Plastic Knockdown Fittings Interlocking Mechanisms Furniture

A roll-top desk combines the best features of a utilitarian workspace with the elegance of a piece of fine furniture. The tambour on the author's oak desk, above, is made with interlocking slats, instead of being the more common cloth-back tambour.

Drawer case back panel

Knee-hole / panel (optional)

A roll-top desk combines the best features of a utilitarian workspace with the elegance of a piece of fine furniture. The tambour on the author's oak desk, above, is made with interlocking slats, instead of being the more common cloth-back tambour.

figure 8s are screwed to the top edge of each case, as shown in figure 2. The desktop is then placed on top of the cases and the locations of the figure 8s are marked on it. Then, the top is removed and shallow mortises are drilled with a brad-point bit at the marks on its underside. The figure 8s fit into the mortises so the top will pull down tight to the frames when the screws are driven home. Be sure to angle the screws that go through the figure 8s and into the top so you can drive them in without hitting your knuckles on the side of the drawer case. To complete the base assembly, the center drawer is slid into place on guides screwed to the sides of the drawer cases.

The interlocking slats of the tambour are now fed into the access.grooves at the top of each of the curved side panels. The top board is attached with figure 8s screwed to the roll-top frame in the same way the desktop is fixed to the drawer cases. Finally, with the tambour open, the pigeon hole unit is slid into place. It's made to just fit between the side panels and below the fully opened curtain. Felt glued to its bottom lets it slide easily without marring the surface of the desk. A narrow strip tacked along the back of the desktop acts as a stop to make sure the pigeon holes aren't slid so far back that they interfere with the opening of the curtain.

An optional knee-hole panel can be installed with knock-down hardware between the drawer cases. I prefer the heavy, solid look that this knee-hole panel gives the desk, especially when the desk is used in the center of a room instead of up against a wall. I originally used four regular 90° metal angle brackets to join the knee-hole panel to the drawer cases, but I've since found a somewhat more elegant solution, identified simply as "joining devices" in The Woodworkers' Store catalog. These three-part knock-down brackets, shown in the detail of figure 2 on the next page, provide a tight connection and disassemble and reassemble without having to remove and replace screws. The two brown plastic mating portions of the device screw to the parts to be joined and a metal joining plate slides over them to make the connection.

Making die frames and panels-As you can see in figure 1 at left, the basic building blocks of the desk are all frame-and-panel assemblies, except for the desktop and top board, which can lie glued up and cut to size at this time. You'll need a left, right and back panel

Traditional Furniture Projects 53

assembly for each of the drawer arses and, if you choose, a knee-hole panel assembly to go between them. The roll-top frame requires a back panel assembly and the two curved side panel assemblies. To begin, determine the overall dimensions of the desk you're building based on the size of the desktop, and prepare a parts list. Then, cut out, groove, tenon and bevel all die frame-and-panel parts for die base of die desk and the roll-top frame at one time.

I cut the grooves in the frame members with a dado blade on the tablesaw. These grooves accept the tenons from adjoining frame members and also provide the space to house the floating, raised panels. To simplify machining I've standardized the groove for all the pieces at V-t in. wide by 3/s in. deep. Where the back panels of the drawer cases and the roll-top frame join their respective side panels, a Win. by %-in. tongue, machined the full length of the frame member, is glued into a groove in the inside surface of the vertical frame members of the appropriate side panels. While you're cutting grooves and tenons, don't forget the parts for the drawer guide frames (see figure 2 below), as they also use the same size groove and tenon.

I originally cut the rectangular raised panels with a dado head on the tablesaw, but this was very time-consuming because of all the sanding needed to clean up the bevels. When I tried to make the cut in one pass with a panel-raising cutter on the shaper, I had to reject a significant number of panels because of chipping. Even when I made multiple passes, there was some chipping across the grain. To eliminate this chip out, I switched to a two-tool operation on the tablesaw and shaper. First, I make a '/»-in.-deep scoring cut on die tablesaw at die inner edge of die bevel. Next, with the sawblade set to the same angle as the shaper's panel cutter, 1 saw off most of the waste. Finally, the shaper is set up to make the finish cut.

