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
 
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.

Here's a good, old-fashioned .001 Ohm shunt resistor in series with the supply leads to the motor. I'm reading DC voltage across the shunt so the current will be the voltage reading / 0.001. It reads 6 mV (i.e 6 Amps) at low volts and 4.4 (i.e. 4.4 Amps) when set at 13.3 V. In all complete honesty, the 4.4 picture was usually showing 4.9 making the current just under 5 amps but when I snapped the picture, it grabbed 4.4. It was kinda hard to hold the leads and snap the picture at the same time.

FWIW, I observed similar behavior when playing around with one of these little gear motors....

https://www.ebay.com/itm/12V-3RPM-T...2233342920?epid=706769864&hash=item465e82d3c8


IMG_20180523_170446.jpgIMG_20180523_170924.jpgIMG_20180523_171001.jpg
Ray
 
Thanks for the follow up. Makes no sense to me.

Well, what now makes sense to me, is that I purchased WAY more power supply than what was needed. The specs on that motor were showing levels of 25A at max torque and 13A at 400in-lb. I did some measurements and came-up with a torque measurement at the crank handle. I was leery of the measurement technique and I inflated the estimate -a little bit too much. So, here I am thinking I needed a miniature welding unit to drive this thing and it turns out I bought 6 times more PS than necessary. On the flip side, the difference in price was about 12 bucks so... oh well....

Ray
 
That is a good mod and will pay off the first time you have to go from top to bottom and back again, cranking gets monotonous. Looks like a nice job on the gears.
 
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