Birch Ply alternative and thickness for dob base

[Ratlet]

Afternoon all.

I'm wanting to build a nice looking base for my 10" solid tube dobsonian.  The plan is to rebuild with plywood and go for a swish looking varnished finish.

Most of the guides online recommend birch plywood however the prices for it are currently astronomical (£200 per sheet for the 18mm) does anyone have any recommendations for sheet material that would be a suitable alternative at a more reasonable price whilst still giving a good looking finish once varnished.

Scope will be kept in an unheated shed, out of the elements most of the time.

[Peter Drew]

My 30" Dob base is made of MDF which has a good native finish to it.  Well painted, it has survived for at least 25 years.     🙂

[Astrobits]

The cheaper the sheet the more gaps you will get in the internal plies. Sometimes they might be filled with any filler that was handy.

You can always fill any holes that show up in your cut pieces with a suitable filler.

Nigel

Edited March 17, 2023 by Astrobits

[apaulo]

Do not be fooled by any low priced marine grade plywood  its rubbish.  You  might have to settle for a decent sheet of exterior ply, and cut it up more or less as soon as you get it, before it warps. 

[CraigT82]

MDF should be fine as long as well sealed from any moisture ingress (including just absorbing moisture from the air in a damp shed/garage), can even get veneered MDF so it will have a nice hardwood finish on the outside which is more expensive but should still be cheaper than top grade plywood. 
 

 

[markse68]

3 hours ago, Ratlet said:

Afternoon all.

I'm wanting to build a nice looking base for my 10" solid tube dobsonian.  The plan is to rebuild with plywood and go for a swish looking varnished finish.

Most of the guides online recommend birch plywood however the prices for it are currently astronomical (£200 per sheet for the 18mm) does anyone have any recommendations for sheet material that would be a suitable alternative at a more reasonable price whilst still giving a good looking finish once varnished.

Scope will be kept in an unheated shed, out of the elements most of the time.

18mm is pretty thick for supporting something that weighs what- 10kg? You could use a thinner ply skin reinforced with timber or laminate layers of ply where necessary

Mark

[Ratlet]

19 hours ago, Peter Drew said:

My 30" Dob base is made of MDF which has a good native finish to it.  Well painted, it has survived for at least 25 years.     🙂

Well, you can't say that and not share a picture you big tease!

It might sound weird, but my old man had a lot of plywood furniture in the house he made during his city and guilds back in the day so there is a serious nostalgia push towards the plywood.  Although I can be convinced of other options.

[Ratlet]

I've spoken with a local timber yard who I've used previously and normally have very good prices.  I'll see what they can do.  If they can't do the birch then I'm pretty certain they'll be able to recommend something suitable.

Cloudynights seems to agree (as much as they ever do) with @markse68 that suitably reinforced 1/2" will be sufficient for the 10" which weights about 15kg.

[Piero]

The rockerbox of my 12" f6 has a thickness of 3/4”. It works but I think it is better now after decreasing the centre of gravity.

In my 16" f4 the boards have these thicknesses: bottom: 1", sides: 30mm, front/back: 15mm. I used dowel joints for the RB.

Personally, I'd go for 3/4” for a 10" and then cut off wood without affecting sturdiness.

Edited March 18, 2023 by Piero

[Piero]

My 12" was designed by Lukehurst. The original mirror and rocker boxes were quite tall and in my opinion the rocker was not particularly sturdy  although it worked. 

I updated it quite a lot last year and now it's much sturdier, lighter, more portable, but most importantly more predictable in terms of performance particularly due to the new mirror cell.

Here's a summary of the changes.

[Mr H in Yorkshire]

If you must have birch ply, although more expensive per unit area, you don't need a whole sheet for a base. Alternatively you even get melamine faced MDF, or as has been proven successful, kitchen worktop offcuts. Just pay careful attention to sealing all cuts and for one sided finishes, glue on a balancer sheet.  Another possibility is to buy a thinner sheet and glue a number of layers together, to get them flat put spare pieces on top, and park your car wheel on the pile. It's been done.

Edited March 18, 2023 by Mr H in Yorkshire

[Ratlet]

I'll see what the timber yard come back with regarding price. 

Lot to consider.  I like the designs with cutouts using the thicker wood.  Also seen some nice use of veneers for a more decorative finish.

[callump]

Personally, i'd just use whatever exterior grade ply you can get and not stress too much about it.

A good coat of varnish will seal against the elements.

All depends on whether you want to look through it, or at it...

/callump

[Mr H in Yorkshire]

Do you have much woodworking experience? I ask because for a good circular finish to a base, assuming you want it circular, you'll want a plunging router - £100 or so but much better than a jig saw. Freehand will be awful and even attached to a radius piece, because the blade tends to flex out of the vertical and create a wonky edge, it will not be good. For a project I'm working on (will post some stuff soon) I just did that - cut some 18mm ply with a jig saw, rubbish warped edge, recut with a router, perfect finish but now smaller diameter than originally planned. Then routed the inner edge of an aluminium bike wheel rim coated with a thin layer of high performance wood filler. The router easily dealt with all these materials, even though my whole workshop is now coated in a layer of white dust and microscopic aluminium shards and that's despite attaching a garage vacuum to the router. As they say, it's a dirty job...

