Wood, nominal sizes, imperial vs metric, what's going on?

[Reggie]

Hi all, I'm starting to look at purchasing wood, just ply atm. for learning certain processes with, although I do expect to buy the real stuff in the nearish future.

I understand the nominal width of wood being to do with expansion/contraction due to atmospheric conditions, storage, finish etc. I'm ok with that (I've got to be, can't change the nature of wood), however, my only source of wood is one of the diy superstores and it's left me feeling quite confused. Everything is marked up in mm but seems to be fairly random sizes, for instance, 18mm thick ply I presume is supposed to be the metric equivalent of 3/4" but it's clearly not, it's at least, 1mm shy of 3/4" before we get any 'nominal' stuff happening due to poor storage. Other thicknesses they seem to like are 9mm or 6mm.

I think what I'm trying to ask is, how do you mitigate for the inconsistencies and how much does it actually matter? For a lot of cuts with a saw then you can do your measurements/marking off the the wood that you've got but if you're working with say a router and you have a 3/4" dado to cut, you're going to cut a 3/4" dado, so how do you mitigate against sloppy shelves etc.?

Lastly, when purchasing wood, what are the sorts of questions I need to ask about the state of the wood, storage etc., what answers should I expect to get?

 

[Jacob]

..... 18mm thick ply I presume is supposed to be the metric equivalent of 3/4"

No it's supposed to be 18mm. You might find 3/4"ply somewhere but I doubt it

at least, 1mm shy of 3/4"

so it should be, 3/4" is 19.05mm. Nominal sizes don't apply to sheet. It should be what it says on the label.

... how do you mitigate for the inconsistencies and how much does it actually matter?

You work to the known size, or reduce to a chosen size. if not things won't fit

For a lot of cuts with a saw then you can do your measurements/marking off the the wood that you've got but if you're working with say a router and you have a 3/4" dado to cut, you're going to cut a 3/4" dado, so how do you mitigate against sloppy shelves etc.?

Use an 18mm cutter if you want to fit 18mm shelves

 

[Random Orbital Bob]

In the process of converting from imperial to metric, the industry has rather let the gold standards slip. What was once 3/4" is now a range of sizes depending on the manufacturer. For routing dado's that give a proper "snug" fit you will need both 18 and 19mm cutters unless you plan to be nudging the fence a mm every time you rout for multiple passes.

Luckily, the router cutter manufacturers are used to nominal sized sheet goods being "undersize" ie 18mm (This is doubtless about profit ie less wood in an 18mm board versus true 3/4") so you can get 18mm cutters.

It reminds me of the early 70's when decimalisation happened and suddenly stuff that used to cost about 2 or three shillings less than a pound miraculously became a pound overnight!!

 

[Reggie]

Hi Jacob and Bob, thanks, I think's that's what got confusing, I've seen 18mm listed as 3/4" equivalent, I don't mind getting 18mm bits either, if that's what it takes for a snug fit, but I had no idea that the nominal sizes don't apply to sheet materials, I take it we're talking any composite materials such as ply, oriented, mdf?

 

[Jacob]

Nominal sizes usually are associated with sawn timber, which may be sawn and dried intended to end up at at say 5" x 1" but may be smaller or, more rarely, larger, but still sold as 5x1. But if it is planed it's called 5"x1" nominal, or ex 5x1 but will be a lot smaller (about 6mm each way, typically)

 

[Random Orbital Bob]

.....which as Jacob says, started out life as 5x1 rough sawn...but when planed true end up quite a bit smaller. You'll see that timber referred to as either PAR (planed all round) or PSE (planed square edge).

So you buy a bunch of 4x2 and get it home only to find its actually ex 4x2 which is around 31/2 x 21/2...which can be very annoying. To make life simple I always have a tape on me when at a saw mill or timber yard. Different yards also have machine fences set at different tolerances etc...the variation is huge as to what constitutes a piece of wood that's marketed as 3/4"

 

[The Bear]

If you want snug dados in 18 mm ply, I wouldn't bother with 18 and 19 mm cutters. Use a smaller cutter and an exact width dado jig. Much better job.

Lots of examples on you tube and on this site


Mark

 

[nanscombe]

Here is the Woodwhisper's version of an Exact width Dado jig for some inspiration.

 

[Reggie]

Thanks for the links, so do they actually do 3/4" plywood, or is it all 18mm for around that thickness?

