I am interested in getting a milling machine such as the one below:
http://www.woodstockint.com/Products/M1005
or this one:
http://www.grizzly.com/products/G9959
My initial objective was to use it for woodworking projects. However over the past 6 months I have become more interested in machine and metalworking projects as well, and as a result I would also like to use it for cutting a larger variety of metals and steels. THe machine already has a belt drive system, with a speed range between 400 to 5000 rpm. However would it be possible to add a variable speed drive to this mill, in order to obtain a wider variety of speeds? I would love to be able to change speeds simply by dialing it in with a handle or knob. That way I could switch from low speeds for metals, and then jack it up for the high speeds that work well in cutting woods. How difficult would this be? What kind of a speed range could I get? What difficulties would I have to overcome?

CocoMedic,
Yes, it is possible to add a VFD to one of these machines IF you change the motor out to a 3Phase motor and add the VFD. Based on a quick check of the web sites, it appears these units origionally ship with 3450 rpm motors. If you want to run metal cutters, I would suggest replacing with a 1750 rpm motor and if it is a proper inverter rated unit, you should be able to run it twice rated rpm with the VFD to keep the high speeds (also useful for small cutters in aluminum). I would also keep the origional pulleys so you can keep the torque curve at low rpm.

Hope this helps.

BTW the VFD can run off 220v single phase in, (3phase out).
I converted both of my shop Excello Knee mills, these had the original 2 pole motors (1750 rpm) and I run these at 120hz which doubles the rpm.
The Varispeed pulley was replaced by timing belt drive.
They already have a two speed gearbox.
Al.