By Martin Harris.

Dust Collection Hoods You Can Build

A dust collection hood is the opening that pulls dust into your duct at a tool or work area. The hoods that work best wrap around the dust source as much as the work allows, sit as close to it as you can get, face the direction the tool throws dust, and have a flange or smooth taper at the edge instead of a raw pipe end. Distance matters most, because suction drops off fast as you move away from the opening.

This page covers the capture rules from published sources, then hood shapes for sanding stations, lathes, bandsaws, and bench work. For how hoods fit into a full system, see my dust collection system design guide and my complete guide to small shop dust collection.

Capture velocity falls off quickly with distance

Two kinds of air speed matter in dust collection. Transport velocity is how fast air moves inside the duct, fast enough to keep chips from settling. Capture velocity is how fast air moves at the dust source, fast enough to pull dust into the hood. A system can have plenty of transport velocity and still miss the dust if the hood is in the wrong place.

Donaldson, an industrial filtration company, explains hood sizing with an equation from the Industrial Ventilation Manual, the standard reference for this work. In plain terms: airflow needed equals capture velocity times (10 times the distance squared, plus the area of the opening). Distance is squared, so it dominates. Donaldson's example uses a 4 inch opening (0.087 square feet) and a capture velocity of 100 FPM:

Source: Donaldson, "Best Practices in Dust Collector Hood Designs," using the Industrial Ventilation Manual equation Q = V × (10X² + A). The 12 and 6 inch rows are Donaldson's worked examples. The 3 inch row is calculated with the same equation.

Donaldson sums it up: cut the distance in half and the air volume needed drops by about four times. Bill Pentz reaches the same point from the woodworking side. He says fine dust needs air moving at least 50 FPM to be captured, and he calculates that a 6 inch duct moving 785 CFM only holds 50 FPM out to about 13.42 inches. A hood a foot and a half from the work is mostly moving shop air.

Flanged vs plain openings

A plain opening, like the cut end of a pipe or hose, pulls air from every direction, including from behind the opening where there is no dust. A flange is a flat collar around the opening. It blocks the air coming from behind, so more of the suction reaches forward toward the work. Industrial ventilation references treat flanged openings as more efficient than plain ones for this reason: the same airflow reaches farther, or less airflow does the same job.

Donaldson lists five hood entry types in order of increasing effectiveness:

1. Raw edge. A plain pipe or hose end.
2. Flanged. A flat collar around the opening.
3. Tapered. The hood narrows gradually into the duct.
4. Conical. A funnel shape.
5. Bell mouth. A smooth rounded inlet, like the end of a trumpet.

For shop built hoods, this gives two easy wins. First, never leave a bare hose end hanging near the work when you can mount it through a board that acts as a flange. Second, where the hood meets the duct, use a tapered or funnel shape instead of a flat panel with a round hole cut in it. A box hood with sloped sides that narrow toward the port does both.

Rules that apply to every hood

• Enclose what you can. Oneida's first fundamental of dust collection is capturing dust at the source, with the hood as close as possible to where dust is made. Each side you close in means less open area pulling room air.
• Aim at the throw. Tools fling dust in a direction set by the spin of the blade, bit, or disc. Put the opening in that path so the dust's own speed carries it in.
• Keep the opening small. A bigger opening spreads the same airflow over more area, so the air speed at the face drops.
• Size the port to the duct. Per Pentz, a 4 inch duct carries about 349 CFM at 4,000 FPM, a 5 inch about 545, and a 6 inch about 785. A large hood on a 4 inch port is limited by that port.
• Add a grate. Strong hoods pull in offcuts and sanding scraps. A grate over the port keeps them out of the duct.
• Seal the seams. Gaps in the back or sides of a hood pull air from where the dust is not.

