- Circles are easy to cut (router, circle hole cutting drill bit, etc.)
- The weight of the telescope presses down on the friction drive, so gravity and mass work in your favor.
- Gear systems suffer from backlash error, arising because the teeth must move a small amount before they engage each other and transfer motion. This error accumulates over time and affects GoTo accuracy. This is reduced in friction drive systems.
gear reduction ratio. As an example, let's say that a motor is connected to a wheel with a 1o cm circumference. This wheel is pressed against a wheel with a 40 cm circumference. Every rotation of the motor and 10 cm wheel causes the large wheel to rotate through 10 cm of its circumference or 25% of the total circumference. This means that is takes 4 motor rotations to turn the large wheel one rotation or a 4:1 gear reduction. Since circumference is proportional to diameter (circumference= 3.14 x diameter), we can use the ratio of the wheel diameters to calculate the gear reduction.
The Autostar motors rotate too quickly to be directly linked to the telescope axis of rotation. Meade supplies the 492 motor kit with a 60 tooth worm wheel. This gives a 60:1 gear reduction, meaning that the Autostar motor must rotate 60 times to turn the 60 tooth worm wheel through one complete rotation. This gear reduction is sufficient for a small telescope, but I wanted a larger gear reduction, which will give greater GoTo pointing accuracy. Following the guidelines at Steve Bedair's DS Mounts Tips page, I decided to build a 200:1 gear reduction.
The above test mock up showed that the Autostar worm wheel and worm could transfer rotation to a threaded shaft, but this configuration would be difficult to mount on the side of a truss tube. I needed to have something compact that wouldn't protrude too far outward. I settled on the design shown below (shown on an early prototype mount).
I retained as much of the original Autostar worm mount as possible (KIS). The Autostar turns a 60 tooth worm wheel, giving a 60:1 gear reduction. The worm gear turns a threaded M6 shaft that turns a 6 cm diameter wheel. The 6 cm wheel is pressed against the rollerblade wheel. The rollerblade wheel only transfers rotation to the telescope sector, so its diameter doesn't affect the overall reduction. The altitude sector is 22.3 cm in diameter. This final gear reduction is as follows: 60 Autostar revolutions to give 1 revolution of the 6 cm wheel. The 22.3 cm sector has a circumference that is 3.717 times the circumference of the 6 cm diameter wheel; this means that the 6 cm wheel must turn 3.717 times to turn the sector (telescope) a complete revolution. Since each turn of the 6 cm wheel requires 60 revolutions of the Autostar motor, the final gear reduction is 3.717 x 60 = 223:1. If I need a different gear reduction, I just replace the 6 cm diameter wheel with a different diameter wheel, making this a very flexible system. The gear box is mounted to slide on the truss tubes. In this early prototype, it was locked into position with a wing nut on the lower mounting bracket: this allowed the pressure between the 6 cm wheel and rollerblade wheel to be varied or completely released.
The final design (below) uses the locking mechanism from the original clothes line rod (used to clad the truss tubes)-KIS. I later fabricated a polycarbonate motor cover from 3 mm polycarbonate hobby plastic.
The cross sectional diagram color code is as follows:
Red = Autostar Motor and Spur Gear
Pink = Autostar Worm Wheel Mount
Grey = M6 Washers
Black = M6 Lock Nuts
Purple = Meade 60 tooth Worm Wheel
Blue = Meade Worm
Green = Washer with Gasket
Yellow = 3 mm Polycarbonate Hobby Plastic
Azimuth Gear Reduction BoxThe azimuth gear reduction box is much simpler than the altitude gear reduction system. Since the rollerblade wheel on the azimuth axis sweeps out a 31 cm radius, it can be coupled directly to the Autostar motor without the need to use the 60 tooth Meade worm wheel. This system couldn't be used on the altitude axis, since the telescope sector was too small (22.3 cm diameter).
The basic idea was to take the Autostar worm and turn it down to a smooth, 3 mm diameter shaft. This shaft would be pressed directly against the rollerblade wheel. Each rotation of the Autostar motor would rotate the 3 mm diameter shaft one revolution. This would move the rollerblade wheel 9.42 mm per motor rotation. Since the rollerblade wheel sweeps out a 194.68 cm circumference, it would take 194.68 cm/0.942 cm = 206.7 motor revolutions to turn the telescope one revolution; this gives a 206.7:1 azimuth gear reduction.
