Joinery for plywood

http://es.scribd.com/doc/27357383/Plywood-Users-Guide-3797
User’s Guide to Plywood
For furniture makers, sheet goods offerstrength, stability, and good looks
BY ROLAND JOHNSON
Furniture makers are most likely to use plywood made with a hardwood faceveneer. Domestic hardwoods are the most readily available as face veneers,although exotic species may be special ordered.
VENEER-CORE PLYWOOD
stays flat, holds a screw well, and is reasonablylightweight. This material is ideal for building cabinet cases. Panels, partitions,and cabinet floors can be cut to size quickly and assembled into strong,light boxes. Veneer-core plywood also can be used to make cabinet tops andtabletops, but they’ll need solid-wood edging and a durable finish to protect thethin face layer.
COMBINATION-CORE PLYWOOD
has an MDF layer between the inner plies andthe hardwood face veneer, which eliminates voids and ensures that grain fromunderneath won’t cause imperfections in the top layer. Combination core can beused anywhere veneer core is used. It’s not much heavier than veneer core, withsimilar screw-holding ability and less tearout when sawing.
s much as I enjoy building furnitureout of solid wood, there are times when it is not the best choice of material. When I make built-in cabinets or
large tabletops, I turn to plywood. Its sand-
wich structure of thin veneer layers, withthe grain oriented at right angles, makesplywood flat and stable. When faced witha high-grade hardwood veneer, plywoodlooks good and saves time and money.
Plywood also is structurally stronger than
natural wood and has excellent screw-
holding capabilities. It is also ideal for doorpanels, frame panels, drawer bottoms, and
cabinet backs. As a shop resource, ply- wood makes strong and stable jigs andcolor variations. Grades for face veneerbegin with AA for the best quality and rundown to grade E, which can include un-limited color variations and patches. The
back veneer is number graded, with grade
1 being the best and grade 4 allowingknotholes, splits, and other defects. You also should select plywood with acore that’s as uniform and free of voidsas possible. In the HPVA grading system, J signifies the best core material with no voids in the plies. Cores step down inquality to grade M.This system occasionally changes, andother types of plywoods, such as Balticbirch, or ApplePly, use their own grading
systems. When in doubt, ask your distribu-
tor how the product is graded.
When I order plywood, I prefer to let thelumber dealer determine the correct grade
fixtures that are inexpensive to build andeasy to modify.
How plywood is graded
The best way to buy plywood is to selectsheets individually from a dealer with agood inventory. If that is not possible, ithelps to know what to ask for.If you’re ordering plywood sight unseenfrom a distributor, you’ll want to specify several things, including the quality of the veneer on the sheet’s face and back (theface is generally better looking) and thecomposition and quality of the plywood’sinner core.The Hardwood Plywood and Veneer Association (HPVA; www.hpva.org) setsstandards for grading hardwood-veneerplywood based on how free the surfacesare of defects such as knots, patches, and
Options for hardwood-veneer plywood
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Photos, except where noted: Steve Scott; this page, facing page: Michael Pekovich

VENEER CORECOMBINATION CORE
A
Numerouslayers of birch,alder, or both yieldexceptional stability,strength, and density. Balticbirch is especially suited forbuilding drawers. Check sheet sizesbefore ordering; several manufacturersproduce only 5x5 sheets. Similar productsare known as ApplePly, Europly, Russian, orPolish birch.
BALTIC BIRCH
Core consists of alternating layers of hardwood such as poplarHardwoodface veneerLayer of MDF belowface veneer






for my application. I tell him what I need,such as cherry veneer-core plywood withplain-sliced veneer, both sides good. Mostlikely the product will be A1, VC (veneercore) Cherry PS (plainsawn), but that may be simply how my sheet-goods distributorhas it set up in the inventory system.
Tips on cutting big sheets safely and without tearout
Plywood sheets are awkward and heavy.It’s always a good idea to work with ahelper while making your first cuts in afull sheet. If help is not available, set upsawhorses with height extensions, mov-able workbenches, or whatever it takesto support the sheet both before and afterthe cut.The worst kickback I have experiencedoccurred when I tried to rip an edge off a half sheet of plywood without adequatesupport. The tablesaw sent the half sheetand me flying to the shop floor with damage to both me and the plywood. The floor survived intact.For ripping or crosscutting plywood cleanly, use a triple-chip blade with a high tooth count (80 teeth on a 10-in. blade isnot uncommon). Be aware of the rotation of the blade and the good veneer face of the plywood. When cutting on a tablesaw, the good veneer should face up; with
a circular saw, the good veneer shouldface down.For especially delicate face veneers thatare prone to tearout when crosscutting, Isometimes scribe the face veneer with a sharp utility knife before cutting. This pro-cedure is very fussy, and absolute accuracy is a must, but it does work. A handheld circular saw will quickly reduce full sheets of plywood into moremanageable sizes. If possible, cut thepieces about 1⁄ 8 in. to 1⁄4 in. oversize and
Bending plywood can conform to atight radius without splitting becausethe grain in all of the veneer layersruns in the same direction. Thismaterial is indispensable for makingcurved doors and panels. Oncelaminated or veneered (a vacuumbag is best), it will hold the intendedshape. Thicknesses range from 1⁄8 in.to 3⁄8 in., and species include poplar,birch, okume, and lauan. It comes in4x8 sheets with grain running eitherthe length or width of the sheet.
MDO combines a veneer core witha top layer of kraft paper impregnatedwith waterproof glue. This provides a flat,smooth surface favored by outdoor signmakers; it is absolutely waterproof. MDO isgreat for furniture panels that will be paintedor that don’t require the look of wood grain. Itis ideal for painted outdoor furniture.
MEDIUM-DENSITY OVERLAY (MDO)
Elegance in sheet goods.
The back and door pan- els of this cherry cabinet were made from
1⁄2 in.-thickveneer-core plywood with a book-matched face.
Veneer can be taken off the log in several different ways, and each method yields itsown distinctive look. The most common types of face veneer are rotary cut; plain, or flatsliced; and rift cut. There are also several methods for matching the veneer on a panelface: book matching, slip matching, and random matching.
CHOOSE FACE VENEERS BY CUT AND MATCH
BOOK MATCHING is accomplished by turning over every other sheet of veneer for a facethat resembles the opened pages of a book, with opposite patterns identical.
SLIP MATCHING uses progressive veneer sheets joined side by side, with the same sidesfacing up. Book matching accentuates the grain, while slip matching tends to appearuniform, more like solid wood.
RANDOM MATCHING is as it sounds. Sheets are randomly assembled, with the chanceof veneer from several logs on one face. This method can lend a very real laminated look,but it also can lead to multiple color and grain patterns in one face.

