If you have followed many of the projects in this blog
you will know that I frequently use a sacrificial fence when cutting tenons or
making rabbits. The usual method is to
clamp a scrap piece of plywood or similar to the table saw rip fence and bury a
part of the dado blade in it. For
example, if I needed a 15/32” wide rabbit a ½” wide stacked set of dado blades
would be used with 1/32” of it buried in the sacrificial fence. That looks like this.
The problem with that is over time the sacrificial fence
can get chewed up as different depth and height dados get cut. To get around wasting fence material plus
adding some straightening, tapering and pattern cutting functionality I decided
to make an L-Fence using one described in Fine Woodworking as a starting
point. Here is what the SketchUp 3D
model looks like with an exploded view on the bottom. In operation the center section gets adjusted
up and down depending on the need.
More on the functions later.
To use, the jig gets attached to a fence straddling
assembly I made some years ago for another purpose. The assembly fits over the rip fence and is
held in place with a couple of toggle clamps.
You can see the clamps in the right photo (green arrows). The assembly also has seven attachment points
for bolting jigs to it. In the photos
the red arrows point to “T” nuts at some of the bolt attachment points.
In the top drawing below the L-Fence has been bolted onto
the fence straddling assembly and the center section has been adjusted so the
table saw will cut a rabbit narrower than the dado blade stack. The bottom drawing is a closer view showing
how the board rides along the edge of the L-Fence while clearing the blade
(dark red).
In these working drawings I did dimension all the pieces
but they are more for guidance rather than targets that need to be hit
exactly. Most important is the nesting
of the parts, clearances and the need for aligning holes between pieces. To accomplish that I start with the “L” part
of the fence and generally work my way back out toward the back mounting board
fitting pieces as I go.
The “L” part of the fence will be made out of red
oak. Sorting through my leftover bin of
scrap oak I pulled out a stack of potential candidates shown in the bottom photo for making the glue-up . There is more
here than needed but some have flaws like splits or cracks that may not be able
to be worked around.
After excluding those pieces with flaws there was still plenty to use.
Fortunately, none had any twist or any real cupping. However, about half of them had a bit of
crook or a sideways curve to them. That
can be taken care of by clamping my 8’ level to the table saw’s rip fence and
using it as a long reference surface.
The high side of the piece is placed away from the level as shown in the
left photo. That edge is then cut giving
me a straight edge. The piece is then
flipped so the new true edge is against the level and the bad edge is cut staight. There are a couple of constraints
with this method. First, the piece can’t
be longer than half of the length of straight edge used or with my 8’ level no
longer than 4’. That’s because the high
corners have to be in constant contact with the straight edge to make a true cut edge. Second, the piece cannot rock when
placed against the level. If it does the
cut edge will not end up straight. The
right photo illustrates the single point of contact at the end. The other end provides the second point of
contact.
With the crook removed next is gluing the first two
pieces together. Most of the time when
gluing up multiple pieces it’s the narrow edges that get glued together like in
the top photo below from the Porch Swing/Glider. In this case two wide pieces are glued edge
to edge. Because of the pieces width
they are stiff enough the clamping won’t distort the pieces so the glue line
and edges will remain straight. As
pieces get narrower, they get more flexible and aren’t stiff enough to keep the
glue joint straight. The bottom photo
shows rockers for a Maloof style rocking chair made from ten thin layers glued
and bent around a form.
My point is these pieces while thicker than the layers
used in the rocking chair it is still the thin dimension getting glued
together. As such I felt it was possible
that the overall straightness could be compromised during clamping. To make sure they stayed straight while
gluing I decided to use the top of the workbench as a flat reference
surface. That’s done by first setting
down a piece of 6-mil poly sheeting to keep the pieces from being glued down to
the workbench if I get carried away with the glue. Once the glue is applied the pieces are
clamped together using the front rail of the workbench as a clamping surface. Shown is the first of three sub-assemblies.
There is one other quirk to this sub-assembly
glue-up. It didn’t cause a problem in
gluing them together but was different.
That’s because the two pieces used here are cutoffs from a tapered leg
so they have a taper running from end to end.
In the top photo I drew a pencil line along the joint so you can
see what I mean. The bottom photos show
the left and right ends of the piece.
Since the taper in both pieces is the same the finished top and bottom
surfaces ended up being parallel.
Two more two-piece glue-ups finished the
sub-assemblies. Once these
sub-assemblies are cut to a consistent thickness the three of them get glued
together using the same process as before.
After the glue cures overnight, the blank gets run through the thickness sander to flatten out any irregularities and the ends get trimmed square. As mentioned earlier the dimensions for most of this project are guides. That’s true in this case as this piece for the “L” bracket was listed as ¾” thick but ended up at 1 1/16”. I had hoped for a 1” thick slab but this will work out better.
Next Up – Stopped Dado, & Slotting Adjustable Backer plus Beveling “L” Piece
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