really best used to clean up dust rather than to capture it at its source. A shop vacuum works well for light-duty dust collection from a router or sander. But with its modest air flow and small hose diameter, it can be overwhelmed by the chips and shavings disgorged by a planer or jointer. And a shop vacuum's universal motor is less powerful than the induction motors used on the dedicated dust collectors described below. Also keep in mind that a universal motor has a shorter service life, on average, than an induction motor.
Single-stage collectors—Single-stage blower-and-bag units are the next step up in price and performance. These units have the power to handle large quantities of dust and chips, and the more powerful among them can be used to run a central dust-collection system.
A single-stage dust collector typically uses a pair of bags. The bag on the bottom gathers large debris before the air stream exits through the upper filter bag. (See drawing, opposite page.)
The chief drawback to a single-stage collector is that dust and chips pass directly through the impeller.
causing wear and tear on the parts.
Two-stage collectors—Two-stage blower-and-bag collectors commonly have a collection drum with the blower mounted on top. (See drawing, below left.) The major difference between two-stage collectors and their single-stage cousins is that on two-stage collectors, large chips and shavings fall into the drum and do not pass through the impeller. This reduces wear and tear on the blower. Fine dust collects in the filler bag.
Small two-stage collectors make-ideal roll-around units, and more powerful models can run a central dust-collection system.
One drawback to two-stage collectors is that you have to lift the heavy blower housing in order to empty the waste drum.
Cyclone pre-collectors—In this type of dust collector, the air-stream first enters a tall metal "cyclone" housing where the debris is spun around in a vortex as it drops down the sides. (See drawing, below right.) This cyclone action separates most of the debris and deposits it in the waste container below. Then the air stream reverses direction to exit through the top of the cyclone. Only the finest dust passes through the blower and into the filter bag.
Cyclone pre-collectors are ideal for central dust-collection systems in larger shops and can handle large volumes of dust and chips.
(For more detailed discussions of all four kinds of collectors, see AW »37.)
Dust collectors are rated by the volume of air they can move (cfm) at a given static pressure (resistance to air flow measured in "inches of water"). Unfortunately, catalogs for many small-shop dust collectors don't list cfm and static pressure (sp) realistically. T hey'll list the collector's maximum or -free-flow" cfm with no resistance at all (when it's not connected to a duct system—0 sp), and they'll list the collector's maximum sp with the inlets blocked so that no air can flow (0 cfm).
The key to a realistic rating is the symbol For example, if a collector is rated "500 cfm @ 8 in. sp,' that's what it will deliver under real load conditions. ▲
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