Step 13: Timing Belts
3x MXL timing belt
6x 3/16" x 2" eye bolts
12x 3/16" nuts
24x #8 washers
6x 6-32 x 5/8" machine screw
6x 6-32 nuts
The timing belts are what converts the rotary motion of the stepper motors into precise linear motion along the axis. The belts are clamped onto eye bolts and the tension is adjusted by tightening the nuts on the eye bolts.
13.1 The first step to assembling the timing belts is to locate the two holes per end needed to clamp the belts in a loop. Test fit the components and mark where holes should be located if looped around the eye as in Photo #1. I purchased closed loop belts and simply cut them open using an utility knife.
13.2 Drill/punch the holes into the timing belts at the points marked. Use a 1/8" drill or an awl to form the holes. The material will be flexible and the resulting hole will be smaller than the bit diameter. The holes should be similar to those in Photo #3.
13.3 Assemble the timing belt anchors by looping the ends of the timing belt around the eye bolts and closing the loop using 6-32 machine screws (Photos #2,3). The holes will be smaller than the screws but twisting them while pushing will get them though the hole. The grooves should be on the inside of the loop and clamped shut making sure the grooves are meshed. The completed assembly should look similar to Photo #5.
13.4 For the Y-axis, loop the belts around the pulley and idler bearings and then bolt the eye bolts to the steel belt clamps (Photos #7-9). The Y-axis was designed to be the correct length for the belts used so there is no excess. The slots in the belt clamps are oversized to allow for adjustments in the belt positioning; adjust them so that the belt lines up with the idler bearings and the eye is far enough away from the base that it clears the gantry side when it is at the extent of its travel.
13.5 For the X-axis there is excess length in the belt. Instead of cutting the belt to length I tied the excess out of the way as shown in Photo #10 in case I wanted to make the axis wider in the future. It is a tight squeeze to get the belt around the idler bearings and motor mounting bolts and onto the pulley. Don't try and snake the eye bolt through because it wont fit. Make a loop in the middle of the belt and feed it up past the idler bearings and over the pulley. Position the eye bolts so that the belt is in line with the idler bearings which should also be close to in-line with the X rail. Photos #10,11.
13.6 Finally tension the belts by tightening the outside nuts on the eye bolts. Try to apply equal tension on the belts. Once you are satisfied with set-up, tighten the inner nuts to prevent the eye bolt from rotating.