ODDS AND ENDS.

Backlash in Worm Drives.

No backlash is wanted in a worm drive especially if the telescope is used for photography. Bachlash is a problem in all machine tools and certain procedures have to be adopted to counteract it, such as always approaching a setting from the same direction; not going past it and coming back. In the gem faceting machines I manufacture there is an index wheel for dividing the facets on the stone. if facets do not line up. where does the fault lie? With the machine or the operator. As I have no control over the latter I have to ensure that the fault is not in the index wheel. For this a very accurate dividing head is required. I purchased an "expensive for me' Dividing Head. A test card came with it showing its error in seconds for a full circle as tested with a "Carl Zeiss' high precision digital index texter. It had a maximum total error of 42 Seconds of arc. However as I found minute backlash at certain points. I adjusted the worm engagement to eliminate this and then found that at other points it was too tight. A friend of mine who attended an International Machine Tool Exhibition discussed this my problem with a Japanese manufacturer. They said they could make me a dividing head for $40,000 which would not have this problem. This was out of the question. As I cut enough index wheels in ten hours to last me a year I considered the $2600 I paid for my dividing head more than enough.

Now I do not know if my worm drives have periodic variations and wouldn't have a clue as to how to test them. But a smooth drive for the whole circle and no backlash I was able to make. Fig 2. below Shows my design. A 114° diameter shaft supported by and running on ball bearings carries the worm. This is adjusted up into engagement with the wheel so that there is no back lash. At the shaft flexes over this length it can maintain an even pressure of engagement with no backlash and no tight spots. Simple as that.

Tube End Rings.
A tube without end rings has an unfinished look. They must be a problem as the first thing a couple of other telescope makers asked me on seeing my telescopes was how did I make my end rings. This was not a problem for me. I cast and machioned them out of aluminium. Holding such a thin ring in the lathe to machine it is difficult. However I machined both top and bottom rings from the same casting. As the bottom ring forms part of the mirror cell and is flanged, a firm drip in the chuck is possible. Fig. 3 illustrates this. Recently I came upon a rubber moulding that made excellent end rings. Illustrated is a cross section of it fitted to a tubeend. I purchased it from a local rubber shop but they were unable to tell me who manufactured it.