Circle cutting guide?

[Dusty]

Help I need to cut some holes through a 5mm thick steel bumper, finnished size will be 94mm dia. I have a cut 40, so plasma cutting will be the order of the day, but any suggestions as to how to make a suitable cutting guide???

When You have to cut nice round holes HOW do YOU DO it?

I've not needed to cut largeish holes in wood/metal before so don't have anything more than a set of jobbing drills and a few flats for wood, an electric jigsaw etc.
I have in the past cut rough holes in ply sheet using a bar clamped in the fence mount of the jigsaw and a nail as the piviot!
But they aint pretty

I've seen ? TANJANT ? guides for gas torches, heard they make em for plasma too ! ? anyone?

Strikes me that to make holes in wood first is complicated enough!
where can I get a guide that fits my plasma torch; cut 40 / pt-31
Some thing that would allow me to cut 35mm dia up to 150mm approx.

Thanks Dusty

 

[shenion]

I cut a larger hole in plywood and clamp over work; then run torch around inside.

I have wanted to try mounting a screw in the center, then a loop of wire to set the torch length. Or a piece of flat bar with a hole to fit the torch tip and a center hole drilled.

 

[snowcat]

They are very easy to make, a trammel bar with a circle for the torch instead of a point, Iv always bought mine as there pretty inexpensive, but you could easily mock something up..

 

[Hitch]

A short bit of tube that the torch fits snuggly in. tacked on the end or a bit of 10mm bar, then a block with a clamp screw, and a centre point on the underside.
As snowy said, theyre easy to make, but cheap to buy...but it depends if you can get one to fit on your torch.

 

[Dusty]

Well, pottering about in the shed last night, I had an Idea; some odd scrap's of mdf and nuts/bolts. IT'S Only a proto type!

By changing the bolts round I should be able to cut the two different size holes. The dome head bolt keeps the tip a set distance from the work, so hopefully it wont get too hot!

I'll try it out later and post a pic if it works!

Have a Good day and an even better night!
Laters...

Dusty

 

[Wozzaaah]

Good work Dusty, a couple of things I've thought of;
slot the arm so you have variable hole diameters and you aren't limited to just two sizes.
If you are cutting a circle to keep the 'round' then you'd end up with a bolt hole in the centre to fill. This circle cutting kit uses a magnet as the pivot point.

 

[Hitch]

Looks good dusty....
Wouldnt fancy its chances with a lot of holes mind

 

[Dusty]

Just to prove it works! Ok so I had to fiddle about making the torch hole a bit deeper to get it arc! and its a sod trying to go smoothly round the smaller one! IT WORKS Hurrah, mind you Woz, when I can afford it that kit looks to be the way to go. This was to solve a one time prototype problem. so Hitch you may be right, it might not last very long!

I only need to do 4 large and 4 small holes on this job, so No prob

custom job on my car not for resale, well not unless people want the same o'course...

Pic's show both holes cut and a lamp held to show fit.

Laters Dusty