Is there any kind of "standard" for variation of plywood thickness?

[Wayne Jolly]

Hi all,

I've started building a router table using plywood that I bought at Home Despot and Lowes. One sheet was a 3/4" maple ply from Lowes, and the other was a 3/4" birch ply from the despot. My design calls for about a dozen dadoes for different parts so I used my digital calipers to measure the thickness of the birch plywood (.724"). I then set up my dado in my table saw and made a test cut. This fit my sample piece of birch ply, and the maple ply perfectly, so I cut the remainder of the dadoes.

Well wouldn't you know. Apparently the birch ply sample I tested was the thinnest piece on the entire sheet because not a single piece of the cabinet parts fit into the dadoes. Not one, and not even close. I got my calipers out again and as it turns out, my test cut would not even fit the other end of my birch sample. It was off by .020". I measured some of the other pieces and the thickness varied by as much as .030" thicker than my sample. One piece even measured just over a real 3/4" at .755".

I have cut quite a few dadoes in plywood before, and I have had to tweak some of them, but nothing like this. So now I have a dozen dadoes that I need to tailor a few thou for each piece. I started by taking a piece of 1/2" mdf and gluing a piece of 120 sandpaper to it. That would work but it might be Xmas next year before I finsihed them all. I think I will go to Lowes and get some 60 or 80 grit to see if that would be faster. I did try one of those drywall sanding sheets. They are pretty aggressive, but they really don't last very long. I don't have any other hand tools that would be appropriate for the job.

I have a dado jig that I built for my old Porter-Cable 690 router, but I gave that router to my son. I would have to run some tests to see it it will still work properly with my Milwaukee 5616, Bosch 1617, or Bosch 1613.

Such is wood. Cut and learn.

X

[Jeff Duncan]

Short answer….no:o

Plywood is made up of thick veneers of wood. As such even if it were the same thickness end to end when it was made, it will not stay exactly the same over time. Add to that the difference between different materials from different factories and forget it. What can help is cutting the edges off sheets before starting and using sheets in order from the same lift when possible. But even then you'll still have a bit of play that you have to live with.

Or when possible go with a veneered mdf or pb core which will stay flatter and more consistent in thickness IMHO;)

good luck,
JeffD

[Bob Lang]

This is the state of plywood in the 21st century. It is now a commodity product that has been "value engineered" into being close to unusable. A few years ago, I conducted tests for Woodworking Magazine, where I cut a bunch of sheets of plywood into small pieces, then measured the thickness of each piece. I found significant variations within pieces that were less than one square foot. The "rules" allow for this, and the rules are written by a trade organization of plywood manufacturers. These problems are not the nature of stacking layers of veneer, or metric sizes, they are the result of poor quality control on the part of the manufacturers.

Forget about the "odd-sized" router bits that claim to work, the variations within most sheets mean sloppy somewhere and too-tight somewhere else. The solution I see is to make a consistent, smaller dado (say 5/8" or 1/2" for 3/4" plywood), then mill a shallow rabbet that will leave a consistent thickness to fit.

Bob Lang

[Curt Harms]

Where possible and practical, I use a router jig. It's essentially 2 'jaws' held together with slotted pieces perpendicular to the jaws. The 'jaws' are at least 1/2" thick. The magic is using a top bearing or pattern router bit 1/2" X 1/2". You could also use a top bearing mortise cleanout bit. Clamp one jaw on one side of the dado, put a piece of shelf cutoff against that jaw then move the other jaw against the scrap and secure it in place. Remove the cutoff, set the router bit depth so the bearing rides on the 'jaws' and route changing depth as required. The dado width is determined by the material being fitted into the dado. To address the thickness variation mentioned by Bob, you could use the actual piece to set the width. Another way to address the variable thickness is to not buy Borg plywood:). I wanted flat sawn oak ply for a project so bought from 'real' supplier. Precisely 3/4" thick, sheets 49" X 97" to allow for saw kerfs, no voids, and it didn't warp or twist.

One limitation with this jig is this only works on dados slightly wider than the router bit. My dados are mostly cut for 3/4"+/- material so that hasn't been an issue. The wood whisperer did something similar using a short bushing & a straight bit instead of a pattern bit, that'd work to cut narrower than 1/2" dados.

[Peter Quinn]

The plywood is all over the place in thickness, end to end, middle to edge. I've seen more than .030" variation in thickness in a single sheet from a reputable manufacturer. The solution practiced at work is the same as Bob suggests above, make the dados 5/8", make the plywood edges fit the dados by trimming one edge essentially creating a shallow rabbit, but I would add do so in a manner that leaves a consistent thickness to this tongue. We use either an over arm router or a tall fence on the table saw or router table, so the arrangement is fence---plywood---blade. If you don't get the stock captive between table or fence and cutter it results in using the wrong reference face and merely mimicking the dimensional inaccuracies. You must cut the opposing face of your reference face. Or you could run a split tenon head on a shaper or stacked dado with spacer, but the lose one face as a positive distance marker if that matters to you.

