I have to cut about 60, 160 mm diameter holes in 3 mm Al plate.
I have done this successfully before for a few holes using a router but its quite a slow process and handling the 3 mm plate pieces around a router is a PITA.
So I decided to use a plasma cutter.

I borrowed the home made plasma torch holder for cutting circles from the mens shed but was not happy with the lack of smoothness and overall accuracy so I decided to make my own. This has taken me far longer than it should have but after doing a few test cuts I'm really pleased with it.

The basis of the jig are 3 bearings.
The axis (a bolt attached by a 4 mm CS screw to the centre of the work) uses a 22 x 8 x 6 mm bearing, while the torch holder uses 2 scooter headset bearings (41.8 x 30.2mm ID) inside a brass and SS holder.

Also shown are a couple of outrigger wheels which might be useful for larger circles.
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top View
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Below is a better view of the torch holder.
B1 and B2 are the scooter headset bearings recessed into a brass inner collar and held together by 3 mm SS rings by brass screws passing through those copper tubes. The tubes prevent the bearings from popping apart if the brass screws are over tightened.
A pair of 4 mm grub screws (G is one) through the brass collar grips the ceramic head of the torch.
The head rotates super smoothly.

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Here's what I'm cutting/making.
These gizmos are 150 mm Blast gates for the Mens shed dust extraction system.
The holes have to match the OD of ~150 mm PVC pipe which is 159.8mm
I've started cutting the holes 159 mm wide and by the time I remove the dross with a bobbin sander they clip neatly over the PVC where they will be PU glued in place

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Here is what they will look like - this one is in my shed and is made from 6mm PVC.
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The reason we are using Al at the mens shed is because we have a heap of 3mm Al sheet.

I also want to have a crack at using some argon instead of air to see if I can get a cleaner cut.