It depends on how familiar you are with machining. I used set them up in the mill and a boring head and turned the knob down, then threaded it with a die. I now have a fixture that holds them in the lathe and I single point thread the end. Much faster, easier and nicer looking than the mill but the fixture took some time to get right. I've seen a few guys who have use a bench grinder to grind the original knob down then ran a tap on it. The threads might not end up perfectly straight but it will work. But if you don't feel confident in doing it yourself give me a call I'd be glad to put a knob on for you.
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Thankyou to everyone for your fast and great feedback, I am going to buy the knob off of ebay and do it myself. I actually am going to have the machinist at my work do it, I have seen him do amazing things with metal, this should be a breeze for him.