2" shaft mounting Pt.49: Counterweight retaining bush.


While I was on the lathe I turned a hefty, brass, counterweight retaining bush for the end of the declination shaft. However, it would require rather long screws on typical plastic tightening knobs. Unless, of course, I add a wide spacer to ensure finger clearance. Or used hex socket head screws which can be tightened with a hex wrench. Having struggled with hex wrenches on the Fullerscopes MkIV weight retaining bushes over the years I am not very keen. It can be critical to get the clamp on and tightened before one's strength fails and all the counterweights slide off onto one's feet! Which is why I would much prefer tool-free tightening and free-fitting weights and clamping bushes. It was always a struggle to remove the MkIV's shaft clamping bushes.

Simple thumb screws were used on the Fullerscope MkIII mounting bushes. They attracted rust like a magnet due to to both components being made of steel. Wing screws need only a little clearance for the fingers but aren't normal stock items in most DIY outlets. So the search begins again. I could use normal, plated, wing nuts and thread lock a stud into them. They would rust too unless I can obtain stainless steel wing nuts. I can feel another search coming on unless I buy them online.

As a temporary measure I drilled and tapped the bush at 120° intervals anyway. Three radial screws seemed like a better option than the usual one. Then I used plated steel M8 screws and wing nuts. The socket head screws needed the undersides of their heads tapered in the lathe to fit inside ordinary wing nuts. This gave the option of finger tight or hex wrench for extra grip, while still looking quite smart. Now I have had some practice I shall buy stainless steel screws and wing nuts in the same size for a more permanent [non-rusting] job.

Wing nuts provide plenty of torque so there is no hurry to get out the wrench to retain the counterweight. Though I would want to be absolutely certain the weights would stay in place when the declination shaft is vertical! I can see the good sense in radial drilling posher counterweights for individual screw locking. This would not be possible with my cheapo dished 'bodybuilder's' counterweights. I may need to dimple the shaft with a drill to provide extra security for the weight retaining bush screws. First I need to know how many and where the weights will be normally positioned to balance the OTA. An A-frame prop would aid weight fitting and removal.

To complete the day's efforts I drilled out all six altitude pivot holes to 16mm. A painless exercise using the lathe in back gear, the drill in the 3-jaw chuck and the tailstock providing the necessary pressure. Trying to drill large holes in under-powered, DeWalt rechargeable drills, or cheapo bench drills, is a complete waste of time and effort. The speed is far too high so they chatter and grab because there isn't enough pressure even if you lean on them.

Click on any image for an enlargement.


2" shaft mounting Pt.48: Declination/saddle bush reinforcement.

The Tollok bush which holds the saddle onto the end of the declination shaft had little or no room for an expansion ring. The supplied steel ring would not fit inside the saddle channel section without surgery. This would involve thinning the ring considerably with the potential for disaster. The Tollok bush relies on opposed tapers to grip the shaft and simultaneously expand into something tubular which offers serious resistance. The grip on the shaft relies on the physical containment of the outer, flanged bush. 

I decided to turn a full length cover for the Tollok designed to provide the vital resistance to expansion. Unfortunately I have no round aluminium stock in the necessary diameter. So I was forced to use brass. This is much heavier than I wanted for this situation and will require more counter-weighting. The advantage of the sleeve is that it protects the bare metal of the Tollok bush from unsightly rust. It also overlaps the flange bearing projection.

The images show the general idea. The piece of scrap, brass bar had been discarded by an educational establishment after a couple of knurling exercises. I left the knurling in place for decoration. The middle image shows the Tollok bush sunk flush with the brass sleeve. All much as I had done with the much large [7" diameter] aluminium cylinder on the polar axis.

Having the entire Tollok bush enclosed maximizes its ability to compress strongly onto the declination shaft without local stresses or flexure. I reasoned that since both halves of the bush are  split, the full length enclosure would remove any chance of local flexure. Far better, surely, than a short ring?

The narrow front lip, which accepts the inner cone flange, set the maximum and minimum diameter which would just go inside the saddle's channel section. I deliberately allowed a little extra clearance here to avoid stressing the thinner lip by internal expansion. The much thicker, main body was made a closer fit where there was over a cm of solid brass to resist expansion.

I had bought some oversized, stainless steel washers to fit the Tollok, clamping bush screws. The idea was to spread the loads from the ten replacement, longer, stainless steel screws. Due to the screw spacing I had to turn the washers down in diameter to just clear each other. This ensures the maximum area and of the saddle is sharing the pull from all ten screws. Adding a load spreading plate under the screw heads would be pointless unless the plate was bonded on. It would also demand much longer screws with the potential for flexure.

Click on any image for an enlargement.


2" shaft mounting Pt.47: Firmly planting the base fork.

