Q: Variance in Plywood thickness

[Mike Dembroge]

For those of you that use a lot of plywood in your cabinets, casework, etc, what do you find as an acceptable amount of thickness variance in your sheet goods?

My last 3 projects all used plywood of various species (A-1 Mahogany, A-2 Maple, C-2 Maple), and all was purchased from the same source: PALS in Oakland, CA. However, I found with each project that the thickness of the plywood varied not only between sheets, but within the same sheet as well. On my last project, I measured .009" difference in about 25" of length in a sheet. While this doesn't sound like a lot, it was enough to create a sloppy dado at one end and a too-tight-to-fit one on the other.

Am I being too fussy, or is this normal? I spend a fair amount of time setting up the stacked dado set to produce a clean-fitting dado that is not too sloppy and not so tight that it distorts the material. So, when I make several cuts and some don't fit, it's a PITA.

Mike Alameda, CA

[Upscale]

No variance is acceptable as far as I'm concerned, but it's a fact of life and just one that you're going to have to adapt to. We've got it worse up here in Canada since changing to the metric system several years ago. How does 19.5 mm grab you? 17 mm even? Another 17 mm sheet of plywood that is not the same thickness as the first?

I think it's all a conspiracy by the router bit manufacturers to sell more bits.

[boorite]

Or a conspiracy by the RTA hardware manufacturers to make us all switch to butt joints and cross-dowels or cam-locks or whatever. :-/

[B A R R Y]

Two suggestions:

1.) Don't "fully house" the plywood. For example, cut the dados 1/4" or 3/8" wide and top and bottom rabbet the shelf ends to match. The double rabbet can be quickly and easily cut using a "tongue" bit in a router.

2.) Try a simple router jig that uses an piece of the actual material to set the dado width. Leon occasionally posts a drawing to the version he uses.

[Swingman]

IME, it has always been that way and you need to learn to live/deal with sheet good variances, just as you do with measuring inconsistencies.

When I cut dadoes, it is usually a bunch and I batch cut, so I rarely use anything but a dado stack in the TS ... routered dadoes are just too damn time consuming for batch cutting dadoes in 30+ cabinets at a time.

So, with that in mind:

One of my solutions is to buy plywood (an entire project's worth from the same stack if possible) that closely fits the "project" dado size.

To do this, I keep a number of marked dado "story sticks" in my truck. These are simply cut off a reject/extra dadoed rail or stile from past projects and are of a known, repeatable dimension.

down the edges of a sheet of plywood, they give an excellent indication of what I can expect to deal with from that sheet. (I've been known to do this to every sheet in a pickup load before it went on the truck).

I also use a Freud dial-a-width dado stack, and I ALWAYS keep the CLEAN chippers stored in the same order when not in use so that they go on the TS in the same order everytime.

That way the only thing I have to change when fine tuning a dado for a particular batch of sheet goods is the number of clicks and the direction ... (the +7 in the photo above is the number of "clicks" I need to "increase" a 3/4" set of chippers to repeat that particular width).

I also shoot for tight over loose. A quick hit with 80 grit on both sides of that 1/4" edge of plywood is usually enough to "force fit" a piece that is a bit too thick without having to setup a piece of machinery to deal with it.

Nothing is perfect, but the sum of the above can save a lot of time and can be done quickly, and without much thought/effort necessary.

[redbelly]

Mike, I have two comments:

1. Are you sure the slots were cut to the same width? Might be worth checking them with a pair of calipers, for consistancy in cutting width.
2. If the less than 0.009" gap results in an acceptable amount of slop, could you insert a thin shim to fill in the gap? It wouldn't need to fill in along the entire length, just a 1/2" wide piece every 6 or 12 inches or so.