[Ratlet]

@Mr H in Yorkshire I'm going to borrow the father in law's router,which I believe is a decent model.  He's very much a buy once, cry once kind of person.  I've a bit of wood working experience, mostly joinery so I'm planning on taking my time with this, measure twice and cut once and all that.

@Piero those are absolutely beautiful.

@callump going the be honest and say both.  If I'm going to go to the effort of building a better base mechanically I want to go to the effort of making aesthetically pleasing.  It'll be a bit limited by the fact it's going to support a black solid tube, but I'm hoping to get something at the end that looks the part. 

[Piero]

If it helps, here is the album for the design and construction of my 16":

https://photos.app.goo.gl/DAZ8CqeRtJGe83jF8

That was my first wood / metal work.

[skybadger]

21 hours ago, callump said:

Personally, i'd just use whatever exterior grade ply you can get and not stress too much about it.

A good coat of varnish will seal against the elements.

All depends on whether you want to look through it, or at it...

/callump

I use a 15% or so thinned varnish as the primer varnish coat - penetrates more and seals all the edges before you start putting normal thickness coats on it. 

I have a few scopes treated this way, including parts built from kitchen worktops. 

[PeterStudz]

I made a base for my 8” Dob about 2 years ago. I would have loved to have used birch plywood but even then (wood has really increased in price) I just could not justify it. One of the main points of building a Dob base is that you can make something decent at a reasonable price. If you need to buy everything from screws and bolts up (I didn’t) it’s surprising how the costs can mount up.

I ended up making mine from cheap 18mm plywood from my local Wickes. At the time it was £18 per sheet & I needed two. I even had some left over to make most of my EQ platform. It was more than adequate for the job. I made mine “organically” without a plan and it’s a little over “engineered” and maybe a tad heavy but it’s as solid as a rock and extremely steady. One important thing - whatever material you decide to  use make sure that you inspect it carefully and make sure it’s dead STRAIGHT. Most of the cheaper wood, even some of the expensive stuff will be warped, even if ever so slightly. I went through all the 18mm sheets at Wickes (yes, I was that guy) and hand picked sheets that were dead straight. Even a slight warp in your ground board will cause it to have annoying “sticky” points.

I painted mine but you could easily make one with a decent plain varnished finished with the plywood that I used . You just need to take time and extra care cutting and finishing off. Which you’d need to do with more expensive wood anyway. The edges on cheap plywood will have occasionally gaps which will need filling. I started off filling mine with filler but quickly got bored with the necessary sanding. In the end I used the old model aircraft technique of sealing the edges with tissue paper and dope/sanding sealer. A surprisingly easy, cheap and efficient method. If you want something fancy for a varnished finish you could seal the edges with iron on veneer furniture edging. Although I’d glue it on - far quicker, easier and secure. In fact you could veneer the whole base (not difficult) which would give you something different, interesting and hopefully nice. A bit like my dew shield. 

'

[CraigT82]

6 minutes ago, PeterStudz said:

I made a base for my 8” Dob about 2 years ago. I would have loved to have used birch plywood but even then (wood has really increased in price) I just could not justify it. One of the main points of building a Dob base is that you can make something decent at a reasonable price. If you need to buy everything from screws and bolts up (I didn’t) it’s surprising how the costs can mount up.

I ended up making mine from cheap 18mm plywood from my local Wickes. At the time it was £18 per sheet & I needed two. I even had some left over to make most of my EQ platform. It was more than adequate for the job. I made mine “organically” without a plan and it’s a little over “engineered” and maybe a tad heavy but it’s as solid as a rock and extremely steady. One important thing - whatever material you decide to  use make sure that you inspect it carefully and make sure it’s dead STRAIGHT. Most of the cheaper wood, even some of the expensive stuff will be warped, even if ever so slightly. I went through all the 18mm sheets at Wickes (yes, I was that guy) and hand picked sheets that were dead straight. Even a slight warp in your ground board will cause it to have annoying “sticky” points.

I painted mine but you could easily make one with a decent plain varnished finished with the plywood that I used . You just need to take time and extra care cutting and finishing off. Which you’d need to do with more expensive wood anyway. The edges on cheap plywood will have occasionally gaps which will need filling. I started off filling mine with filler but quickly got bored with the necessary sanding. In the end I used the old model aircraft technique of sealing the edges with tissue paper and dope/sanding sealer. A surprisingly easy, cheap and efficient method. If you want something fancy for a varnished finish you could seal the edges with iron on veneer furniture edging. Although I’d glue it on - far quicker, easier and secure. In fact you could veneer the whole base (not difficult) which would give you something different, interesting and hopefully nice. A bit like my dew shield. 

