Stability of timber

[PGC]

When building baseboards, what is the most stable (i.e. expands and contracts etc) timber to use? It's assumed that MDF is completely stable when, in reality, it's not, so is it more stable than ply or hardwood, or is one of those more stable than MDF, or is there something else more stable?

 

Thanks

 

Phil

 

[kandc_au]

Phil,

FWIW, the layout I am currently doing is pine frame and mdf on top. I am using mdf because that is what was put on the frame 10 years ago.

If I was building it today I would use ply as a top board.

I have not real complaints re the mdf...BUT I have painted it before doing anything to seal the stuff to help stop absorbing moisture.

I do the same when making buildings in card. Coat of paint seals the card or you could use shellac as in days of old.

 

Khris

[PatB]

No timber product is truly stable in the long term in the way that, say, steel or plastics can be. However, some do better than others.

 

As you note, MDF isn't as stable as is commonly believed. It's not bad dimensionally (unless it gets really wet) but it's not very stiiff and will tend to sag over time unless very thoroughly supported. I built a workbench a few years ago which used 18mm MDF on edge to support the top. In spite of making the beams a generous 200-250mm deep, over time they sagged quite a lot until I replaced them last year with ply. I'm no longer a fan of MDF. Because of the short fibres it's  made from it's not stiff enough to rely upon (IMHO) and it's staggeringly heavy.

 

Of the man-made boards, I favour plywood, preferably of decent quality. It doesn't have to be marine but a good, A-C (meaning one good face and one rather less good face) exterior ply to a recognised standard (BS if I was in Britain, AS/NZS for us Antipodeans) used sensibly is going to be very hard to beat for stiffness, lightness and stability over time. If you want to be really fancy, the webs of ply beams should  be cut from the sheet diagonally so that all the plies can play a structural role but that's probably over the top detail for baseboards.

 

Natural timber will only be acceptably stable after it's seasoned for several months. I haven't come across anything in a hardware store in 20 years which didn't exhibit ~5% shrinkage as it dried out. Not really acceptable for baseboards IMHO. OTOH, any timber which has been sitting around for a while and is still acceptably straight will probably remain so indefinitely as long as it's not subjected to to severe a change of temperature or humidity.

 

If I had to use natural timber for baseboard framing I'd look at L-girder construction which has the advantage of built-in adjustability so any movement in the materials can be allowed for. Only suitable for permanent layouts really but then, if I was building for portability I'd use ply anyway.

[tamperman36]

MDF and chipboard are both particle boards made up of pulped wood and a bonding glue which could be of a moisture resistant, fire resistant or standard type, all are normally stable however can suffer from swelling and movement in damp conditions like storage in a garage.

 

Plywood is made up of multiple layers of wood veneers 3 or more bonded together with glue, again of several different types like moisture resistant, fire resistant and standard. Ply is quite stable but can suffer problems in extreme confitions and damp, it can also suffer from delamination occasionaly.

Timber covers all types of wood both soft and hardwoods. Do not make the mistake of thinking that softwoods are soft snd hardwoods are hard as Balsa wood is a hardwood. The general definition is Softwood comes from an evergreen tree and Hardwood comes from a deciduess tree. There are some exceptions however.k

 

Softwood ie general sawn and PSEs (planed square edged) timbers sold in a timber/diy store can suffer from twisting and warping/bowing, it also quite often has knots in which can create weakness. It will suffer in extreme conditions.

 

Hardwoods such as Oak, Ash etc are generally once dried out (fresh cut is wet you can get kiln dried) is more stable however not absolutly rigid. Hardwood is expensive so makes it uneconomical for using to build a layout.

 

When deciding to buid a layout think about where you are going to keep it like in a garage or a central heated home and will you be making it mobile or static. Softwood can suffer more in central heated areas whereas standard ply can suffer in damp like garages.

Ply is these days often used for making moblie layouts as it is makes light weight boards which are generally stable.

[Ron Ron Ron]

Stability depends on many factors.

Humidity, exposure to moisture, temperature are the obvious ones; but not all ply is equal and neither is MDF.

For example, high quality furniture grade Birch Ply is far more "stable" than cheap Ply; also there are high grades of water and moisture resistant MDF which are more "stable" than cheap Ply.

(Note: Such high grade MDF will not be found at your local DIY, or at most timber merchants).

 

Solid wood, whether hardwood or softwood, is the least stable in terms of expansion, contraction, warping and twisting.

 

One of the most "stable" baseboard composition methods, that may be worth a try, is to combine high grade Ply framework with rigid extruded foam for surfaces.

The benefits include lightness, rigidity, resistance to expansion/contraction and warping and the ability to be easily sculptured. 

 

 

 

 

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[The Stationmaster]

The big problem with softwood is that it tends to be kiln dried and not very thoroughly so it will bend and twist, especially in the smaller cross sectional sizes.  If time allows just leave it in the place where you are going to use for a few months of mixed weather and it should stabilise to that atmosphere.  Ply is more stable to start with but can still suffer although teh better grades should be ok.

[Mike Storey]

In humid or fluctuating temperatures or both, for baseboard tops, always go for good quality, marine ply (or even better, birch faced ply if you have the funds) and seal all the exposed edges, either by heat (singeing until blackened) or copious amounts of preservative, as well as the main surfaces, with outdoor PVA or similar.

 

The supports (in wood) least liable to flex or warp are ply sandwiches (two strips of ply with a ply or softwood spacer at intervals in between. But well seasoned and sealed softwood will often be just as good provided you use enough cross braces and good joins. I have used both methods in lofts, a shed and now an unlined barn, with very few problems.

 

Just beware that many DIY places will tell you that their ply is not marine but is good for outdoor use. I have found that statement, despite sealing as above, to be often highly optimistic. You are much better off going to a proper timber yard, if there is one near you.

[Bernard Lamb]

Talking to a chap the other day who restores old cars.

He is currently working on the body of a Lagonda and is having to use American Ash as good quality British Ash cannot be obtained for love nor money. It is a museum quality restoration so cost is of fairly minor importance.

There is a chap on Orkney who makes chairs and the wood is kept to dry out for at least three years. The cost comes in around the £1k mark for a basic chair.

Another friend is building a canal boat using Cherry finished ply for the interior. Cost per sheet is rather eye watering.

It depends on how stable you require your timber to be. All the above examples will cost a large amount of cash but will give excellent long lasting performance. How long and for what purpose will a model railway baseboard be needed. I would go for a reasonable quality ply as the best compromise.

Bernard

[Pacific231G]

Homebuilt aircraft are often built out of ply so though it will be an even higher (and therefore more expensive) grade than marine ply it might be worth checking out. Lightness, strength and stability are all vitally important but those are useful attributes for layouts as well so structures are generally based on fairly thin ply made up into various types of box girder.  The Light Aircraft Association would be a good source for information about suppliers. 

[PGC]

Thanks for all the responses. The reason for my original question is that I need to make a sector plate, and I have been wondering what material to use. Obviously I don't want to use a material that will expand or contract (especially on it's length) as this may cause sticking of the plate and I need to to run smoothly.

 

From reading the responses, what I think I will do is make a sandwich of ply outers with softwood inners that will hopefully be fairly stable, and I will also use some very thin (5 thou) plasticard on the bottom (after sanding the bottom layer of ply very smooth!) to give a very smooth, slippery surface to make the plate slide freely.

 

Again, thanks for the responses.

 

Phil

[28XX]

For a sector plate I would consider using an aluminium box section frame clad with ply. For the surface it slides on consider melamine faced ply or chipboard, ideally from a 40 year old wardrobe, far better than new from Homebase (other purveyors of overpriced tat are available).