HandyAndy
professor emeritus
Reged: 01/11/08
Posts: 650
Loc: West Midlands and around
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Hi,
The problem with trusses is the funny angles the tubes join at and to the mirror box and top ring.
I have thought of having a sphere of hard plastic which just fits into the truss tube. The sphere has a hole drilled in for the bolt to the required tolerance. The tube has an oversized hole drilled through at the correct length and roughly in the correct orientation.
When two tubes are bolted together to the mounting point the sphere can revolve to the right orientation.
An alternative to get the centre points of the tubes to meet is to machine off just over half the diameter of the end of each tube so they can be bolted on to each side of the hole in the sphere. I think this would give the stiffest triangle as there is no offset.
The sphere cut be cut in half across the hole with the cut the thickness of a lug attached to the mirror box or top ring. The bolt would then pass through a truss end, one sphere 'half', the lug, the second sphere half and the second truss end. All the connection axes will then be co-centric.
Cheers. Andrew.
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Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 8" VISAC: (GP2)
Car: 6" F5 MPCC: SP, 5" 127mm F7.5 (GP2)
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
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polaraligned
sage
Reged: 12/26/08
Posts: 229
Loc: P. R. of New Jersey
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Spherical truss ends is a good idea and has been used by a number of commercial manufacturers.
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Don W
Postmaster
Reged: 05/19/03
Posts: 14621
Loc: Wisconsin, USA
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Ayup. Moonlite sells them.
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Don Wyman
Obsession 18" f/4.5 #1166
W/Argo Navis DSC and Torus Primary
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Owen
professor emeritus
Reged: 06/21/07
Posts: 514
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Just a little thought experiment... whilst you are considering the truss connections...
Is a sphere actually necessary to obtain the same level of 'flexibility' in positioning of the truss..?
Owen
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Howie Glatter
Vendor
Reged: 07/04/06
Posts: 190
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>Is a sphere actually necessary . .
Only a segment of a sphere is necessary, but it must be a sphere.
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Owen
professor emeritus
Reged: 06/21/07
Posts: 514
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Hi Howie...
are you really sure that it needs to be a segment..? (thats a clue)
Owen
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HandyAndy
professor emeritus
Reged: 01/11/08
Posts: 650
Loc: West Midlands and around
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Hi,
I was thinking of a sphere which in the simplest option fits inside the truss tube. The bolt goes through an oversized hole in the tube and a fitting hole in the sphere. This is a simple self aligning bearing. I am not thinking of a round knob on the end of a truss tube.
In the second option I have one sphere with two reduced section truss tubes with one hole in so one can be bolted on to each side of the hole in the sphere.
Between the bolt head or nut there needs to be a washer, possibly a short half section of a larger tube with the inside diameter matching the outside diameter of the truss. Or maybe a soft washer to conform to the indent of the larger clearance hole in the truss tube.
Cheers. Andrew.
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Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 8" VISAC: (GP2)
Car: 6" F5 MPCC: SP, 5" 127mm F7.5 (GP2)
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
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jasonharris
sage
Reged: 09/16/06
Posts: 248
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Doesnt it depend on the design of the telescope? I use the moonlight connectors on my 16" I built following 'the book' and the balls certainly make it easy to put the top on the scope but the purpose isn't to get the angles right is it?
A ball and socket has huge advantages when the trusses have compound angles but most dob scopes only have one angle to contend with and thats mostly important at the connectors down the bottom which I have never seen a ball used there.
At least thats how I see it on most dobs.
JAson
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Roy McCoy
professor emeritus
Reged: 10/13/05
Posts: 642
Loc: Glendale, AZ
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This was a small chore to build but it works very well. The rod ends give 17 degrees of movement.
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Roy
MN65
114NT/500
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Owen
professor emeritus
Reged: 06/21/07
Posts: 514
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A simple self aligning bearing, doesnt need to have a 'sphere' at all, thanks to geometry...
Another clue - # The contours and plane sections of the sphere are circles.
This property defines the sphere uniquely.