'

Very nice work that, rally like it. Did you make the stool too? Also…. Have you got enough cider!? 

[PeterStudz]

1 minute ago, CraigT82 said:

Very nice work that, rally like it. Did you make the stool too? Also…. Have you got enough cider!? 

Thanks! The stool was a £10 from Gumtree. It’s an old Ikea screw stool. Sadly no longer made.

The telescope has your old primary mirror. Well I’m sure that I bought it from you!  The OTA was given to me free (without a base) as the primary was damaged. 

I never have enough cider  As you know I make my own but cider varies and it’s difficult to get anything decent near me.

[CraigT82]

1 hour ago, PeterStudz said:

Thanks! The stool was a £10 from Gumtree. It’s an old Ikea screw stool. Sadly no longer made.

The telescope has your old primary mirror. Well I’m sure that I bought it from you!  The OTA was given to me free (without a base) as the primary was damaged. 

I never have enough cider  As you know I make my own but cider varies and it’s difficult to get anything decent near me.

Oh yes that’s right! Well I’m super pleased to see my old mirror living its new best life in your great looking dob😊

[Mr H in Yorkshire]

2 hours ago, skybadger said:

I use a 15% or so thinned varnish

Some years ago I did a DIY floor sanding job and bought some lacquer that you mixed with sawdust to seal the gaps. Been using the lacquer ever since as a first coat - it has very low viscosity and is very fast drying being thinners based. Very useful properties.

[Ratlet]

3 hours ago, PeterStudz said:

I made a base for my 8” Dob about 2 years ago. I would have loved to have used birch plywood but even then (wood has really increased in price) I just could not justify it. One of the main points of building a Dob base is that you can make something decent at a reasonable price. If you need to buy everything from screws and bolts up (I didn’t) it’s surprising how the costs can mount up.

I ended up making mine from cheap 18mm plywood from my local Wickes. At the time it was £18 per sheet & I needed two. I even had some left over to make most of my EQ platform. It was more than adequate for the job. I made mine “organically” without a plan and it’s a little over “engineered” and maybe a tad heavy but it’s as solid as a rock and extremely steady. One important thing - whatever material you decide to  use make sure that you inspect it carefully and make sure it’s dead STRAIGHT. Most of the cheaper wood, even some of the expensive stuff will be warped, even if ever so slightly. I went through all the 18mm sheets at Wickes (yes, I was that guy) and hand picked sheets that were dead straight. Even a slight warp in your ground board will cause it to have annoying “sticky” points.

I painted mine but you could easily make one with a decent plain varnished finished with the plywood that I used . You just need to take time and extra care cutting and finishing off. Which you’d need to do with more expensive wood anyway. The edges on cheap plywood will have occasionally gaps which will need filling. I started off filling mine with filler but quickly got bored with the necessary sanding. In the end I used the old model aircraft technique of sealing the edges with tissue paper and dope/sanding sealer. A surprisingly easy, cheap and efficient method. If you want something fancy for a varnished finish you could seal the edges with iron on veneer furniture edging. Although I’d glue it on - far quicker, easier and secure. In fact you could veneer the whole base (not difficult) which would give you something different, interesting and hopefully nice. A bit like my dew shield. 

'

That looks really good, even before the paint.   Sort of the look I'm thinking of.  Only with a bit of walnut on the sides.  Thinking of drilling a star pattern on it like yours through the walnut to show the lighter wood underneath.

Edited March 19, 2023 by Ratlet

[Bentley]

Hi Everyone,

There is a product called Colorfin that was developed for concrete forms ( Colorfin (Koskisen - KoskiDecor). It is made of Finnish birch plywood, both sides are coated with a colored translucent phenolic film. Only the edges need to be finished, the T-edging would work nice. In the US it's available from Roberts Plywood, I don't know if it can be had in the UK.

 

Bentley

[PeterStudz]

17 hours ago, CraigT82 said:

Oh yes that’s right! Well I’m super pleased to see my old mirror living its new best life in your great looking dob😊

It has provided, amongst other things, some stunning views of Jupiter, Saturn and Mars. And my daughter is especially fond of the planets. Picture from back in 2021, at around 4am at the end of a 2 hour session on Jupiter and Saturn. The first time Alice had seen these. Base obviously not finished.

@Ratlet, the crescent moon has a function. I use it as a handle to carry the base on its own. I also put in a round cabinet light. Useful when setting and packing up. 

@Bentley, I haven’t heard of that product, although it might come under a different name here in the UK. One of the issues that I’ve had when trying to order this kind of thing is that often the suppliers here assume that you are some kind of builder working on a house/building and consequently have minimum orders of silly amounts. And if you can order a small quantity equally silly minimum shipping charges.