By slightly changing the settings on the tablesaw and shaper, you can bevel the V-i-in.-thick drawer fronts right after the panels are made. Experience has taught me to belt sand the outer surfaces of the drawer fronts and panels before cutting the bevels to avoid rounding the crisp edges of the raised panels. To sand the cross-grain bevels, I clamp the panels to the workbench and begin sanding with 80-grit paper on an electric block sander. Oak panels usually don't need to be sanded finer than 120-grit;

Heavy Curtains For Canopy Bed Frame

Stop strip for pigeon holes

Fig. 2: Basic construction

Escape groove for inserting tambour

Screw through elongated holes to attach roll-top frame to desktop.

Detail: Frame-and-panel joinery

Knee-hole panel

Groove for back panel

Use figure 8s to secure top board.

All grooves and tenons are standardized at V* in. wide by 3/i in. deep.

Scrapwood frame member, covered by base board

Frame member, % in. thick

Drawer guide frames are mortised and tenoned together from stock, 3/t in. by 13A in.; front piece is oak, and runners and back piece are secondary wood.

Figure 8 \

desktop fasteners are screwed to top of drawer cases and relieved into underside of desktop.

Center drawer runner

Holes in desktop for blocks that activate locking mechanism

Knock-down joining device for attaching knee-hole panel

for locking mechanism.

333/« Dust panel optional

Stopped dadoes for drawer guide frames, / V* in. deep

Knock-down joining device

Raised panel, % in. thick

Fig. 3: Roll-top frame, side panel

Groove for back frame-and-panel assembly

Stop strip for pigeon holes

Block, V* by % in., to activate locking mechanism

Groove for back frame-and-panel assembly

Stop strip for pigeon holes

walnut and birch are usually sanded to 220-grit.

Work out the design for the curved frame-and-panel sides for the roll-top frame full scale on paper. When the tambour is open, only the handle slats should show and when the tambour is closed, there should be only one or two slats hidden behind the lip on the front edge of the top board (see figure 3 above). To test this relationship, lay a string along the path of the tambour groove on your drawing, and mark the length of the string when it's lying in the closed tambour position. Now, move the string to the open tambour position and compare the distances. Because the depth of die desktop Ls already established, you must vary the height of the side panel assembly and the slope of the curve until the two distances are equal.

Once you've established the profile of the roll-top frame, you can bandsaw the curved frame members and sand them smooth with a drum sander chucked in the drill press. Then, rout the groove in the inside edge with a bearing-guided '/t-in.-wide slotting bit. With rasps and files, I fit the shoulders of the tenons to the curve where the bottom frame piece of the side panels join the curved frame. After the frame pieces are complete, use them to lay out a pattern for the curved panels. Bandsaw the panels to shape and bevel them with a horizontal panel-raising shaper cutter in conjunction with a guide bearing. If you don't have one, you can use a bearing-guided router bit with a profile that will remove most of the waste, and then finish up with chisels and sandpaper to match the bevel on the other panels.

After all the frame-and-panel parts are grooved and tenoned, assemble them into the modules that will make up the desk. Take care when gluing the joints to avoid locking the panels to the frames; the panels must float freely so they can expand or contract with changes in humidity. In addition, be sure you have a flat area to lay out your clamps or you might end up with twisted case sides. After the cases are glued up, I run a baseboard around them, mitering it at the corners and screwing it to the case from inside the bottom rail. Since the first 3'/2 in. of the bottom rail gets covered by die baseboard, I save oak by leaving a gap and using scrapwood or secondary wood at the very bottom (see figure 2 on die facing page).

After removing the clamps from the frame-and-panel assemblies, sand the frame pieces with a small electric pad sander to about 100-grit. Keep the sander on the frame members and be careful not to round over the crisp corners of the raised panels. I go over the cases again with 100-grit after they are glued up and then usually finish-sand them to 120-grit.

