Homemade Circle Cutting Jig

[JHCC]

One of the reasons I got my oxypropane torch is to cut circles of sheet steel for making bowls. I've been looking at circle cutting jigs, but decided to make one for myself. Here it is:

The scrap assembly that eventually became the base of the stand for a new anvil probably started life as some kind of wheel, so it had a piece of 1/2" plate on either side with a ball bearing press-fit into a hole  in the middle. I started by torching out a circular piece around that hole with a bit of a tab on either side (the second tab was intended for a design element that I ended up scrapping, so that got cut off):

The next step was to get an extra of the nut that holds the cutting tips onto the torch:

The nut is filed to go inside the bearing, with a very tight press fit:

Flats are filed into the tip of the nut for a wrench, and a 11" piece of 1/2" square bar is welded onto the remaining tab for a beam:

The pivot point is made from a piece of garage door spring, bent to shape, sharpened, and with the tip hardened and tempered:

The pivot is held by a block that slides along the 1/2" square beam; both pivot and beam are locked in place with thumbscrews:

Another view, without the pivot:

The block and its thumbscrews:

The hole for the pivot is drilled through, and the holes for the thumbscrews are drilled and tapped. Those holes and the thumbscrews are threaded to 12-24 UTS. The square hole was made by punching a 3/8" round hole, which was then squared with a drift made from the same 1/2" square stock:

The jig is set up with the torch by putting the appropriate tip in the nut and tightening it onto the torch with a 5/8" wrench:

The pivot is set at the appropriate radius, and we should be good to go. Unfortunately, I discovered that I'd run out of oxygen, so I haven't had a chance to test it yet. I'll keep you all posted.

 

 

 

 

[Scott NC]

  Nice job of that.  I like your thumbscrews.  We used to use one of those a lot for cutting large holes in thick plate and when you blew the hole through to start out it always left a crater that later had to be refilled and ground down.  A guy in our shop devised a a circle cutter with a hinge attatchment so the torch could be angled away from the cut line to poke the hole and then lowered back into position to finish the circular cut.  IIRC it had a small cam mechanism to lift the tip slightly when angeling it away from the cut line.  We cut a lot of holes....   Circle attachments work well with plasma too.

[Frosty]

Nice circle cutter John. I  made one using lab bar clamps and steel tubing. There's no bearing to let the torch maintain orientation as you cut a circle but I was so used to turning the torch too it's not an issue for me. The problem I've always had was maintaining the correct distance from the work and the little steel wheels on commercial circle cutters never seem to work very well. I just bent a piece of 1/4" round in half so there was a rounded nub to ride the plate. 

I started drilling or free handing a starter hole to prevent the blow through crater you describe Scott though they aren't as bad with oxy propane rigs.

I look forward to how yours works John, that's quite a piece of work, you get a double thumbs up from me.

Frosty The Lucky.

[JHCC]

I like the idea of the rounded guide. I will give that some thought. 

[Frosty]

You could drill and tap yours for a piece of uplated all thread with a round end. 

Frosty The Lucky.

[JHCC]

With a locking nut, perhaps. Interesting idea. 

[bubba682]

As long as it works good and helps cut smooth not jerky and clean is all that matters i went from torch to plasma cause acetylene costs a fortune up here now. Alot of initial cost but in 4 years its saved me alot of cash in acetylene/propane and oxygene.I bought a circle jig its got a metal wheel mounted on the torch holder it really helps smooth the cut out take a look on the web it might be worth a try.

[Scott NC]

On 3/26/2022 at 1:29 PM, Frosty said:

I started drilling or free handing a starter hole to prevent the blow through crater you describe Scott though they aren't as bad with oxy propane rigs.

  Unfortunately, it was semi-production work and we had no time for drilling.  And free hand made it tricky to get the circle cutter back in the center punch hole while the cut was going.  It was 2-3" plate.  They gave the guy a bonus that made that rig, for cutting costs and improving productivity.  We used acetylene.  Eventually they started a computerized plasma and torch pantograph department that took over all that type of work.  The slugs made great workbenches and stands at scrap prices for employees.....  They had a tubing department as well with drops for leg material....  

[Frosty]

Ahhhh, a job with benefits is a good thing.

Frosty The Lucky.

[JHCC]

Well, finally put it to work. I’m still learning hire to cut in the first place, so things were a bit ragged at times. Nonetheless, it worked quite well cutting out a half-dozen bowl blanks and one for a frying pan. 

[George N. M.]

John, what  thickness/gauge do you use for your knitting bowls and what diameter are the blanks?  From the photos I would guess about 6-8".  I know several knitters who would appreciate them.

Thx.

GNM

[JHCC]

More like 8-9” in 16 gauge. Now that my wife is getting her yarn shop (named “For Ewe”) up and running, it looks like we’ll be selling these through her website. 

[Frosty]

Getting there John. Going fast is the secret but it takes practice. 

Frosty The Lucky. 

[JHCC]

Getting the torch properly adjusted helps too.  

[Frosty]

Oh yeah. I never did get fast enough to really make my torch work like it can.

Frosty The Lucky. 

[JHCC]

Update: The center-point holder turned out to be not particularly accurate and prone to shifting. I tried doing a small divot center-punched directly into the metal, but that wasn’t great either. Yesterday, I had a bit of a brainwave and built this:

This is the magnet off a Harbor Freight magnetic part dish (although a small speaker magnet would work equally well), with a piece of 1” bolt ground to a tight friction fit. The bolt has a divot in the center to take the pivot pin from the circle guide:

Since this raises up the point of contact rather significantly, I’m not using the dogleg side of the pivot pin, but instead sharpened the straight end to fit the divot. It works very well, and I can just stick it on the side of the cutting table when not in use. 

[Frosty]

A good way to prevent a magnet from shifting on flat material is to squirt a bit of silicon calking on it and spread it very thin. It forms a very non-slippery surface, it's a trick I learned from a mate on a State Ferry ride to Kodiak. The run is known for heavy seas but the dishes never slid off the tables, everything but the flatware was wiped with silicone calking on the bottom. It's dishwasher safe too.

Frosty The Lucky.

[ThomasPowers]

I've suggested using a magnet with a center piece instead of making a divot in the workpiece several times over the years here,  Some speaker magnets have a central  hole in the keeper plate already.