[Wayne Jolly]

I have used a bit of plywood in the past, and the tolerance for thickness has never really been an issue. Certainly nothing like the variation I got in this last sheet. I've cut sheets into the pieces for a project, cut a test dado, and all of the pieces fit into their proper places without much tweaking at all. Maybe one piece might need a little gentle persuasion to get into its dado (tapped in, not sledge hammered), or an edge sanded a little bit more.

But what I am essentially hearing and seeing now is that the quality of almost all wood products is declining. American made products are getting more and more difficult to find, and when you do find them one of two things will result. One, the quality will be superior, but so will the cost by a large margin. Or two, The quality will be crap just like all the foreign junk and the price will still be on the high side. Plus, the manufacturers are saying "If we make it they will have to buy it because they don't have any say in the matter, and it wouldn't matter if they did."

Jeff - mdf or pb? Eeesh. I hate 'em.

Bob and Peter - I hear ya. I watch The Woodsmith Shop on TV. They call that tongue-and-dado joinery, and they use it a lot. One thing that gets me is that it seems to me they always insert the tongue on top and the rabbet on the bottom. I think I would put the rabbet on top and the tongue on the bottom. I'm just sayin'.

I MAY employ that technique in the future, but for now I think I'm kinda screwed. It would be difficult to cut a shallow rabbet that might be .005" deep on one end and .015" deep on the other. I realize that .010" over 26" is probably mouse nuts, but if someone were a little too anal about it . . . (and I ain't sayin' just who that might be. ;) ). I will also tune up my router jig to work with one of my current routers too.

Curt - attached is a picture of my dado jig for my router. But as I mentioned, it was made for a PC 690 router that I no longer have so I will have to look into fitting it to one of my existing routers. It won't work for dadoes longer than 26" though. I don't use many that are longer than that, but I have.

Thanks for all the replies,

X

[Charles Coolidge]

Agree with Peter the Lowes/Home Depot junk isn't even a consistent thickness within a single sheet. I have 1/2 a sheet of Russian/European birch ply in the shop, its much higher quality than Lowes/Home Depot but I measured it just now and even it varies in thickness .020.

[Jeff Duncan]

Jeff - mdf or pb? Eeesh. I hate 'em.
X

That's only b/c your not trying to make money. If you were in business and trying to feed your family you'd love them like the rest of us:D

In reality these are the quality products of the day, consistent thickness and modern hardware is designed for use in them. Plywood….well that's what you use the sheathe your house:) Pro shops are almost completely opposite of hobby shops though. If I get a job calling for AWI premium quality I could not use Plywood for the casework, mdf or pb only. Ok I do still use plywood fairly often as well, but there are many applications where it just isn't flat enough to work. Of course I don't use dados either. CNC drill and dowel screws for me….time is money;)

good luck,
JeffD

[Wayne Jolly]

Here we go again. Dowels - I hate 'em. :D

X

[John Coloccia]

Yep, plywood is what it is. I clamp a straight edge to one side, lay a piece of plywood against it, clamp a straight edge to the other side, and router with a bearing bit. No use knocking your head against the wall dealing with inconsistent product.

The MDF I get is pretty good, though! Not perfect, but pretty reasonable consistent.

[J.R. Rutter]

"Plywood standard" is an oxymoron. Best to plan on the undersized groove/dado and trim the sheets to fit. I spec'd my custom cab door cutter inserts for a 6mm groove (0.236") so that I could get by with 1/4" mdf core sheets. For some species, we do have to skim the panel edges with a back cutter on the shaper, but most just slip fit with varying degrees of tightness. I routinely get suppliers trying to pass off 5.5 mm sheets (apparently, plywood manufacturers caught on to the trick of using a 5.5 mm groover and just decided to save some pennies by manufacturing to that new target) as 1/4".

[Bill Neely]

I'm lucky, I live about an hour from Eugene and buy States Industries (makers of Appleply) plywood at a supplier in Springfield. Nice stuff, I can get shop grade maple for ~45 a panel that is really nice cabinet stock, flat and has a uniform thickness.

**I don't think there is an association that applies standards for hardwood plywoods. The APA is concerned with construction grade ply and issues stamps to compliant manufacturers. The Hardwood Plywood Veneer Association sets voluntary standards but that's just hogwash.

[Richard Wagner]

You hate too much. Relax.

[Jamie Buxton]

I use biscuits to position the parts, and glue to hold the parts together. One of the nice features is that variations in the plywood thickness don't matter. It is also faster to cut biscuit slots than router out dados. I do use a cabinet back to strengthen against wracking. I've never had a joint like this come apart.

[Chris Friesen]

Assuming you want to use plywood, one simple option is to cut the dado/groove thinner than the ply thickness and rabbet the ply to fit on the router table or tablesaw. The key is that you need to run the plywood between the reference surface (fence or tool table top) and the cutting edge, with something to press the ply against the reference surface. That way the remaining "tongue" will be even thickness, while the depth of the rabbet on the ply will vary with plywood thickness.