Before gluing up die drawer cases, you need to lay out and cut die dadoes on the inside of the case sides to receive the drawer guide frames. I cut diese stopped dadoes on the tablesaw, and then use a router to clean them out to within about 1 in. of die front, and right up to die groove for the back panel assembly at the rear of die case sides. Then, notch the front corners of die guide frames to fit die stopped dadoes (see figure 2). I use the tablesaw to ensure square and accurate notches. When die guide frames are all notched to fit, glue up die drawer cases. Make sure the cases go together square or you'll spend hours fitting the drawers to out-of-square openings.

With the base of the desk in clamps, you can turn your attention to the roll-top frame. Before gluing the curved side panels to the upper back panel, you'll need to rout the tambour groove, as shown in figure 3 above. The easiest way to do this is with a '/2-in.-dia. pattern cutting bit and a Masonite template. Pattern cutting bits come with interchangeable bearings that fit on the lA-in. shaft above the cutting portion of the bit, and they are available from Trendlines, 375 Beacham St., Chelsea, Mass. 02150. The top bearing makes it possible to use a Masonite template bandsawn to fit right up to the inside edge of the desired groove. Clamp the template and side panel to the top of your bench and rout the Va-in.-deep groove. The next step is to mark the location of the escape grooves tor inserting the tambour slats and rout them freehand. Now you can glue the side panels to the upper back panel and attach die roll-top frame to the desktop. The top board can't be screwed in place until the tambour is installed, but this is a good time to mount the figure 8s to the side panels and mark and drill to recess them in the underside of the top. As shown in the photo on p. 53, a thin strip is glued below the front edge of the top. The purpose of this small lip is to fill the gap created by the curve of the tambour as it goes under the top. However, this piece should be made and glued in place only after the tambour is installed to be sure it doesn't interfere with the tambour's motion.

Drawers and locks-Each of die drawer cases that compose the base of die desk contains diree drawers and a pull board, as shown in figure 1 on p. 52. Both bottom drawers are deep file cases diat can hold either manila folders or Pendaflex hanging files. The fronts of the file drawers incorporate a visual ploy common to this style of roll-top: a false double-drawer front. The narrow, center drawer runs on wooden guides screwed to the side cases (see figure 2 on the facing page). Additional drawers, as many or few as you prefer, can be made to fit the pigeon holes.

My desk drawers have dovetails on all four corners and are flush

  1. 4: Locking mechanism
  2. 5: Shaping the interlocking slats
  3. Flute, Va in. dia., on shaper to create "neck," Vs in. thick on ball.
  4. 4: Locking mechanism

Remove cotter pin and cut 'A in. off spring, 2Vi in. long, to reduce its tension.

Do not attach bracket below spring.

Latch mounting brackets are attached to blocks mounted on vertical frame

Detail: Cross section

Commercial locking mechanism modified to be activated by tambour

Activating lever Latch rod

Detail: Cross section

Commercial locking mechanism modified to be activated by tambour

Remove cotter pin and cut 'A in. off spring, 2Vi in. long, to reduce its tension.

Latch mounting brackets are attached to blocks mounted on vertical frame

Do not attach bracket below spring.

Unlocked

Block for pivot, 5/s in. thick

Outdoor Sectional Locking Mechanism

Block to activate locking mechanism

Hook bracket

Drawer back

Drawers should come to within */» in. of the back frame.

Unlocked

Block for pivot, 5/s in. thick

Block to activate locking mechanism

Blocks for latch mounting brackets, V* in. thii1

Drawers should come to within */» in. of the back frame.

Hook bracket

Drawer back

3. Flute, Va in. dia., on shaper to create "neck," Vs in. thick on ball.

  1. Begin with stock, 3Ax3'/4x54 in., to get two slats from each piece. Dado, V* in. wide by V« in. deep, to begin socket.
  2. Core-box bit, Vi in. dia., with router mounted in table completes socket.

mounted, meaning they fit within the case frame so their front edges are flush with the case. Flush drawers make it imperative that both the cases and the drawers be built perfectly square; the fit around the drawer front is right there for all to see. The wooden drawer pulls are easily made if you begin with a long piece of stock. Clamp this long board in a vise and lay out for several pulls, marking both the length of the pull and die location of the finger notch for each. Then, with the long piece still in the vise, rout all the notches with either a 45° bevel or a core-box bit. Bevel the front face of the pull stock on the tablesaw, and sand this beveled face. Now, crosscut each handle from die long stock and round the corners.

