Hal, Good questions and very appropriate for this group I think. You sure are correct in that stiffness is super important in machine tools. You apply a force and a deflection occurs in the direction of that force.
Strange things start to happen if the force is applied in rapid short oscillations though. Take a rubber band and hook it to a weight like a pencil or something. Hold the free end of the band and slowly move your hand up and down. When you hand moves up, the pencil dangling from the other end moves up as well. Now move your hand faster up and down. At some point you will observe that when your hand is moving up the pencil is moving down! We call this a complex conjugate pole pair, but the gibberish doesn't matter. The important thing is that you are commanding an up movement and the pencil goes down. If this is a cnc machine tool the fact that the motion is opposite of that desired creates massive instability in the control system. It's a real bad thing. To make the frequency that this happens as high as possible and thus less a problem is very desirable. This is done by maximizing SPECIFIC STIFFNESS among other things. Like I said, steel and aluminum are about the same in this parameter. I do use aluminum a lot for some things because it's lesser elongation often makes it easier to machine. But I use steel for machine structures... It's cheaper, more thermally stable, and as you said three times the elastic modulus of aluminum. Les Leslie M.Watts L M Watts Furniture Tiger Georgia Main page: http://www.lmwatts.com Engineering: http://www.lmwatts.com/shop.html Cnc surplus for sale: http://www.lmwatts.com/forsale.html Carved signs: http://www.lmwatts.com/signwp.html -----Original Message----- From: Hal Eckhart [mailto:[EMAIL PROTECTED] Sent: Monday, October 25, 2004 8:51 PM To: [EMAIL PROTECTED] Subject: RE: [CAD_CAM_EDM_DRO] Re: sourcing motion control parts - steel vs aluminum On 10/25/04 8:28 PM, Leslie Watts wrote: >Steel has about three times the modulus of aluminum, but it is also >three times the density! > >Specific stiffness is the ratio of those two parameters, so they are >about the same. Okay, I'll bite. What exactly does "specific stiffness" have to do with the rigidity of a frame? (no sarcasm intended; I'm just curious) Hal Eckhart - Casa Forge - Minneapolis MN - <http://www.casaforge.com> 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. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........ bill List Mom List Owner Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