My initial plan to add angle profiles on each side of the fork base would not provide vertical stability. It badly needs some downward pressure which might as well provide some rotation for polar alignment at the same time. This will require a division between the fork base and the large bottom plate. Unfortunately I only bought one of the these large 10mm [5/16"] plates from the scrap yard. So cannot simply duplicate the base plate with a central pivot and familiar screwed azimuth adjusters. I do have a little more of the 15cm x 10mm plate but wont have much in reserve if I duplicate the fork tines with doubled plates. Nor will the plate provide enough resistance from the compression of a large, central pivot screw.  

The fork is now sloping backwards by 20° to provide more clearance for the wormwheel. The cross studs and vertical pivot ought to be centrally placed relative to the fork tine base. Which doesn't provide much room the higher they are placed.

I was thinking about having a really sturdy crossbar between the fork tines to resist the compression loads. A large nut and washer would provide the downward pressure via a vertical pivot stud or bolt. I do have limited milling ability on my lathe but would rather avoid milling slots in the fork tines just to fix a crossbar between them.

The original idea was to bolt the entire mounting together without machining. The cylinder rather dented the original plan but was at least optional. I just happened to have a chunk of 7" round stock. Alternative means could have been found for effectively supporting the Declination axis.

Two sturdy, spaced, 15mm studs between the tines would allow a vertical stud to pass between them with a 6" plate resting on top and sandwiched between the tines. The stiff cross studs would help to support the cross plate and simultaneously compress the fork tines together against the edges of this horizontal plate. Or plates, if a bottom plate is added.

Here is an image of the mounting supported from the new 1T lifting strop and 1T chain hoist. The steel hook allows more rapid movement along the ceiling joist than the former, multi-looped cord.

Front and rear sandwiched plates would make a closed box out of the fork and it would all be under heavy compression from the twin, horizontal cross studs. Plus the vertical compression from the central, pivot stud. The tops of the tines are compressed against the sides of the Dec housing by the altitude pivot stud. All helping to avoid flexure anywhere in the fork.

However, first I have to decide on the final fork dimensions and angles. I must avoid contact between the large RA wormwheel and the base plate, fork or large diameter pier pipe.

Materials shortage over. Another visit to the scrap yard produced more aluminium plate in various thicknesses up to 10mm. I now have a second 10mm [5/8"] base plate to bond to the first. A pack of Loctite 'Metal' is on its way in the post.

I have received some useful information on filing and "grinding" aluminium. Files pick up the metal which causes scratches and slow removal of material. Using ordinary bar soap on the file is supposed to help. I've just bought a bar of soap to give this a try. Lamp oil had little or no effect despite being recommended [and highly beneficial] for turning aluminium. The lamp oil had an amazing effect when I was turning the 7" diameter cylinder yet had no obvious benefit on my files. I wonder whether soap would help on the 36 grade angle grinder disks? Might be worth a try.

With the fork slope decided I could duplicate the tines. They are now 20mm thick [0.8"] and look far more substantial. As does the 20mm thick base though it has yet to be trimmed.

Open fibrous abrasive disks have been recommended for cleaning up rough aluminum edges and surfaces. I tried the local DIY chain store but they had nothing like it. Another online search and purchase is obviously  required. 

A 44 mile ride on my touring tricycle produced some goodies to speed up smoothing the aluminium. The card, file brush [bottom] will help to clear aluminium from my files.

I spent another hour trying to level the sawn edges of the duplicated fork tines. Rubbing dry soap on the disks and files is a definite improvement.

The Loctite 'Metal' epoxy should arrive today to fix the plates firmly together. Plates simply held together with nuts and bolts tend to behave more like leaf springs under bending loads. The epoxy should make them behave more like the thicker material the laminations pretend to be. I shall have to have all my G-cramps ready to squash the plates hard together.

Click on any image for an enlargement.



2" shaft mounting Pt.46: Going down below.

A trial assembly with the RA wormwheel at the bottom of the Polar axis made far more sense. It also showed that I should move the altitude pivot upwards along the PA housing. This would better balance the rather top-heavy appearance. It would also push the wormwheel further away from the base plate, pier and fork. Perhaps even allowing considerably shorter fork tines.  I have sawn off the studs while still leaving a small reserve for unforeseen changes.

Am I a robot?

Rain forced me into the workshop today but at least I had the chain hoist to help me lift things into place. The arrow in the image shows the direction for a new altitude pivot point. If I removed the nearby furniture screw it could rise even further. The [much larger] pivot stud would provide far more compression than the smaller one. So there would be no sacrifice of strength. Once the new pivot point is properly established I can then measure how far I can shorten the fork tines. I am trying to maintain the full radius curve on the top of the fork tines for appearance and maximum friction.

