Ply Prices.

I maybe preaching to the converted here, but anyway, it's worth being aware. For many years I bought 18mm ply in 5x5's, so long grain or cross grain didn't apply. Now that I don't use such large quantities, I buy 8x4's instead, which because of it's WBP content is, and has always been, more expensive per sq. ft. than Russian 5x5's. I currently pay £36.00 per board for 8x4's. A cross grain ( 4x8 ) is £6.00 per board cheaper. This week I asked why the difference, because obviously, the wood content is the same. The answer is :- larger mills produce 4x8's which are the most popular by far, and therefor, cheaper.
I bought the cross grain ply, because nothing I am currently making is more than 4' tall. So if you are buying 10 boards you will save sixty pounds on the job. Out supplier is Timbmet, who have recently joined forces with Silvermans. They are located in Basildon.

I'm feeling I should know the answer to this but what practical difference does the grain make? I thought they were alternated in plywood anyway and if you're not seeing the bare wood does it matter?

Chris, acoustically, there is no difference. Long or cross grain has only ever been an issue with regard to how you cut on the saw, and how you want to finnish your edges. e.g. If you are making a scoop bin where the size is more than 4' high, you would want to rip down the long grain when cutting the sides. This becomes more of an issue if your saw blade is a little blunt and it starts to make a mess when cutting across the grain. We always optimized the way the grain should be. This is more important if on occasions, as I did, made some cabs with just a clear finnish. Our cutting sheets always contained the long grain size first followed by the cross grain dimension.
Regarding cost of ply, I posted this to remind people to ask there supplier for 4x8 instead of 8x4 and to compare prices. Hopefully some money can be saved, without compromising the build.

I've never had any problems getting clean cuts either way, but that might be to do with the saw I use. Most of my cabs are cut up in the workshop of a mate who runs a flightcase business. Their bench saw has a very fine tooth blade for cutting laminate cleanly. They also get good prices on ply from Lathams which is quite useful!

I do my cutsheets just taking into account whichever way is easier!

If it doesn't matter which way the grain is, you should certainly go for cross grain and save some money.

I shall have to ask my mate in the flightcase shop which one he buys, if he knows. Cheers for the info.

I'm feeling I should know the answer to this but what practical difference does the grain make? I thought they were alternated in plywood anyway and if you're not seeing the bare wood does it matter?

Panels in a cabinet are stiffer when the longest unsupported dimensions are along side the grain. The more so when the thickness of the panel decreases.

The strength of the panel is ussually also given along side the grain and on a 90 degree angle towards it (perpendicular IIRC). The latter is lower. There are some different figures floating on the web, but it gives an idea, like these (higher is better):

For 18 mm:

Eb = 3400 N/mm^2 perpendicular to grain
Eb = 4600 N/mm^2 parallel to grain

Eb shows how stiff a material is when bended. Higher means stiffer and thus less flexing.


Wkr Johan

I think the obvious point is being missed here. I'm only joining in again because it always amazes me how different people see things in different ways.
The point about using a board of cross grain ply is not to swap the way you would normally use it, so once more, (but only to clarify what I was banging on about.) If, for instance, you have a side panel that is less than 4' high you cab, then buy a cross grain board (4x8) and cut across the board instead of ripping down the length. When stood upright you will have you long grain facing the right way up, and happy in the knowledge that satisfactory results were obtained from a board that has cost at £6.00 cheaper than an 8X4 board.

Here's a question. Have you ever been able to make wood noticably stronger using coatings eg paint, varnish, plastic etc.

I wondered about a thin layer of fibreglass, pressed hard onto both sides of the wood, so the epoxy resin goes into the wood a bit. That ought to reinforce for compression and tension at the surfaces where the forces are highest. You might be better off even if the wood is nasty B&Q wood (or "cat's p*ss" wood as I call it due to the unique smell of the glue they used to use).

There are some types of flightcase material available which uses a thin fibreglass skin on the plywood. Makes it very resilient to impact and abrasion. Often used for very large cases for mixing desks etc.