Dome rotation (1)

Having tackled the mechanisation of the two shutters I thought that although manual rotation of the dome was  simple,  fast  and easy it would be cool if I could motorize that as well. Certainly, were I to decide on a fly-by-wire remote control of the observatory in the future, hands-free operation of the dome window orientation would be necessary.

My first attempts involved rubber wheels attached to low voltage motors and attempts at rotation of the dome by friction from the wheels applied to the undersurface of the doughnut beam at the base of the dome. Sadly this proved unreliable and beyond the capabilities of the motors I had at hand. I must confess also that I had little faith in the arrangement from the start and although I have seen similar systems work on some sites I visited on the net, it struck me as a rather crude way of providing mechanical coupling,  liable to slippage and jams.  

A “rack-and-pinion” sort of system or one that provided drive via a chain and sprocket coupling would provide a more secure and controllable linkage between motor and dome base. The problem of course was that a chain like the one I had used on the shutter would not be easy to mount on its side and therefore some sort of belt had to be used that provided holes all along its length for the sprocket wheel to sink its teeth in and drag. 

Commercially these systems are available for purchase and for mounting on certain ready-made domes but they are quite expensive and I was not even sure I could get one to fit my precise needs. Looking for some ready made alternative punched steel flat bar I had no joy and so once more I had to make my own.

Like any DIY metal worker can tell you, a jig is a great time-saver!  So if you are planning to make equally-spaced holes, half-an-inch apart all the way around a hoop of  15x 4 mm thick flat bar over 7 meters in length  you had better take your jig-making seriously.

This is what I came up with…


Although perhaps it may look all a little daunting at first (and the backdrop of jumbled workshop ware does not help!) it is actually quite a simple contraption. It consists of a channel 15 mm wide made by welding two short sections of angle iron with the gap in between that allows lengths of 15mm flat bar to be run through beneath the vertical drill. At the exit end of the channel a sprocket wheel (salvaged and modified from the bicycle gear set) was mounted on a lug welded to one of the angle bars and held there by means of a bolt and a lock-nut that act as a pivot allowing the sprocket wheel to turn freely. The hole for the bolt is made in the form of a slot to allow adjustment of the sprocket wheel enabling it to just about reach the bottom of the channel as it turns. This allows the sprocket to dig into the flat bar at each hole, thus checking that  they are properly spaced. 

To further ensure that the drill-bit is at the right place and held there while it turns, two adjacent spring-loaded bolts were installed between the sprocket wheel and the drill and spaced exactly 3 hole lengths away from each other and six hole lengths away from the drill and from the sprocket at its lowest.

The spring loaded studs were simply made from 6 mm round bar that was threaded 2/3rds of its length and fitted inside a “U” made from a piece of flat bar. Compression springs were inserted in the section of stud within the ”U” and their tension adjusted by a nut and washer threaded below them. Another nut was threaded at the top of the stud around the bit that extended from the top of the “U”. The picture above shows the arrangement. When both  bolts slammed down into the holes beneath it meant that the distance between the holes was perfect and the next hole can be drilled. Movement of the sprocket wheel with advancement of the holed belt underneath further confirmed proper hole alignment.


The arrangement was very sensitive to minor deviations in alignment and altogether this jig was to a large extent the main contributor to the project’s success. Some sections were deemed unsatisfactory and had to be discarded. A total of over 500 holes were drilled using this gadget and when one got the hang of it, it became a breeze! 


The belt was built up from several sections of punched flat bar screwed to the inside perimeter of the doughnut beam at the base of the dome, making sure the joints themselves were as low profile as possible and performed in the gaps between holes but keeping the spacing in sync. The holes themselves were slightly countersunk to allow easier release of the sprockets and prevent them ‘catching’ inside. Small 3 mm countersunk holes were further made at intervals of each six holes in the spaces between adjacent holes to allow 15 x 3mm screws to be inserted to fix the belt sections to the wood underneath.




















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