Here I have moved the pivot further up the PA. The highest point was not the most favourable and I had cross studs to avoid.  So I moved it 3/4" down on the same line. I think the whole mounting looks much better balanced now. It is suddenly looking rather impressive [in real life] thanks to the large scale. The tape measure shows the overall height to the top of the saddle is now 40" or 100cm from the base plate.

The mounting's overall balance is now backwards or clockwise seen from this viewpoint. So I had to make up a stub axle which could hold a weight and be locked against falling by tightening the flange bearing grub screws. Adding the OTA, declination axis shaft and counterbalance weights will overcome the imbalance. Allowing for a slight bias to be overcome by the turnbuckle for fine altitude adjustment.

As the fork tines are sloping I have to be careful that the large RA wormwheel does not get any closer. Just cutting off bits of fork would bring the wormwheel nearer. I could cut a more acute angle on the base of the tines. This would tilt them over more and bring the mounting's CofG further within the fork base. While simultaneously moving the wormwheel slightly further away from the base plate and pier. I have made up a cardboard "set square" for 55° to aid rapidly setting the mounting at the correct angle.

Now I ought to make the large pivot stud to get rid of the G-cramp and allow free access to the fork. I'm thinking of boxing in the fork with front and back plates held by cross studs, along much the same lines as the bearing housings. This will help to stiffen the fork more than simply adding channel profiles to the sides. It will also allow greater freedom to fit the dual angle profiles. I used a 12mm [1/2] galvanized stud for the pivot. The nuts look a little 'understated' against this scale of mounting. Some larger washers will help. Perhaps I should step up to the 15mm stud size I used in the bearing housings.

Click on any image for an enlargement.



2" shaft mounting Pt.45: Forked base.

I have been putting off the base while I sought heavier [scrap] materials. They were not forthcoming so I am going to use what I have in 150mm wide strip x 10mm [6" x 3/8"] aluminium plate.

I shall build a simple support fork for the polar axis bearing housing. Which will pivot the housing to allow fine PA altitude adjustment via a sturdy, stainless steel turnbuckle.

I carefully marked out where the 50mm [2"] shaft resided inside the casing as well as marking the position of the heavy studs. [All threads] There seemed no particular purpose in moving the pivot any further north than half way up the housing. I may change my mind over this but at present it allows room for the RA wormwheel if I should move it to the bottom of the PA shaft.

I then drilled small holes in opposite plates and supported the polar axis on two nails. The G-cramp [C-clamp] keeps it all steady while I admire the initial layout to see if I have overlooked anything important. Which I often do these days. The worm support plate will be relocated on the side of the housing to allow a control rod to be brought back to the eyepiece.

The "nail" holes will be opened out to take the largest possible threaded rod [stud or all thread] to tie the fork tines rigidly to the PA housing. This continues the compression idea to make the housings as solid and stiff as possible.  Otherwise I could easily have used captive pivot bolts pointing outwards through the side plates. This would not have provided the desired compression as the side plates would be held only by the smaller cross studs.

I try to imagine flexure modes exaggerated into catastrophic failures. Hence the through stud rather than two loose bolts. Offsetting the pivot holes between the axis shaft and large studs offered two options. I could pivot the PA housing above or below its center line. The higher position caused clearance and appearance difficulties for the fork tines. Though the PA housing could still be flipped over if desired for a much lower housing position. But then the RA wormwheel won't fit under the housing.

Oblique view showing the fork more clearly. Once the studs are finally sawn to length below the lower flange bearing there should be room for the RA wormwheel. For the moment the studs make useful handles for carrying the hefty PA housing around.  They also provide convenient propping points for mock-ups to see how things look in practice.

The fork will be fixed down to the large 10mm [3/8"] base plate via sturdy aluminium angle both inside and out. A length of scrap angle is resting against the workbench in this image all ready to be sawn up. 

Even at 10mm [3/8"] thick the fork tines look extremely flimsy. So the sides of the fork will be further reinforced against flexure with channel section aluminium carried to full height. I have propped up a short length of channel to show the basic idea. You will have to imagine the channel rising up both fork tines and having a radius applied to the tops to match the rounded fork plates. I am considering using more furniture screws to hold the channel in place.

Perhaps I should double the fork tines to 20mm [13/16"] thickness? I still have enough 6" wide strip for this job. Epoxy between the two layers would bond the sandwich into a solid mass with far greater stiffness. I don't want to follow this route until I have finally decided on RA wormwheel position as longer fork tunes consume a lot of scarce material.

Should I decide to leave the RA wormwheel at the top of the PA axis then the fork tines could be considerably shortened. This is where a bolt-together assembly is handy for prototyping different ideas.

Click on any image for an enlargement.