Splined Edge Joinery

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One of the best edge-joint reinforcement techniques is perfectly suited to the router. It's the spline. You cut grooves (or call them slots, if you prefer) in the adjoining edges, extending them nearly the length of the boards. Fit a strip of plywood or hardboard into rhe joint as you glue it up.

The spline strengthens the joint by expanding the glue area and by providing a mechanical lock. It also makes the boards a little easier to align and clamp during glue-up. If done properly, the spline will bring even slightly bowed boards into flush alignment.

The table saw might seem to be the fastest cutter of the necessary slots, and it probably is. But spline slots are usually stopped for the sake of appearance—who wants to sec the spline at the edge of a tabletop. for example. A sawed slot that's stopped will have a 4- or 5-inch section at each end that won't accommodate the spline, because the slot isn't full-depth.

So do the job with a slot cuttcr and hand-held router, or a straight bit on the router table.

Slotting for Splines

If you want—or need—to do the job with a hand-held router, use a slot cuttcr. You can rest the router on the face of the workpicce, and even use an offset baseplate to help avoid bobbles and tips. The job can be done in a single pass, as opposed to having to make two or three passes to get it to depth.

Moreover, those slightly bowed boards can be forced flat and held that way with a few clamps. The router will have a flat surfacc to slide across, and the cutter will make that kerf a consistent distance from the surface.

Rather than get too worked up about trying to center the slot, just mark a rcfcrcncc face of each board. Always keep that face up—through all machining and assembly stages. The slots will all be the same dis-

To rout thejirst curved edge, the fcncc has to be positioned on the workpicce with its undulations in sy nc with those cut into the workpiece. Then the second piece has to he positioned in sync with its mate-to-be. Here, the walnut edging for the panel shown on page 252 is being trimmed. Because of the narrowness of the piece, the cut has to be started and stopped three times so the clamps can be shifted out of the router's path. The challenge is to shift them without jostling th e workpiece in the least.



A slot cutter is usually an assembly, consisting of an arbor, a cutter, and a pilot bearing. The arbor includes a nut that secures both cutter and bearing on the threaded end of the arbor. Often, the arbor will include a slinger or two, and several spacers.

The cutters themselves are available in two-, three-, and four-wing designs. With only two carbide cutting edges, the two-wing model is least costly, bur rhe plate holding the carbide is smaller and thus less stiff. The cutter won't cut quite as fast as those with three or four cutting edges.

The size specified for a cutter is the width of the kerf it makes. The standard anting depth, regardless of the cutter size, is Yi inch.

Byrom makes an extensive selection of slot cutters. Shown below are the three- and four-wing cutters on Y-t-inch and Yx-inch shanks. All the cutters can be used on either shank size. The two-wing cutter from MLCS is less costly, but the cutter is fixed on the shank.

In the manner typical of piloted bus, you can change pilot bearings to alter the cutting depth.

Using the spacers supplied by-some vendors allows you to alter the position of the cutter on the arbor. This enables you to extend the range of your router's bit-adjustment mechanism. Remember that changing the router's bit-height setting adjusts not the cut's depth but the cut's position.

These assemblies aLso allow you to put the pilot bearing either above or below the cutter, according to the job. You can even use two bearings on some long arbors, one above and one below.

Because you arc dealing with an assembly, you can save by buying a single arbor, a single set

of bearings, and a selection of cutters. Buying a single assembly as separate components never saves you money, though.

To give an example, a M&-inch three-wing assembly costs $26. H from one supplier. The cutter, arbor, and standard-diameter bearing purchased separately cost $30.72. For one cutter, the assembly is cheaper, clearly. But if you buy five different-sized cutters QAtr, Ya-, 5^2-, Mft-, and Winch) as assemblies, the cost totals SI 31.60. Buy an arbor, a bearing, and just the cutters for those sizes, and the total is $104.18. If the convenience of having a separate arbor and bearing for each cutter is worth more than $25.00 to you. you know what to do.

An interestingaddition to any bit drawer is this adjustable slotting assembly, sold by Amana as the Quadraset (above). It's a slot cutter stack set—conceptually like a table-saw dado set—that includes Ya-inch, %2-inch, yi6-inch, and Yt-inch two-wing cutters, a Yz-inch-shank arbor with a pilot bearing, and a handful ofsj>acers, washers, and shims. You can use the cutlers individually on the arbor, or you can combine two. three, or all of the cutters on the arbor. Thus you can cut slots that range in Va-inch increments from Yk inch wide up to inch wide.

An interestingaddition to any bit drawer is this adjustable slotting assembly, sold by Amana as the Quadraset (above). It's a slot cutter stack set—conceptually like a table-saw dado set—that includes Ya-inch, %2-inch, yi6-inch, and Yt-inch two-wing cutters, a Yz-inch-shank arbor with a pilot bearing, and a handful ofsj>acers, washers, and shims. You can use the cutlers individually on the arbor, or you can combine two. three, or all of the cutters on the arbor. Thus you can cut slots that range in Va-inch increments from Yk inch wide up to inch wide.

