Home-Made Circle-Jig for my Plasma Cutter

[tomrscott]

In the continuing saga of my reborn Hypertherm 600 Plasma Cutter. I justified getting it largely to cut up a giant chunk of steel into tractor wheel weights for my deep dish R4s, so I needed a circle-cutting jig.

You can buy these for one or two hundred bucks from Hypertherm and other companies, but that would be almost what I paid for this the whole plasma cutter (remember that it was not working at the time).

So I "thunk" on it a while and came up with this: Mind you "pretty" was not at the top of my list of requirements, but I have 42 disks 16" diameter to cut out of a large plate of 3/8" steel plate, so it needed to be robust, and functional.

I decided that I wanted the torch head to swivel in the fixture so that it would be easy to follow around the circle, so I started with a large ball bearing that was a snug fit on the 1" torch barrel. Then I added a couple of snap rings to hold the torch barrel in place, welded a couple of 1/2" roll pins to the bearing, some 1/4" threaded rod through the pins, a block of 3/4" plexiglass on the other end of the rods to insulate it from ground, with some bronze bushings that are really overkill, and welded a threaded Tee-nut into a hole drilled in a large speaker magnet for a swivel anchor.

The next several pictures will show a couple views. I will probably have adequate weather to try it out tomorrow and I'll try to post some pictures of it in action then. Dry runs tonight seem like it will work great. /forums/images/graemlins/smirk.gif

 

[tomrscott]

Here's another view:

 

[tomrscott]

And here's a view with the whole contraption flipped upside down so you can see the other side. In the lower left corner you can see the tool I have to use to pry the magnet up off of a plate of heavy steel. Those magnets are really strong!

When I welded the roll pins to the ball bearing, I clamped the ID sleeve of the bearing between two big one inch nuts and clamped it down to a huge chunk of aluminum for a heat sink. I also soaked the bearing in solvent a week earlier to get all the grease out of it cause I didn't want it to boil out while I was trying to weld to it. I used some 6011 rod and high amp setting and almost spot welded it just long enough to get a puddle started, in two places for each sleeve, letting it cool down between. Managed to get away with welding to a ball bearing without destroying it. Still spins fine. I wouldn't enter it in any beauty contests, but it provides a real stout anchor for the torch head.

 

[rswyan]

</font><font color="blue" class="small">( You can buy these for one or two hundred bucks from Hypertherm and other companies, but that would be almost what I paid for this the whole plasma cutter (remember that it was not working at the time). )</font>

Nice job - let us know how it works. Yup - these type of accessories from the mfg. always seem way overpriced for what they are.

Loved hearing about your score on the plasma cutter.

Flame on !

 

[johnbilt]

Hi Tom

Looks like it will work well. Great fabrication idea.

I bought a cheapo dial indicator mount from Grizzly a few years back. It has a magnetic base with a release lever/knob. Kind of slick. Might be something you can get cheap that will make pulling the magnet a bit easier.....

Have you put any thought into a cnc setup ? I've been doing some research on putting together an inexpensive x-y gantry table with a stepper motor drive....

Glad to hear you are working along with your project.

John

 

[PineRidge]

Tom facinating stuff, and lots of food for thought.

 

[tomrscott]

John,

Yes I have admired CNC setups. I've probably got all the pieces I could need, and I know some real expert motor control electronic designers. My "day job" is as a component engineer at Xerox making solid ink color office printers which have a couple dozen motors in them.

CNC would be real fun, but it will have to be well down the queue before I get to it. It will probably require a project that really justifies it. CNC really makes the best sense if you are going to either be cutting a lot of the same thing, or some one thing that is too elaborate a design to easily control by hand. I've even got an old laptop that would make a good Linux platorm for a control system. I'm just not sure what I want to build that really justifies CNC.

The torch head portion of this circle jig could be used in other kinds of fixtures, such as attaching it to a straight edge guide using a ball bearing drawer slide.