There are only two key holes in the desk, as shown in the photo on p. 53: one on the bottom slat of the tambour to secure it to the desktop and one for locking the center drawer. The side drawers lock automatically and simultaneously when the curtain is closed, and conversely unlock when the curtain is open.

I couldn't find a locking system designed to be activated by the tambour of a roll-top desk. But Selby Furniture Hardware Co. (321 Rider Ave., Bronx, N.Y. 10451) has a mechanism (part #L-7CTRDSK U) that locks all the side drawers when the center drawer is closed. With a few modifications, I made the mechanism work off the weight of the tambour. Metal rods with spring-loaded latches are screwed to the inside back of each of the drawer cases, and activating levers are connected to the top of each of these latch rods. To work off the center drawer, as the mechanism is designed to function, the latch rods are placed so the activating levers extend over to each side of the center drawer. Each lever is screwed to the back frame of the drawer cases so it will pivot when its free end is forced up by triangular metal brackets screwed at the back of each side of the center drawer. So, when the center drawer is closed, the brackets push up on the lever and the lever's pivoting action forces the latch rod down, engaging the latches with hook brackets on the back of each drawer and locking all the side drawers simultaneously. When the center drawer is open, a spring at the base of each latch rod pushes the rod back up, disengaging the latches and unlocking the drawers. However, this system is cumbersome because the center drawer has to be left partially open for the side drawers to be unlocked. By modifying the mechanism, as shown in figure 4 at left, 1 made it work off the weight of the tambour.

I reversed the position of the activating levers so that instead of the brackets on the center drawer pushing up on the levers to lock the drawers, the tambour will be pushing down on them. This means that the springs at the bottom of the latch rods, instead of pushing down on the bottom mounting bracket, will be pushing up against the first mounting bracket from the bottom. Since the bottom mounting bracket moves with the latch rod, it must not be screwed to the frame of the desk. Although the springs were easily squeezed by the force of the brackets on the center drawer, the

The last slat has no socket.

The first slat has no ball.

  1. Bullnose, V* in. dia., to round "shoulders" of socket.
  2. Roundover, 3/v in., to radius corners of flutes.
  3. Modified bead, 3/a in. dia., to define ball.
  4. The third pass with the modified bead will separate the two slats.
  5. Fourth and final pass with modified bead will be on the separated slat.

File one leg of beading cutter to preserve "neck" of ball.

  1. Bullnose, V* in. dia., to round "shoulders" of socket.
  2. Roundover, 3/v in., to radius corners of flutes.
  3. Modified bead, 3/a in. dia., to define ball.

The last slat has no socket.

File one leg of beading cutter to preserve "neck" of ball.

  1. The third pass with the modified bead will separate the two slats.
  2. Fourth and final pass with modified bead will be on the separated slat.

The first slat has no ball.

weight of the tambour was not sufficient to compress them. To remedy this 1 cut Vi in. off the 2'/2-in.-long springs and moved the fulcrum point of the activating levers closer to the lifting point to increase the levers' mechanical advantage.

Now that I've installed a few of these mechanisms, I find it's easier to mount the lock on the desk back before gluing the drawer case together. The lock is mounted with the curtain contact point 1 in. above the drawer case. A 1 '/¿-in. square is cut in the desktop and blocks glued to the last slat of the tambour activate the lock through these openings. The final step is to mount the hook brackets on the back of each drawer for the spring-loaded hooks to grab.

Shaping the tambour slats-One of the keys to shaping the interlocking joint on the edges of the slats is beginning with 3%-in.-wide boards. This width allows you to get two slats out of each piece, which speeds up the process, and gives you a substantial piece of wood to work with, which keeps your hands away from the cutter. The 3'/->-in.-wide boards should be milled % in. thick and crosscut a little longer than the finished length of the slats. I prefer to sand the surfaces of the stock before machining the slats so the only clean up I have later is on the rounded-over corners.