JOINERY FOR PLYWOOD
Cutting accurate dadoes
Plywood rarely measures up to its stated thickness, sostandard dado-head widths can yield sloppy joints. Fora snug dado, start by gauging the thickness of the shelfor partition. Then install enough chippers to get closeto this mark, and use shims to fine-tune the width ofthe dado. Another option is to cut dadoes with a router,using a straight bit sized specifically for plywood. Anumber of manufacturers offer undersize bits to accom-modate common plywood widths.BISCUITDADORABBET
Drawings: Bob La Pointe
MAY/JUNE 2005
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Plywood cases can be assembled using a variety of joinery techniques, including rabbets,dadoes, and biscuits. A rabbet joint’s structure makes a stronger corner than a simple butt joint. Dadoes provide great strength in edge-to-face joinery. Biscuits work well for edge-to-edge or edge-to-face joints. They aren’t as strong as dadoes or rabbets, but they’re useful forkeeping adjoining surfaces properly aligned.
Shim for a perfect fit.
Because plywood isoften narrower than its stated size, shims areneeded to create a dado of the correct width.





FINE WOODWORKING
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Hiding unattractive edges
SOLID EDGING
Biscuits align solid edging.
Usecard stock as a shim when slot- ting the plywood(above), but not onthe edging. Milled slightly thicker,the edging will stand proud of thetop. Plane eachedge flush before gluing (right).
Use biscuitsfor alignment.
Miteredcorners
IRON-ON VENEER
Iron-on veneer edge-banding.
The ma- terial may be bonded with the use of ahousehold iron. Trim the edges flush witha handplane or specialty trimmer.
There are a few ways to hide the wood-sandwich edges of plywood. Solid-wood edging isappropriate for high-wear edges, such as on tabletops, and can be profiled with a routerafter installation. Iron-on veneer tape is adequate for shelving or case partitions. A faceframe will give a more refined look.
Adhesive-backedveneer tape




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covered with face frames. Solid or veneer
edge-banding usually is applied to shelves
or tabletops.Edge-banding with a heat-sensitive glueback can be purchased in most domes-tic and some exotic woods. Shopmadesolid edge-banding usually is attachedto plywood with yellow glue (aliphaticresin). When using thick edge-banding,apply glue to both the banding and theplywood because the end grain readily absorbs glue.Edge-banding typically is wider than theplywood is thick, so it must be trimmedflush to the plywood’s edge. Sounds easy enough, but the process is always a nail-biting experience because of the fear of damaging the plywood. Most often, I trimthe edge-banding flush with a well-tunedblock plane, working from the sheet outacross the banding. With a little practice,
it’s relatively easy to trim the edge perfect-ly flush without gouging the face veneer. A
router with a flush-trimming bit will workfine, but it’s often not as handy to use asa block plane. A clean surface is an important startingpoint if the cut edge will be banded. Theedge of a sheet of plywood is often dam-aged or dirty. Try to cut away these dam-aged edges as you’re cutting the plywoodto size. Start by ripping a little wider thanneeded, and then trim the edges for afinal cut.I used to use a belt sander to level edge-banding with the plywood, a risky proce-
dure because it’s easy to ruin the plywood
face with this aggressive machine. My technique was simple. I scribbled on theplywood with a graphite pencil, makinglines that came up to the back edge of thebanding and extended 5 in. or 6 in. intothe plywood. These reference lines let meknow whether the belt sander was staying
flat or tipping into the plywood, and saved
me lots of veneer sand-through.
Sand with care
Generally speaking, I’d stay away fromsanding plywood with a belt sander. If
you’re not careful, you could sand throughthe face before you know it. On the edges,
the sander can tip over easily and erase
the thin veneer there. I use a random-orbit
sander instead. Always follow the grits in order, and
don’t skip any. Start with P120 grit to clean
up marks and small dings, work quickly up to P150, and finish with P180.Sand with relatively slow, even strokes.Swirl marks are hard to remove from thethin veneer without doing damage.

Roland Johnson is a contributing editor.
Build the faceframe oversize.
To create an evenoverhang on each side, use a card- stock shim whenbiscuiting the case(above left). Glueand clamp the faceframe to the case(above right). Use ablock plane to bring the face frameflush with the case side (left).
MAY/JUNE 2005
CasesideFaceframeFace frameshouldoverhang case sideby
1
⁄
32
in.
FACE FRAME