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What wood to use for a Dob mount?


Uplooker
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Hi All,

I am after peoples suggestions and opinions on what wood to use for building the Dob mount for my 300PDS. Shane (Moonshane) uses a variety of thicknesses from 12 -18mm, but AFAIK it is all Birch plywood.

Now i appreciate that cost is not everything, but I am trying to fathom the advantages or not of using Marine, External or Birch plywood - all 18mm.

A few notes before i continue:- BB =good surface finish used for joinery, CC= not so good finish used for construction i.e. shuttering, WG= well glued - any knots are removed and filled / well glued

BB/BB is most expensive, BB/CC is less expensive, CC/CC cheapest.

Birch Ply.

  1. Multiple layers / laminates - between 7 & 9
  2. Generally available as BB/WG for £50 - 55 per 2240 x 1220 sheet
  3. Good overall surface finish, although one side is better than the other
  4. Soft woods used for the laminate with birch (hardwood) on the external layers
  5. Not many "defects" giving it greater longevity

External or Eastern Ply

  1. Not as many layers / laminates - between 4 & 5
  2. Generally available as BB/WG for £30 per 2240 x 1220 sheet
  3. Good overall surface finish
  4. Hard woods used for all the laminates
  5. Fewer "defects" giving it greater longevity again

Marine Ply

  1. Not as many layers / laminates - between 4 & 5
  2. Generally available as BB/WG for £50 - 60 per 2240 x 1220 sheet
  3. Good overall surface finish
  4. Hard woods used for all the laminate
  5. Fewer "defects" giving it greater longevity again

Now my main question is which to go for and why. I do not need anything to take major structural loads. They all have pretty good surface finishes, provided i go for BB/CC, and it does not take too long to sand the poorer surface.  I do not really need the expense of marine ply as it will not be subjected to huge amounts of moisture and i plan to paint it when constructed.

I am thinking that I can use External / Eastern Ply. Not because it is the least expensive, but because it satisfies my requirements at a price i do not need to go any higher on.

So to all the ATMs out there your advice and guidance please.

Ian

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Greetings

Fareastern ply would be my first choice because the finish is first rate when a good quality varnish is used as the finishing coating, use atleast 4 coats for a rich deep colour.

Andy

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Shane made the dob mount for my 12" F/5.3 from 18mm Baltic ply which then had several layers of protective stain applied. Its been in use for over 12 months now and there is not a mark on it and it's the most stable and smooth mount I've used. 400x plus is no problem for it and a rap on the tube damps out in just a second or two. The mount weighs around 10kg and the scope 16kg so the whole lot weighs around the same as a Skywatcher 10" dob. It wasn't the cheapest option but I think the cost difference was about the same as a BST Explorer eyepiece and that sort of difference evaporates when you are at the eyepiece getting exciting high power views of course :smiley:  

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I used exterior plywood from B+Q, 18mm for everything. My first mount used cheaper, thinner plywood, and well after a couple of times being left out it fell apart. It also warped quite badly when not in use. My new base hasn't warped at all since July. Looks really nice when it is varnished.

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I would look at marine ply, there is a lot of moisture at grass level and it is produced with aspects like this in mind.

The others should be fine if treated right and sealed, but I would still look at marine.

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I made my 30" Dob from MDF :eek: , still OK after 29 years, some parts need bracing to avoid warping. This is not a recommendation, the whole thing had to be built quickly and to a very tight budget in time for the return of Halley's Comet in 1985.  :smiley:

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Birch ply contains no softwoods it's birch right through. This was what they also called aircraft plywood because of it's superior strength.

For me it's a no brainer, use this.

You nay be able to buy ply wood with a birch veneer and softwood innards but this is not birch ply.

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Marine ply would be my first choice!, But if you are looking to save money then go for Birch ply!,

If you want a DURABLE weather proofer for what ever wood material you  use then use Danish oil,

It will out last any varnish or any other paint product on the market, :grin: .  

