The G0704's spindle motor was originally swapped with a treadmill motor using Hoss's plans and that worked okay for a while. After a few months the treadmill motor started to arc under load or high RPMs. Having seen other people successfully do the conversion I am convinced that the problem was a bum motor even though it was said to be brand new. Knowing that the TM motor was on it's way out it could be replaced or upgraded with something a little more industrial.
After browsing automation direct a 2HP 3450 RPM C56 3 phase motor and 2HP GS3 VFD was on order. Surprisingly, shipping is quick and free for orders over $60. I think the total invoice was around $430. Could save $50 bucks by going with the GS2 drive.
Dove into some 3D modeling to figure out how to make this thing fit. Most people would probably do that before ordering but I was determined to make it work. Fortunately it fit even if it was just barely. Using a 30 tooth drive, 18 tooth spindle pulley, and 45 tooth belt (L pitch) there was roughly a quarter inch of clearance between the front and back of the motor. On paper this worked out to be a spindle speed of 6100 RPM at 63.7 Hz.
The new standoffs are a bit larger than what they need to be given the material on hand and putting minimal run time on the dying treadmill motor. In the end it worked out. For the 30 tooth pulley to clear a section of both stand offs had to be hogged out .250". So having the extra meat available made me feel better. The extra material extending down past the top of the head will also double as a tramming aid with the addition of a few screws.
The original spindle pulley attached using four set screws at 90° and didn't much care for how some seated in or on a spline. The new pulley has six 1/4"x20 set screws at 60° intervals. When tightened the screws make contact at the base of each spline. The hub of the 18t pulley had to be machined off to avoid collision with the motor's face plate. This also makes room for a simple spindle wrench. When changing cutters my left hand would some times slip and sliding down the end mill when breaking the draw bar free. A scar serves as a reminder for each incident... The motor pulley was pretty straight forward. Broach a 3/16" internal keyway and 2 set screws at 90°. Broaching the keyway was the biggest PITA of the whole project though.
The new motor is 35 pounds heavier than the treadmill motor so there was no expectation that the current Z axis arrangement would work. Surprisingly the 570 oz/in stepper and stock ACME screw worked just fine as is. If I ever get around to finish turning the last ball screw and nut mount it will be even better.
The electrical integration was pretty simple and clean. Mach3 controls the drive via a modbus link over RS-485.
With the drive set to V/Hz mode the spindle was running roughly 5945 RPMs at 60Hz. A little faster than the pencil whipped values but there is was no load. When in sensorless vector control mode it over rev'ed the spindle around 15%. Don't really know why but that is a problem for another day.
Still hasn't made its first chip yet so the jury is still out on performance. Don't anticipate any major problems though.
Here are the part numbers and suppliers if any one is interested.
Econo belt-
30t pulley - QPELA75030F20
18t pulley - QPELA75018F20
45t belt - QB-L01688-075
Automation Direct-
Motor - MTR-002-3BD36
VFD - 1GS3-22P0
Some pics and a video. It looks ridiculous but it can't be too big so long as it fits, right?