Attaching the Toroid to the Secondary Coil

I have been considering how to attach the toroid to the secondary coil. This involves a physical connection to hold a rather expensive gorgeous hand spun aluminium toroid and an electrical connection between the toroid and the secondary coil. I would like to be able to quickly remove the toroid for transport and access purposes but without risking damage to any electrical connections. I have come to the conclusion that the secondary coil should have a permanent threaded conducting protrusion on its top cap which is electrically connected to the secondary coil. The toroid simply slots over this protrusion and is secured by a threaded knob.

Here's the top of the secondary, the central hole is 10mm and was drilled originally to mount the secondary coil onto the winding jig shaft (a 10mm threaded rod). I thought 10mm was a little excessive as a mount for the toroid so decided to bush the hole down to 8mm with a piece of brass rod.

I turned a flange on one side of the insert. This would sit on the under side of the secondary top cap.

Here's the bush fitted, admittedly the wrong way, just making sure it was a snug fit.

Ok, so I now have the top cap ready to accept some kind of 8mm bolt that will protrude upwards. I need to do 2 more things. Firstly, I want to add a little height to the top of the secondary to lift the toroid,  secondly, I need to make this protruding bolt captive so when I undo the fastening for the toroid this bolt doesnt spin or release and drop down the inside of the secondary. To cover off both these requirements I am going to use two pieces of perspex. The first is a 100mm diameter 15mm thick clear disc ordered from my favourite plastic supplier Trent Plastics.

As you can see I have masked the disc up and drilled a central 8mm hole. This disc will sit on top of the secondary coil top cap. The second piece of perspex required is exactly the same size except it has a 25mm central cut-out.

These two discs will be bonded together as shown in the mock up below.

The larger hole in the top disc will provide clearance for the nut which will make the toroid mounting pin captive. OK, now for the mounting pin.

I cut a length of 16mm diameter copper rod, started off with about 10cm. The shank of the pin needed to be 65mm long and 8mm in diameter.

Turning the copper rod down to 8mm was down using a type 5 parting tool. Probably not the best tool to use but I always have the most success with this type. The material was removed in 0.4mm passes, I didn't want to risk any deeper cuts as it's quite a long shaft to turn and copper is pretty soft. I had an experience in the past where the tool dug in and the work was completely destroyed.

The toroid will be secured in place with an M8 threaded knob so I needed to cut some threads into the pin. There's quite a lot of info on the web about cutting threads onto copper rods, lots of comments about poor results and differing views on cutting methods and lubricants. I used plenty of WD-40 and patience. Don't try to cut too much thread in one go, I found it best to cut 1/8 of a turn each time and reverse the die 1/4 turn back every time. Use plenty of lube and clear swarf from the die when needed.

Here's the assembled mounting pin.

The copper lug will provide electrical connection to the secondary coil. The nylon M8 nut in the top pic will be replaced with a stainless steel version.