Figure 5 above shows the progression of cuts I use to make the joint. The socket for the elongated ball-and-socket joint is begun with a '/i-in.-wide dado cut on the tablesaw and completed on the router table using a Vi-in.-dia. core-box bit. You need to set the router table fence so the core-box bit will center on the dado groove to begin cutting. Once you're in the groove, the Win.-dia. shaft of the core-box bit will guide itself along the dado.

The ball portion of the joint is formed on the shaper. First, I make four cuts with a Vs-in. flute cutter to define the '/«-in.-thick "neck" of the ball. Then, I change to a multiple profile cutter, as shown in step #4 of figure 5 above, which has the next two shapes I need: a 3/i6-in. bullnose to contour the "shoulders" of the socket and a 3/i<s-in. roundover for what will be the outer edges of the slats (step #5 of figure 5). The bullnose cut is made with the stock flat on the table; all the other passes are made by running the stock on edge. I readjust the fence and the height of the cutter in between the bullnose and roundover operations. The final ball shape is formed with a Vs-in. beading cutter that I modified by grinding back one of the protruding cutting wings so it would leave the neck of the ball. To avoid weakening the piece prematurely, make the first two cuts on opposite sides from opposite edges. The third pass will separate the two slats (see step #7 of figure 5) and the fourth and final cut will be done on an individual slat.

The whole operation results in a significant amount of chips and sawdust because the process removes approximately 40% of the blank. It may be desirable to hog out as much of the waste as possible with a dado head on the tablesaw; some woods are less forgiving than others and won't tolerate heavy shaper cuts without splintering. Depending upon the quality of the wood, you should machine 5 or 10 extra slats to ensure that you end up with the 25 to 30 good slats needed for a desk. Don't forget to make a wide slat with only a socket for the handle slat, and make another with only a ball for the last slat. Also, the small blocks that fit through the holes in the desktop and disengage the locking mechanism when the tambour is open are glued to the last slat.

I don't crosscut the slats to final length until after I've screwed the roll-top frame to the desktop so I'm sure to get an accurate measurement. The slats should be about % in. shorter than the distance between the curtain grooves. Then, I cut a rabbet on the back side of both ends of each slat to leave a 7i6-in.-thick tenon. Calculate the length of the tenon to leave the back of the slats Vn in. shorter than the distance between the roll-top side panels. The tenons for the handle slat have to be somewhat thinner and rounded so this wider piece will negotiate the curves.

Building the pigeon holes-I build the pigeon hole unit after the rest of the desk is complete so it can be dimensioned to fit between the side panels with very little to spare. When laying out the pigeon holes, consider how the desk will be used and design the compartments to suit that particular purpose.

All pigeon hole pieces are cut to size and sanded. Make the top and end pieces a little wider than the shelves and dividers so they can be rabbeted for a back piece. The grooves for joining the parts are all dadoed on the tablesaw. To ensure that the compartments all come out square, it's important to lay out and cut all the grooves from the same end of the horizontal shelves. This can be tricky, so take your time and check your spacing before you glue up the unit. Don't be surprised if you have to trim some of the pieces to length to account for the depths of the dadoes. Gluing up the pigeon holes is a delicate process because some grooves are only '/•> in. wide by '/» in. deep.

After all the component parts have been built and fitted together, the desk should be disassembled and the parts finished separately. 1 apply up to seven coats of polyurethane to the writing surface to completely close the pores. The other portions of the desk get three or four coats of polyurethane, and I sand between coats. The tambour slats are finished individually with a liberal amount brushed into the interlocking sockets. Once the finish is completely set, all wearing surfaces, such as the tambour's ball joints and the tambour grooves in the curved panels, as well as the drawer guides, are coated with a hard carnauba wax to ensure free and easy movement. □

Kenneth Baumert is a mechanical engineer and woodworker in Emmaus, Pa.

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Responses

  • wegahta haile
    How to figure out tambour groove on sides ?
    1 year ago

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