There are a number of very good VFD's, the lower priced ones (under $200) tend to lack features and also some (like the L510) do not support an external braking resistor. Recommended would be the 3 HP single phase input Teco E510, Hitachi WJ200, Automation Direct GS21 and Yaskawa GA500 series, I have done installs with all these models and they have all performed well. Availability has been an issue, the Hitachi WJ200-022SF has been out of stock with most vendors, although it is probably the most commonly used VFD in the 935 mills. Price wise they are all within $100 range of each other. How they operate and the program parameters are quite different, this also comes into play if you want to use the JOG function with what is called 3 wire control. I am working with another forum member who is setting up a WJ200 for his 935 with both 3 wire and JOG as well as a back gear sensor, need to see if it actually works when he gets his VFD. Tech. support is not very helpful when I asked if the JOG inputs work with 3 wire control. There are ways to do it, just depends on the VFD model and the circuit design/programming.
On the mill I typically use 3 wire control which is a momentary switch for START, STOP and a sustained switch for REVERSE. Also mills with back gear forward is switched to reverse in back gear, I add a small switch activated in back gear that reverse the the VFD directions in back gear. I have a number of different designs but they vary by VFD manufacturer. David's system (as well as my mill) use 2 wire control with momentary buttons that latch relays, it is a more complicated design. The run buttons are in a pod attached to the knee. I do not use a JOG function on my mill because I use spiral taps and thread in one pass, otherwise I just hand tap (it fits into the chuck). You also can just turn down the speed pot, power tap through and then just flip the FOR/REV switch and back out the tap. I have setup mine to auto reverse, ideally you want to use a shorter braking time when tapping, otherwise it is 3 seconds unless the E-Stop is pressed.
I can provide you with suggestions on a parts list. On most modern era 4P mill motors, I run them from 20-120Hz and vector motors to 200Hz, so I usually like the 3 turn mil-spec pots over the wider range. I also find that a lot of the cheaper single speed pots to either fail, or not hold speed stability as the vibration from the machine can bounce the wiper. I usually use mil-spec. speed pots made by ETI, Spectral, etc. that are wire wound/composite film and rated for millions of cycles. I have a 10 turn on my mill, it came that way from the factory. Also problem with a single turn (in some cases) if you change the speed too quickly you can get a VFD fault from an over voltage spike trying to change the speed too quickly. There are some program parameter changes which can minimize these issues.
ETI MW22B-3-5K 1W 3 5Kohms Turn Wire wound Potentiometer
https://www.ebay.com/itm/201057249723
ETI SP22E-2K 1W 2Kohms 1 turn conductive plastic
https://www.mouser.com/ProductDetail/ETI-Systems/SP22E-2K?qs=wuXwK9wHm9tJD5TtBzWetA==
You can read up on the VFD's, when you get closer to ordering/building, I can provide some designs and parameters. There are lots of posts on the 935 and VFD installs, I would review what others have done and then go from there.