A pilot bearing isn't the only means of governing a slot cutter's depth of cut. If you need to cut a shallow slot, a quick and cheap guide block made from a scrap of Winch stock can be attached to the baseplate with double-sided carpet tape. On the band saw or with a saber saw, make a cutout for the slot cutter, as shown. When you stick the block to the baseplate, set it so the cutter can only cut the depth you want.

The slot cutter is an unusual hit, in that it cuts at right angles to the hit axis. It's great for cutting slots for splines, since the router can rest squarely on the broad part of the workpiece, yet be cutting into the narrow edge (top left). A hand-held router can keep a slot positioned even in mildly bowed stock, since it has a relatively small contact patch that can follow the board's contour. On the router table (bottom left), the work must be very flat, or the slot position can shift.

tance from that lace, so when the splines arc inserted and the boards joined, their reference faces will be (lush.

The slot cutter can also be tiscd on the router table. The advantage of the slot cutter over a regular straight bit is that you can lay the workpiecc flat on the tabletop. Save yourself the time it takes to position and clamp hold-ups and hold-ins, not to mention hold-downs. Fit the cutter in the router, adjust the height, set the starting pin. and rout. Be consistent about whether you have the reference face up or down when you cut. and you'll get a good fit.

Are one or two of the boards slightly warped? Just force them flat with a few clamps. Bowed boards are the province of the hand-held router.

A final note on the slot cutter. It's a din generator. If you have a dust collector accessor)', by all means use it.

Grooving for Splines

There are other approaches to use if you have a straight bit but not a slot cutter. The best of them involve the router table or the horizontal router table. In a pinch, you can do the job wirh a hand-held router.

If your working stock is good and flat, cutting the grooves for the splines is the same as cutting any stopped groove on the router table. Mark your starting and stopping points on the table and the work-piece. Guide the cut with the fence.

Using the horizontal muter table gives you the advantage of having the workpiece flat on the tabletop. If the boards are broad and fairly long, this is a plus that should not be discounted.

Hie singukir disadvantage in both approaches is that you have to make two passes to get to a reasonable depth, or you have to settle for a very narrow spline. To save time, make a first pass in all the boards before resetting the bit height and doing the second cut.

If your working stock includes a board or two with some bow to them, you may need to replace the fence. It may be too difficult to press the bowed boards flat against the fence (or the horizontal table's top). The groove for the spline may end up an inconsistent distance from the reference face, a woodworking disastcr.

Instead of the fence, use a pivot


Here's how to cut stopped spline grooves on the horizontal router table. The Iward is flat on the table-top, held there by the feather board. Sight down on the hit to align it with the start mark on the workpiece. Push the hoard onto the bit to start the cut. teed left to right.

block to support the stock. As shown in Pivot Block, you make the pivot by gluing and screwing a support into a groove plowed down the center of a base board. The support projects beyond the base by a couple of inches at one end. Line up this end of the support just shy of the bit. The distance between the bit and the pivot, of course, is the distance the groove will be from the working stocks face. Clamp the pivot to the tablctop. Then set a board at right angles to the pivot, bracing it so that pressure in the proper feed direction won't force the pivot to swing out of position. Clamp the brace to the tabletop.

The use of this pivot is self-evident. Keep the board tight against the end of the pivot. At any time during the operation, it's irrelevant where the ends of the workpiece arc, so long as its reference face is tight against the pivot. But you must keep the work perpendicular to the line between the bit and the pivot. Don't swivel the work, or the groove will wander.

If for any reason the job must be done using a hand-held router and a straight bit, you've still got an option or two.

Balancing the router on the work-piece's edge is pretty dicey, but you can trap the workpiccc between two edge guides. That'll keep the router pretty much centered and upright. Another approach is to clamp the workpiccc to the edge of the router bench—or a similar work surface— with its edge flush with the bench top. Use an edge guide to locate the

A pivot block, much like you would use to support stock during resawing on the band saw, also supports bowed stock for splitting 011 the router table. And if you need to groove a workpiece that's curved by design, this is the way to do it. The brace clamped to the table beside the pivot block keeps it front twisting out of position. And note the featherboard holding the workpiece against the pivot block's nose.

groove, and enjoy the extra router support the bench top affords. See the chapter "Dadoing and Grooving" for more details on both of these approaches.

However you producc the slots lor the splines, completing each joint is a matter of cutting strips of spline stock and gluing it into a slot in one board. Then bring the mating board into alignment. Pit it over the spline, and clamp the joint closed.

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  • rudolph
    How to cut a stopped edge spline?
    9 years ago

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