Regards,

Mark

[Wayne K]

I just finished drawing up some plans for some file cabs made with "3/4" plywood. Trying to draw to these dimensions is impossible. I built in a little extra clearance to compensate for the missing wood. Frustrating. It's started me thinking why? Why can't they make 3/4" plywood 3/4"? Because they can get two veneer layers where they used to get one? Because it is measured in millimeters when everything else is is imperial? (who came up with that brilliant scheme.) Why can't they anticipate the thickness before sanding. Slight variance within the sheet is acceptable and understandable. But dealing with these minute dimensions in the design stage is very frustrating. Rant mode off.

[CW]

Why?

[Wayne K]

I guess I should qualify that statement. Most of my drawings are really glorified sketches to 1/4"=1" scale on 81/2 x 11 graph paper. I find it very tedious to work to the 64th of an inch. Although I could if need be. My drawing board is not in my shop (no room for it). I prefer to draw in my shop, more insperational. Why cant 3/4 " plywood actually measure 3/4"?

[CW]

Why is a 2x4 not a 2x4? Less wood for you, more for them. The more for them, the more money they make.

[Lew Hodgett]

Wayne K wrote: > I guess I should qualify that statement. Most of my drawings are really > glorified sketches to 1/4"=1" scale on 81/2 x 11 graph paper.

Way back when, engineering drawings were done with vellum and pencils along with the trusted pink pearl eraser.

Back then, most drawings carried a few notes, some general, some specific.

An example of a specific note: NTS (Not to Scale)

An example of a general note: H&FTF (Hammer and file to fit)

Either one handles odd ball plywood, IMHO.

BTW, NTS applies to most of my free hand sketches which covers at least 90% of my sketches.

Lew

[Mike Dembroge]

Thanks for the info Swingman. I use a Forrest stacked dado set for dados. I don't like using a router. That's just me though.

The one that got me was the piece of plywood whose thickness varied in the same sheet. I can understand it when they vary between stacks and/or manufacturers, but within the same sheet? Oh well, the consensus is that it's just standard operating procedure.

[Mike Dembroge]

The slots were cut on the TS using a Forrest stacked dado set. The problem was with the plywood thicknesses.

The problem with using shims is that they look pretty sloppy. One project was a Mahogany desk for a yacht, so it really had to be right.

[Mike Dembroge]

Wayne, I design everything in AutoCAD as well, but I just draw everything as

3/4"stock and haven't had a problem. Can you elaborate on the specifics of the problems you've had? I'm curious.

[Wayne K]

I really don't have problems, just irritations, My dad always said, "if it fits on paper scaled down, it should fit in real life." He was a well respected tool & die maker. He also designed, or modified many of the machines in the plant where he worked using his drafting and machining skills. So when I am doing a scale drawing I really want it to measure out when adding up the dimensions. Lest I feel the smack on the back of me head. When you use autocad, I take it you have to compensate for your overall height and width, even though it may be less than 1/32"? I realize the difference may be negligible, and we are working with a medium that is difficult to keep exact dimensions, but it is still irritating to me.

[John]

I have been in the veneer business for a long time. Remember, we are dealing with living material, subject to expansion and contraction with temperature and environmental changes. When rotary cutting at 1/42 veneer, it will range off the log at 1/41 to 1/43. That's a result of the wood being wet, then run thru the veneer dryer. Now take that error and multiply it by 9 or 11 or whatever crossbands in a sheet of plywood.

If you have to have dead flat XX dimension, then you can order calibrated plywood. Specify your thickness and your supplier "SHOULD" run it thru a multihead sander and bring the entire panel down to .73 or whatever thickness you desire. This way, you will ALWAYS have perfect plywood, at least as far as thickness goes.

Naturally, you will pay more for calibrated core

John

[CW]

Consistently small instead of small with variations. OK.

[Wayne K]

I understand slight variances. But that does not explain why the thickness of the veneers cannot be compensated to end up with a finished product of stated dimension.

[Patriarch]

"Wayne K" wrote in news:EdidnVYcg8XuBGLZnZ2dnUVZ [e-mail]:

You seem to expect an awful lot from a manufacturing process so highly automated...

That's real wood in those layers, unless you're using MDF. Then it used to be real wood.

Patriarch

[CW]