Lock miters are strong,
attractive joints that make
assembly easy. So why the
heck don’t we use lock miters more?
I think it’s because they can be a pain
to set up.Well,no more excuses.Here’s
a clever technique, sent to us by Jim
Rodgers of Martinez,California. Give it a
try and you may become a lock miter
What’s a lock miter?
A lock miter router bit cuts a 45-degree miter with a
tongue and groove. When correctly cut, the parts go
together at a perfect 90-degree angle and the interlocking
tongues and grooves make for lots of mechanical strength
and glue surface area. Lock miters are also great at keeping
parts aligned during assembly.Use this joint on drawers,
boxes or even hollow columns like newel posts.You can
cut a lock miter on end grain, as shown in our photos, or
on the long grain.Almost anyplace you’d use a miter, you
can successfully use a lock miter.
Lock miter bits come in a range of sizes.The size you use
depends on the thicknesses of your wood.Even the smallest
lock miter bit makes a substantial cut, so I prefer bits
with a 1/2-in. shank. They’re more stable and result in
smoother cuts.Expect to pay $50 to $100 for a bit,depending
on the size.
Note: The maximum size lock miter bit you can run in
a 1-1/2-hp router is the 2-in.diameter.Larger bits must be
run in a 2-hp or higher machine.
It is essential that you run these massive cutters at the
right speed—about 10,000 rpm. Your router must have
variable speeds so you can slow down for these big cutters.
The perfect set-up
Follow the sequence shown in Photos 1 through 7 to produce
perfect lock miters on your router table.Remember
to have on hand the material required for your project plus
six test pieces. It’s critical that the test pieces be the same
thickness as the project pieces because the bit set up is specific
to the thickness of your material. As you get more
familiar with the set up procedure you’ll need fewer test
Lock miter tips
If you follow the steps you’ll be able to cut perfect lock
miters. Here are some tips to make it even easier:
Use wide pieces. Your project
parts must be cut to the
right length before cutting the
lock miters, but they can be
any width. Leave them 1 in. too
wide, and cut them to final
width after you’ve done the
routing. They’ll be easier to
handle, and the “blowout” you
get on the back of the cut will
be cut off when you machine to
final width. If your project calls
for narrow pieces, 2 in. to 3
in. wide,machine pieces 6 in. to
7 in. wide and rip them to the
size you need.
Use a tall fence. Holding a
piece vertically against the fence is a lot easier if the fence is high. Use one that’s about
Use a “zero clearance” fence. Having the opening surrounding
the bit as small as possible helps prevent your workpiece
from slipping into the opening.
Precut your parts. A 2- or 3-hp router will hog all the
material off in one pass, but it’s going to be a lot easier to
machine the lock miter if you precut your parts on the
tablesaw with a 45-degree bevel.Trim off only about 3/8 in. on
Use push blocks. Neoprene-padded push blocks will make cutting
lock miters easier and safer (see Sources, below).
Make a permanent set up piece. When you have all the setup
done, machine a piece and keep it for your next project. If
you want to set up the whole operation for the same thickness
of material again, use the set up piece to set the thickness of
your work, the height of the bit and to position the fence.You’ll
still need to do some test cuts and some final tweaking, but you’ll
be darn close.
Fig. A: Centering the Bit
Fig. B: Fence Position
(Note: Product availability and costs are subject to change since original publication date.)
Eagle America, 800-872-2511
Woodline Arizona, 800-472-6950
Eagle America, 800-872-2511
This story originally appeared in American Woodworker April 2000, issue #79.
April 2000, issue #79
Purchase this back issue.
Click any image to view a larger version.
A lock miter
First, center the bit on the material.
1. Center the bit on your workpiece by eye. Be sure
your router is unplugged.
2. Adjust the router table fence by eye to its
approximate position.Three points must be aligned.The
top of the workpiece, the face of the fence and the 45-
degree angle of the cutter (Fig. B).This is just a preliminary
set up.You’ll perfect the fence position later.
3. Test the height of
the router bit
by cutting two
piece flat on
4. Assemble the test pieces. When the cutter is perfectly
centered, the faces of the two pieces will be aligned.Adjust the bit
as needed.You must have the bit centered on the material
before you start working on the fence position.
Now, set the test position.
5. Examine the test cuts to determine if the
fence is correctly positioned. If
the cut looks like A, the cut is
too shallow and the fence must
be moved back. If the cut looks
like B, the cut is too deep and
the fence must be moved forward.
Adjust the fence until the
cut looks like C; producing a
perfect knife edge on the cut.
Finally, cut your parts.
6. Machine your parts. One part is held flat on the
table.The mating part is held vertically against the fence.
7. Assemble the pieces. Your careful