by Sandor Nagyszalanczy
Butt joints can be used in a lot of different ways to join two boards or panels: edge to edge, end to side, end to edge, etc. To make a simple square or rectangular frame, cabinet, box or drawer using butt joints is very easy. For example, to make a simple picture frame, cut four parts, each with a square end. One opposing pair of sides is cut the same length as the length of the frame. The other pair of sides is cut to the width of the assembly, minus twice the width of the other pair of sides. Because stock is often not exactly the width that's specified in plans, I like to actually measure the width of my pair of long sides, then subtract the measurement from the length of the other pair of sides.
But before cutting any parts, it's wise to make sure the saw you're using for cuts (compound miter saw, table saw and miter gauge, etc.) is cutting smoothly and precisely. I like to take a test cut or two and check the cut ends and edges for squareness with an accurate try square. For a perfectly square assembly, it's crucial that each opposing pair of sides be cut to identical length. Use a fence and stop with your miter saw (or table saw and miter gauge) to assure consistent lengths.
After parts are cut, the easiest joint assembly is to simply glue the parts together. This works just fine when joining solid-wood boards edge to edge, to make a wider panel or shelf, or when gluing parts up edge-to-face, with long grain glued to long grain. You can also get away with gluing butt-joined plywood carcasses, as some side-grain-to-side-grain gluing takes place. An easy way to strengthen carcass assemblies is to add nails or screws, to hold the joints together while the glue dries. This is fine for utility drawers, boxes and cabinets that aren't subject to much stress and strain during use. However, such assemblies have very little resistance to racking-the tendency of the frame or box to flex diagonally. Never use glue and nails to assemble an end-to-edge butt-joined solid-wood frame: The wood's porous end grain doesn't provide any strength when glued, and any flexing or stress easily breaks the weak glue joint and pulls the nails out, which have very little withdrawal resistance.
Reinforcement of some kind is necessary for strong, long-lasting butt joints. Not only does reinforcing the joint add strength and durability to your assembly, but also helps keep parts aligned during glue-up. Fortunately, there are many different possible ways of reinforcing a butt, from easy-to-add glue blocks and braces, to pocket screws, dowels, biscuits, splines and butterfly keys.
Glue Blocks and Braces
For butt joint assemblies with interior surfaces that won't be seen, the very easiest way to add reinforcement is to apply glue blocks. A glue block is just what it sounds like: a small block or strip of wood that's glued into place. Glue blocks are commonly used inside plywood and MDF assemblies, like cabinets, dressers, armoires, etc. They can also be used on solid wood projects, as long as the glue block's grain runs parallel to the grain of the assembly. No need to clamp the blocks in place: Just spread glue on the block and press it in place, using a back-and-forth rubbing motion. You'll feel the block "suck" into place, as it gets harder to move. For greater strength, add drill and drive screws through the glue block into both halves of the butt joint. Alternatively, you can screw a brace across the inside corner of a butt joint, for example, a table apron. The brace can be a metal L bracket or corner gusset (available at hardware stores), or a wood brace made from a scrap of hardwood with 45 degree mitered ends, cut as you would cut a picture frame member.
A popular way to create reasonably strong butt joints is with pocket screws. This system uses steeply angled, counterbored holes drilled into one butt joint member - a frame rail, cabinet side, carcass bulkhead, etc. Screws driven into the holes pull the parts together and secure them firmly. Pocket screwed butt joints are quick to assemble and require no glue, so there's no squeeze-out or messy drips to clean up afterwards. Also, pocket screwed joints can be disassembled, in case you make a mistake or want to take your project apart in the future.
While it's possible to drill the angled holes necessary for this joint using a regular electric drill or drill press, you'll get cleaner, more precise holes by using a special jig. The Kreg pocket hole system uses a special jigs with drill bushings, clamps and a stepped drill bit to bore clean, perfectly angled and spaced holes in your workpieces. Screwed to a plywood base plate, the jig shown holds frame members or panels firmly as they are drilled. A pair of bushings in the jig lets you drill two closely spaced holes without having to reposition the workpiece. A special adjustable end stop positions subsequent same-sized workpieces in the jig. This allows you to drill holes in say a few dozen cabinet frame stiles without having to measure and mark each individual part. For frame members, holes are always drilled in the ends of the rails, rather than through the sides of the stiles. This way, screws are driven into side grain where they have lots more holding power than if driven into end grain.
A tried-and-true (if a bit old-fashioned) method of adding strength to basic butt joints is doweling using cylindrical dowels fitted into holes drilled in both joint parts. Made of hardwood, dowels are ribbed or spirally fluted along their length, to allow glue to spread around them during assembly, resulting in a strong bond. The trick to getting good dowel joints is to drill the holes in exactly the same location both parts; any discrepancies in location and the dowels won't fit into both parts or the joint will be misaligned.
The easiest way to accurately position dowel holes is to use a special dowel drilling jig. Simple jigs, like the Rockler doweling jig shown have hardened steel holes that guide the drilling of one or more holes. Other jigs automatically center holes in the thickness of the stock or allow you to adjust hole position on the stock, thickness wise. Reference marks found on all dowelling jigs show the centerline of the holes, so you can position the jig in the same location on mating workpieces and space dowels along the length of the workpiece.
