The saga of the Bird Mouth Bit #2: Last preps for the cut

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Blog entry by splintergroup posted 01-08-2021 05:02 PM 299 reads 0 times favorited 1 comment Add to Favorites Watch
« Part 1: Setting up the material Part 2 of The saga of the Bird Mouth Bit series Part 3: Assembly with a clamping jig »

Last entry left off with a pair of strip-veneered panels.

Karelian birch and zircote over a poplar core.

These are each about 12+ inches long, 6” wide, and a bit over 3/8” thick + the veneer thickness. Grain of the poplar runs along the long dimension, the veneers are laid out cross-grain on both sides.

With veneer and miters, the edges are very susceptible to chips and other thing that can happen , especially when crosscutting the veneers grain. My goal was to form a “perfect” hexagon and in general I like to add details to items like this and where these miters will eventually join is a perfect place.

First I re-trim the long edges of each panel, this cleans off any glue squeeze out from the veneering process and gets my future mitered pieces to have parallel ends.

Some masking tape along the top and bottom edges where these trim cuts happen will reduce and veneer chipping, but I’m not too concerned since they will be covered.

The plan is to boarder the edges with some walnut.

I first run the walnut through the drum sander to remove a small warp.
Concave side down with shims supporting the center. Light passes (180 grit) until the top is flat. The part is then flipped and the concave side is made flat.

The DS is probably my second most used tool behind the table saw. With the sled, I can remove twists, warps, and other defects easily.

Four approximate 1/2” x 1/2” slices are cut, then returned to the DS to clean up the saw cuts on each side.

I will cut a centered, shallow (1/16”) groove along the length of each part to get a tight fit over the sides of my panels. The box joint blade makes these easy, although multiple passes with a FTG rip blade will also work.
Cut one pass, rotate 180 degrees, cut again. Perfectly centered.

To the bat saw! (apologies to Batman fans)

I roughly center the first cut, measure the thickness of the panel and my groove width, and determine the amount I need to move the fence.

A dial indicator on a magnetic base is a super accurate and quick way to move the fence a perfect amount.

There are gizmos you can equip a fence with to do the same thing, but I haven’t jumped down that rabbit hole yet 8^)

Grooves are done!

Notice I still haven’t cleaned up the ends of these panels, that comes later.

I final sand the veneer now when it is easiest (down to 320).

Glue the strips to the panels and clamp it all up

Cauls are used to keep it all square and the veneering cauls are used again as plates to keep the assembly flat.


I’m OCD enough to do a light skim cut on each side to ensure straight and parallel.

The Bird Mouth Cut

Yeah I know, about freaking time!

This is a large bit compared with my usual selections

Given the out of balance of this Yonico, I run my router at about 12000 RPM.
Bit setup requires the “point” of the bit to be at 0.5 of the panels thickness. Protrusion from the face of the fence should be 0.866 of the panels thickness.

Having the six individual side parts as a single panel will allow the BM bit cut to be done for all six sides at the same time. Much easier to do it this way then struggle with passing narrow parts past the bit and keeping everything square.

The 0.5 setup is easy enough. Measure the panel with some calipers, divide by 2 and use a ruler to set the height

1/64” below 1/4”. Router lifts are very handy!

Actually you just need to get close enough by eyeballing it.
The easier way is to move the fence so the point just protrudes enough to nick the panel as it is passed through.

It’s a lot easier measuring this bit-scored line than measuring the bits point above the table! If it is not exactly 1/2 the thickness, you can tweak it or just cut the birds mouth a bit deeper than planned. This is one reason I don’t usually stick to dimensions unless the item needs to fit something else. The sides will be about 6”, but if I lose 1/16” due to needing a deeper cut because of a slightly offset bit setup, who cares!

Now for the depth of cut into the panel.
Bit setup is spec’d at 0.866 of the thickness, which is theoretically ideal. In actuality you just cut until there is a sharp corner on the outside edge of the panel.

(The panel is routed with the eventual outside surface facing up)

I cut the BM a little at a time. makes for a cleaner cut and less stress on the bit.

Almost there!

A worthwhile trick:

With these bits (including the 45 degree lock miter bits), the cut is finished when you have a sharp edge. The problem is if you cut slightly too deep, the workpiece will not ride flush with the fence on exit and the cut will be ruined. If you cut just right, the workpiece will be riding the fence with a sharp edge that will end up crushing (screwing it up).

The solution?
Double side tape a straight edge, cut from hardboard, to the workpiece!
I place the panel edge against my table saw fence, then drop on the hardboard and press the two together with the tape in between. This gets them perfectly aligned. Don’t use too much tape, you only need enough to hold things in place.

The bit depth is advanced until there is a sharp corner. In fact, you probably want to be cutting slightly into the hardboard to be certain you have a proper cut. The remaining flat edge on the hardboard will keep the cut running true.

You only need to cut one side of the panel.

All done!

Next up is to slice up the panels to create the six side pieces and glue it all up.

Hopefully see y’all there!

1 comment so far

View JimYoung's profile


399 posts in 2563 days

#1 posted 01-08-2021 08:31 PM

Good information and tips on setting up this type of bit. Thanks!

-- -Jim, "Nothing says poor craftsmanship more than wrinkles in your duck tape"

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