Hood for a sanding station

Downdraft tables are the classic sanding hood, a box with holes or slots in the top that pulls air down through the work surface. Bill Pentz is critical of them. His downdraft page says they compromise and let much of the most dangerous fine dust escape. He works out that a 2 foot square downdraft table needs at least 1,000 CFM and at least a 6 inch port, which is more than most small shop collectors deliver. He recommends a portable hood connected to your dust collection system and set right next to the work, and he says to wear a good NIOSH approved respirator whenever you use a downdraft table.

A backdraft hood is a practical build for a garage shop: a box on the bench, open toward you, with sloped sides that narrow to a port at the back. You sand right at the front edge, a few inches from the opening. Here is how I would build one:

1. Size the opening to your parts. Make it just wider and taller than the pieces you usually sand. A smaller face means more air speed at the work.
2. Cut two tapered sides, a top, and a bottom. Slope the top and sides inward from the front opening toward the back so the box works as a funnel.
3. Mount a flanged port at the back. Use the largest port your duct and collector support, 4 inch at minimum on a collector.
4. Add a wide flange around the front. A strip of plywood around the opening blocks air from behind and to the sides.
5. Add a grate and seal the seams. Hardware cloth or a slotted panel over the port catches loose sanding discs and scraps.
6. Clamp it to the bench and test. Sand close to the front edge and watch the dust. Move your work closer if dust drifts away.

For random orbit sanders, a hose on the sander itself is still the first line of defense. The bench hood catches what the sander's own pickup misses. My sander dust collection guide covers on-tool extraction.

Hood for a lathe

A lathe is hard to collect from because the work spins in open air and flings dust in a wide arc, and the spot you are working changes as you move along the piece. Sanding on the lathe makes the finest dust. A lathe hood needs to be close, and it needs to move with you.

• Mount it on a movable post. A hood on a post that slides along the bed or on a flexible arm lets you keep it a few inches from the area you are sanding.
• Put it behind and slightly below the work. The top of the work spins toward you and the bottom spins away, so dust thrown off the bottom heads toward the back of the lathe. A hood behind the work at about center height catches that stream without getting in the way of your tools.
• Use a clear or open front. You need to see the work. A clear plastic hood or a wide funnel keeps your view open.
• Consider a ready made hood. The Rockler Dust Right Mountable Dust Hood has a 14-3/4 by 11-1/4 inch opening, and Rockler lists it for lathes, miter saws and other tools with poor dust collection. Mount it on a post or bracket and it covers most bowl and spindle work.

Pentz's fine dust target for stationary tools is around 1,000 CFM, and an open lathe is one of the tools where that target makes sense. Most small shops cannot reach it at the lathe, so pair the hood with a respirator while sanding. My lathe dust collection guide has more on lathe setups.

Hood for a bandsaw

A bandsaw blade moves straight down through the table, so it carries dust down into the lower cabinet. Most bandsaws have a port on the lower wheel housing. The weak spot is right under the table, where dust comes off the blade before the lower port can reach it, and it spills out around the lower guides.

A small hood under the table, as close to the blade as the lower guides allow, fixes that. Build a narrow box that wraps behind and beside the blade below the table, with a 2-1/2 or 4 inch port. Keep it clear of the guides and the blade tension and tracking adjustments. Tie it into the lower port with a wye, or run it to a separate shop vac. For above the table, a small flanged pickup mounted behind the blade guard catches dust thrown upward during resawing. See my bandsaw dust collection guide for port upgrades.

Hood for bench work

Power carving, small router jobs, and detail sanding happen at the bench, where there is no port at all. A small box hood clamped to the bench, with its opening a few inches from the work, is the simplest answer. A flanged hose end through a scrap of plywood works too, and it takes five minutes to make.

For small bench hoods, a shop vac is often the better air source. A small opening on a narrow hose needs suction more than volume. A dust collector suits large hoods with 4 inch or bigger ports. Check what your collector actually delivers with my CFM calculator before you build a large hood around it, and see my guide to building a miter saw dust hood for a full size box hood build.

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