Motor duty cycle / advice on a non-FIRST project

Technical Technical Discussion
1

Hi all,

During the build period (once upon a time), someone on my team made a comment, "THat's crazy, if you looked at the poewr output of the Chippy and the new drill motors, they are almost identical.  Why is the chippy soooo much bigger?"

I ventured a guess that the drill motor is probably not designed with continous duty in mind, while the chippy, well, I don't have the info handy, but could probably run longer without overheating.  Are there any other theories or explainations out there?

Reason I asked, is I am considering building a new milling machine spindle for our TAIG CNC mill.  Design criteria for this milling machine spindle:  Different gears (can be done with belts, just like the Bridgeport J-Heads), to give different speed.  I want high torque at lower RPMs to cut steel (we would like to cut our own gears next year), but also retain the ability to run the spindle at 10k RPM if not higher for PCB routing and for high speed machining of aluminum.   Because it's a small machine that we'll potentially use in the 2004 season, it *has* to be able to run almost continous duty.  The motors I've considered are:
  • A replacement motor for a Chinese-made mini-mill, such as those on littlemachineshop.com. Model shown below is rated for roughly half horsepower

http://www.littlemachineshop.com/products/product_view.php?ProductID=1311

  • A drill motor. Hey, after 2 build seasons, I know the trannies in and out. This baby looks beautiful, from DeWalt (if only if we can get that in our kit!) :slight_smile:

http://www.botparts.com/product_info.php?products_id=195
(2 HP motor, 24VDC, 68Amp nominal, 192 in-lbs stall torque)

  • The CIM / Chippy motor. It’s not too shabby at half horsepower, and it seems pretty closely match to what Sherline is using default in their mills, which is actually fairly respectable a motor.

  • Wheelchair motors. They don’t seem to be made for high RPM though.

Any comments / suggestions? How is duty cycle determined?

-=- Terence

2

*Originally posted by SiliconKnight *
**During the build period (once upon a time), someone on my team made a comment, “THat’s crazy, if you looked at the poewr output of the Chippy and the new drill motors, they are almost identical. Why is the chippy soooo much bigger?”
**

The curves I have show different specs than what you describe. The chalupa peak efficiency is at 4000 RPM @ 142 watts out while the Bosch is 9000 RPM @ 376 watts out. If you compare the two motors at 2000RPM the chippy is making way more power at 321 watts while the drill is only making 184 watts. If you are looking for a high RPM motor than the Bosch appears to better suited for you needs.

3

I don’t know too much about duty cycles, but I do know what the CIM motor is applied to By Atwood in this case. It is a trailer tongue jack. This is for leveling trailers. When you level a trailer it all happens in about 30 seconds.
If you want a longer duty cycle, you are wise in wondering how long this motor will run before it over heats. Notice that while its bigger, it has no vent holes. (outdoor usage)
The Bosch motor has its own fan and so it can be smaller.

They aren’t expensive. My suggestion is to test one yourself.
In our 130 # robot, we used 4 motors and 4wd. The CIM’s never overheated in a 2 minute contest. (our adaptation of the Atwwod gearbox was not effective, though)

Charlie Affel