My 2 cents on the subject, no particular order:
1) Never mind torque, never mind RPM. Think Watts. Watts is power and power is what gets things done. 2) Power is torque times RPM or linearly, force times velocity. Specifically, Watts = RPM * in-oz / 1351 or linearly, Watts = IPM * Lbs / 531 3) 1 Watt can push (or lift) 1 LB at 531 inches per minute or it can push (or lift) 531 Lbs at 1 inch per minute. Mix or match; you see the point. 4) 1 horse-power is 746 Watts. 5) Steppers or servos? A stepper (NEMA34) tops out at 200 to 250 Watts (1/4 to 1/3 HP) mechanical. A servo goes past 1,000 Watts (Using 80VDC rated drives). Both are equally accurate and reliable as positioning motors. My recommendations? If you need 100W or less, think steppers. If you need 200W or more, you have to go with servos. In between, either will do. 6) How do you estimate how many Watts you need? The best trained Olympic atheletes can generate a sustained 200 Watts of power output. I'm talking about Marathon runners, long distance swimmers or Tour de France bicyclists. This is documented in the lead- up to the human-powered Gossamer Condor human powered flight across the English Channel. I have in my experience been on the shop floor more than a few times. The machinists I remember didn't look like Olympians and they weren't working up a sweat running manual Bridgeports. It's safe to say less than 200W was being put into the hand-cranks of these machines. Start by figuring what torque you need on the lead-screw. Measure the diameter of the hand-crank of a manual machine. Use your experience as a machinist of how much force you would apply to that handle before you figured you are doing something wrong. 5 Lbs, 10 Lbs, or both hands to get over 25 Lbs? Multiply that number by 16 to get ounces, then multiply that by the radius of the hand-wheel. The result is the torque in in-oz you apply to the lead-screw. If you apply 10 Lbs on a 5" diameter crank, you should get 400 in-oz of torque. Calculate the screw RPM for the speed you want. If you have a 5 TPI screw and you want 120 IPM, you get 600 RPM (IPM * TPI = RPM). Multiply screw RPM by in-oz, divide by 1351; that is the power you need. Using the above, you get 177 Watts. 7) Steppers are better than servos for milling CNC applications. Steppers have a speed-torque curve that makes it ideal; lots of torque at low speeds where work gets done, miserable torque for rapids where nothing is needed of it. Think of a stepper as a motor with an infinitely variable automatic transmission gearbox. Trade in torque for speed. 8) Gearing is very imortant. Steppers or servos, each has their "sweet point" for best power delivery to the load. Steppers should be run between 900 to 1500 RPM to deliver max power. Servos should always be run near 80% of no-load RPM for same. 9) Never have a power supply voltage higher than what you need when using a stepper. It is absolutely insane to run a 5A NEMA34 step motor at 80VDC when the motor will never turn faster than 500 RPM. All you get for your trouble is a very hot motor that could deliver decent power at 2,000 RPM but it never gets run there. What a waste. Use a lower voltage. This particularly applies to people using rack and pinion transmissions. 10) Don't replace a stepper with a servo of the same frame size and expect things to work OK. They won't. Steppers are high torque, low RPM motors. Servos are the opposite. They have little torque but they develop that at high RPM. They need reduction gearing to to match the load. Think of being in a sports car with a manual transmission. The light turns from red to green; which gear do you want to be in? 1st or 5th? Same engine, same power. Were it a desiel, (stepper) the gearing would be different than if it were a turbocharged 4-valve, 9,000 RPM red-lined 4-banger (servo). If transmissions matter so much on cars, they matter just as much on your choice of motors. Steppers and servos are entirely different animals and must be treated that way. I could easily do (11) thru (99), there is so much territory to cover but that's enough for now. Mariss Addresses: FAQ: http://www.ktmarketing.com/faq.html FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/ Post Messages: [EMAIL PROTECTED] Subscribe: [EMAIL PROTECTED] Unsubscribe: [EMAIL PROTECTED] List owner: [EMAIL PROTECTED], [EMAIL PROTECTED], [EMAIL PROTECTED] Moderator: [EMAIL PROTECTED] [EMAIL PROTECTED] [EMAIL PROTECTED] [Moderators] URL to this group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO OFF Topic POSTS: General Machining If you wish to post on unlimited OT subjects goto: aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if you have trouble. http://www.metalworking.com/news_servers.html http://groups.yahoo.com/group/jobshophomeshop I consider this to be a sister site to the CCED group, as many of the same members are there, for OT subjects, that are not allowed on the CCED list. NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM. 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