Wire Inlay

Make Delicate Designs in Wood with a Plunge Router, a Template, and a Tiny Bit.

Wire inlay is the art of setting narrow ribbons of metal into grooves cut in a wooden surface. Traditionally, it was done with hand tools to decorate gunstocks and boxes; modern applications are limited only by your imagination. My technique uses a plunge router, templates and tiny machinists’ micro-end mill bits to cut the grooves. It’s a simple and accurate method that gives me a lot of creative freedom. In this article, I’ll discuss the materials, tools and techniques I use in my work.

Tools and Materials
Metal-Although this technique is called wire inlay, I actually use narrow (1/8-in.-wide) strips of metal, of varying thicknesses. I have used three metals in my furniture: pewter, silver and brass, and I recommend them in that order.

Pewter is a good metal for inlaying because it is relatively soft. In fact, it’s so soft that it’s generally not available in strip, or wire, form. But I’ve found that I can rip pewter sheet into strips on the tablesaw with a metal-cutting blade. I use a 6-in.-dia., 190-tooth, high-speed-steel metal-cutting blade (see Sources, page 47) and a zero-clearance throat plate. (See top photo, page 46.) If you don’t feel comfortable ripping sheet metal yourself, a local metalworking shop can cut it for you on a metal shear.

Silver is another fairly soft metal. Fine silver (unalloyed) is easier to work than sterling silver. You can buy silver in sheet form, but it is also available in precut strips-which I prefer-or in longer, coiled strips commonly called flat wire. (See Sources.)

Brass is more difficult to work with. You’ll have to buy it in sheet form, and I do not recommend ripping it yourself; it is too hard to rip safely on a tablesaw. And because of its relative hardness, brass is also more difficult to inlay.

The thickness of the metal determines the width of your inlay. When you’re designing the inlay pattern and choosing the metal, keep in mind that while metal thickness is measured in gauge, micro-end mill bits are made in 0.005 in.-dia. increments. (See chart) It’s simplest to choose a metal gauge that coincides with a micro-end mill bit diameter-such as 18, 22, 24 or 30 gauge. This will give you a perfect fit.

Bits-Machinists’ micro-end mill bits are like straight router bits, only much smaller. The ones I use have a 1/8-in.-dia. shank that tapers into a tiny cutting end with straight sides and a flat bottom-perfect for routing delicate grooves.

Micro-end mill bits are available in high-speed steel or carbide. (See Sources.) I prefer carbide bits. Several designs are available; for this procedure, you’ll want a two-flute, single flat end bit. Choose an end diameter that matches the metal thickness you plan to use. (See chart.)

Bushings-Aside from the bits, I need a bushing to adapt my 1/4-in. router collet to the 1/8-in.-dia. Bit shank. I also need a template guide bushing to guide my router along the template. (See Sources.)

Making Templates
Router templates give me smooth, accurate grooves. Making the templates requires a little forethought. I have to offset the profile of the template to account for the distance from the outside of the guide bushing’s rub collar-which rubs against the template-to the centerline of the bit. This works out to half the outside diameter of the rub collar. So, with my 1/4-in.-o.d. guide bushing, I need an 1/8-in. offset. (See drawing detail.)

The template material should be at least as thick as the depth of the rub collar on your guide bushing. (See drawing.) I make my templates from 1/4-in.-thick Baltic birch plywood; it provides a clean edge when worked with hand tools.

An easy way to make templates is to mark the centerline of the desired groove on the template blank, set a compass to the offset distance (see drawing), mark an offset line parallel to the original line and bandsaw almost to the offset line. I refine the edge with a spokeshave, a scraper and sandpaper.

With the template/guide bushing arrangement, I can’t see exactly where the routed groove will fall on my workpiece. If you’re routing a simple pattern, the template alone provides sufficient guidance. For more accurate results-especially on complex curves-I glue a layer of maple backing veneer to the bottom surface of the template. Then, with the guide bushing riding firmly against the template, I rout through the veneer with the micro-end mill bit. The resulting edge of the veneer shows exactly where the groove will be, so I can position the template exactly where I want it on the work. (See drawing.)

Preparing the Wood and Metal
For my inlay surface, I usually use a thick veneer (l/16 in. to 1/8 in.) pre-glued to its substrate. Thick veneer allows some leeway for the flush-off process. You can also lay metal into solid wood, if lines don’t travel far in the cross-grain direction. Wood movement can cause cross-grain inlay to loosen over time.

I seal the wood with a wash of four parts water to one part liquid hide glue before routing the groove. This helps prevent darkened pores later, if epoxy squeezes out of the groove when I glue in the metal. Open-grained wood like oak is especially prone to darkening; I recommend experimenting on a sample
with the epoxy, a sealer and your finish.

