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We all know that a 3/4" thick piece of plywood is rarely 3/4" thick. Router bit
manufacturers have made "undersized" bit sets just for this reason, but chances
are good that they won't fit exactly, either. The problem is that these panels will vary
and they can even vary from board to board. You can go nuts because of this, or you can
learn this quick and very accurate way to make exact dados. I have been using it for years
now (but that is not to say I am not nuts.)
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I asked
Andrea to rummage through the scrap bin and find several examples of 3/4"
stock. She measured each and found quite a variety. The Lyptus was a full 7/8", the
MDF was 3/4", a piece of plywood was almost 1/8" less, and the 3/4" pine
was 13/16".
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She
uses a 3/4" straight bit to view the discrepancy. It is hard to tell this way but if
we make dados using this bit, the various samples will not fit.
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To start
my procedure, Andrea starts by installing a 1/2" straight bit. We could also be using
a 1/2" up-spiral bit. The very same procedures would be used. We will be using the
Festool OF 1400 router.
Andrea asked why we use a 1/2" bit rather than something closer to the
final dado. My answer is that since we need to make the dado in two passes, we can use any
smaller bit we want. The 1/2" bit just makes for an easier first pass since we will
be removing less material.
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She plunges the router until the bit just touches the workpiece and locks it
at that point.
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She then
uses a 3/8" spacer and locks the plunge stop to this depth. I like to make
dados 1/2 the thickness of the board itself.
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Now
Andrea installs a guide rail system that we have put together from other Festool
accessories. The large knobs allow us to adjust and tighten the stops at points along the
edge guide rods. Her left hand is on the nut that locks the rods in position on the
router.
We will be using the Festool guide rail system in this example. The
method can also be used with other routers using a clamping straight edge.
See also my review of a year or so ago of the Accurate Guide. Here: "New Product: "The Accurate Guide" for Precise Dados
and Sliding Dovetails "
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She will
first set the router guide so that the first cut is 3/4" away from the guide rail.
This is somewhat arbitrary but works well for us. The rail is clamped to the workpiece.
This would also be a wonderful use for the MFT, but there are times (like now) when the
work table is more accessible.
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Andrea
uses the outside stop to move the router closer or further away from the guide rail. The
inset shows the view she has of the router bit aligned to the pencil mark she made.
Once that is set, the outer knob is untouched so that the router can be positioned at the
3/4" mark without sighting or marking the line.
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To this point, she has made all her adjustments with the dust extraction and power
connection not made — a great safety feature. Now she can connect these. They are
coming down from the boom arm.
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