For anyone with an Optimum mill. As you've already found, the mill comes with a whitworth threaded drawbar and most arbors have metric threads. You can use a long M12x1.75 pitch bolt and knock the arbor out by tapping on the head of the drawbar, however be careful as the quill gear, apparently is plastic. Or, you can do as I've done: I bought a long M12 x 1.75 pitch bolt, turned the head down to 17mm and drilled a 5mm hole through the head to a few mm. I turn down a piece of 19mmm stock to 17mm, then turned down one end to 5mm long enough to fit in the hole in the head of the bolt. The other end I turned down to 9.5mm and milled flats on either side to fit a spanner. The 17mm section needs to be no longer than 24mm so it will fit under the cap nut. I then inserted the 5mm spigot into the hole in the bolt head, welded them together then turned the welded area down to 17mm. It came out a little under 17mm, but that's not critical. That done then it was just a matter of dropping the drawbar down the spindle, screwing on the cap nut and fitting the arbor. To remove the arbor just fit a spanner over the flats on the new drawbar and turn anti clockwise. In retrospect I should have turned a 5mm spigot on one end of the 19mm stock,, welded that to the bolt head and then turned down the hole lot to 17mm. As it turned out the bit I welded onto the head turned out spot on, but that's called a fluke I think.