If I’m cutting my own pewter or silver strips on the tablesaw, I set the metal-cutting blade low and run the sheet metal slowly over the blade. I run the bulk of the sheet between the blade and my wooden auxiliary fence, and I hold it down with my jointer push stick, which I’ve shimmed up with veneer, double-faced tape and a layer of sandpaper for a better grip on the metal. (See photo) The push stick has a “heel” on the back, so it contacts the sheet on one side and one edge. I cut one strip, then adjust the fence and cut the next strip. Strips should be wider than the inlay groove depth, so you can do a test fit and still remove them to glue. I rout my grooves 1/16 in. deep and cut my strips 3/32 in. to 1/8 in. wide.

Routing the Inlay Groove
Next, I position my template on the workpiece and clamp it in place. (See photo.) I do a “test run” without the bit plunged down, to get a feel for the template.

I set the plunge depth to about 1/16 in., keeping my strip width in mind. If you have a variable-speed router, set it to the fastest speed. Keeping the guide bushing against the template and the stop, I plunge into the surface, and feed very slowly-about 6 seconds per inch, or slower on short or curvy sections-to minimize strain on the fragile bit.

Little cleanup is necessary with my technique. I remove any debris with a mat knife or by blowing compressed air into the groove.

If two lines meet at an acute angle or with little grain between, I’ll inlay one line and flush off the inlay before cutting the groove for the second line, to avoid short grain breakage in the corner.

The hole in a standard router sub-base (see drawing) isn’t always perfectly centered, which can create a wobbly fit if you inadvertently rotate the router while cutting. It helps to keep the router facing the same direction as you guide it along the template.

I wanted even greater accuracy, so I replaced my router’s standard sub-base with a piece of 1/4-in.-thick acrylic. To get a perfectly centered hole for the guide bushing, I mounted the acrylic on my router base and plunge-cut a hole with a bit the same diameter as the guide bushing’s threaded section. On the underside of the acrylic, I counterbored (going only partway through) with a bit whose diameter matched the guide bushing’s widest section. (See drawing.) Then I attached the guide bushing and remounted the acrylic right side up.

Laying In the Metal
Once the groove is cut, I snip a strip of metal slightly longer than the groove with wire cutters and file the end to fit. Where inlay lines intersect, I bevel the end of the strip as necessary. I ease the bottom edge of each strip slightly with sandpaper so it goes into the groove more smoothly, I test fit, I pre-bend the strip to the curves of the groove with needlenose pliers. (See photo.) This makes the final fitting much easier. When the strip fits well, I remove it and scuff lightly with sandpaper to improve glue adhesion. Then I clean it with acetone.

I glue the inlay in place using West System Brand Epoxy 205 hardener with the 105 resin. (See Sources.) Epoxy adheres well to metal, but it can darken the pores of the surrounding wood. While sealing the wood ahead of time limits epoxy penetrating, for extra insurance I often lay masking tape next to the groove to catch squeeze-out.

I spread the epoxy along the groove with a splinter of veneer or whatever will fit. (See photo) Then, I place the metal in one end of the groove and press it in gently with a hammer head. You don’t have to bear down too hard; just make sure the strip is seated firmly in the groove. I wipe off squeeze-out before it dries.

Flushing Off and Finishing
I remove most of the metal standing above the wood surface with my router and a 1/2-in.-dia. carbide straight bit. I block up the router to span the metal (you can use your templates to work one narrow area at a time) and set the bit to just above the surface of the wood. You could use a single-cut mill file instead; just be sure to tape up the ends of the file so they don’t mar the wood.

Once I’m close to flush, I finish up with a scraper or a hand plane-unless I’m working with brass. The only way I’ve found to work brass is with sandpaper wrapped around a hard block. Even then the metal provides much more resistance than the wood, and a slightly undulating surface can result. You can finish the inlaid surface with any product that protects the metal from tarnishing. Traditionally, lacquer was used. I’ve used Waterlox and shellac successfully thus far.

American Woodworker, April 1996

Sources:

Pewter, silver and brash sheet and silver wire:
T.B. Hagstoz and Son, Inc.
(800) 922-1006 Circle #601

Precut fine silver strips:
Hoover & Stron
(800) 759-9997 Circle 602

Metal-cutting tablesaw blades:
J&B/Eastern Saw and Tool Service
(215) 467-2245 Circle #605

Machinists’ micro-end mill bits:
MSC Industrial Supply Co.
(800) 645-7270 Circle 604

Bushings to adapt a 1/4-in. collet to 1/8-in. bit shanks:
MLCS Ltd.
(800) 533-9298 Circle # 605

Template guide bushings:
CMT Tools
(800) 531-5559 Circle #606

West System Brand Epoxy:
Gougeon Brothers, Inc.
(517) 684-7286 Circle #607

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