I see a lot of questions about VFD's and some statements of personal expectations of them. I want to put this out there to set reasonable expectations for VFD's. What not to expect and frankly their shortcomings. Hope you find it informational. VFD's are actually a new spin on an old technology. Servo drives and DC servo motors have been around for years. And variable speed 3 phase motor controls are nothing new either, but the older ones used contactors and resistors to limit the current and voltage to the motors to slow them down. The excess power was just burnt off as heat. Now with newer technology the VFD or Variable Frequency Drive will control a 3 phase motor in a much more efficient manner. WHAT THEY ARE USED FOR IN INDUSTRY While VFD's are in some folks eyes are a God sent for the Hobby Machinist. They were not designed for the purposes that they have adapted them for. The two main applications for industry are air handling fans and material handling, IE conveyor drives. They can be found elsewhere, but this is the mainstay of them. They provide a 50% or so reduction in the operating speed of the controlled motor and this of course is significant with these applications. Fans are fans. I will not go into the specifics of them. They run slow, fast, or in between. Conveyors are what shows the biggest strength and weakness in machining for a VFD. Remember that it's a 50% reduction. Some can get a bit more but that's a reasonable expectation. Now, with a conveyor the motor is NEVER coupled directly to conveyor, there is always a gear reduction that with a decrease in RPM increases torque dramatically. But these conveyors are setup to run a specific speed or range of speeds or be stopped. If a conveyor is designed to run at 100 foot per minute, it will never be expected to run at 1 foot per minute. This is where the VFD on your mill will let you down as well. You can't expect to set your mechanical drive gearing of your mill to 2000 RPM and then use the VFD to slow it down to 200 RPM. And even if you could, you still loose the mechanical torque advantage of the mills gearing so torque suffers greatly. At 2000 RPM with standard SFM cutting speeds and a .5 end mill you are either hogging out aluminum or plastic. At 200 RPM, you are going to be cutting really hard stuff or using a really big end mill on mild steel and not a half inch one. That requires torque and lots of it. So whats reasonable? As mentioned before 50% or so. A great application for a VFD is a cone drive (wide belt) lathe. If you have 4 settings that are 1500 1000 500 and 250 and you need a RPM of 750 what do you do? At 500 you are getting poor machined surfaces and at 1000 you are getting excessive wear on your tool due to heat buildup from the increased speed. So you set the belt for 1000 RPM and slow the motor until 750 RPM is reached. You have the correct speed and are not suffering for as much lost torque as if you were to set the belt at 2000 RPM and then slow the motor to achieve the needed 750 RPM. This applies to a number of vertical mills I have seen as well that used a step pully drive system to get a sort of close spindle RPM for their machines. Of course these are some of the machines that we are using in our home shops and suffer from the close but not quite spindle RPM issues on a regular basis. Point is to not have an expectation that you can simply lock out your mechanical speed control system on your machine because you have a VFD and expect it to work at all speeds. It's not going to happen. Reasonable expectation is key here. A VFD in unison with the mechanical controls of your machine and a optical tachometer will allow you to set your cutting speed correct and spot on every time. But you have to play within the rules to have a good experience.