By Martin Harris.

How to Build a Miter Saw Dust Hood

A miter saw dust hood is an open front box that sits behind the saw and catches the dust the blade throws backward and up. Build it wider than the saw's full miter swing, deep enough for the slide rails and motor, and put a 4 inch or larger port low at the back behind the blade. Close in the sides as far as your work allows, and keep the saw's own dust port hooked up too. That combination catches far more than the stock bag or port alone.

I cover why miter saws are so hard to collect from in my main miter saw dust collection guide. This page is the build: how big to make the hood, how much airflow it needs, how to put one together from plywood, and the flap and curtain tricks that close the gaps.

Why a miter saw throws dust behind and above

Look at which way the blade spins. On a miter saw, the teeth at the front of the blade come down into the wood. That means the teeth at the bottom of the cut are moving toward the back of the saw, and then they rise up behind the fence. Chips and dust ride along with the teeth and get flung out the back and up in a fan shaped spray.

The stock port sits in the upper guard, near one small part of that spray. Most of the stream misses it. A sliding saw adds a second problem: the rails and motor move forward and back, so the port moves too, and the space behind the saw stays open. The dust has nothing to hit except the wall.

A hood works because it gives that spray a place to land. The dust is already moving toward the back of the saw at high speed. The hood only has to catch it and hold it long enough for the collector to pull it away.

Why the hood has to be close and wrap around

Suction falls off fast as you move away from an opening. Bill Pentz works this out on his dust collection basics page. Fine dust needs air moving at least 50 feet per minute (FPM) to pull it in. By his math, a 6 inch duct moving 785 CFM at 4,000 FPM only holds that 50 FPM out to about 13.42 inches from the opening. Past that, fine dust drifts where it wants.

Donaldson, an industrial filtration maker, shows the same thing with the hood equation from the Industrial Ventilation Manual. In its worked example, cutting the distance between the hood and the dust source in half cuts the airflow needed by about four times. For a miter saw, the lesson is to bring the back and sides of the hood as close to the blade as the saw's movement allows, and make the hood surround the spray instead of sitting behind it like a small window.

Sizing the hood

There is no single published size, because miter saws vary so much. Use your own saw to set the numbers:

• Width. Swing the saw to its full miter angle in each direction and measure the widest point the motor and handle reach. The inside of the hood needs to clear that, with a little room to spare.
• Depth. On a sliding saw, push the head all the way back and measure from the fence to the back of the rails. The hood has to swallow the rails at full travel. A chop saw without rails needs much less depth.
• Height. Raise the head to its full up position and measure to the top of the motor. The hood's top edge should clear it. Tilt the saw to its full bevel and check again.
• Opening. Keep the front opening only as large as the saw needs to move. Every extra square inch of opening spreads the same airflow over a bigger area.

For a sense of scale on commercial hoods, the Rockler Dust Right Mountable Dust Hood has a 14-3/4 by 11-1/4 inch opening, and Rockler lists it for miter saws, lathes and other tools with poor dust collection. That size works well behind a compact chop saw. A 10 or 12 inch sliding compound saw usually needs a shop built box that is much wider and deeper.

Port size and CFM

WorkshopCalc's reference chart puts miter saws at around 300 to 350 CFM. That is a chip collection number. Bill Pentz argues that many stationary tools need closer to 1,000 CFM when the goal is capturing fine dust. A miter saw hood is a big open box, so more airflow always helps it.

The port size sets a ceiling on how much air you can move while keeping duct velocity up. Pentz lists how much airflow each duct size carries at about 4,000 FPM:

Source: Bill Pentz dust collection basics, duct capacity at a 4,000 FPM target. Match the port to what your collector can deliver through your duct. A 6 inch port on a collector that only moves 350 CFM at the saw drops duct velocity too low. My 4 inch vs 6 inch dust collection comparison and my CFM calculator help you check your own setup.

How to build a plywood miter saw dust hood

Most shop built hoods are five sided boxes: two sides, a back, a top, and a bottom, with the front open. Half inch plywood is light and stiff enough for most hoods. One published build from the Saws on Skates blog frames the hood in 1x4 pine, lines the inside with 1/8 inch hardboard for a slick surface that sheds dust, mounts a 4 inch fitting in the bottom, and angles the top to shrink the inside volume and steer dust toward the port.

1. Unplug the saw. Keep it unplugged the whole time you are measuring and fitting parts around it.
2. Take your measurements. Record full miter width both ways, full slide depth, full raised height, and full bevel clearance, as described above.
3. Sketch the box. Draw the sides with a sloped top edge that runs from the front opening down toward the back. The slope reduces the empty space above the saw and aims dust toward the port.
4. Cut the sides, back, top, and bottom. Cut both sides together so they match. If you want a sloped back as well, bevel the edges where the back meets the top and bottom.
5. Cut the port hole. Place the port low and centered behind the blade, where the spray lands. Cut the hole to fit a 4 inch or larger flanged fitting, the kind with a flat plate that screws to the panel.
6. Assemble with glue and screws. Glue and screw the panels together. Line the inside with hardboard if you want the smoother surface.
7. Seal the inside seams. Run caulk or aluminum foil tape along every inside joint. Gaps at the back pull air from behind the hood instead of from the front where the dust is.
8. Mount the hood to the station. Set the saw in place, slide the hood behind it, and cycle the saw through every position by hand, unplugged. Nothing on the saw should touch the hood, and the lower blade guard must move freely. Then fasten the hood to the bench or wall.
9. Connect the duct. Run the hood port to your main with a blast gate. My guide to dust collection fittings covers the flanges, reducers, and wyes you may need.
10. Keep the saw's port connected. Tie the saw's own port into a small hose with a wye into the hood branch, or run it to a shop vac. The two pickups work on different parts of the spray.
11. Add flaps and test. Plug in, turn on the collector, and make a few cuts. Watch where dust escapes, then close those gaps with the tricks below.

Curtain and flap tricks

Even a well sized hood leaks dust out the front. The builder of the Saws on Skates hood says a good portion of airborne dust still escapes from the front, and suggests adding sliding or adjustable doors on the sides. Here are ways to close the gaps:

• Side flaps. Hang strips of heavy clear vinyl from the front edges of the hood sides. They fold out of the way when you swing the saw and fall back to narrow the opening.
• A top curtain. A clear vinyl strip across the top of the opening, hanging down to just above the raised motor, keeps dust from spilling over the top edge.
• Hinged or sliding side doors. For a saw that mostly makes square cuts, a door that closes in each side of the hood down to the width of the saw makes a big difference. Open it for wide miters.
• Brush strip at the fence gap. Stock brush strip, like the kind used on door sweeps, can fill the gap behind the fence where the blade passes, while still letting the saw move.
• A grate over the port. Offcuts get pulled into a strong hood. A simple grate or a few dowels across the port keeps short pieces out of your duct.

Keep every flap and curtain out of the blade's path and away from the guard. Use clear material so you can still see the cut line. Never remove or tie back the saw's lower blade guard to fit a hood. If the guard hits the hood, rebuild the hood.

Shop vac or dust collector for the hood

A big hood opening needs volume, which is what a dust collector moves. A shop vac moves much less air, but at higher suction, which suits the saw's small stock port and a narrow hose. That is why the split setup works well: collector on the hood, shop vac on the saw's port. My dust collector vs shop vac comparison explains the difference. For more hood shapes for other tools, see my guide to dust collection hoods you can build, and for planning the whole shop, my complete guide to small shop dust collection.

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