The below photo shows a first test at turning down a Mead Autostar worm with a drill press and a file. This first test used the hob made for cutting nylon worm wheels. I later found that it worked a bit better to use my high speed masonry drill (this was also faster). The right photo shows a regular Autostar worm and the prototype azimuth worm. The final worm design retained the threaded sections and just turned down the center section to a 3 mm shaft.
During functional testing, I
during transport, and the turned down Autostar shaft snapped-the brass
was just too soft for a 3 mm diameter rod. I ordered a replacement gear
set, but decided to tryfitting a steel shaft into the Autostar spur
Autostar Shaft CouplerThe nylon Autostar spur gears accept a tapered shaft (the tapered Autostar worm shaft is shown in the below photo to right). I experimented with different ways to make a straight shaft fit this tapered gear socket. I placed a M12 threaded rod in my drill press and rotated it against a metal file. This "poor mans" metal lathe worked, but I couldn't turn down the threaded rod to accurately fit the tapered spur gear socket. I tried threading the turned down rod and tapping out the inside of the spur gear socket with the same thread, but this also failed. I made a few prototype worms, but they were all slightly off centered in the Autostar spur gear and didn't work.
A good site dealing with Autostar shaft couplers is Christopher Erikson's website; here you will find ideas for shaft couplers and diagrams that can be taken to a machine shop for fabrication. I took a look at the diagrams on Christopher Erikson's website and tried turning down a M6 threaded steel rod to fit the Autostar gears. I placed the M6 rod in my power drill and spun it against a file. The below photo shows (from top to bottom) the broken Autostar worm, the prototype, and the final replacement part (before cutting to length).
Following the instructions on Christopher Erikson's website, the first prototype shaft coupler worked fantastic; the threaded rod sits perfectly centered in the spur gear and the steel is a much stronger material than the brass Autostar worm. The Autostar spur gear is held in place by a 2.5 mm bolt. I didn't have a 2.5 mm tap, so I drilled a 2.5 mm hole and tapped it for a 3 mm bolt (this worked just fine).
I think that my initial problems in fabricating a shaft coupler were due to beginning with a M12 threaded rod. When I started with a M6 threaded rod, tapering the end required removal of very little metal and was both fast and easy. The problem of coupling a shaft to the Autostar spur gears is easily solved by just spinning a 6 mm diameter rod against a file, and the prototype and final fabrications only required about an hour each!I fabricated a replacement part using M6 threaded steel rod, which was turned down to a 3 mm shaft using a high speed masonry drill and a file.
I tapered one end to 5 mm diameter, drilled a 2.5 mm hole in the tapered shaft end, and tapped this to fit a M3 threaded bolt. I had to drill out the 2.5 mm hole on the spur gear to fit the M3 bolt. The ends of the M6 shaft were wrapped with electrical tape to give a snug fit in the Autostar worm mount (which normally accepts a 7 mm diameter worm shaft). The final 3 mm drive element and the Autostar worm mount are shown in the below photo (top and bottom, respectively).
As with the altitude drive, I retained the Autostar worm mount. A small section of the circular plastic was cut away to accommodate the rollerblade wheel. I cut a scrap piece of pine to fit around one of the rollerblade wheels and drilled a hole to act as a pivot point. The Autostar motor motor and worm mount was screwed to the underside of the pine board, the assembly was slid over the rollerblade wheel mount, and the rollerblade wheel was installed. The below photo shows side and bottom views (top and bottom, respectively). When the front of the assembly (end with the worm) is pressed down, the worm shaft presses against the rollerblade wheel and transfers power from the Autostar motor to the rollerblade wheel. I still need to fabricate a locking mechanism (I currently ram a screwdriver between the ground board and pivoting azimuth drive).
The below left photo shows a top view, looking downward onto the 3mm worm shaft and the rollerblade wheel. The below right photo shows the high-tech azimuth drive locking mechanism. I will probably fabricate something better to do this job, however the screwdriver works very well and it's handy to have an extra tool along for the ride.