The funny thing with this circle jig was, deciding how elaborate I should go. I could have gone a lot fancier, used the lathe, etc., but was trying to build it robust enough for the job at hand and not too much more than that. I might not ever use that tool again after this initial job. It is always easy to over-engineer anything, and there is nothing wrong with that to a point, but sometimes the real challenge is engineering it "just enough" for the job at hand.

Life is short. I hope to finish a few things I've started already. /forums/images/graemlins/smirk.gif

 

[tomrscott]

Correction, the tool for lifting the magnet is in the lower right corner... Well you probably figured that out... /forums/images/graemlins/crazy.gif

Hummm, I don't know if the weather is going to cooperate for me to cut steel outdoors today or not. There is no way that pieces of steel is coming indoors. I would be almost easier to build another pole barn around it! /forums/images/graemlins/crazy.gif

Might do a bit of practice welding today. Really should stop to "learn how" before doing a lot more welding projects. I guess it is kind of like "stopping to ask directions," sometimes I end up trying to figure it out myself. Eventually I would like to take a welding class, but I can't do that for awhile, too many comitments already. (Finishing a bachelors degree at the local college already. Imagine finishing my bachelors degree a couple months before my 52 birthday!)

I've got some cutting and welding to do on the hydraulic cylinders for my TNT project. First I need to cut some ends off the cylinders which will be pretty easily done. I want to take some pictures of the cylinders before I begin so that I can have some "before and after" shots. I have all the ends I need to get started though, so if the weather doesn't cooperate to work on the weights, I'll probably do that.

 

[tomrscott]

Well the weather cleared up nicely while I was having my morning cup of coffee Saturday. Turned into a nice sunny day, so I got quite a bit of plasma cutting in and a bit more today.

I should describe this plate of steel I am cutting up a bit better. Forgot to get a good picture of the whole thing but this will give you a pretty good idea:

It is about 19 feet long, by 58 inches wide. One long edge has a four inch edge rolled down, and the opposite edge has a four inche edge rolled up, but tha tedge has a about a five inch deep notch for about six or seven feet in the middle of the long side. Welded to the bottom side of the plate are three long pieces off angle iron, 3" x 3" x 1/2" thick, about sixteen feet long, and welded across one end of the plate were two pieces of angle stock, 3" x 3" x 3/8" thick, 56 inches long. This was one well-reinforced piece of steel! Fortunately, the angle stock was only welded on with stitch welds, about two or three inches of bead every foot or two.

In order to take the angle stock off, I first gouged out most of the weld bead with the plasma cutter gouging tip. This removes pretty much all of the weld bead, but leaves a thin smear of a weld down in the root of the joint. In order to help separate this, I took a 1/16" thick x 3" diameter air grinder and gouged a bit more of this weld smear away and then was able to get a 3" inch wide broad chisel into the crack with a hand sledge. After a few whacks, there was a loud "BANG!" as the last of the weld popped apart. From then on, once I had some prying force into the gap, I could go back and gouge out more weld and each weld would "BANG" itself apart, sometimes requiring a bit more encouragement from the local application of a broad chisel and sledge.

Using that process, I removed one of the 56" cross angles the other day, and this moring I removed the other one, and also removed one of the 16 foot long pieces from the underside with the whole plate lifted up on cinder block stacks of two blocks high. I was very careful to have everything well blocked before getting under it to work, and they used a lot of caution as that 16' length of 1/2" thick angle iron started to separate. But with all of those pieces removed, it left one long edge free to begin cutting circles.

I did take some pictures of the circle cutting. The first three are taken through a piece of plexiglass, so they are a bit cloudy. The first two shots are from about three feet away, and the third is about eighteen inches from the torch (all with self timers). The fourth shot is of my work setup with three disks cut out. I went on to cut three more disks Saturday, and four today for a total of ten so far.

I took a picture of the large plate of steel after cutting the first six disks out and removing some of the angle stock.

This project is not only going to yield a lot of wheel weights for my tractor, but a bunch of valuable steel for fabricating other projects.

 

[tomrscott]

Rereading that last post, I realize that some of it was written offline Saturday evening while TBN was down, and then I added to it today Sunday. Sorry for the confusion.

Here's another shot of cutting...