Power Z Conversion For PM45

I have an early model PM 45 mill that has the manual hand crank for Z elevation. Since I just reorganized my shop and a toolbox is placed nearby, I'm going to attempt to convert it to powered elevation using a heavy duty gear motor that I've had for a long while.

I'm not 100% sure this will work to my satisfaction. The odds are maybe 70%... We'll see. I'll outline the design considerations as we go.

Here's some pictures and explanations...

Here's the mill with crank handle removed. I put a wrench on the shaft flat and roughly measured the torque needed to raise the head. Holding the wrench about 1 foot (little less actually) from the point of rotation, I measured about 8-10 lbs to move it. This was measured by setting weights on the wrench which is a little inaccurate. Anyhow, I'm assuming it needs 10 ft.lbs to keep it in motion.

IMG_20180505_084318.jpg


Here's a gear motor purchased at a surplus shop that I've had laying around for years. This motor is used in mobile-home campers to slide-out the extension walls and rooms from the sides of the campers. It's very heavy duty and purchased new, they cost over $300.

http://www.cshincorporated.com/rv-power-gear-slide-out-motor-pn-523900-k01531a300/
  • Speed 6 RPM @ 850 in-lbs, 25 Amps
  • 10 RPM @ 400 in-lbs, 13 Amps
  • 12 RPM (no load)
  • Voltage 12 DC
  • Torque 850 in-lbs
  • Holding Torque 500 in-lbs.
  • Rotation Reversible
  • Duty Intermittent
  • Enclosure TENV
  • Mount Four bolt face mount on 3.38" x 3.38"centers
  • Dual Shafts 3/4" diam. x 1-1/4" with 1/4" crosshole (no keyway)
  • Size 9-1/4" x 3-7/8" x 6-1/4"

IMG_20180505_100601.jpg

So... 400 in.lb is 33.33 ft.lb but, this is at 10 RPM. One full crank raises the head 1/8" so, it would take almost a minute to raise 1". -Too slow... Gears are needed!

With a 3:1 gear ratio, I'll only have 1/3 available torque, which happens to be 11.11 which is really darn close to the 10 ft.lbs I think is needed to move the head. With a 3:1 gear ratio, I'll get 30 RPM instead of 10. This means the head will raise 3.75 inches in 1 minute. That seems terrible but, it's not. Manually raising that head with the crank is a royal pain in the butt and is a very slow process. For the convenience of flicking a switch... I can live with slow.

A simple 2-gear solution immediately came to mind. For many, many years, I would dream-up ideas and just slap things together on the fly but not anymore. I no longer kludge things as I go. Time is too short and precious and I dislike getting 50% done only to find I'm boxed in a corner where my choices are ugly.

Here's a CAD model. The gear and shaft diameters truly represent the pitch circle. Given this, I can use CAD to take centerline measurements of the shafts and design an enclosure that will hold the gears at the correct spacing.

CrankMotorModel.JPG



In the model, the gear sizes represent 85 and 35 teeth (respectively). This is a 2.43:1 ratio, not quite the 3:1 ratio I was thinking about before. This is because I'm using scrap pieces of metal for the gear blanks, using-up existing metal in the junk pile. 2.43:1 will be a little slower but, will give a little more torque.

So... Toss out questions, comments and suggestions. I'm going to dig-around the junk pile for more stuff and think this over before I start cutting gears.

Ray
 
So... I ordered the 40A version of this 12V power supply and I happen to have a left-over PWM speed controller from another project. I'm guessing this power supply functions as a current source so the speed controller should tame it down if needed.

https://www.ebay.com/itm/Universal-...var=422931826550&_trksid=p2057872.m2749.l2649

That power supply says it's for LED strip lights. Here's a question for y'a. Who in hell is going to power 480 Watts of LED lights? Somebody is trolling for UFOs? What am I missing here?

Anyhow, I wanted to make sure the gears were meshing properly because it's hard to adjust that mechanism due to visibility. Everything sounded OK and looked good. No signs of grinding etc... I ran a strip of writing paper through there to make sure there was a good impression without shredding at the root or tips of the addendum. It seems OK. I'll have to do it over once this is finalized.

We all know the dangers of gears but I gotta tell you, watching that paper get slowly churned through those teeth... Oh, you could make somebody talk with that setup.

