Table Saw Dados
Getting clean dado cuts from your table saw and dado blade is not as easy as it might appear. More often than not, you'll discover that the dado blade has left a fairly ragged edge on the shoulders of the cut. This can happen in both softwood and hardwood - and is even more of a problem with plywood. The good news is that with a few simple techniques, you can virtually eliminate rough edges and tearout with dadoes.
First let's take a look at what causes tearout with dado blades. The structure of the blade itself is mostly to blame. A dado blade has to move a lot of wood debris out and away from the cut while it's cutting. The only way to make this happen is to have wider gaps between the teeth and broader gullets. Unfornately, this type of blade design leaves a much rougher cutting edge than more conventional table saw blades. What's more, the cutting action of the blade itself tends to beat up a wood board more than usual, and this can also create a rougher edge on your workpiece.
Solutions to the problem:
Cut Dadoes on oversized board
Most of the tearout trouble caused by dado blades happens along the edge of the board - where the wood fibers are most fragile. One simple solution to avoid this problem is to first cut the dadoes on an oversized board, then trim the board down to the size you need - removing whatever rough edges the dado blade may have left.
Score the edges
The cleanest cut you can get on just about anything comes from a simple razor. One of the easiest ways to ensure clean edges on a dado is to score the edges first (with a utility knife) before you head to the table saw. Just a few strokes with the knife will keep the dado blade from roughing up the edge. This method works particularly well with plywood.
Lay down masking tape
Putting down a strip of masking will keep your dado blade from tearing up wood fibers. This works really well along both the cut line and the end of the dado. When removing the tape, keep in mind that the wood fibers are still pretty fragile at this point, so pull the tape towards the dado - not away from it.
Use a zero-clearance insert
Most table saws out of the box come with a fairly large gap around the saw blade. This helps shoot wood debris and dust away from the blade, but it also leaves the delicate edge of a board hanging on its own while the blade tears away at the fibers. This setup might work fine for most work on the table saw, like simple rip cuts and crosscuts, but with more delicate work - like dado joinery - woodworkers have already figured out that a zero-clearance insert is pretty much an essential in the wood shop. As the name implies, a zero clearance insert replaces your factory-installed insert, with a much smaller gap for the blade. This means more support for your wood pieces where it counts the most - up close to the blade.
Use Clamps and Hold-downs
The less your workpiece shakes or vibrates, the cleaner dado cut you'll get. That's why if I have the room to do it, I like to clamp my workpieces down before sending them across a dado blade. This keeps my boards nice and snug against the table and fence while the dado blade churns away at the wood. You can buy manufactured clamps and hold-downs (like featherboards from Bench Dog Tools) that attach to your table saw fence.
Take multiple passes
There's nothing that says you have to cut the entire dado in one full swoop. Sometimes you'll get a cleaner edge if you take it slow, and make several gently passes over the blade. The first pass is the most critical - as this is where you'll determine just how clean and sharp of an edge you'll have on the shoulder of the dado.
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