This is a story of a mod., with a bit of knowledge and a lot to learn. Unfortunately I didn't take any photos of my progress as my camera was broken, but now that I have a new one there will be a shot of the finished item.
I bought a tool holder at an Autojumble (my first, I'll definitely go again). It had a 25mm square shank, my lathe tool holder only takes 16mm. No problem, just pop it in my homemade milling vice on my homemade milling machine. Using a 12mm HSS end mill I tried my first cut ( Huh, I need to get on and finish my Quorn!) I placed an order for a Carbide end mill, this was positive, I live in a holiday area and the seller has a caravan just down the road, so the cutter was delivered next day. I popped it in the mill and off I went. What a difference! Needle sharp splinters of steel were sprayed in all directions. I pushed it further, red hot splinters flew everywhere. Then it stopped, no warning no smoke, no nothing. It just stopped. I was quite worried, the wiring of this machine had stretched me to the limit. Trouble shooting was possibly going to be a step too far.
Next morning I switched it on and joy oh joy, it worked. It must have been an overload in the speed controller. Although it was now working I was finding that the rate of metal removal was quite slow and I was worried that I would ruin the carbide end mill. Then I had a revelation, I ordered some 1mm thick cutting discs for my angle grinder. They arrived after 2 days, 30 minutes work had the tool holder roughed out and another 30 minutes in the mill had it machined nice and square. Phew, what a palaver! At least next time I will be able to do it more quickly.
I tried it out on a piece of mild steel, rotten finish, it is too late now I'll sort it out in the morning.