Gene, Thanks that is some good real-world data. My plan with this Mini Mill is to do the conversion in steps.
1) Add the z-axis The factory rack and pinyin z-axis setup is not very good and if nothing else a motorized screw based z-axis is a big improvement. 2) Do a quick and simple X, and Y axis conversion using the stock lead screws and 3D printed parts 3) Re-do #2 using metal parts and zero backlash ballscrews 4) Add a 4th axis I want to document the process so if it works someone else can duplicate it. I ended up reading a lot of data from the belt supplier's web sites. Yes GT2 profile should be used in every new design. Only use trapezoid profile teeth as replacement parts. The newer profile almost doubles the system performance and as you said, it uses a much lower belt tension too. The major different I find between ball and lead screws is friction. A ball screw converts about 95% of the applied motor torque to thrust while a lead screw converts only 40% to 50% of the torque to thrust, the rest of the power goes into heating the screw and nut by sliding friction. So in theory I should see at least a doubling on the z-axis performance you have. I don't know, we shall see. My cheap ($190) 3D printer has me spoiled. It routinely operates at 100 mm per second. (in Imperial units that is 230 inches per minute.) It can reliably move 30% faster. My eye cannot really follow motion that quick. It can print anything I can draw. Now I want milling machine like that. BTW, I looked at your web site. Noticed the wooden gear model that gets about 60:1 reduction. That same principle is further developed by a guy who uses the name "gear down for what" in Thingiverse. He has a very well engineered system of modular parts. You can assemble high torque systems with no tools. He uses ring gears that are thin enough to be flexible, you wrap it around the planet gears then press fit a round collar over the ring gear that stiffens it up. At first look it seems more complex than a 15-speed automatic transmission but after some study is its very simple. He uses herringbone gears which are like two helical gears, one backwards glued together. Herringbone gears are very strong and have zero side force but are near impossible to cut in metal without a high-end 5-axis mill and tiny ball end. A spur gear version would be much easier. In any case his innovation was to cut all the sun and plant gears on ONE SHAFT. And not to use any kind of gear carrier or bearings. The lack of carriers and bearing makes in very buildable. He has versions that go from 20:1 all that way to 1200:1 all based on the same design. It is very compact, no wasted space. https://www.thingiverse.com/thing:2731585 Writing g-code by hand might be a problem for design this complex. My brain would explode if I tried to write the code for just one helical gear. But it's trivial to go to the Boston Gear web site and download a CAD file for one of their stock helical gears. Then I convert that to g-code. I don't have to understand involute curves. I just download a stock part from their catalog. The McMaster Carr catalog works too. The whole process takes 10 minutes and I end up with a 3MB g-code file. On Sun, Feb 18, 2018 at 5:17 PM, Gene Heskett <ghesk...@shentel.net> wrote: > On Sunday 18 February 2018 18:53:59 Chris Albertson wrote: > > > Question: What would be the preferred timing belt for a new design? > > > > Here is the application. I'm making a z-axis control for a Harbor > > Freight mini mill (Sieg X2). In this design a 5/8 diameter, 0.200 > > pitch ball screw is fixed and the nut spins. The nut is driven by a > > timing belt "about" 50 teeth and the motor has an about 25 tooth > > pulley. I'm thinking a 400 in/oz. NEMA 23 stepper should work. > > Assuming the 2:1 reduction each 1.8 degree step lifts the head 0.0005 > > inches. By 1/4 stepping I get 0.000125 per pulse. > > > > When looking for a good design for the mini mill I was looking for a > > good place to put the ball screw. This design places it very close to > > the dove tail, almost touching it. Maybe 0.1 inch in front of the > > dovetail. The ball screw goes right where the current rack it the > > original design. I will have to remove the rack to fit the screw. > > > > So I need to select a belt type and size. My initial guess at this a > > the GT2, 3mm pitch and 9mm wide. > > > > Some one else used a XL type 3/8 wide and 0.2 pitch but I'm reading > > that XL is not the best for new designs as GT2 has a much improved > > both profile that eliminates backlash. > > > > I started to calculate torque and load on the belt then thought "Why > > bother?" the actual force is going to be whatever the motor can do as > > I'll set the acceleration limits until it start skipping steps then > > back up by about 30% or so. I might go with a NEMA 34 1100 in/oz > > motor if I need to. So I might choose a belt that can handle whatever > > the 1100 in/oz motor can do. Just looking to double check GT2, 3mm > > pitch and 9mm wide is reasonable. > > > > So I guess the best way to ask the question is what kind of belts are > > working well with motors in the 400 to 1100 in/oz. range. > > > > If there is any interest I'm modeling this in Fusion 360. and can > > share the CAD files > > Its been a while since I did my micromill, but ISTR I used the XL belt > and its quite tight. Like you, I spin the nuts, doubled nook bronze > ones, and my backlash stays below 3 thou. the screw is in front of the > post by about 1/2 the distance between the post and the edge of the > gearbox cover, which has been turned 90 degrees to get that clearance. > the motor is an 8 wire good for a bit over 300 oz/in. Works well. you > can see pix of it on my web page in the sig. The counter springing has > been reworked a bit but the rest of the pix are still valid. > > Not a cad artist, so what you see came straight out of my head, quite a > few years ago now. I write my own gcode. > > -- > 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> > > ------------------------------------------------------------ > ------------------ > Check out the vibrant tech community on one of the world's most > engaging tech sites, Slashdot.org! http://sdm.link/slashdot > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > -- Chris Albertson Redondo Beach, California ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
