AW Extra 7/10/14 - Making Curved Doors - Crown Millwork

AW Extra 7/10/14 – Making Curved Doors

Making Curved Doors

Kerfkore flexible panels
handle curves with ease.

By Brad Holden

Curved doors and panels add elegance
to any project, especially
kitchen cabinets. Making these
complicated parts requires careful planning
and building. The task can be made much easier
by using a flexible panel product called
Kerfkore (see photo, right, top). It follows the
same principle that cabinetmakers use when
they saw multiple kerfs on the back of a piece
of plywood to make it flexible. A disadvantage
to this saw-kerfing method is the risk of the
face veneer cracking or kinking at the kerfs,
spoiling the curve’s smooth surface. Kerfcore’s
advantage is that that the kerfing is done for
you. Its flexible paper backing
provides a smooth
surface to attach
your veneer.

There are several important tricks and techniques
to working with Kerfkore but it’s a surprisingly
easy material to work with. I’ll walk
you through the main steps of making a curved
door with Kerfkore from layout to final trimming.
Information on using Kerfkore in other
applications can be found at the Web site
www.kerfkore.com.

What is Kerfkore?

Kerfkore is a very
flexible
panel made
of 3/8-in.-wide
ribs spaced 1/8
in. apart on a
paper backer sheet.
The spaces between
the ribs allows the board to flex. The flexible backer sheet
provides a smooth surface for veneering. The ribs in different
Kerfkore products are made of particleboard, luan
plywood, poplar plywood, MDF, fire-rated treated particleboard
or lightweight styrene foam. These different cores
have different weights, strengths and screw-holding characteristics.
The particleboard core works well for generalpurpose
doors, such as those for kitchen cabinets.

Kerfkore with paper backer on both sides is also
available. This makes the material more rigid and
somewhat easier to handle and enables you to create a
curve that turns into a straight run. To make the twosided
variety bend, you cut the backer on one side with
a utility knife where you need the bend to occur.

Kerfkore comes in 4-ft. by 8-ft. sheets in 1/4-in., 3/8-in.,
1/2-in., 5/8-in. and 3/4-in. thicknesses and costs
between $40 and $125 per sheet (see Sources, below).

Click any image to view a larger version.

Kerfkore flexes
easily in both
directions.

Doors made with Kerfkore have
a smooth, even curve.

Pick your veneer

1. Your three main veneer choices are two-ply wood
veneer (shown here), phenolic-backed (plastic-laminate)
wood veneer or vertical-grade plastic laminate. The twoply
veneer is the most flexible of the three and is easily
cut with a utility knife or scissors. The phenolic-backed
veneer and plastic laminate must be sawn or scored and
snapped. The cut edge of the two-ply veneer leaves a dark
line that may show on your finished door, depending on
the wood species and the finish you apply. The phenolicbacked
veneer and plastic laminate both leave a black
edge line.

Make a full-size drawing

2. Use a top-view drawing to determine the size of the
Kerfkore panel and the angles at the edges. The width
of the Kerfkore should be the outer circumference of the
curve minus 1-1/2 in. to 2 in. for the solid-wood edges. The
measurement is easy to take using a flexible curve.
Calculate the angles of the edges by drawing a line tangent
to the front curve at the corner of the door and then
measure the angle with a protractor.

Apply the front veneer first

3. Use contact cement and work flat when applying the first
sheet of veneer. Working flat may seem odd, but when you
do so, the contact cement remains flexible and a good bond is
ensured. The panel will gain a bit of stiffness but will remain
flexible enough to form to its final shape later on.

Add solid-wood edges for strength, appearance

4. Solid-wood strips provide durable finished edges
when the door is done. Each strip can be up to 1 in.
wide. Leave a 1/8-in. gap between the solid-wood strip
and the adjacent rib to maintain maximum flexibility.
Attach the strip with contact cement. After the edge
strips are attached, use a small router and a shopmade
edge guide (Photo 8) to trim the veneer flush
around the entire panel.

Cut the edges to the required angle

5. It’s easier and safer to cut the angles on the panel
while it’s flat, rather than after it has been curved.
Cut at the angles you measured on the full-size drawing
(Photo 2). The panel is sufficiently stiff at this stage that
it saws much like a normal piece of plywood.

Attach the back veneer with yellow wood glue

6. Yellow glue dries stiff and helps the curved door hold
its shape. The folks at Kerfkore recommend yellow
glue for both two-ply and phenolic-backed veneer. The
grain on the back of two-ply veneer runs 90 degrees to
its face side and adds stiffness to the door when the
glue dries.

Attach the back veneer with a vacuum press

7. A curved form—made from two curved ribs and a spare
piece of Kerfkore covered with plastic laminate—provide
the shape. A vacuum bag provides clamping pressure.
Mark centerlines on the form and on the ends of the
door. Line up the marks prior to clamping to ensure the
curved door ends up straight after the glue dries. With
the whole assembly in a vacuum bag (see Sources, below), pump the air out. Netting in the bag prevents
air pockets from forming. Let the glue dry completely
before you remove the door.

Trim the curved edges

8. This shop-made edge guide
(see photos, right) simplifies
routing the veneer on the
curved edges. The guide fence
has an angled opening and
clearance slot that allow it to
trim around the curved overhanging
veneer. It works equally
well on the convex and the
concave sides of the door. You
can use either ball-bearing guided
straight bit or a standard
straight bit with this guide.
The edge guide is slotted so its
fence can be adjusted flush with
the edge of the router bit.

Veneer the ends and trim flush

9. Apply veneer to the end with contact
cement or yellow glue and trim flush.
When all edges and veneers are trimmed, the
doors can be installed. Hanging a curved door
is not difficult. Butt hinges go on the same way
as they do with a flat door. For European-style
hinges, support the door on the drill-press
table so the hinge-hole drill bit drills square to
the surface of the door. If the hinge-cup screws
land in a gap between the wood ribs, squeeze
in some epoxy glue as a filler and anchor.