a dozen disks out (about 300 inches of cutting) it would begin to get a bit fuzzier but I could still use it for another half dozen disks or so. Twelve to fifteen disks seemed to be about the limit for one electrode and nozzle (that would be about 600 to 750 inches of 3/8" steel cutting).

The circle jig really worked slick. I think this job would have been a pain to do any other way.

Now I think I'm going to weld up a stand for my bench grinder so I can run it in the driveway and keep the grinder dust out of the garrage. Going to do a lot of clean up on these disks. In addition to the edges, there are some surface welds that I want to clean up so the disks will stack nice and flat.

Once I get the faces and edges cleaned up a bit, then I've got to make an accurate drilling template with one and start drilling 168 half inch mounting holes. I guess I will drill about a 3/16" hole first to minimize the center drift that can happen with a large drill. These plates need to line up pretty well if I am going to stack 21 of them on each wheel.

I've also decided that I am going to take one plate for each wheel and weld a lot of scrap to the back face. You see the first weight will stack on top of the wheel bolts and they stick out a little more than 5/8". So I can fill up most of the back side of the first plate as long as I leave room for these wheel bolts. I need a lot of stick welding practice anyway, and this is something that can be ugly and no one will know. I can weld scrap on that over hangs the edges, and trim it with the circle cutter. Then when it is completely full, I can even practice just running beads for what my welding text calls "padding". This way I can get a lot of welding practice without wasting anything.

The other by-product of this job is that I've got a real nice stack of heavy-gauge angle iron and a lot of scraps of 3/8" plate. I have a total of about 50 feet of 3"x3"x1/2" angle iron, almost 12 feet of 3"x3"x3/8" angle iron, and almost 30 feet of the rolled edges of the original plate that had a 4" edge flange rolled with about a 3/4"" radius. I tried to leave as much plate attached to those flanges as I could when I was laying out the disks. Some of that rolled edge has 2"x4"x3/8", about 12 feet of it has 4"x4"x3/8", and about four feet is 4"x6"x3/8".

I am already planning a project with this scrap to make a 3pt frame that will do double-duty for a back blade and a trenching plow.

I will post some pictures when the wheel weights are all finished and mounted.

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[PineRidge]

I'm curious how well the bearing in the circle cutter jig held up after all the plasma cutting? Does it still spin pretty freely?

 

[slydog]

Great job Tom! Let me know how the drilling goes. I guess I just don't know what I'm doing, but I have the worst luck trying to drill a nice round hole in a piece of steel. Even with a brand new titanium bit! Keep the pictures coming.

 

[tomrscott]

Yeah it still works fine.

I thoroughly degreased it before welding to it, so any crud that got in it didn't stick. I just blew it out whenever it started collecting stuff. I would still like to make a washer/shield out of aluminum can material to protect the race, but even after about 2000 inches of circle cutting, it still works just fine.

The bearing made handling the torch MUCH easier than if I had to keep the handle tangent to the circumference or something.

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[tomrscott]

Slydog,

My experience with drilling steel is that you need a sharp bit, some cutting oil, and I wouldn't try to take on that many holes without a solid drill press. Go slow and let the bit cool down once and awhile.

I've also always found that a small drill (something like 1/8") follows a center punch better than a larger one does. Once you've got a pilot hole drilled through, the larger drill will follow that pretty well. Sometimes I have even drilled large holes in three or more stages starting with 1/8" from the center punch, and then about doubling the size with each drill.

What I think I will do in this case, since I've got so many holes to drill, is to very carefully layout and drill one disk (maybe in three stages), and then use it as a template to drill the others by clamping together a thick stack at once. Since these are 3/8" thick, eight disks would only be 3" thick. Not counting the template, six sets would do all 42 (6x7=42). If I use a square to carefully line up the edges and then clamp them together I should be able to get pretty good alignment. With 3/8" steel plate, I don't think the template holes will get opened up much. I really want the holes to be pretty snug on the bolts to minimize slop, but that requires that everything has to line up very accurately. /forums/images/graemlins/smirk.gif

 

[PineRidge]

<font color="blue">I really want the holes to be pretty snug on the bolts to minimize slop, but that requires that everything has to line up very accurately. </font>

Tom I don't think the tight fit to your bolt is critical. The cast wheel weights that I purchased from New Holland use two 5/8 inch bolts to mount each. the hole size is 3/4 but using a large washer as the instructions recommended has always made for a snug, tight fit. And each successive weight is daisy chained off the weight before it. Makes it real easy to pull off a single weight without disturbing the rest of the stack.

