Earlier this year I bought the 'Gaunt' pillow block eq mount from Matt aka Kunama. This beautiful mount takes my 6" f12 refractor with ease. Its all there except the RA setting circle, RA gear and tangent arm for dec control.
I'm well on the way to rebuilding all these parts, at least theoretically!
So far I have partly built the tangent arm and have just made the RA setting circle.
Pics below show what it is. Two discs of 6mm brass with a 3mm aluminium disc sandwiched inbetween, to match the exsisting Dec circle. I need to bond the circle together. Will JB Weld to the job? Any other suggestions? The RA circle will then be bolted to a 12mm thick aluminium disc which in turn is secured onto the shaft via grub screws....This aluminium disc will then become a component of a friction clutch system still in the pipeline.
Photos show the mount and brass discs.
Of course there is engraving the 8" circle with the hours which is another hurdle to get over....ideas thoughts on this?
Matt
How about drilling and tapping for some countersunk brass screws?
You should only use brass screws with aluminium if you don't want to dissasemble it later. Brass expands with age and the screws will become tightly bound.
I use JB weld a lot, and it is incredibly strong and resilient, BUT...
Keep with the style of the times Matt - JB Weld didn't exist when this was made. Brass screw it together, or even brass crush rivets/pins (over-length pins crushed down in a shop press to friction fit TIGHTLY, then the head ground back and re-centrepunched to re-expand a bit)
Or, you can solder brass and alu together - I know some can do it with conventional tin/lead solder, others use tin/zinc, or you can use silver solder (probably more authentic?). You can even buy Alumiweld to solder it.
Food for thought. I just would not go JB Weld. I have had it last years, but it will eventually fail.
Brass screws it is. Good info on JB weld I didn't know, thanks Lewis. It's not a strict restoration and I want the mount to be easily usable, keeping with what was used, hence the Tangent arm and hand slomo control on the RA. Might add a motor to the RA one day, then again I might not.
I'll replace the aluminium disc now painted gold with a brass disc one day, so screws will be the way.
A photo of the original Dec circle, now to do the same on the RA with the hours and mins.
More to come on this....
Would there also be an issue with galvanic corrosion between Al and brass? Could you not use a brass inner ring?
Quoting an article I read: Contact between copper, bronze, brass and different kinds of steel alloys (passive and active) and aluminum can cause severe corrosion so it is advisable to provide insulation between the two metals
Hmmm might be making a trip down to George White (brass supplier) soon and getting another sheet of brass for the inner ring, which should be 4mm.
Corrosion I had not even thought about, might as well do it right from the start. I need 0.5mm thick brass for the slip clutch anyway, unless anyone can think of a material thats better like Teflon maybe?
More to this than meets the eye…thanks Lewis and all.
I am going back to the old carburetor days, a brass swivel pin through the throttle butterfly. If you did not use the car for a while the throttle would start to seize, a mechanic told me that the brass pin would expand with age. I am wondering if the brass was absorbing oils etc and that was what caused the expansion? I had some expandable brass collars on the wheels of my old pier mount, one of which completley siezed in the aluminium leg so tight that I had to drill the steel pin out to remove the wheel.
I now get worried when I hear people talking about using brass fasteners on aluminium components especially in situations where there is moisture. Galvanic action is bound to occur.
To give an example:
Last year I made a concrete pad for the equatorial platform. To help in setting up the EP for each observing session I concreted in four aluminium plugs in the pad in which a positioning jig is bolted. To stop dirt filling in the 8mm threaded holes in the plugs between observing nights I screwed in brass set bolts.
Four weeks ago I went to remove the brass set bolts to set up the EP. (The pad had not been used for two months owing to the weather.) When I came to remove the brass set screws my troubles started. The head of the screws sheared off owing to galvanic action between the brass and the aluminium.
To cut a long story short I ended up drilling out the shank of the screws and rethreading the holes. I replaced the screws with those made from marine grade stainless steel.
I learnt my lesson to never use brass and aluminium together.
To cut a long story short I ended up drilling out the shank of the screws and rethreading the holes. I replaced the screws with those made from marine grade stainless steel.
I learnt my lesson to never use brass and aluminium together.
well you shouldnt use stainless and aluminium either the better choice is mild steel and aluminium but if you use something like Loctite anti seize you wont have many problems . Mild Steel and Aluminium being closer together on the galvanic scale. Penrite has a grease for situations where galvanic corrosion may occur and I use it on my mounts as they have Aluminium , brass,chrome, mild steel etc.
Do you have a name for the Penrite grease Roger?
I'm shortly doing a thread on a friction clutch and 360 tooth gear and worm on the RA on the Gaunt mount which combines brass aluminium and SS bolts etc
Sounds like it might be corrosion heaven
Matt
Do you have a name for the Penrite grease Roger?
I'm shortly doing a thread on a friction clutch and 360 tooth gear and worm on the RA on the Gaunt mount which combines brass aluminium and SS bolts etc
Sounds like it might be corrosion heaven
Matt
Have a read and see if its for you, I could well be declared a heritic for suggesting a non Dino grease. It used to say galvanic protection now says oxidation inhibitor, or perhaps I looked further into it some years back. http://www.penriteoil.com.au/pis_pdf...JAN%202010.pdf
Could use a brass solder to join the parts and give it a good polish after, as for the engravings any good machine shop should have an indexing head and milling machine, it can even be done in a metal lathe with fairly good accuracy.
Doug is 100% correct. You cannot use brass and aluminium together in any combination, whether it be a brass and aluminium plate side by side, or brass screws through the aluminium, whenever there is any likelihood of exposure to the elements in any way. The zinc in the brass has a galvanic reaction with the aluminium, to the extent that over time the brass will dissentegrate and crumble. This galvanic reaction is typified with the zinc sacrificial anode fitted on outboard motors. Most outboard motor legs are made from cast aluminium and they have a zinc sacrificial anode fitted just under the cavitation plate. The idea of this is that over time the anode gets eaten away by galvanic reaction thus preserving the integrity of the aluminium in the leg housing. When the anode is almost eaten away after a few years you just replace the anode, while the aluminium in the leg remains in good condition. When you see outboard legs where the aluminium is corroding badly, you will usually find the anode was eaten away ages ago and never replaced. Similarly, you should not use stainless steel with aluminium as the chromium in the stainless will cause the aluminium to corrode.
In regard to your project, if you want to do it right, I would replace the aluminium plate with a brass plate and then rivet the plates together with homemade crush rivets, made from some 4mm or 5mm, or thereabouts brass rod. You will need to get the drill size spot on when you do this and you then just grind the overhang on the rivets off flat with both sides of the plate. If you do it right, it will never come apart, short of drilling the rivets out and it will give you an aesthetically nicer finish than using machine screws.
I forgot to mention that you could also use copper rivets which would give a nice 2 tone aesthetic effect. You could also use stainless steel rivets, but they would be very difficult to crush and grind off properly with hand tools.
the better choice is mild steel and aluminium but if you use something like Loctite anti seize you wont have many problems . Mild Steel and Aluminium being closer together on the galvanic scale. Penrite has a grease for situations where galvanic corrosion may occur and I use it on my mounts as they have Aluminium , brass,chrome, mild steel etc.
always learning something coming in here .
