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Milling Machine Motor Replacement, Tach, and VFD


The final machine to receive a variable speed motor is my mill.  I started this project by trying to find a good solution for replacing the motor and by installing a MachTach.

Tachometer
The tachometer posed a problem, as there was no obvious place to locate reflective strips or magnets for the speed sensor.  I thought about placing reflective strips on the flat areas above one of the step pulleys, but to avoid interference when changing belt speeds I would need to place it inside the belt loop between the pulleys.  There just is no room there.  I finally decided to abandon the very top step of the spindle pulley and place a ring of magnets there.  I have not used the top 3 pulleys, and the overdrive capability of the VFD will allow the next step down to reach a higher speed than the top step with a normal motor.




I machined an aluminum ring which is a snug fit around the top pulley, and epoxied
4 magnets into counterbores around the outside.  I then added a block to mount
a sensor bracket.



This is the bracket I made to mount the sensor.  The slot in the end is to position the sensor.


  
I bent the leads of the sensor a short distance from the body and using JB Weld, epoxied it into the slot.  After the epoxy hardened, I carefully dressed
the holder and epoxy to just expose the outline of the sensor.



The sensor is mounted in place.  There is a slight gap between the sensor and the ring.


I then procured and built another MachTach kit, the same as I have done for the lathe and the drill press.  I set it aside until getting to the control box part of this project.


Motor Replacement

There is a problem trying to replace the motor on this machine, as the original motor has a 4 inch long shaft.  It is very hard to find "off the shelf" motors with that long a shaft, and I have not been successful doing so.  My alternate options were to buy a standard shaft motor and machine a shaft extension, which would then mount the standard pulley, or to machine a whole new pulley, designed to attach to a shorter shaft.  I delayed doing either, although I did design a custom pulley with 3 of the original 6 pulley steps.  I figured that with the overdrive capability of the motor, and the fact that I have never used the top steps of my existing pulley, that that would suffice. 

While I was contemplating these other options, I located a member of one of the Internet forums who had an original Rockwell 3 phase motor he was willing to sell, and I bought it.  It was in fairly rough condition, as it had not been used for some time, and showed some weathering.


  
This is the motor as I received it.  I connected it to my lathe VFD and it ran fine, although there was some bearing noise.  When I opened it up, I
found a wasp nest on one end of the armature!  It had not dislodged at my trial up to 3400 RPM!  I guess I should have opened the motor first!



After a thorough cleaning and repainting, and a new set of bearings, the motor is
looking quite respectable.

VFD and Control Box
I purchased the same model VFD that I have on my lathe.  It is rated for a 1 HP motor, and this new motor is 3/4 HP, so it is fine.  My original single phase motor was 1/2 HP.

I decided to mount the VFD, the MachTach, and the control switches and buttons in a single enclosure on the side of the mill.  Home Depot had a very sturdy 12 x 12 x 6 inch plastic enclosure.  I made an aluminum panel to hold the various controls and the tachometer, and cut clearance holes in one end of the enclosure where this mounted.  All the other components were mounted inside the box.




The control box holds pretty much everything.  The control panel is on the right with the power switch and the buttons
on the top, and the MachTach and speed control pot are on the bottom.  The mounting board holds the VFD,
the fuse, a terminal block, and an AC outlet.  The tach power supply plugs in there.

Putting everything in one box really simplified the wiring.  There are only 3 external cables, the AC power cord, the cable
driving the motor, and the tachometer sensor cable, which attaches using a connector. 

I tried to keep the wiring systems separated.  The incoming AC is all at the top of the case with the control wiring at the bottom center,
going directly to the VFD.  The MachTach power and sensor cables are at the very bottom, and the motor 3 phase output is also at the
bottom  so I kept the power and the tach cables twisted, and after installing the box, separated the tach cables as much as possible
from the 3 phase.



The control panel is mounted to the enclosure, which is mounted to the mill.
This seems to put all the controls in an easy to reach location.  I kept the STOP
switch at the top, as it would be the last position to be blocked by a large
fixture or workpiece.



The final installation allows easy access to the VFD for setting parameters and for viewing
the frequency to the motor.  The controls are handy and the tachometer is easy to see.

Additions of 6/3/15

Ever since I bought the original (50 year old) factory 3 phase motor, I have been worried about running it on a VFD.  Most current motors are "inverter rated" meaning they have better insulation to withstand the high voltage spikes the VFD produces.  They also tailor the design to have better performance and efficiency with VFD use.  I am probably being paranoid, since it is running just fine, and there are many Rockwell mill owners running them on VFD's without any problems.  However, I decided to replace the motor with a new "inverter rated" motor, as I was prepared to do when I saw the offering for the used factory one.

I purchased a new 3/4 HP 3 phase motor with the correct mounting on the face, but with a shorter shaft than needed for this mill.  I had already designed a shaft adapter to make this type motor work.  I had also defined several other changes I would like to make to the control box.

I had originally decided to place the stop button on the top with the start button below it, figuring if I had a large workpiece it would be less likely to block the top position.  I have fought this from day one!  The stop button (in my mind, anyway) belongs BELOW the start button.  I made that change while I had the box off.  I also mounted two duplex outlets on the rear of the box, switched by the main power switch to plug in my light, power feed, and DRO.  This way when I turn off the power, I turn off ALL the power.  I have often left the power feed on and occasionally the DRO.  If I should decide to keep the DRO powered on between sessions for a specific project it is easy to replug in into the wall outlet.



This is the shaft adapter I needed to achieve a 4+ inch long shaft on my new motor.  I
had  to cut about 3/8 off the motor shaft for everything to fit.  I used 5 blunt nose
set screws in the shaft keyway instead of trying to machine an internal keyway.  I was
originally going to make the adapter keyway 180 degrees from the set screws to minimize
runout, but found that between the slight runouts of the adapter and the pulley, the
present location, coincidentally in line with the set screws, gave me the least overall runout.
 The pulley now actually runs slightly truer than when it was directly on the motor shaft.



I added 4 outlets on the rear of the box, switched with the main power switch.
Now, it will be easier to verify that I turn off all the accessories.
I clamped the box to the mill table for ease of work.  Since it is wired to the
motor, I could not easily take it to my work bench.



Here is the completed modification including the new motor, the revised control positions,
the outlets on the rear, and a plate for mounting the work light



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R. S. Mason   12/2013