[Peter Quinn]

Wayne, for me the rabbit is always on the blind side, and most of what I build has an obvious blind side. For lowers in kitchens or built ins for example, we use a lot of pre finished ply, bottom shelf rabbit faces down, mid shelf or stretcher rabbit faces up (yes...we occasionally have to use. Solid 3/4" "dust panel" as the mid stretcher). Top stretcher has no rabbit, the plywood edge gets matched to the plywood as well as possible. In drawer banks, take your pick, I usually go rabbits down and keep it consistent.

Jeff, is your PB core material melamine or veneer faced? Most custom shops near me stick to plywood for top quality work unless required. Flatness aside, many customers don't want to see plastic insides of custom cabs. Of course others demand it for " easy washability".....just what are they doing to their cabinets? Every region seems to have their own idiosyncrasies regarding what is high end custom or not. I'd rather lift a sheet of plywood than PB

[Wayne Jolly]

Wayne, for me the rabbit is always on the blind side, and most of what I build has an obvious blind side.

Actually, up or down for the rabbets for this project wouldn't be an issue since all of the edges are going to be covered with a Maple face frame anyway. For the immediate project it just comes down to two choices. One - rebuild my router jig to fit my current routers (which I need to do anyway), or two - build a tall fence for my table saw (HEY! That might be a good use for some of that MDF junk. :D ) and shave a rabbet on the pieces.

Well, today is going to be a good day to do some woodworking. Well . . . maybe not good for woodworking but still the time I like to do it the most. Temps in the mid 60's, light winds, . . . and raining. I like to open the garage door and work when it's raining. It could be a bit warmer but those days are probably in the near future too. And I just KNOW that the wood will really LOVE all that extra "moisturizer".

X

[glenn bradley]

Even if there was a standard, the irregularities over a given length of plywood would negate it. I would not use plywood for a tool top. Well supported MDF will serve you much better IMHO.

[John TenEyck]

Actually, there are specs., or at least I've seen them for some of Columbia Forest Products plywood products, I just can't seem to find them at the moment. But even the best plywood is not as flat or consistent in thickness as MDF due to whatever combination of factors. I recently bought some A1 cherry veneered Columbia MPX plywood, at about $140 + tax at retail. It's pretty nice, but it's not as nice as the same veneer on MDF. I choose it because of weight and structural considerations. Otherwise, it would have been veneered MDF, as others have said.

[Jeff Duncan]

Jeff, is your PB core material melamine or veneer faced? Most custom shops near me stick to plywood for top quality work unless required. Flatness aside, many customers don't want to see plastic insides of custom cabs. Of course others demand it for " easy washability".....just what are they doing to their cabinets? Every region seems to have their own idiosyncrasies regarding what is high end custom or not. I'd rather lift a sheet of plywood than PB

Short answer is….it depends. As I said for some commercial cabinetry where "premium" is spec'd, (like what I'm bidding now), plywood is not an option. For residential kitchens I still use plywood that's usually either Columbia or Norbond. Melamine is pretty much standard fare even on the so called "high end" kitchens….Poggenpohl and the like. I think your right about small shops using a lot of ply, however if you walk into most large architectural millwork shops plywood is much more scarce.

Anyway most of the stuff I do is residential and as I mentioned it's still pre-finished North American sourced veneer cores for kitchens and some vanities. For custom closets it's basically all melamine with pvc banding. Other jobs it depends and I choose the best material for the application. Veneered mdf is great for door panels and drawer bottoms.

I think the thing is to go with what your comfortable using. There's no real right or wrong here, though there are a lot of misconceptions about sheet materials in general. I've had plenty of them when I first started out as well and probably still have a few lingering ones. As you grow and learn in this business though you get educated to what actually matters when making a quality cabinet. Also having the right equipment to use different materials. Melamine for instance works best with confirmat fasteners which may be difficult for small shops to employ? FWIW I've seen plenty of 40 year old melamine kitchens built with 5/8" material that are still going strong! It's usually the drawer slides and hinges that fail from use as opposed to the material;)

So my advice is to use what works for you. And as far as lifting goes PB may be a bit heavier than ply….but it's still better than mdf! That stuff is miserable to lift and even more miserable to machine. No matter how good your dust collection is it gets everywhere:(

good luck,
JeffD

[Wayne Jolly]

I would not use plywood for a tool top. Well supported MDF will serve you much better IMHO.

The project I am working on is a router table, and truth be told I did use two layers of 3/4" mdf covered with high pressure laminate for the top. I don't like mdf, but I will use it if it is better for the job at hand.

I am also a little bit disappointed that today didn't turn out to be such a nice day for woodworking after all. Temp isn't going to get much past the mid 50's, but the biggest reason is that the wind is up bringing in a storm. Rats.


X

[Rod Sheridan]

I am also a little bit disappointed that today didn't turn out to be such a nice day for woodworking after all. Temp isn't going to get much past the mid 50's, but the biggest reason is that the wind is up bringing in a storm. Rats.


X

LOL...........It's -15 today, going down to -22 tonight/tomorrow............Maybe you can send me some of your weather...........Rod.