Thanks for the idea about taking a tape :-D I had taken one with me for but it doesn't help if the sawn wood you're measuring isn't what's really on the label, now I know that they will all invariably be out, it's a bit easier to cope with.

 

[ColeyS1]

I'd just ignore feet and inches as it makes things so complicated. I only heard a tale of someone who wanted a shelf 5 foot plus 9 mm lol. A pair of electronic caliper thingys are useful for checking

Sent from my GT-I9300 using Tapatalk 4 Beta

 

[bugbear]

I understand the nominal width of wood being to do with expansion/contraction due to atmospheric conditions, storage, finish etc.

Well, that's part of it.

Here's a reasonably accurate and amusing article on the subject, sadly it's American, so it's in archaic units.

http://www.straightdope.com/columns/rea ... our-inches

BugBear

 

[Random Orbital Bob]

So I was right....profit was the motive....well there's a surprise!

 

[Jacob]

So I was right....profit was the motive....well there's a surprise!

Not really it's more about the practicalities of supplying an innately unstable material which may subject to a variety of conditions and also treatments.
One odd side effect is that your own self-planed 2x4 is likely to be fatter than a bought piece of PAR 2x4. This isn't a fiddle, in both cases sawn 2x4 is used but you are going to cut to length and select the straightest for the longest lengths. Which means you will get less waste than the timber yards - who will be bunging whole long lengths through.
Another effect is that you very rarely find whole inch sizes in old woodwork - most of it started out as sawn whole inches but is reduced by 1/4" throughout.

 

[Reggie]

It seems like it's six of one and 1/2 a dozen of the other, I can agree with nominal sizes, that's fine, however, it seems that there is more than one factor to the madness that seems to be wood sizes, profit in certain circumstances (spending less time setting up tooling, supplying less wood) the only thing I reason I can think of for keeping such a poor numbering system is to allow timber merchants to identify people that don't know what they're talking about :-D

In certain circumstances, I can see that size isn't necessarily so important, if it's hidden and has the strength to do the job that you want for instance, however, with things like furniture, if you're building to spec, the only way around it that I can see is to start bigger and plane/sand etc. the wood to the correct sizes. so for instance, if I want something to fit in a 3/4" dado, I'm better of looking for an XX by 1, or if I want to fit something to a 1" dado it'd need to start off at XX by 2 and potentially shave 1/2 an inch.

That's what I really needed to know, thanks guys, it's not so confusing any more and having a couple of strategies to deal with it will be very helpful.

 

[Jacob]

There isn't real a problem once you realise that timber supply is very variable in so many ways.

if you're building to spec, the only way around it that I can see is to start bigger and plane/sand etc. the wood to the correct sizes. so for instance, if I want something to fit in a 3/4" dado, I'm better of looking for an XX by 1, or if I want to fit something to a 1" dado it'd need to start off at XX by 2 and potentially shave 1/2 an inch.

That's it!
But there are strategies - frinstance if you wanted 2no 4" wide timbers you might buy one 9" and split it down the middle, rather than 2no 5" sawn.
You can of course modify the design to fit the available timber. That's the beauty of the "rod" - you can design with it at the beginning and then modify it as the work goes along.

 

[johnwc812]

Hi All
About a year ago I visited my local timber yard and asked for a length of 50mm x 25mm sawn softwood.
The "getting on a bit" in the brown coat said "Oh you mean two by one".
3 days later I went back for another length, being a quick learner I asked for a length of two by one.
The very same brown coat said "Oh you mean 50mm x 25mm"
So much for the customer is always right.
Cheers John

 

[Random Orbital Bob]

next time ask him for 4 candles

 

[longinthetooth]

Actually - it's 25 x 50.

 

[Eric The Viking]

Is this the 5 minute argument, or did you pay for the full half hour?

Of course, a lot depends on how sharp your plane blade is, and getting it sh......... [no carrier]

 

[tomatwark]

What gets me is that the sheet size is imperial and the thickness is metric when buying timber based sheet materials in this country.

EXCEPT

Chipboard flooring which changed in the mid 90's to completely metric.

I found this out half way through laying a big floor which I needed the flooring dropped of in a couple of lots due to storage.

The first lot arrived and I set the joists out for 2440 x 610 sheets, the second lot arrived and it was 2400 x 600, it turned out the first lot was the last imperial stuff they had. :roll:

We had to add extra bits of joists to make the floor work,

Tom