IMG_20180513_112431.jpg

Oh, Redmech: Check Surplus Center for those motors. The show-up there once in blue moon. I think mine cost around 75 bucks about 8 years ago. https://www.surpluscenter.com/Elect...ors/DC-Gearmotors/?page_no=1&page_length=9999


Ray
 
BTW: The power supply, reversing switch and on/off toggle switch showed-up today. No sign of the enclosure yet but I still wired it for a test run. The power supply is amazingly good. I bought one rated for 40 Amps because it wasn't very expensive at all. It has a little pot for voltage adjustment. I set it for 12.50 volts under no load. Under full load moving the head up it keeps the voltage rock solid at 12.50 and can moves the head up without straining at all.

https://www.ebay.com/itm/Universal-...var=422931826550&_trksid=p2057872.m2749.l2649

Just waiting on the enclosure and this project is done.

PowerSupply.jpg

Ray
 
I am curios to see how slow you can go and still move the head. With that setup.

Pretty darn slow.

Since this is not fully setup with inline fuses, I only did limited testing with 15 second intervals when testing at low voltage. At the lowest voltage (10.4V), it was 1.2 IPM (ballpark). The head moved steadily and it was not balking or straining.

At 12 V the motor is supposed to spin 10 RPM. The gears were 80:30 which is 2.6666 ratio x 10, divided by 8 (Z leadscrew is 8 TPI) so, it should go 3.333 IPM. I'm setting the supply at 12.5 V and measured 3.5 IPM with consistent results on 3 tests. This worked out well and is right in the ballpark of what was hoped for. The velocity is essentially the same going either up or down.

The max voltage on the power supply is 15.5 V. I do not plan to test it at that voltage.

BTW, I have it wired with an On/Off toggle switch, a polarity reversing switch, and a debounced "momentary on" toggle switch. Giving it a quick "blip" on the toggle switch, at 12.5 volts, it consistently moves the head 7 thou -same distance either up or down. When transitioning from up to down (or down to up) it consistently displayed 3 thou total movement (i.e. 4 thou backlash).

During normal operation at 12.5V, when the switch is released, it has no carry-on momentum; that is, as soon as that switch is released, it probably travels about 7 thou inch before coming to a stop. This is expected since it's an ACME screw and not a ball screw.

Ray
 
Sounds like should be able to bore with it then. I have the G0704. I need to find one of those motors.

The speeds line-up so you should be able to pull it off. Be aware, that particular motor is rated for intermittent use so, I wouldn't try to do many pieces in a row that take a long time to finish. Also, the motor was designed to run at 12 V nominal and was intended for use in mobile home campers. Automotive and RV batteries are typically 13.3 Volts (despite being called 12 V batteries) so, things will run with slightly lower amperage. Since power is conserved, running the motor at lower voltage causes a corresponding increase in current. With my limited experience with this motor, I don't know how it will react to longer-term operation at lower voltages. At normal voltages, the motor easily moves the head up & down and a PM 45 head probably weighs 2x as much as the 0704. Doing that at low voltage would heat it up for sure -which is why I would not do the experiment without fuses in place.

Not trying to be a nay-sayer... Just don't want you pay good $$$ and accidentally burn the motor up. If I were in your shoes, I would look around for some kind of stepper motor and a simple speed controller kit. Maybe there's an Arduino-based kid that can do this out of the box.


Ray
 
Although “power is conserved,” that does not mean a motor operates at only a given power level. Take for example a simple brushed DC motor, which has a certain winding resistance. With a lower drive frequency, there is less current in the winding. Power is lower than at a higher voltage. In that example, power is V^2/R.
 
The power supply shown is a straight-up DC regulated supply not a synthetic PWM DC drive. My comment about conservation was with respect to the parts used in this project. In this case, operating at increasingly lower voltage will increase current to the max the motor can draw. I did in-fact run it at low volts for several brief periods and I could feel the 12ga wires warming up more-so than at 12.5 V.
 
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The power supply shown is a straight-up DC regulated supply not a synthetic PWM DC drive. ... In this case, operating at increasingly lower voltage will increase current to the max the motor can draw. I did in-fact run it at low volts for several brief periods and I could feel the 12ga wires warming up more-so than at 12.5 V.
Not sure what relevance the power supply has to motor current. Also not sure what you mean by synthetic PWM DC drive; your power supply is a switch-mode power supply, which uses PWM to control voltage. But that's beside the point.

I don't believe there are many logical explanations for the motor drawing more current at a lower drive voltage, unless the load conditions on the motor were different. If you used a very low PWM frequency to drop the voltage, then current would contribute to heating.
 
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