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Old 15-02-2019, 10:10 AM
Sunfish (Ray)
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Revisiting miniature rat cage.

The ascending order is,
100mm x 6mm leftover gal CHS
10mm scrap gal welded steel plate pier top
12mm alloy riser plates and 80mm solid alloy riser made by a friend.
8mm alloy pier top plate aligned for the C8 wedge from the scrap yard.
8mm base plate for the HEQ5 and 3x8mm M8 316 bolts with wing nuts.
Two threaded drilled 8mm plates and 4x 316 M8 bolts for the miniature rat cage.

I am hoping I can loosen the Main 10mm bolt through the slot to adjust alignment and adjust tilt with push pull tension on the 4xM8 nuts and bolts.. Gives 0.2mm or so movement. I can not see the Pole so the whole thing is drift aligned and level within 5 -10min.

I can add a third layer of 8mm plate in the sandwich if this is too flexible. Have some left over. I also wonder how much tension is enough on the main 10mm bolt. Hand tight with the socket head enough?

Is this going to be rigid enough? Perhaps the M8 should be countersunk hex tops.
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Old 15-02-2019, 06:21 PM
StuTodd
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IMHO, you can make it super rigid by doing away with the rat cage altogether.
People either love them or hate them and I'm with the latter. Far better to sit your EQ5 on a solid base.
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Old 15-02-2019, 07:29 PM
Wavytone (Nick)
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Looks wobbly to me.

The rat cage is not necessary when the mount already has altitude and azimuth adjustments, and only weakens the overall structure. I'd remove it. The cage is a relic from the past when EQ mounts had either none, or a most rudimentary adjustment in altitude.

A better solution is a single flat plate welded to the top of a large pipe. If the mount requires a single central bolt, the solution to that is to cut one or two holes in the side of the pipe sufficient to get a socket in there to hold the head of the bolt or nut (depending on mount).
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Old 16-02-2019, 12:16 PM
Sunfish (Ray)
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I did not mention that the 6mm thick 100mm CHS is braced at half height with 45 degree 35mm pipe.

So the pier size and the 80mm solid tapped riser is not a problem.

The stiffness of the mount connection is a function of the moment of the 100mm hollow mount base and the tension of the central bolt irrespective of the size or diameter of the plate it sits on. Hence my question. How tight is tight enough before there is a danger of stripping the alloy thread.?

I agree that rat cage bolts could have a slenderness ratio less than the 100mm hollow alloy base. Depends on the bolts. That can be calculated.

Plates are required to allow some azimuth fine tuning and to provide a 25 mm socket for the mount base. The ground here moves more than 1mm a year.

I can add a plate to achieve a 25mm solid base but the question of tension remains. Perhaps tapping the mount base for three screws from the bottom of the plate is the only way to increase the moment of resistance without overtension of the bolt.
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Old 16-02-2019, 01:45 PM
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RobF (Rob)
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I love my rat cage. Its over engineered and still leaves handy spot for cable management and knick knacks to keep them off the ground.

Compared to a tripod you have to be miles ahead in stability anyway.

People never talk about Focal Length before getting into this whole rate cage debate. Unless you're running a heavy RC at >1.5m FL its probably pretty moot to your tracking and Arcsec/pixel.
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Old 16-02-2019, 03:52 PM
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Peter Ward
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Quote:
Originally Posted by Sunfish View Post
....

Is this going to be rigid enough? ....
This old chestnut. Rat cages serve no function. They simply reduce the rigidity of a system down to its smallest component....i.e youve opted for the rigidity a M8 bolt over a +100mm diameter pier.
Perhaps a better way to think about how best to proceed is: think of how it would go if made of plasticine....youd want the biggest blobs you could get...hence Id suggest you toss the cage away.
Connect the EQ head directy to the pier.
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Old 16-02-2019, 06:05 PM
Sunfish (Ray)
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Actually 4 stainless steel bolts with a combined cross sectional area of 200sqmm . The bolts are still required to bolt the 25 thick base to the base plate. The cross sectional area of the cast aluminium mount base is about 1000sqmm. Perhaps a poor idea to call it a rat cage. More a bee slot.

Still no idea of maximum tension required.

According to some rules of thumb an alloy thread should be 3x diameter , so for the 10mm bolt , 30mm which is about what is available and can take 1500 mpa or 90kn.That should exceed the strength of the mild steel or 316 bolt which yields at 250 mpa or 9.8 kn for M10. Sound right?

Better buy a better bolt with the full 30mm or so engagement.

http://tribology-abc.com/calculators/default.htm
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Old 19-02-2019, 12:07 PM
Sunfish (Ray)
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It works, azimuth tuning.

I can fine tune my azimuth drift alignment using the mount adjustment screws on the mount.

Not a rat cage I suppose as the all of the four 8mm plates are bound together by the central mount bolt and fixings top and bottom. More a 25mm thick 100mm square alloy column with a slot, cheaper and more solid than a proprietary pier extension.

I can make the mounting bolt hand tight and loosen or tighten with the captive joined shaft nut and spannner for azimuth adjustment after a quick check of the drift.


Now if I can get this made in stainless with countersunk bolts and a fabricated spanner .....

Thanks for the comments which encourage improvement.
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Old 20-02-2019, 03:58 PM
Sunfish (Ray)
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Going unplugged

Works also when going unplugged. Another 200mm height in solid aluminium.

Not wobbly.

Although I might have to lose the C8 wedge alignment plate or get longer arms.

Great how the German GIRO 2 fits on a skywatcher shaped pier and accepts a Japanese clamshell direct . Although the giro centre bolt size is 3/8 instead of 10mm and everything else on the mount is metric.
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Old 21-02-2019, 06:51 AM
Sunfish (Ray)
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Rob,

Thanks for the comment.

I can see how a rat cage with 4 x 20mm bolts would exceed the cross sectional area of the heq mount base itself , and then in HT steel.

So if the bolts are short , well tensioned and fixed to the correct depth it should be just as good as the mount itself and better than a thin poorly fixed tube.

I think piers only need to big at the bottom where the bending moment and vibration is large cantilevering out of the ground. Ideally tapered. Thus the use of fins or , like me, braces. There is no advantage to the top being larger than the mount plus fixing room. Unless you want to cut a big hole in your your big thin column at the top where it is not doing much work, so all the same really.

Nice to keep your stuff on the big plate though.

So keep those rats caged.

Cheers
Ray


Quote:
Originally Posted by RobF View Post
I love my rat cage. Its over engineered and still leaves handy spot for cable management and knick knacks to keep them off the ground.

Compared to a tripod you have to be miles ahead in stability anyway.

People never talk about Focal Length before getting into this whole rate cage debate. Unless you're running a heavy RC at >1.5m FL its probably pretty moot to your tracking and Arcsec/pixel.
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Old 21-02-2019, 08:19 PM
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RobF (Rob)
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And if we win the lotto this weekend and then look to set up 20" RC on an AP1600 in remote dome, promise I won't use a pier with any rats
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Old 21-02-2019, 08:50 PM
Sunfish (Ray)
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Yep. A great steel tapered mount like the great pyramid with a vast rotating observing chair.

They dont build them like that any more.

My sons friend works for astronomers looking for fast radio bursts where the tools are mostly wire. Rats and the shrubbery are still a problem.

Cheers
Ray

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And if we win the lotto this weekend and then look to set up 20" RC on an AP1600 in remote dome, promise I won't use a pier with any rats
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