Tolerances with buying dimensioned timber?

[paulc]

Hello,

I ordered my first pieces of dimensioned timber from 2 timber suppliers. (I've only ever bought rough / PAO from builders providers & rough from timber suppliers before)

Anyway, what was supposed to be 18mm thick is 17.6mm in one instance (walnut) and in others what should be 20mm is 19.5mm (oak & beech)

So 0.5mm out in thickness - Is this usual in your experience?

I find it really annoying, obviously 0.5mm over would be much better.

Thanks

 

[Jacob]

"Dimensioned" timber usually known as PAR (planed all round) is for convenience of those who can't do their own. It's usually sold e.g.as "ex 1"x6"" - i.e. the sawn size 1" x 6" as it comes out planed up, which will be variable - typically losing 1/4" smaller each way but may be more, rarely less.

 

[paulc]

Sorry, to be clear this was bought as 1000MM X 45MM X 18MM (the walnut) and 1200mm x 70mm x 20mm (the oak and beech).

 

[katellwood]

did you have them specifically machined, usually I would call this PSE (planed square edge) as opposed to PAR. as both pieces are within .1mm off then what machinery does the supplier have,

If their machines have seen better days or have been setup wrong then I doubt they would have dropped a digital vernier on the wood.

In the days prior to the availability of cheap digital verniers then you would have taken the timber and not noticed the difference and remember timber moves, it was probably stored outdoors but under cover at the timber yard. you have got it home taken it indoors and its moved.

 

[AndyT]

If I was buying it, I'd accept that as near enough. There may be a good reason for it - perhaps if the planer setting had been .5mm less the surface finish would have been poor. Or it might have been necessary to go up to the next thickness to get the full 18mm, which would have been expensive and wasteful.

When I have bought hardwood planed all round, what mattered most was that all the pieces were the same thickness, not whether they exactly corresponded to a written down number. I struggle to think of any ordinary furniture making where the exact numerical size would matter, unless you are trying to match up with something else already made or part made.

Everything gets a final clean up with a smoothing plane and/or abrasive paper any way, so the final dimension will never be what you start with.

 

[custard]

+/- 0.5mm is an acceptable tolerance, it's a tolerance that's widely used in woodworking generally.

If it's critical you could request "no less than 19.0mm", the yard may or may not accept the order but at least you know where you stand. Another option if it is critical is to hand select 25mm rough sawn boards and machine your own. A bigger problem with PAR is the degree of cupping, warping and crook that you get, if you're say a shopfitter getting a delivery on site and with no facilitates for machining, then PAR is pretty much all you've got, but for most people PAR is more trouble than it's worth.

 

[Jacob]

+/- 0.5mm is an acceptable tolerance, it's a tolerance that's widely used in woodworking generally.

If it's critical you could request "no less than 19.0mm", the yard may or may not accept the order but at least you know where you stand. Another option if it is critical is to hand select 25mm rough sawn boards and machine your own. A bigger problem with PAR is the degree of cupping, warping and crook that you get, if you're say a shopfitter getting a delivery on site and with no facilitates for machining, then PAR is pretty much all you've got, but for most people PAR is more trouble than it's worth.

Specifying "no less than 19.0mm" could mean paying for sawn 1 1/4" or more, depending on their stock.
You'd have the same problem if you were planing/thicknessing yourself from your own stock

 

[paulc]

You're quite right. I was in fact trying to match the walnut as trim to 18mm BB ply and thought, as these were small lengths specified to the mm, that they'd arrive a little over if anything. Next jump up in thickness was quite a bit and didn't want to buy dimensioned stock only to dimension it.

Having up until now brought all wood to final dimension by hand, 0.5mm off is still something to me, but I can see that with machining this may be outside of an incremental range.

 

[AndyT]

With luck, your 18mm ply will only be about 17.5mm ... it's worth checking!

 

[paulc]

Cheers Andy, will do!

 

[custard]

Ply is sold as "nominal", with a large tolerance. It catches a lot of people out as they may use an 18mm router bit to cut the housings for 18mm ply shelves, and then find it's a really sloppy fit.

The big problem is that ply will vary in thickness not only sheet to sheet, but also within an individual sheet! Consequently if you're applying lipping to ply the best method is to size the lipping to be wider by 1-2mm and then flush it off after the glue has dried. There are specialist tools for flushing lipping (like the Lamello Cantex), but if you're only doing a bit a block plane run along with it's heel on the main workpiece works great, or for slightly bigger quantities a router can be pressed into service.