On Sunday 18 February 2018 20:17:06 Gene Heskett 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.
That was me, Gene, and in retrospect the GT2 is probably the better choice, as I have the belt tension right up to pretty hard on the motors bearings. Anything less and the backlash gets bad quickly. 17 teeth on the motor, 42 of the nut(s). nema 23 motor, double stack or around 270 oz/in. With a damper of the rear of the motor, I can exceed 32 IPM on a 10 tpi nook acme screw with only 28 volts to the motors. Mounted in front of the post, similar to what you've described but a little farther from the post. Pix of my cobble job can be seen on my web page in the sig. Add emc to the base address or click on it. Pix 28 shows it moderately well. A 3 ft, 1.5x.250 strap was cut in two and made the stand above the post for the motor, a 3+ inch of 1/8th strap was bolted up the back long enough to get it welded, with a motor mount between the straps, and the whole thing bolted about 3x up each side to the post. Two tabs were welded on to project forward to hold the nuts bearings. Pix 33 shows it before mounting the motor or being installed on the post. The gearbox was turned 90 degrees to make room for the screw to pass and be bolted into the top of the sled casting. You can see the shopmade damper sticking up from the rear shaft of the motor in some of the pix. Since those pix were taken, I have added a pair of 1/2" thick alu pieces around 7" long, with the center cut away, bolted to each side of the sled, carrying a skate bearing on the end to each "leg" to ride the face of the post, extending the effective wheelbase of the sled on the post by several inches, and as long as the post has a coat of vactra on it as I start the days use, all traces of stiction are gone. With that short a contact on the post, stiction is a huge problem, needing a dose of vactra (68) about every 30 minutes to even think about controlling it. > > 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 250 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