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  • 3 weeks later...

Hi Ian,

In Australia we have plywood called plasply, or formboard.

You will probably have the same suff in the UK with different brand names.

It is the ply used for concrete formwork.

Very water resistant, does not need painting except to seal the cut edges.

It,s not cheap but as they say, you gets what you pays for.

Have not used it for a scope mount but used it to make a trailer in 1982 that

has never been in a garage since then and the wood is as good as new.

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I'd go with external ply.  Solid birch ply sounds like nice stuff but I don't think you need the extra strength for the 300 OTA.  Perhaps if you were trying to get the weight down by using a thinner ply the solid birch plywood would be important, though I don't think mounts should be too light.  Top quality plywood is probably more pretty too though.  I made an oversized base for my 250 with BnQ 18mm ply and there was never an issue with strength.  Going into the store an picking the sheets I wanted was a benefit but whatever you use is going to last longer than a skywatcher spongeboard base.  My feeling is that what you do with it when it gets home is what determines how good your base is.

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Thanks for all the replies. I am still undecided as to what to go for. I need to make a decision soon or I will never get round to it. I have nearly finalised the dimensions, almost sorted the laminate, am just waiting on the PTFE turning up and I have borrowed my father-in-laws router in prep for getting stuck in :grin:

Ian

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for a whole scope you might save £20-40 buying cheaper materials. for me there's no comparison. I'll only ever use birch/baltic ply.  that said, my 16" base is still rock solid and straight after three/four years - it's painted which hides a lot of the defects you'll get in other materials.

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I used an old sideboard (£16 on ebay) , it was 35mm vaneered fibreboard(like an early chipboard but better made) when made, I filled the rough edge with filler then exterior under and top coats then matt black final coat, its had two years use and soon to be outside permanently, no sign of ingress (and the base is a home made lazy susan with 180 ball bearings. the back was made from solid beech ( the doors) and the feet cut from 35mm board and fully sealed. The other doors made the tube support. There may be prettier and lighter options but as a mount it is super smooth and super solid.

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Ian

Solid birch ply all the way for me, please don't use marine ply especially made from mahogany as its killing the rain forests!

Birch is strong and sustainable and what the three scope's I'm building are going to be made from in 12 and 18mm.

Oh and if your using power tools leave the whiskey bottle inside ha ha

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Id definately use an 18mm exterior graded ply wood in birch or some other hardwood.

Marine ply is possibly an overkill but then if you only intend on making the mount once then I cant see there being an issue.

Dont skimp and go for 12mm as there will be too much flex  and dont get cheap interir ply as the damp will kill the bond between the layers and cause delamination..... and if this happen in the base boards then you will ruin any smooth running you have.

Cheers

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Well everyone,

Thank you for your posts and suggestions. I have decided that I am going with 18mm Birch Ply BB. I just need to finalise my dimensions for the mount so that i can get the yard to cut the board into more manageable sections - not least so that i can fit it in my car. i hope to start just after Easter.

Ian

Edited by Uplooker
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  • 1 month later...

Hi All,

I am sorry to revive this Zombie. Well I have finally decide on the majority dimensions for my Dob base. I have placed the order for a sheet and provided the dimensions for relevant cuts. This is primarily down to having spoken to and gained knowledge from a very experienced Dob mount builder (resides on SGL).

i know people have asked for me to provide some dialogue and photos of my build, I will try, but I am not that good at it. The reports that people give on here are outstanding. It will not be a quick process - due to other demands on my time. It is definitely my intention to have everything finish for the start of the dark nights :grin: I have assembled 95% of the components that I need , just a couple to go.

I will keep you updated.

Ian

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good luck Ian

once you start it's not as scary as you think it might be. just take your time, as you said, measure twice (or several times) and then cut. think ahead and about what the implications of the dimensions you think are right will mean for eyepiece height, balance point and stability. in the end it's a compromise so don't aim for perfection, just 'rightness'.

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