This Old Mill

The journey of a guy getting an old CNC milling machine up and running in his garage…

Powering the Beast?

While having a mill in your garage is impressive…imagine how much more impressive it could be if it actually worked?  Well, to make it work, it’s gonna need power, and plenty of it.  Ya see, dang near all these old milling machines require 3-phase power, including mine.  In fact, nearly all industrial grade machines use 3-phase power, its just a more efficient way of providing instantaneous power to machines that have big electric motors.

Unfortunately, almost no residential neighborhoods in the U.S. offer anything but single phase power.  The good news is, there are ways around this issue.  The typical way of dealing with this is by purchasing a phase converter, which is exactly what I originally planned on using to make my machine go.  A phase converter is used to convert single phase power to 3-phase power.  The most common phase converters are rotary phase converters.  As I understand it, these are essentially a 3-phase induction motor with the two hot wires (240VAC) from single phase power connected to two of the three legs of the 3-phase induction motor.  The remaining leg of the 3-phase induction motor generates power as the motor spins – creating the 3rd phase.  I don’t know why, but being the kind of person I am, I never warmed up to the idea of a phase converter.  Just knowing that this motor is gonna run whenever power is connected to it, regardless of whether the mill is on or not bothers me, it just seems inefficient.  On top of this, they ain’t cheap.  My mill has a a 3HP spindle motor, and a new – very basic rotary phase converter will start at about $600, as go as high as $1000 for a machine this size.  Crazy eh?  Dang right its crazy, especially given I didn’t spend $1000 on the entire mill.  I suppose someone reading this is thinking, ‘yeah, but you can build one much cheaper’.  You’re right, you can, but I won’t.  I’m not ruining my machine or worse, burning my garage down, messing with that crap.

There has got to be a better way, and in my opinion I have it, but please bear with me as I explain.  Now all of the following didn’t come to me overnight, I had some figure’n to do, and questions to ask folks a lot smarter than me (Tony Coffman – Master Electrician).  Additionally, I was lucky enough to receive my mill with some of it’s original wiring diagrams.  The following image is a picture I took of one of the power wiring diagrams, and I’ve overlaid some highlighted areas to help relate what I write to the image.


In short, if you follow the highlighted 3-phase power input section, you can see it becomes distributed as highlighted by the Phase distribution section.  Three of the phases pass through the fuses labeled FU1, and contactors M1F and M1R and eventually connect to the spindle motor M1.  So there’s no way around needing 3-phase power to run the spindle motor on the mill.  At least not without replacing the 3-phase spindle motor with a single phase equivalent – but that’s cost prohibitive.  Moving forward, if you follow the other branch leaving the Phase distribution section, you’ll notice they pass through some some fuses FU2 and FU3, and end up connecting to the Servo Transformer and Control Transformer  respectively.

What’s important to recognize here is that only two of the legs of the 3-phase power ever connect to these transformers.  This means these transformers are powered by single phase power!  This is huge, and is what gives me the option to do something other than a phase converter.  The other obvious thing for me to tell you is that I know the guy I bought the machine from was running the machine  off a 240VAC 3-phase connection to power.  Additionally, the descriptions shown in the highlighted Servo Transformer and Control Transformer section indicate these transformers are fine with 230VAC inputs.  By the way, if you’re paying attention, the power diagram references 230VAC.  As I understand it, no power companies ever supply 230VAC, they supply 240VAC.  This is just some sort of convention that is followed due to some weird old 220/240VAC European standardization problem.  See Google if you need a better understanding.

This page is getting long, so I’m gonna close with this.  I have plenty of single phase 240VAC in my garage, so for sure I can power the Servo Transformer and Control Transfer from this.  However, I still need to convert the single phase 240VAC to something the 3-phase spindle motor will be happy with.  For this, I chose the awesome Variable Frequency Drive.

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