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Shim Cutting Jig – Toolbox

As a finish carpenter, I have gone through god-knows how many shims in my career, and I have a feeling I’m going to need a few more in the years ahead.

I have tried a few different variations: the shingle split into 1 1/2-in. pieces, the composite, the solid plastic, the “stack shims” which come in varying thicknesses, and, of course, the pre-cut cedar shims (of which I have probably gone through a pallet-load). All have their place, and I still use a few of the solid plastic shims for exterior projects, but for the past few years I have been cutting my own. This may seem like an exercise in the mundane, but the cost of high-quality shims became prohibitive, and fumbling with poorly-cut cedar shims finally became too much trouble.

First of all, I want to point out that the jig I designed is not an original idea. I’m sure some of you out there already have a similar setup. I simply tweaked the design to be a little bit easier to operate and, in my opinion, a bit safer, too.

Just for basic understanding, this jig is simply a fixed tapering jig. I started with a piece of 3/4 birch plywood that is roughly 10 in. x 10 in.. Why birch? Because that was what I had on hand.
Next, I attached a piece of 3/8-in. x 3/4-in. hardwood to ride in the miter gauge slot of my table saw. Use staples, brads and glue, 1-in. screws, whatever’s handy.
I attached the piece parallel to one side, roughly 6 in. from the edge.
For the handle, I grabbed my shop-made push shoe and traced it on another piece of 3/4 ply.

Really, the style of the handle is up to you: The first shim jig I ever saw just had a chunk of 2 x 4 screwed into it. Just make sure your handle is at a comfortable working height, that it is far enough from the blade for your safety, and that the angle of your grip allows you to press down and forward in the same motion (you can see why I just copied my push shoe).

After you cut out your handle, round over the edges.
I just ran in a few screws to attach mine.

Check your saw before attaching. The distance from your blade to the miter gauge is all you need to know. It’s not imperative that you be exacting or even exactly square with the edge.

All you need is for the base to ride in the miter slot, and for enough of the base to hang past your table saw blade.
Once fixed in place, you simply set the jig into the miter slot and run the base through the blade. Now the edge of the jig is exactly square with the guide.

The shims I like are edge grain, 9 in. long, and taper to 3/8 in. at one end. This is worth noting because the next step sets up your desired length and amount of taper. If you like a shim that is 8 in. and tapers to 1/2 in., or say you prefer 7 in. shims that taper to 1/2 in., the process is still the same regardless of dimension.

I begin by making a mark 1/2 in. from the leading edge of the jig along the same line where I just made the squaring table saw cut. I then make another mark 9 in. from my first. This represents the length of stock I use. Next, I make a line 3/8-in. square to the edge of the jig. If you connect the end of the 3/8-in. line and the first mark that is 1/2 in. from the front of the jig, you will see the exact size of shim this jig will produce. However, I go a bit further.

At the end of the 3/8-in. line, I make another line that is 3/8 in., parallel to the side of the jig and pointed towards the front, then another line that is 3/8 in. and square to edge like the first. This allows you to cut shims that are either tapered to 3/8 in. or 3/4 in. (for those times when you need a SHIM) depending on where you index the stock. At this point, I draw a line with a straight edge from the end of my final 3/8-in. line to my first mark 1/2 in. from the front. I cut out all the lines with a jigsaw. At this point, you are technically ready to cut, but now for the safety part.

My biggest gripe with this jig was that once you passed your stock through the blade, it would occasionally send the freshly cut shim flying because it would catch on exiting the blade. To remedy this, I took a piece of 3/4-in. hardwood that is 1 1/4 in. x 3 in. Off of one corner, I created a haunch that is 7/8 in. x 7/8 in. I installed this piece so that the haunch hangs over the new shim, but not the rest of the shim stock. This acts as a hold down, and will keep your shims from becoming missiles (see drawing below for more detail).

(Click to enlarge) The small hold-down at the rear of the sled is a critical safety feature and prevents the shim from slipping into the saw blade after it’s cut. Notch the hold-down so that it applies just a bit of pressure to the top of the stock, and chamfer the leading edge so it’s easy to slip the 2x under the hold-down.

As for technique, I take my shim stock (which is usually a 9-in. off-cut of 2 x 6 framing lumber) and nest it against the front of my jig and into either of the notches.
Pass the stock through,

then flip the stock so that the front faces back and the top faces down. Then you repeat this process, “cut, cut, flip… cut, cut, flip…” If you just “cut, cut, cut,” you’ll end up with a chunk that looks like a piece of pie (I know from experience). Of course, you won’t be able to utilize the whole block, but I usually have less than 2 in. of waste for every piece of stock I use.

I have always just cut edge grain shims, but I know there are some carpenters who prefer end grain shims. Supposing you wanted to cut shims from 2 x 6 framing lumber, all you would have to do is change the 9-in. dimension to 5 1/2 in. and use the same notch setup that I described, or use a different layout that suits your fancy.

The only thing I would do differently is change from a ripping blade to a combination. It would probably produce a better and smoother surface.

Finally, a word on safety. We all know that woodworking can be dangerous. Don’t try anything that makes you uncomfortable. Start with a blank, and only cut 6 or 7 shims till you feel comfortable with the process. Also, you can raise the riving knife to where it extends over the top of the blade as an added bit of safety. It won’t interfere with the function of the jig; you’ll just have to take off the anti-kickback mechanism.

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Please don’t try anything you see in THISisCarpentry, or anywhere else for that matter, unless you’re completely certain that you can do it safely.


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