 

[tomrscott]

Pineridge,

You see that's the difference, I am not going to have each weight bolted to the one before. That's a real nice design and I can see the advantages, but it is just too many threads to cut. Very nice feature, but I don't really anticipate changing the weights that often. I am planning to mount the entire stack of 21 plates using the four 1/2" carriage bolt square holes that are provided in the wheel face. I've got some hot galvanized 1/2" carriage bolts 10 inches long. With that much moment arm at the end of the stack, there is going to be a lot of side forces. If each plate is a real snug fit on the bolt shanks, especially the ones on the unthreaded portion of the bolts, it will provide better support. Ultimately it just has to be real tight and not shift around.

The ideal, if you were casting these, would be to have some dovetail features cast into the weight surfaces, male features on one side and female on the other side, so that each weight nested into it's neighbors. That way the bolts shank would see almost no shear forces, almost all the force would be tension. I thought about trying to put some sort of features into the plates like that, but decided it was too much trouble.

Trying not to make a career out of this. /forums/images/graemlins/smirk.gif

 

[jimgerken]

I just read about your need to clean up the disks after cutting them out with plasma. In my experience, to attack the dross (slag produced by plasma process) with a grinder heats it and makes it stick much harder than it is before you start. Here's a faster easier way. Hit it with a hammer, with a shearing action. In other words, hit the edge of the circle, with the dross side up, hitting just the very edge line of the circle part, and what you are doing it shearing the dross off horizontally. It should pop off easily and in large chunks. If some wont come off this way, it will have to be ground. If it is on both surfaces (the bottom and the top) it will be much harder to deal with. The bottom side dross will come off easily with the above method. Most of it anyway. I see some dross on top once in a while when I am too far away or too close to the workpeice when cutting. Good luck , hope this helps, try it. /forums/images/graemlins/grin.gif

 

[jimgerken]

Drilling suggestion: Layout one of them and drill it with the first small bit, like 1/8". Use that to transfer to each other one, at the 1/8" size. THEN drill each one up to final size, in one or two more steps whatever you can perform with your drill press. The more steps, the more accurately located the holes will stay. Maybe this is what youy were thinking but I understood you were going to drill one to final large size and then transfer drill thru it-- problem with that method would be that you cannot penetrate the steel with the final size drillbit. You want to penetrate each one with the small bit first. Make sense? Hope it helps...

 

[tomrscott]

jimgerken,

Regarding chipping off the dross with a hammer rather than grinding it. I guess I wasn't clear, but yes I completely agree. In fact I have already done that chipping with about half of the 42 disks. No, the grinding I was referring to was smoothing out some of the jagged cuts that resulted from my failure to control the torch with a smooth motion. Also, sometimes when I was getting to the end of the life of the torch consumables, the cut would get a bit ragged.

Finally, there is a fair amount of surface weld remnants from the angle iron brackets that I removed by gouging out welds. These welds left some remaining weld bead that would prevent the disks from stacking flat, so I need to grind these off fairly flat.

As for the slag, for the most part, I don't have any slag on the top surface, and the little bit of slag on the bottom surface chips right off very cleanly with a very light tap. Plasma cutters are great that way. After I had some experience cutting, I finally discovered that this weak slag would often keep the cut disks from popping out. When I was beginning, I would go back over the parts that were still stuck, but that tended to make the cut even more ragged. After awhile I figured out that when the circle was cut all the way around I could take a hammer and whack it a couple times and it would generally come loose just fine. Anyway, with some experience, the results were much better, but I still have to clean up those first ones when I was learning how. /forums/images/graemlins/smirk.gif