Tripod dreaming

[AstralTraveller (David)]

I'm writing to see whether people think this plan is feasible and how it can be improved.

I need a tripod to hold a Losmandy G-11 / Celestron C-11 combo. Eventually these will be on a pier in an obs so I don't want to shell out for a genuine Losmandy. Perhaps the tripod will be re-purposed, or retired or just used for the occasional trip to the country. I have a 'tame' fitter machinist / welder who would make me one but it would be more satisfying to DIY. That means no welding and no lathes etc. I have a drill press, a mitring drop saw and a good selection of hand tools. So I think I'm better off to build it out of timber.

I haven't got decent drawings yet, just pencil sketches and cals, so I'll try to describe what I'm planning and see how loud people laugh. I'm modelling it to an extent from the Losmandy in that I have an upper pier section and a lower tripod. The pier section will be a triangular prism orientated vertically with the legs attached to the faces of the prism.

The prism will have a triangular top and bottom plate with rectangular sections for the sides (I can get the chamfered edges cut on a table saw) and be glued and screwed together with beading at all joins. I'm thinking of 18-20mm thick ply end plates and perhaps 12-15mm for the faces. The faces will be 250-260mm wide and it stands 500-600mm high (I'll explain why soon). The bolt that holds the Losmandy pier adapter will actually be a threaded rod that runs the length of the pier section, so it is under compression - not tension.

The faces will have mounting holes with 'T' nuts on he inside. By having several sets of holes along the face it will be possible to attach the legs at different spots, thus allowing height adjustment (though not after set-up). I want this partly because I'm trying to work out the optimum height for the pier in the obs and partly because it makes it easier to re-purpose the tripod.

The legs will be two pieces of timber joined to have a 'T' profile thus giving them strength in the vertical and horizontal plains. The top end will have the mounting plate and the bottom end leveling adjusters. The legs will cant out at about 30-35 degrees from the vertical. If the vertical section of the legs is about 100mm deep the join to the mounting plate will be about 180-200mm long. As this join is under tension it will be re-enforced with gussets and beading. I might even need a brace from the base of the pier section to the lower section of the leg. I'm thinking that the vertical section of the leg might have to be solid timber rather than ply. The base of the legs will be about 500-600mm from the central axis of the mount, with one leg pointing south. The counterweight shaft protrudes 380mm from the central axis so I think this is a big enough footprint. For user convenience I'd like it smaller but I also think 'safety first'.

So how does that sound so far? Not strong enough? Overkill? Weak points?

I expect to have lots of supplementary questions later but the first one is: PVA wood glue or liquid nails?

[ZeroID (Brent)]

PVA and spire screws, those hyper sharp Japanese self drilling things. I pilot drill first then drive them in with a power driver.
Liquid nails never goes really hard, it's ok for wallboard and similar but for real wood construction use the right glue for the job.

[JA]

Hello David,

Interesting project. If it were me I would make the triangular prism column from 25 stacked triangular pieces of say 20 (19)mm ply glued and screwed together for your 500mm prism length. Yes the prism would be heavier -calculated at probably around 9kg Vs 3kg for your fabricated version. When has mass not been a good thing in mounts? (Ok- heavier to carry!) You can add the internal T-nuts you want (good idea) by cutting a say 75-100mm hole for internal access (with hole saw) in the centre of every 4th triangle for around a 80mm vertical hole spacing, or finer spacing as required.

I'm not over-joyed by the timber legs (t-section or not). I'm not sure i'd bother with the t-section legs, just use 190 or 140mm x 45 KDHW (if using timber VS steel) and with say an 8-10mm thick aluminium plate screwed at end to act as a mounting plate (with holes) to the centre column. It would be much easier to make (and be stronger). For added lateral stability you could use a triangular or 3 spoke stretcher type base to join the legs.

As for adhesive, PVA is good for timber of course (esp. furniture), but get the external type by Selleys. You can also use Araldite which is excellent and allows you to form nice large "fillets" where necessary, outside the joint surfaces to effectively increase the bonding area.

Gee... I think I now should make one

Best
JA

[Wavytone]

JA the problem with that idea is the stacked plywood is mechanically very weak compared to using thinner ply running vertically up/down the sides of a triangular prism.

I built a very solid fixed wooden tripod before which took the form of an egyptian pyramid, where I used two jarrah floorboards butted edge to edge along each edge of the pyramid. Planed them by hand to produce the desired bevel, then glued with marine epoxy and inconel (bronze) wood screws, and varnished. If I had to make another I would do the same again.

As solid as a tree stump, it carried an 8"f/7 newtonian on a german mount. You can see it in one of the photos of the ATM section of S&T of June 1981.

[AstralTraveller (David)]

Thanks for the comments. Looks like I use spiral screws and outdoor PVA, or Araldite if my cutting isn't too neat . I'm assuming plywood is the best material for the pier but I don't know about grades of plywood. I've got lots of ex-packing crate stuff that I scavenged but I don't know if it good enough. I also think I'll go with 19mm side panels.

The timber legs are gone, replaced by RHS bolted onto each face of the pier. Instead of trying to get the legs to protrude square to the face they are now parallel to it. The makes construction much easier and stronger. I'm still drawing it up but I expect the legs will now be at 45* to the vertical (or horizontal if you prefer ) and I'm thinking of 50x25x3 steel. At that angle the mounting bolts can be about 250mm apart. I'l actually use 3-4 mounting bolts on each leg so as to maximise the grip between the legs and the pier. That way the load isn't wholly born by the T nuts. I'm even thinking of putting a layer of thin rubber between them to maximise grip. I've got a couple of ideas for the feet, all of them pretty simple.

Apart from the end caps, the triangular pier will have two internal brace plates, held apart with lengths of threaded rod. [During construction they will also act as formers.] The end plates will be made from two pieces glued together - one internal triangle and a larger external triangle.

That's where planning is now. Once again, any comments ......

[Wavytone]

If you're going with RHS for the legs might as well dispense with the wood altogether, use RHS for that.

Borrow a welder and learn to weld on some scraps then tackle the real job - it isn't hard. If you know someone who can show you a case of beer should settle it.

If its to be portable even easier - you'll need a drill press, taps for threading holes and some nuts & bolts.

[JA]

Originally Posted by Wavytone

If you're going with RHS for the legs might as well dispense with the wood altogether, use RHS for that.

Borrow a welder and learn to weld on some scraps then tackle the real job - it isn't hard. If you know someone who can show you a case of beer should settle it.

If its to be portable even easier - you'll need a drill press, taps for threading holes and some nuts & bolts.

+1 .... as they say

Best
JA