On Saturday, February 21, 2015 08:16:07 AM Tobias Gogolin wrote: > So after understanding that a linear driven delta 3D Printer is the > most attractive tool to have in ones repertoire me and a friend, who > has already previous cnc mill building experience decided to get > serious, and there is one disagreement, he thinks he would be fine > with inexpensive steppers, I am suggesting small geared brushless > motors, so I am hoping for an expert opion, and also groups, an wikis > where maybe lists of the available motion controllers are offered? > > Thanks! > > Tobias Gogolin
The basic design might look similar, but for smaller stuff (under 10,000 lbs sitting on the floor) steppers are the economic winner hands down. The disadvantages are that they will continuously burn from 25% to 50% of their running power even if being driven by smart controllers that can reduce the drive when they have stopped. And the discrete steps they make, which with smart controlers can be subdivided by microstepping, look like a prob lem but in the world of flying swarf, not terribly important. But unless the current mapping in the controller is exactly matched so that a 1/8th step is exactly 1/8th step, there will be a cyclic positioning error. Miniscule, its less than .001" at any point when direct coupled to a 5mm per rev ball screw. Best match I would guess is when the motor is being run at exactly its nameplate amperage. With commonly available stepper drivers having a dip switch current setting that typically changes in 10 to 15% increments, this isn't as easy to as it should be. OTOH, who among the likes of me, has the gauges to actually meaure this non-linearity in the real world. I have one dial that reads in .0001" increments, and when I put it on the table and start looking for those errors, they are completely buried in the stiction and the machines frame flexibility. So while its a theoretical error, its effectively masked by the other sources of noise between the motor shaft and the tables motion. That hasn't kept me from making something with a tolerance of a thousandth or better as long as I have both eyes open. Step into the servo world, and you immediately have to add the expenses of adding a precision position readout, and interfacing that back to the axis driver in order to obtain a desired position vs actual position error that is then used to drive the motor to reduce the error, which can never be zero because there is no error to drive the motor then. Servo's can probably be made to maintain a higher accuracy but the difference in a well geared stepper system and an equally well balanced servo system are for the most part buried in other measurment noise. Either can be made to move from point AB to point CD at an arbitrary angle that stays well within .0005" of a straight line. Depending on driver and psu for the servo, it can move faster than the stepper can, this because the steppers own inductance prevents the available torque from being maintained as the driving frequency is raised. OTOH, a few weeks ago, I tested a new 48 volt psu for my mill, and successfully drove a 425 oz triple stack nema 23 motor at over 3400 rpm. That top speed limit I got the impression, was set more by the speed of the opto's in the 2M542 driver than anything else as the function generator was then running at nearly 375 kilohertz. Even 2000 rpm will move my tables at 100+ ipm. And to even get that speed will take a 5i25, my parport is all done at 30 ipm. At a base thread of 30 u-secs, thats 33 kiolhertz, 1250 rpms and 10% of what the function generator was doing. If my math is correct, 33k/8/200*60 where the 8 is microstep, 200 is a rev, and 60 is the rps to rpm multiplier. If I didn't blow the math... If I was to make a bid and win that K&T thats on ebay with a starting bid of $1k, and figure out where to put its 12,500 lbs when it got here, I would probably put some geared down nema 42's on it. With a carton of tailgate hatch struts under the knee. But then I would have to find some work for it to do just to pay the electric bill to run its spindle, 50 horse IIRC. :( Work, that's a 4 letter word at my age. :) Thanks for reading this far, stay warm guys. We're collecting more white stuff this morning. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ Download BIRT iHub F-Type - The Free Enterprise-Grade BIRT Server from Actuate! Instantly Supercharge Your Business Reports and Dashboards with Interactivity, Sharing, Native Excel Exports, App Integration & more Get technology previously reserved for billion-dollar corporations, FREE http://pubads.g.doubleclick.net/gampad/clk?id=190641631&iu=/4140/ostg.clktrk _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