To make a dowel joint for a frame, first dry assemble your butt joint assembly. Then, draw lines across the face of each joint to show the centerline of each dowel hole. Use a sharp pencil or a mechanical pencil with a narrow lead, so that your lines are very precise. Align the dowelling jig's reference mark with this line and clamp the jig in place on the stock before drilling.
On all but the narrowest frames, mark at least two holes per joint, as the dowels keep the parts from twisting. Dowel diameter should equal about half the thickness of the stock. For example, use 3/8 in. dowels in 3/4 in. or 13/16 in. thick stock. Now line up the reference mark on your dowelling jig with your pencil mark, fasten the jig to the part, and carefully drill. The depth of the holes in both parts should add up in depth to the length of your dowel, plus 3/32-1/8 in., for glue space. To assure that holes are drilled to the right depth, add a stop ring or flap of tape to the drill bit. When drilling deeper holes, pull the bit out occasionally, to clear sawdust and keep the bit from overheating.
To install dowels in the corners of a butt-joined carcass, first use your dowelling jig to drill holes in both ends of two opposite sides of the cabinet, box or drawer.
Next, insert metal dowel points in each of the holes in one end of a part and use them to transfer the exact hole positions to the other half of each joint.
Align the two parts to form a corner, then use a rubber or dead-blow mallet to tap the parts together.
The dowel points will make clean indentations you then use to center the point of a brad-point drill bit when drilling the matching holes on a drill press.
Invented in Europe more than five decades ago, biscuit joinery (aka plate joinery) uses small, football-shaped wood biscuits glued into slots to form strong, quick-to-make joints between butt-joined wood parts. Carpenter's glue swells the compressed wood biscuits, hence making a very tight, strong joint. The biscuits are a great reinforcement for butt joints used to join cabinet carcases, chests and boxes, drawers and trays or end-to-edge joined frame members 2 inches and wider. A portable plate joinery machine uses a small, horizontally spinning blade to cut semi-circular slots in both parts for the biscuits, which come in three sizes: #0, #10 and #20. Number 20 biscuits are best for 3/4 inch stock; the smaller sizes are for thinner stock.
As with dowel joinery, the first step to making biscuit joints is to mark lines across the butted parts. These are used to align the plate joiner when cutting matching slots in the joined members. The height of the machine's front fence is set to cut a slot roughly centered on the stock's thickness. For wide workpieces, plates should be spaced roughly every few inches.
For end-to-face joined corners of a box, chest or cabinet, a slot is cut in the edge of one part, as shown below. The mating part is then slotted on the face of the joining side by holding the plate joiner vertically, using its fence to space the slot the correct distance from the end.
An alternative to biscuits, splines are narrow strips of wood inserted into slots cut in the surface of a butt joined assembly. These slots can be cut using a table saw and dado blade or a router fitted with a kerf-cutting bit. You can either rip splines from thin plywood (1/4 in. Baltic birch ply works well), or cut solid-wood splines with their grain running in the short direction. Splines can be used to strengthen butted frame joints, but are more often employed on butted carcass joints, such as used to build a chest or cabinet. To be most effective, the spline should run the full length of the joint.
A decorative way to reinforce edge-joined butted parts is with butterfly keys. The hour-glass-shaped key, cut from thin wood, is inlaid into a recess cut into the face of the joint, with the narrowest part of the key centered on the seam between the two parts. When glued into place, the key adds mechanical strength to the joint and keeps it from pulling apart.
Although you could create a butterfly-key joint using a chisel to chop out the recess and a fret saw to cut out the key, it's much easier to use the router template method. This requires a plunge router, a key-shaped template, a straight bit and a special router guide bushing, (available as a kit from Rockler) The guide bushing mounts on the router's base plate and the 1/8-in.-dia. spiral-fluted bit, chucked in the router, plunges through the bushing during routing. The bushing actually has two guide rings, one that fits over the other. To rout the recess for the butterfly inlay, you use the larger guide ring pressed onto the bushing. Clamp the template over the workpiece, with the narrow waist of the butterfly centered on the joint line. Set the router over the opening and plunge the bit down and rout out the recess, working the router back and forth to remove the waste in the middle of the butterfly. The depth of the routed recess should equal about 1/4 to 1/3 of the thickness of the workpiece.
Next, use the router and template to cut out the butterfly key itself. First, prepare stock for the key by planing stock down to a thickness that's 1/32 in. thicker than the depth of your recess. Remove the large guide ring from the router bushing and increase the bit's depth of cut by about 1/32 in. Now apply a layer of double-stick tape to a scrap backing board cut slightly bigger than your key stock, and press the stock down onto the board. Clamp the butterfly template over this sandwich, orienting the key with it's long direction the wood's grain. Set the router over the template with the guide ring in firm contact with the inside edge of the butterfly cutout. Start the router and plunge the bit, then slowly and carefully rout along the perimeter of the butterfly, keeping the guide pressed against the template. When you've routed all the way around, withdraw the bit before switching the router off, then carefully peel the butterfly key off the backing board. Glue it into the recess and sand the joint surface flush after the glue dries.
Sandor Nagyszalanczy is a furniture designer/craftsman. writer/photographer and regular contributor to Woodworker's Journal. His books are available at: http://sandorsworkshop.com/Books.html
©2008 Woodworker's Journal