# Converting a knee mill to 3phase/VFD



## hman (Oct 4, 2020)

I've recently bought a Precision Matthews PM-835S.  Advertised as a "small" knee mill, it's still amazingly massive - and TALL.  I've had to build a platform to stand on (re-using some of the the 2x4s and OSB the mill was crated with), just to be able to reach some of the controls and levers atop the head.  Luckily, my darling bride ordered me a power drawbar kit for my birthday!  In addition, when the knee fully raised, the tabletop is 56 1/2" above the floor - puts the top of the mill vise very close to eye level!



Next on the agenda is to replace the single speed motor with a 3 phase motor and VFD.  The original motor is INCREDIBLY smooth and quiet running, and I dearly love it.  But I often do power tapping on my mills.  Unfortunately, the lowest available speed on the 835 is a bit faster than I'd feel safe with when tapping.  And being a capacitor start motor, there's no quick and easy way to stop and reverse the motor to back out the tap.  Luck continues to be with me, however, in that I have a 3HP 3 phase motor and matching VFD already on hand (palanned for a future project that's failed to materialize).  I noticed that the mill column has a nice, large hole in the side of the casting - just right to squirrel away the VFD and braking resistor.





Next up was the operator control panel.  Note that I'm using a "safety switch" (described elsewhere on HM) for stop/start.  The toggle switch under the shield selects forward/reverse.




One "feature" of these Bridgeport-type mills is that the head includes a back gear, giving twice the spindle speeds.  Unfortunately the back gear reverses spindle rotation.  So the forward/reverse switch would be "backward" in one case or the other.  Not desirable, given that the reverse toggle is shielded.  I'd originally planned to add a second switch atop the control box, to select between direct drive and back gear.  But, being lazy, I'm always looking out for ways to do things automatically.  Istarted by digging around through my stock of miscellaneous switches and found an "industrial" roller-lever switch.



The switch housing has very nice versatility for various setups - you can rotate the “head” by 90º increments and the arm by 45º increments.  Thought it would be nice to mount the switch on the mill head and activate it with the back gear lever.  Last night I got busy on the switch mount.  Started with a sheet of cardboard laid on the base of the drawbar power unit.  Penciled in, measured, then laid out the radii.  Transferred to a chunk of 1/4” aluminum.  Bandsawed, filed, belt sanded and Scotchbrite smoothed the curves.  Extended the activation bar of the switch wiith a shoulder bolt and plastic cylinder.  Positioned the switch by eye, clamped it in position with a Kant Twist, transfer punched for the holes, then drilled & tapped the aluminum plate for a permanent mounting.  The photos show the the switching action being verified with a meter as I shifted the back gear lever.







The final step in the process will be to fabricate a motor mount.  Bridgeport-type mills have a unique mounting style - a flat 1/2" thick plate on the shaft end, that has two threaded holes for belt tension adjustment.  The 3 phase motor I have is a foot mount.  Not too difficult to work up an adapter (shown as a CAD rendering, next to the existing motor).  Once I have that fabricated I'll be able to remove the pulley from the original motor, ream it out from 22mm (0.866") to 7/8", tap the end of the motor shaft for the pulley retainer, and Bob's my uncle!


----------



## erikmannie (Oct 4, 2020)

Wow, that is a well planned, large project.

I wonder why the mill is so tall. Do you like to work with the quill mostly retracted?


----------



## hman (Oct 4, 2020)

Thanks for the kind words.  I kinda get the feeling that the Chinese style of knee mill is inherently taller than a Bridgeport because of the way the knee elevating screw is set on the base.  Need to get some good side-by-side photos to verify this.  But you can see that the base casting of my mill is something over a foot high ... taller than the base on a Bridgeport.

As for the quill being retracted, no reason to do so.  Maybe just a bit of hesitancy on my part, being so used to my round column mill, where the quill is not as precisely held as on the PM.  On the old mill I tended to keep it retracted as much as possible.  The quill on the PM is very precise, so there shouldn't be any reason not to let it out.  And you might have noticed from the photo that the milling cutter was actually "inside" the jaws of the vise!  I'd simply raised the table to its upper extreme for this shot to illustrate what might go on if needed to clamp a thin workpiece directly to the table and use a short end mill.  And though most work would NOT be at eye level, it would still be high enough off the floor that I'd want something to stand on when setting up the hold-downs, etc.


----------



## erikmannie (Oct 4, 2020)

I was working on a Chevalier today (which doesn’t seem too much different than the Bridgeport sitting next to it). I could not see the top of the drawbar when I had to tap it with a hammer to change collets. I am 5’ 10”.

Standing on a box doesn’t seem like a dealbreaker. You could add a fatigue mat for another 3/4” “riser”.

I like your z-axis power feed. How do you go about raising the knee just a few thou? I can understand why you got tired of cranking that z handle.

What do you use the yellow bucket for?

A woman who gifts a power drawbar would get a tremendous amount of goodwill!


----------



## hman (Oct 4, 2020)

I'm a bit more vertically challenged - 5' 6".  Thought about a fatigue mat when I made the riser, but had a BUNCH of the interlocking 2' x 2' x ½" rubber squares on hand, left over from my previous shop.  Used Liquid Nails to hold two squares to the top, then cut around the edges once the goo was well set.
Regarding fine adjustment - see post #17 at https://www.hobby-machinist.com/threads/z-axis-knee-power-feed-recomendations.56729/#post-775637
The yellow bucket (repurposed kitty litter container) is a waste can.  It's for non-oily swarf, paper towels, wire clippings, etc.
And yes, my sweetie is a sweetie!


----------



## hman (Oct 16, 2020)

I just finished machining the frame that will adapt my 3 phase motor to the face mount.  I’ve previously posted about using use the relay outputs on the Teco VFD to turn an auxiliary cooling fan on whenever the motor is on.  This is to prevent it from overheating when run at less than full speed.  








						A happy discovery
					

Lots of folks will tell you that a 3 phase motor can have problems when run at less than full speed, because the internal fan isn't running fast enough for good cooling.  I’ve been thinking about adding an external fan - possibly a 12 volt computer fan - atop the 3 phase motor on my mill.  But...




					www.hobby-machinist.com
				




Figgered out a simple way to mount the fan … the long screws that hold the motor together are threaded 10-32.  I got some coupler nuts with the same thread to replaced the original nuts.  This plus a ring of thin aluminum gave me a platform on which to mount the fan.  The OD of the ring is the same as the motor, so I might even add a sleeve between them if I want more positive airflow.  Not visible in the photo, there are some nice airflow openings in the end bell of the motor.



Then I had a Eureka moment.  I’d designed the motor mount with vertical slots, to permit fine-tuning of the motor's vertical position (lining up the drive pulleys).  But the motor weighs about 43 pounds.  It would be a real chore to try to adjust and and then lock its position while at the top of a ladder, leaning over the top of the mill.  By adding threaded holes to the adapter base and the top edge of the back wall, I can screw in some lengths of threaded rod.  By removing the fan from the coupler nuts and temporarily replacing it with a plate, I could suspend the motor from another threaded rod, in turn supported by a crossbar above.  Fine-tuning the motor position is thus a simple matter of tightening or loosening one nut.  Once the level is correct, I'll tighten the motor mount bolts on the back side of the mount, remove the threaded rods, and replace the fan.


----------

