On Jun 4, 2009, at 8:26 AM, Kirk Wallace wrote: > On Thu, 2009-06-04 at 23:04 +1000, Erik Christiansen wrote: >> The mill-drill which I'm starting to convert to CNC has 0.2mm >> (0.008") >> backlash in the Z feed, making it a much better candidate for stepper >> control than servo, IIUC. There is though, sufficient in-built upward >> spring-loading for the quill to retract if the feed worm is >> disengaged. >> I wonder if that would permit servo control, without hunting? >> >> My brand new rotary table has almost 0.1° backlash, so I guess it >> has to >> be a stepper axis? > > In my opinion, the biggest issue with backlash has nothing to do with > the type of axis motor system. The problem is that your axis control > will put your table or spindle into position, you will start or change > your cut direction and the cutting forces will cause an uncontrolled > movement within the backlash, which the motor controller will usually > never see. Backlash compensation can improve your initial position, > but > the table or spindle can still have uncontrolled movement with in the > backlash. With servos you can move the feedback closer to the > cutter by > having a linear scale on the table, but the backlash movement can be > hard for the feedback system to deal with.
IIRC JMK did a configuration for Stuart (MPM) which used a linear scale connected to the I in PID This drove final position. That said a linear scale when used alone on systems with backlash are difficult to tune. In addition the expense counts up quickly. Ask me how I know. ;-) It may be possible to get more bang for your buck by purchasing a better ball screw ( ebay or HIWin). > > Bottom line: steppers or servos, get rid of as much backlash as > possible. (Has anyone used Turcite or other on leadscrews? > http://www.moglice-turcite.com/ ) > >> It's a 970 kg machine, with a 1m long table, so even when I stump >> up for >> ballscrews, the thought of trying to drive X and Y with steppers is >> daunting, and I fear lost steps, so expect I'll have to go with >> servos >> there. > > I'm not a stepper fan, but they work well (not fast) on my Shizuoka: > http://www.wallacecompany.com/machine_shop/Shizuoka/00018-1a.jpg > http://www.wallacecompany.com/machine_shop/Shizuoka/ > > Both steppers and servos will miss steps or trip a following error if > overloaded. The key here is to size the axis motor system to fit the > load. Trying to evaluate the load is the hard part, which I haven't > figured out yet. It may be best to look at a similar machine that is > proven to work well and copy it. One thing I have been thinking about > recently is to make up an over-sized motor mounted so that I can > measure > torque, speed, voltage and current, mount it to the axis in question, > run it through its paces, then use this data to determine how much > smaller the real motor system can be. If you look at emc1 .ini files they had a set of parameters for at least some of the factors affecting axis motion. I think something was supposed to use them to model motion on the axis. I don't know it that ever worked. Maybe Ray H or Matt S. can elucidate. Direct engineering measurements are always nice but may not transfer well to other machines. > >> I've found some servos with attached encoders here: >> >> http://www.oceancontrols.com.au/motors/servo/servo_motors.htm >> >> but they look rather wimpy. On the other hand, I'd be amazed if I was >> outputting 150W when doing manual rapids, even with the acme >> leadscrew. >> Add some HTD pulleys, to increase the torque, and take what rapids I >> get, maybe? At least the motor compares favourably with the one Kirk >> found recently: >> >> http://www.cadcamcadcam.com/motorservoglobepm-dc.aspx >> >> giving 3.5 times the torque at 2800 RPM. Now, if only the >> appreciating >> A$ had brought the local price down. >> >> Erik > > If budget is the primary problem, I would try to make what you already > have work better, or buy what whatever is least expensive and will > still > work well enough, steppers or servos. Look at what other people are > throwing out and consider how parts might be used for a CNC. Determine > what is the single most objectionable issue and chose the shortest > path > to making it better. I often try to make something perfect on the > first > pass and end up doing something different in the long run, so jury > rigging to prove a design may be a good thing. > > my2c > > -- > Kirk Wallace > http://www.wallacecompany.com/machine_shop/ > http://www.wallacecompany.com/E45/index.html > California, USA > > > ---------------------------------------------------------------------- > -------- > OpenSolaris 2009.06 is a cutting edge operating system for enterprises > looking to deploy the next generation of Solaris that includes the > latest > innovations from Sun and the OpenSource community. Download a copy and > enjoy capabilities such as Networking, Storage and Virtualization. > Go to: http://p.sf.net/sfu/opensolaris-get > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users ------------------------------------------------------------------------------ OpenSolaris 2009.06 is a cutting edge operating system for enterprises looking to deploy the next generation of Solaris that includes the latest innovations from Sun and the OpenSource community. Download a copy and enjoy capabilities such as Networking, Storage and Virtualization. Go to: http://p.sf.net/sfu/opensolaris-get _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
