|
Altitude arm changes and drive:
I had to do a bit of surgery to fit the Nema 17 motor. Three small cuts as indicated in the following two photos. One cut at the back of the stepper motor (which I may have to redo to allow a little more room) and two cuts on the front side of the stepper motor. The cuts are not weakening the structure of the arm assembly. The crooked red lines indicate where the cuts were made.
A 50x50x3 Ali angle is used as a stepper motor bracket. Two holes were drilled and tapped into the existing worm drive assembly for fixing the stepper motor bracket. Two countersunk M3 screws were used to fix the bracket to the worm drive assembly). The 180 tooth gear and worm drive assembly were removed and taped up with masking tape to prevent any dirt or metal shavings from getting into the teeth.
I have used 3D printed GT2 pulleys for the final reduction (48T/20T) mainly because I didn’t want to wait for the Ali-Express order. An idler gear also had to be used to tension the belt (again I had a 160T GT2 belt that had to be used). Stepper motors are 1.68A, 400 steps per rotation. So the steps per degree works out as 400 x 32 microsteps x 180 worm reduction x (48/20 final reduction) / 360 degrees = 15360.
The GT2 pulley of 48T was a little too big for the job and as a result I had to chamfer the side facing the worm drive off to prevent it from touching the main gear of the worm drive. My recommendation is to use a smaller 12T or 16T pulley on the stepper and a 30T to 36T pulley on the worm drive. This will give steps per degree figures of between 14400 and 16000 which is good (as long as it is greater than 12800 and smaller than about 60000 then tracking should be good. Even a 40T/20T combo will do (it works out as 12800 steps per degree). These gears will also allow you to offset the larger pulley away from the worm drive to get a good clearance (my 3D version didn't allow that).
Azimuth drive:
No cutting required here. A 50x50x3 Ali angle is used and fixed to the worm drive assembly in the same manner as described above. The final reduction is also 48T/20T and the same stepper motor was used. Thus, steps per degree is also 400 x 32 microsteps x 180 worm reduction x (48/20 final reduction) / 360 degrees = 15360. The same recommendation applies to use a 30T/12T, 36T/16T or 40T/20T combo for this drive butt a 50T/20T will also do the job well.
|