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Paul Alciatore
01-21-2009, 12:18 PM
In real practical terms, how do you determine the size for a motor, stepper or otherwise, to drive an axis. I want to motorize and ultimately convert to CNC on several different sized machines from a Unimat to a full sized mill.

Dunc
01-21-2009, 01:26 PM
Way out of my league here...

Don't have any formulas but Roland Friestad, in his latest HSM article (Jan-Feb 2009) suggested 200 oz-in for the cross slide and 400 oz-in or higher for the carriage. I don't have earlier installments of his article but the lathe looks to be a good size - 12 or 13 inches capacity.

Bruce Griffing
01-21-2009, 01:48 PM
This question is answered in the FAQ's on geckodrive.com.

vincemulhollon
01-21-2009, 02:01 PM
In real practical terms, how do you determine the size for a motor, stepper or otherwise, to drive an axis.

Conversions are easy, stick the handle on a torque wrench, twist the handle as hard as you'd ever dare on a manual machine, read the torque, add about 50% to 100% for safety sake. Maybe use twice the torque and run the motor at about 75% of "full" current.

You don't need to mount the handle on the torque wrench, you can stick a socket thats about the same diameter as the handle and then twist the socket as hard as you'd twist a similar sized handle.

Or if you have a really big mill handle, clamp the torque wrench in a vise and grab the wrench like it were a handle and wiggle and read off the wrench.

Here is one way to estimate. I figure more than several hundred in-oz on a sherline handle would snap it clean off. So a couple hundred is about right. Here's how I'd estimate that. If I had a handle that were 5 inches across, I'd feel uncomfortable about hanging more than a 20 lbs barbell on it (thats like a very fat housecat putting its entire weight on the handle), and that would be a radius of 2.5 inches times a weight of 320 ounces, multiply to get 800 inch-oz so I'm sure on that machine 800 in-oz would be way way too much. On the other hand hanging a first class letter on the handle is way too little.

The best option is to go by feel on a live machine and maybe scale it up or down as appropriate. So, I've got 300 oz-in on my sherline and frankly the handle is too scary to grab when its running at full power. So, I'd figure thats enough.

If you'd let your kids play with it unsupervised thats probably well under 100 in-oz. If you figure that if you'd grab it and get tangled up in it, you'd lose a finger or less, you need somewhat less than 500 in-oz. If you figure you'd lose an entire hand you need maybe 500 in-oz to maybe 2000 in-oz. On the other hand, if you figure you'd lose an entire arm, then maybe you need one of those giant fractional horsepower servo things.

Or, if you usually use your fingers to turn the knob and they get kind of tired after a few hours, you need maybe 200 in-oz or less. If you usually hog it using your whole arm and your arm gets tired, thats maybe 600 or so in-oz. If your old power feed gets hot enough to burn skin, then maybe you need 1500 in-oz. These estimates are for average middle aged men. Steroid use or old age can invalidate these estimates.

Dawai
01-21-2009, 02:07 PM
Static breakaway torque is not enough..

Acceleration.. a cnc plasma has to go up to +100ipm.. in a relative short time. Meaning it has to have the torque to bring it up to speed, then brake it, all while moving stable interpolating.. Hint, build it light... Or huge motors are needed.

So.. oversize, motors, oversize drives for heat dissipation.

NOW that bridgeport of mine, it has a cross slide that weighs over 200lbs.. a table that weighs another 100, then the indexer I put on top of it at times.. A lot more than twisting a ball screw and a heavily oiled pretty flaky way...

Paul Alciatore
01-22-2009, 04:17 AM
Conversions are easy, stick the handle on a torque wrench, twist the handle as hard as you'd ever dare on a manual machine, read the torque, add about 50% to 100% for safety sake. Maybe use twice the torque and run the motor at about 75% of "full" current.

OK, I did something like that today. On my Unimat I want to take a baby step toward CNC: put a motor on the carriage feed for a power feed with variable feed. I figure I will use a stepper so it will be there for a later CNC conversion. So I borrowed a torque screwdriver and put it on the nut holding the handwheels in place and cranked. Looks like about 10 - 20 in-oz so if I double that I will probably be OK. But it would be nice to have a second method to confirm this.

In this one at least, I don't think I am too worried about accelerations.

OK, another question. I have been reading some specs on steppers and see two kinds of torque ratings: detent torque and holding torque. What do these mean and which one should I go by?

vincemulhollon
01-27-2009, 01:10 PM
OK, I did something like that today. On my Unimat I want to take a baby step toward CNC: put a motor on the carriage feed for a power feed with variable feed. I figure I will use a stepper so it will be there for a later CNC conversion. So I borrowed a torque screwdriver and put it on the nut holding the handwheels in place and cranked. Looks like about 10 - 20 in-oz so if I double that I will probably be OK.

You sure that's as hard as you'd dare twist the handle while machining the toughest stuff as fast as you could imagine with the gibs as tight as you'd ever use? Most folks think a "very hard twist" on a sherline sized mill handle is about 100 oz. Maybe is your torque wrench reading inch-pounds or something?

I use somewhat less than 300 in-oz on a sherline sized mill. I probably could have gotten away with 200 or even less but the price difference was very small. Also I like running at somewhat reduced current (and proportionately less torque and heat) rather than full blast.