Kerf Spacing

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The spacing between kerfs (or the width of the ribs) not only will affect the maximum radius that you can bend, but also how smooth the bent piece will look. The reason kerf spacing is linked to the smoothness of the bend has to do with a problem that's unique to kerf-bending called "flats", see photo above.

QUESTION: What is a flat, and how can I minimize this problem ?

Flats are caused by the difference in flexibility between the thick ribs and the thin webs. As the wood is bent around a form, the webs are flexible and will bend in an attempt to follow the curve. But the ribs are much more rigid and won't bend. This creates a flat surface on the face of the curved piece.

Small flats can be easily removed by sanding. But the wider the flat is, the more sand-ingyoull have to do. To reduce this problem, choose the tightest spacing possible without making the ribs so narrow that they'll crack and break off.

QUESTION: What is the best spacing to use when kerf bending? Are there any general rides?

In most cases, I space the kerfs about W" to V8m apart — even if the radius is large. It might seem like a lot of work to cut kerfs so close together (especially on a long piece), but it's not. Cutting kerfs is fast work. It's sort of a trade-off. If you spend a little more time kerfing up front youll spend a lot less time sanding out flats later.

As a general rule, the closer the kerfs are together, the tighter the radius you can bend, see Fig. 7. But more important, closely spaced kerfs provide a smooth curve.

For example, I found that you could bend 3V-thick stock around a 12" radius by using Vf spacing — but there were noticeable flats, see Fig. 8.

By changing the spacing to W, I was able to bend a similar piece easier, so the flats could hardly be seen, and easily removed with light sanding.

Note: Unless the radius is extremely large, I wouldn't space the kerfs greater than W. Any greater and you risk creating flats that you'll never be able to remove.

SANDING

If you space the kerfs together closely, you may still experience flats to some degree — even if you can't see them. But they may become visible when you apply a finish.

QUESTION: What's the best way to sand away the flats from a workpiece?

I use a hardwood sanding block (not a power sander). Take smooth, gradual strokes following the contour, and constantly check the surface of the work. Stop sanding as soon as all the flats disappear—don't oversand. If you do, you may loose the uniform curve that you've worked hard to create. And, if you oversand. you could weaken the web or even sand all the way through it.

When sanding plywood, there's another problem. Since face veneers on hardwood plywood may be V$2"-thick or less, it's very easy to sand right through the face veneer, see Fig. 8. So sand cautiously — checking regularly to see if the flats have disappeared.

And again, don't use power sanders. They can remove too much wood, too quickly.

FINISHING

QUESTION: What about finishing kerf-bent pieces? Are there any tricks to that?

I like to apply a light coat of sealer to the kerf bent piece and then examine the surface closely. To do this, 1 use a light source from behind the piece. This lets me check the workpiece one more time for flats before applying the final coats.

I also recommend a satin finish rather than a high gloss. The reason for this is if there are any flats (even small ones), they'll show up more with a gloss finish than with a satin or matte finish.

max. spacing for 24" rad. curve

max. spacing for 24" rad. curve

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max. spacing for 12" rad.curve

Kerf Bending Formula Trim Molding

FLAT

max. spacing for 12" rad.curve

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KERF

— HEAVY SANDING CAN CUT THROUGH FACE VENEER

KERFS

3,4" APART

WIDE KERF SPACING, MORE VENEER MUST BE SANDED OFF TO ACHIEVE A SMOOTH CURVE

FLAT

FLAT

KERF

NARROW KERF SPACING, ONLY UGHT SANDING NEEDED TO REMOVE FLATS

KERFS

APART

KERFING TECHNIQUES

There are a number of ways to kerf a work-piece. You can use a radial arm saw, table saw, band saw, or even a hand saw to do this.

But no matter which saw you use, it's best to use an indexing system to keep the spacing uniform. The more uniform the spacing is, the more uniform the curve will be.

The type of indexing system and how easy it is to use determine which type of saw to use for kerfing.

RADIAL ARM SAW

When I need to cut a lot of kerfs in a long narrow board, I choose a radial arm saw. It lends itself best to kerfing since the saw kerf is on the top of the workpiece where you can see it, see Fig. 9. It's also easy to align the kerfs to an indexing line drawn on the fence.

Draw an index line on the fence so the distance from the line to the blade equals the desired spacing between the kerfs. Then align the end of the workpiece with the index line and cut the first kerf. After that, align the edge of the just-cut kerf with the index line to make the next kerf, see Fig. 9.

When kerfing on the radial arm saw, be sure to hold the workpiece flat. Otherwise the workpiece may rise up and youll end up cutting all the way through.

TABLE SAW

On the table saw, the same method can be used by drawing the index line on the table top or an auxiliary fence that's attached to the miter gauge. In either case, it can be difficult to see the mark — so I usually use a different method.

To index the kerfs, 1 drive a No. 4 screw into the auxiliary fence and cut off the head, see Fig. 10. (The shaft of a No. 4 screw has a diameter just slightly less than V&" to fit the kerf of a typical saw blade.) As each kerf is cut, just lift the board and place it over the screw, see Fig. 10.

One advantage to using the table saw is that you can kerf a large workpiece. such as a 4x8 sheet of Masonite.

Shop Note: In most cases, when you kerf a workpiece, you're cutting across the grain. Because of this, I found that a 40-tooth carbide-tipped combination blade worked best for cutting kerfs on either the table saw or the radial arm saw.

BAND SAW

Cutting kerfs on a band saw is sort of a combination of both previous methods. An indexing line is drawn on the fence (as on the radial arm saw). But for the band saw I clamp the fence behind the blade and use it as a depth stop as the board is pushed into the blade, see Fig. 11.

Since the blade is so thin on a band saw, you can cut extremely thin, closely spaced kerfs. This means a smooth curve with minimal sanding.

Plywood Corner Molding Machine

CORNER BLOCKS — A KERFING ALTERNATIVE

There's another interesting way to bend plywood or solid wood around a very tight corner — using a solid wood corner block and a single large kerf (dado), refer to photo on far right. Using this method, the thin outer face of the workpiece is the only part that actually bends around the corner. For strength, it's backed up by the corner block.

To do this, start by cutting a block of solid stock that's shaped to the curve. You can cut a radius on the block with either a band saw or a sabre saw, then sand the curve smooth.

To determine the width of the single kerf (it's actually a dado), first place the comer block on the piece to be bent.

Then, press down on the block to keep it from slipping and rock it in both directions so the end of the block touches the work-

piece, see photo on left. Now make a mark at each end to define the kerf.

Next, waste away this marked section of the workpiece, see center photo. Note: Test the fit of the corner block and sneak up on the final width of the dado by taking a series of cuts.

Finally, apply glue to the corner block and clamp it in place.

Plywood Bend KerfPlywood Bend Kerf

CUT HEAD OFF #4 SCREW AND USE AS INDEX PIN ON

AUXILIARY FENCE

CUT HEAD OFF #4 SCREW AND USE AS INDEX PIN ON

AUXILIARY FENCE

BAND SAW

Wolf Wood Carving Pattern

BAND SAW

CLAMPED BEHIND BLADE ACTS AS STOP

Plywood Bend Kerf

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