On Saturday 13 February 2021 15:40:22 Chris Albertson wrote: > If there is an error, my bet is the parametric pulley designed program > has a bug and/or the author made some simplifying assumptions and the > tooth profile is not perfect. You would have to verify the program > is correct. Do this by comparing a generated pulley to a same-size > pulley design that is downloaded from SPD/SI or McMaster Carr. I > bet there is a difference at the <1% level. Take one design and > overlay it with the other in your CAD system and any difference should > be visible. I'd be surprised if a parametric system was perfect > because the commercial pulleys are the result of much testing and > tweaking.
And sizes are scaled from only .3 digits. Multiply that by 86 and the error is pretty obvious. > The other issue is that 3D printers are not precise in that some of > the surface is printed below the ideal surface and some above so there > is a random texture added to the ideal part. A belt would tend to > ride in the high spots. All gears and pulleys I print need some > amount of post-processing to remove the high spots. #600 wet/dry > sandpaper works well. Printers never make perfect surfaces. > > It is very unlikely that your printer is making (say) 99.9% scale > models of yor parts. It is using stepper motors and toothed belts, > steppers always do the commanded number of steps and timing belts > don't slip and always do the integer ratio of teeth, even if the size > is off. True, But you've forgotten that this printer was near totally fubar out of the box, making smallish 5x5 test blocks, and only feeding it 10% of the fiber it needed for a solid result. I could see thru the first 5x5 I printed, so every step/scale in it has been "adjusted" by moi. > Now back to the pulley you are printing. Why care about a slight size > issue? As long as the belt never slips a tooth the pulley ratio > holds. Only with plain pulleys does a tiny size error cause a tiny > error in speed reduction. Timing belts are like gears, it is the > ratio of the teeth that matter. True, but if you don't want a 1 tooth hop now and then when it reverses, you take out the slack loop on the back side of the pulley. Or you set tensions sky high and thats hard on motor bearings and belts. These pulley's will be driveing the A axis of a metal carving milling machine, and I need all the holding power at what ever angle lcnc tells it to go to. The motor pulley is xl and only 8 tooth, but the axle pulley is quite small and 28 teeth, The gt2-3mm about done is 86 teeth. But supplys of 3mm pitch stuff don't seem readily available. And 2mm is pushing this printer too far. I may have to try it thogh. Even if I use half a spool of PETG and a week to get one useable pulley. Thanks Chris. > On Sat, Feb 13, 2021 at 2:06 AM Gene Heskett <[email protected]> wrote: > > Greeting all; > > > > Messing around with the parametric pulley designer, I have made a > > larger 86 tooth pulley for the big pulley on the A axis supplied > > with the 6040 mill. I'll also change the belt pitch as its presently > > a huge XL. > > > > But the chosen belt, a GT2_3mm, is not a good fit, throwing a just > > detectable slack in the center of the wrap, inicating the pulley is > > about half a red one too small. > > > > Is this a good excuse to add a couple counts to the xy scales in the > > printer, or to play with the variable > > > > additional_tooth_width = 0.2; //mm > > > > in the openscad recipe? > > > > Doing this in PETG, so the ender-3 is at the top of its heater > > range, with a 250 degree nozzle and a 70 degree bed. And around 7 > > rendering hours to try a new fit. > > > > Thanks for any advice. > > > > 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) > > If we desire respect for the law, we must first make the law > > respectable. - Louis D. Brandeis > > Genes Web page <http://geneslinuxbox.net:6309/gene> > > > > > > _______________________________________________ > > Emc-users mailing list > > [email protected] > > https://lists.sourceforge.net/lists/listinfo/emc-users 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) If we desire respect for the law, we must first make the law respectable. - Louis D. Brandeis Genes Web page <http://geneslinuxbox.net:6309/gene> _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