Owen
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Scott Watson
sage
Reged: 05/26/06
Posts: 282
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Roy has it. Spherical rod-end bearings are the key to a good hexapod truss. I've been told several times by members of this forum that this very idea doesn't work. Sorry, but it does. 3 points define a plane and two planes with 6-degrees of freedom (trusses) are all that is necessary for perfect, no slack, properly contstrained collimation. Here is another tip. If you use rod ends with different thread pitches, but the same handedness, your adjustment will be ohhh-soooo-fine. Heck, maybe Art B. will come back if more than two of us push this idea. For those who are still on board, another tip is to use plastic rod ends. They are MUCH lighter, cheaper, and have nearly the same precision and slop as the stainless variety. Good luck...
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Owen
professor emeritus
Reged: 06/21/07
Posts: 514
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http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3404168/page/0/view/collapsed/sb/5/o/all/fpart/1#Post3404802
Take a look - all the advantages of a spherical bearing, with little of the pain/cost...
Have fun
Owen
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Vincent Becker
super member
Reged: 09/16/08
Posts: 198
Loc: France
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Here are some pictures of spherical bearing trusses (about halfway down the page):
http://sites.google.com/site/dobson2005/dobson400serruriercarbone
Owen's link is very interesting too.
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Vincent Becker
10" dobsonian on EQ platform (home-made by my father)
8" string newtonian as travelscope (home-made by myself )
Orion 80ED and GSO 200/1000 on Atlas EQ-G for astro-imaging
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Owen
professor emeritus
Reged: 06/21/07
Posts: 514
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Those are fabulous...
Owen
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Jim Romanski
scholastic sledgehammer
Reged: 01/02/05
Posts: 843
Loc: Guilford, Connecticut
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Here is the setup that I’m using with the 17.5” Scope.
One of the design principles that I was taught with truss tubes was to have the connection butt together in the horizontal plane so that there’s less movement in the vertical. In other words, I didn't want to stick a bolt in from the side when connecting it to the scope to prevent wear and slop. The bottom uses Moonlite ball connectors. The top has Tie Rod Ends or Heim joints from McMaster Carr permanently bolted into an aluminum angle plate. The 1/4-20 thumb screw is captive and screws into the top ring. By having the connections butt in the horizontal plane the vertical length is maintained overall collimation is better retained. Also, this makes for 4 pairs of struts that are easy to assemble in the dark.
I used studs with 1/4-20 thread on one end and 1/4 -28 on the other for the Heim joints since the ones sold by McMaster Carr are only available as 1/4-28 and the tube inserts are 1/4-20. I prefer to use the tube inserts as this also retains the same connection scheme of butting in the horizontal.
The struts are also adjustable.
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Jim
17.5" Dob "Project"
13.1" Coulter
8” Cave
NP 101 on a CG-5
25x100 binos
Naglers, Ethos, etc.
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Jim Romanski
scholastic sledgehammer
Reged: 01/02/05
Posts: 843
Loc: Guilford, Connecticut
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Top tube connectors.
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Jim
17.5" Dob "Project"
13.1" Coulter
8” Cave
NP 101 on a CG-5
25x100 binos
Naglers, Ethos, etc.
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Jim Romanski
scholastic sledgehammer
Reged: 01/02/05
Posts: 843
Loc: Guilford, Connecticut
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Bottom tube connectors.
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Jim
17.5" Dob "Project"
13.1" Coulter
8” Cave
NP 101 on a CG-5
25x100 binos
Naglers, Ethos, etc.
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Jim Romanski
scholastic sledgehammer
Reged: 01/02/05
Posts: 843
Loc: Guilford, Connecticut
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Quote:
Heck, maybe Art B. will come back if more than two of us push this idea.
Come back Art! 
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Jim
17.5" Dob "Project"
13.1" Coulter
8” Cave
NP 101 on a CG-5
25x100 binos
Naglers, Ethos, etc.
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Owen
professor emeritus
Reged: 06/21/07
Posts: 514
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Art is very, very busy at the moment.. but he does pass by now and then
Owen
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MitchAlsup
super member
Reged: 08/31/09
Posts: 184
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Quote:
The problem with trusses is the funny angles the tubes join at and to the mirror box and top ring.
See attached picture. Since the angle only has to be established once, the spherical rod ends are complete overkill. The poles, themselves, were filed to fit the angle brackets and are about 1mm sort on one side compared to the other.
To establish this fittment, assemble the scope with minimal tension in the bolts on the angle bracket to truss tube. Watch how the base of the angle bracket is mismatched to the tube. Find the point of maximum gap and draw this size on the opposite side of the truss tube. Then use a file to remove slivers of the tube until the fit is perfect.
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