Plywood Thickness Tolerance

[Noob99]

Hi All,

Non wood savvy DIYer seeking advice.

I've recently had delivered some CNC cut plywood parts. They're supposed to be cut from 25mm hardwood plywood but the thickness of the ply varies between 23.6mm and 24mm with the vast majority of the measurements that I took coming in at 23.8x mm.
Is this typical for '25mm plywood' or have I been supplied with 24mm plywood?

Thanks

Regards
Chris

 

[Doug B]

Certainly most of the plywood I’ve used the last few years has been undersized, the 6mm I use quite a lot is generally 5.5mm or less so it wouldn’t surprise me to find 25mm ply was thinner by the tolerances you’ve found.

 

[niall Y]

I seem to remember the thicker ply could be out by + or - 1mm. I first queried this with a colleague many years ago when I was building some kitchen cabinets. I had made some 100mm squares, out of the off-cuts, to reinforce where the legs were screwed on. There were two evenly numbered stacks on my bench, and I noticed that they were different heights. After this experience, I never took the stated thickness of board materials as an exact figure

 

[Skeety]

Recently bought 12 sheets of "12mm" and that came out mostly at 11.2mm. Few sheets of 18mm that came out at 17.1mm.

Seems to be the norm these days, i seem to recall hearing that a lot of manufacturers were doing this to save cost.

The 17.1mm ply nearly burnt me on a project as i was laminating 4 layers of it to make a workbench. Came to a critical point of assembly and notices one of my dimensions was 3.5mm out... Managed to rescue it though

There does seem to be quite a lot of variance out there.

 

[HOJ]

I'm inclined to suggest you have 24mm Ply, Myers are the supplier I use, they do 24 & 25 :- Meyer not to say they will be though!

 

[joncooper27]

When I used to work a lot with 18mm and 25mm ply a pack at a time to machine fine limit jigs and machine parts all critical contact points were machined to an exact size and the an aluminium gauge to check. You could get plus or minus 2mm mostly minus thou but some were plus

 

[Sachakins]

Not unusual for ply to be under thickness, I've had 18 as thin as 16.7

 

[Jake]

To be fair it has to be somewhat of a variable process depending on density of the veneers given the pressures involved. And then people will take the water from there to shave some pennies.

'Our 18mm is 18mm when it goes into the press'

 

[ChaiLatte]

I have to ask: if the thickness is critical to the part's function, why do you not obtain a specification sheet from supplier in advance of purchase? Then you can assess whether the manufacturer's published tolerance is acceptable for your design.

 

[Cabinetman]

And of course it may be nearly 26 on another part of the same sheet.
It got me wondering. It’s made from sheets of real wood which will expand and contract in its thickness like all wood. T he sheets of wood are cut from revolving logs and I can’t believe they are dry at that point, and I expect it’s easier to cut when wet. So the manufacturer probably has to make it overthick expecting it to shrink back to a hopefully correct size. No doubt they are cutting back to save money also.
Ian