The plain steel original shafts had been a constant problem with rust forming overnight. This was unsightly and made my previous restoration work look worthless. I had gained previous experience in replacing the MkIII's shafts. So I decided to file away the paint at suitable points on the MkIV castings to find the expected cross pins or shaft fixing screws. Once exposed I simply drilled out the pins and removed the old shafts. New, larger metric screw threads were cut in the original holes in the castings and socket head fixing screws with exposed heads were fitted to allow easy shaft removal in future.While dismantling the mounting to fit the new shafts I noticed some serious alignment problems between the base and the polar castings. When I put a straight bar through the base casting's altitude pivot holes I discovered that the holes did not line up by a full half an inch! The same went for the threaded altitude pivot screw holes in the polar casting! The whole base assembly was badly skewed thanks to the appalling workmanship in drilling these holes. Normally this would not be noticed because the polar axis would not allow a bar to pass right through because the polar axis shaft is in the way. Can one assume that complete amateurs were allowed into the Fullerscopes workshops without skilled supervision? Who was responsible for the truly awful standard of workmanship seen in my MkIV? The holes were diagonally incorrect. Their centres needed 'moving', both along and across, relative to their castings to achieve anything remotely like true alignment.
I decided to file the holes in the castings in the direction indicated by my straight test bar. Once the holes were "moved over" to a better position with a large, coarse, round file I could drill the holes truly round to a larger size. The same work had to be applied to the polar axis casting. Larger metric threads were then cut in the new, larger holes.
The new, oversize, stainless steel, polar altitude, pivot screws look much smarter than the old rusty ones. I also fitted larger washers under the heads of the pivot screws to spread the crushing loads on the "ears" of the base casting. I also fitted very large, galvanised steel, thick, square, roofing washers as packing between the base casting ears and the polar casting. The base cast ears are actually rather thin for the heavy loads they carry. There was already sign of deformation of the flat areas where the big pivot screws had crushed the soft metal slightly. The polar altitude pivot screws now pass through the big square washers into the threaded polar casting. These packing washers were a real struggle to fit because of the concealing flanges on the casting ears. The washers had to be poked gently into place with thin strip of steel and lot of patience with the mounting lying first on one side and then on the other.
Only much later did I realise that I had denied the base casting "ears" their normal levels of friction which they applied to the polar casting. So my modifications made the polar axis sag in altitude when the OTA was removed. I just did not trust the small, altitude locking screws to hold fast in the soft castings. So I could not tighten them with full confidence that they would hold without stripping their fine metric threads. In the end I added a cheap turn-buckle and this has proved ideal. Polar angle can now be adjusted with the fingertips and stays where it is left. No more tools and no need to over-tighten the altitude pivot or locking screws. It might have been simpler to have fitted much large washers under the heads of the altitude pivot screws. Though I would not have the micro altitude adjustment offered by the turn-buckle.
By sheer luck I found an old 62mm, folded steel, car exhaust clamp amongst my junk. This is tightened onto the lower end of the polar axis casting. A fixing hole was drilled and threaded in the steel pier at a suitable point for a strong screw eye. The galvanised turn-buckle is fitted in tension between the two. I think it looks rather neat and it works perfectly for very little effort or expense. The modification cost less than £3 ($5-6 equiv.) in total and took only ten minutes to complete. Smarter stainless steel turn-buckles are available from boating shops but would probably be more difficult to adjust with just the fingertips. Click on the image below for a closer view of the turn-buckle in place.
The image above shows earlier attempts to balance the mounting with multiple OTAs and when the OTAs were removed. I simply added heavy counterweights on the over-long, polar shaft. This proved fruitless. The polar angle still moved up and down and polar axis friction increased far too much for easy pointing with only the leverage available with the short 6" refractor. Normally, when carrying the 6" F/8 refractor alone, the MkIV mounting has the just enough friction to allow easy pointing without the telescope drifting. This is still true even without the slow motion clutches being tightened.
Here is a simple aluminium alloy disk turned in my lathe to house a 1.25" bore ball bearing (shown inserted). The intention is to use countersunk screws to hold the disk onto the top of the polar casting to take the lateral loads. Hopefully with reduced friction compared with the existing bronze sleeve bearing. (which will be removed to allow more clearance) The worm housing will have to be moved upwards and a suitable clutch designed since the wormwheel itself will be moved upwards by the thickness of the light alloy disk.The 1.25" bore, taper roller thrust bearing (sitting on top of the alloy disk) will probably be fixed at the bottom end of the polar casting in a simple housing to reduce the friction from end loads on the polar axis. I just happened to come across these bearings in a garage sale. A fortunate coincidence. A wash out with paraffin and they run as smoothly as new. Given the very low rotational speeds involved in telescope mountings there should be no bearing problems provided the bearings are well protected from the weather to avoid rust. I have since obtained some thin, flat, ball thrust bearings which are much more compact. I am still trying to work out how best to use these bearings without altering the mounting permanently.
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