- Joined
- Feb 17, 2013
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- 4,414
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!
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!
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