This page is only temporary while I construct it properly.
There's a number of different ways to drive a telescope. I chose a worm drive for a number of reasons, mainly cost and ease of construction with limited tools. (I don't have access to a fully equipped workshop).
Various ATMers around the world have had success with a technique of creating embedded threads around the azimuth and altitude wheels using JB Weld or other metal type fillers. This idea was pioneered by Tom Krajci.
I decided against this approach as JB Weld is quite expensive in the UK, and I would require several packs. When experimenting with the idea I couldn't seem to get a decent thread.
Having already routed a groove in my azimuth base wheel, I decided to experiment with placing a nylon threaded rod in there to provide the moving thread, against which the drive rod would mate.
Placing the thread around the Azimuth wheel
The wheel had already been routed by creating a jig to hold the router a fixed distance from the centre of the wheel. This was made around the entire circumference to a depth of around 10mm and a width of about 8mm.
I bought the threaded rod from RS Components International. I chose the M8 6.6, which had an order code of 280-464.
The rod fits into the groove fairly snugly and needs nothing in the way of glue etc to hold it in place, which makes it easy to replace if it becomes damaged or worn. The exception being the ends of the rod which have a tendency to spring straight back out. They were held in place by drilling tiny holes through the wood and nylon and pinning them in place with a small nail.
My wheel is about 69 inches in circumference meaning that I needed more than one single rod to go completely round. I was a bit concerned at where the rods meet, that the threads wouldn't match. This turned out to be less of a problem than I'd imagined. Once the first rod was laid, I butted the end of the second to it, then tested with a dummy "drive rod" to see if the threads met. By rotating the second rod just a little, I got the threads to match. The other end, where the two rods met to form the complete circle, I had to carefully file the rods until they mated correctly. I think there's probably a thread missing between the two, but the drive rod happily ignores this and moves across the junction without causing a problem.
This shows where the rods meet. Two nails are driven from the top to hold the ends in place.
Placing the thread around the Altitude wheel
The wheel had already been designed to run on bearings, so I didn't have a lot of width to play with, and couldn't route a groove into the altitude wheel in the same way. I decided instead to simply route a rebate.
Of course for an altitude bearing we don't need a thread all the way around, as the scope only moves from a horizontal position to a little over upright. Including a bit extra, the thread runs about a third of the way around the wheel. This means that I didn't have to join rods, nor particularly care how they are held at the ends. I simply drilled a hole and screwed the ends down. By doing this at an angle, the rod is held nicely in the rebate. I used filler to glue it into place.
The entire wheel, and a section showing how the end was fixed.
Mounting the stepper motors
I'll add this bit when I get time!
Drive Rod
I had a number of requirements when deciding on what type of drive rod and how to mount it.
Bending the drive rod
My first experiments centred upon bending the drive rod around the wheel. Firstly this would provide a longer distance where the rods contact and provide a better drive, and secondly by applying pressure at the extreme ends of the rods would allow the natural curve of the rod to change a little to compensate for the slight variation in incentricity.
I filed a groove into a wooden block a little larger than the radius of the rod, then cut a slot at 90 degrees to this. The slot provided an adjustable way of mounting the block to the base of the scope and the groove was positioned for the rod to ride in, providing pressure at the far end.
I tried many different positions for the block, relocating the stepper, different pressures and different arcs for the drive rod but could never overcome the main obstacle for this method... wobble. No matter what I tried, I couldn't overcome the rod wobbling in and out as it rotated. This resulted in the azimuth platform rotating at a non-constant speed, which of course would give rise to all kinds of problems.
After a week or two's experimentation, I came to the conclusion that bending the drive rod wasn't going to work!
Straight rod and simple block
For my next experiment I used the same wooden block but placed it adjacent to where the threads meet, keeping the rod straight. This worked after a fashion, but caused binding at some places on the Azimuth circumference slippage at others. Given that the overlap of the two threads is only a millimeter or so, it became obvious that a spring mechanism of some kind was needed to provide adjustable pressure.
Straight rod with complex mechanism
For my next experiments I decided to leave the rod straight but design an adjustable mechanism for holding the rod against the wheel threads in an adjustable way. I came up with a complex mixture of wooden block, shaped metal and springs. This worked after a fashion, but there was too much up/down play where the movable arm protuded and this allowed the drive rod to wobble. Without access to a decent workshop I decided to trash this idea and produce something simpler.
Straight rod and foam backed plastic block
Next I decided to really simplify. I took a plastic block (the kind that you use for fixing together flat pack furnature) and filed a groove in it for contacting the drive rod. Behind this I glued a thin piece of foam rubber, followed by a wooden block with the slot for adjusting the position. The foam was to provide a gentle pressure and gave a few millimetres of play, which I though would be ideal.
Sadly this turned out to be a failure. The foam rubber provided play in every direction including along the length of the rod and up/down. This make the positioning of the rod sloppy and gave a poor drive.
Straight rod and hinge
It seemed to me that the way forward was to provide movement in only one plane (towards and away from the wheel thread) and prevent movement in the others. After a bit of thought, I came up with the idea of a door hinge. The plastic pressure block was screwed to this. Between the hinge and wood I glued a small strip of foam rubber. The same principle as before, but this time the movement was limited to one direction.
This idea worked reasonably well and the rod behaved quite acceptably. Although I was almost there, I didn't think it provided enough pressure. I was certain that a spring was needed and that I could do better.
Straight rod, spring and hacksaw blade
I decided that a hacksaw blade could be used to flex in a single direction, provide a certain amount of spring, and be supplimented with a spring. The final solution is simple and quick to build and provides everything that I was looking for. The photos show the idea better than words can. I lined the plastic block with a piece of teflon salvaged from the old sliders from the bottom of the scope (since replaced with bearings).
Same idea is used for the altitude wheel
(You can see by the mess how many times I've had a go at getting this right! I'll spend some time tidying it up)
I am deeply grateful for the ideas and help and support from other telescope builders around the world. Some of this has come from books, but many ideas have also come from other telescope builder's web sites. I humbly hope that I've added just a little here to the helpful and generous ATM community. When time permits I will be adding a link section to other good sites and the ATM e-list which is a friendly and knowledgeable source of help and encouragement. Thanks folks!
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Page last updated: 8th May 2001
© Mark Rice 2001. All Rights Reserved