Questions about dust extraction come up often by new woodworkers. This is my attempt to try and save regular contributors some time spent answering them. Edited 17/11/16 following feedback from other members.Why have dust extraction?
Most people accept the need for dust extraction when using power tools and machines. However, it’s important I think to understand what our objectives are in dust collection to make informed choices about the right kit. I think there are three main reasons for extraction:1. Health.
Fine dust easily becomes airborne (particularly when generated by machines with high-speed cutters), and then some of this works its way into your respiratory system. This is widely accepted as a cause of respiratory illness. Some materials and some processes are more prone to fine dust creation than others. Also, some materials we work with are chemical irritants, or can provoke allergic reactions. If you care at all about your health, you’ll want to keep airborne dust to a minimum, and the best way to do this is to remove as much as possible from the source.2. Quality of cut.
The work of some tools (mainly thinking the planer and thicknesser) is negatively affected by poor waste removal. Dust and chips get scooped up by the cutterhead and pressed back into the work. The feed rollers also push waste into the newly cut surface, leaving indentations.3. Mess.
Simple really – dust gets everywhere. If you don’t like mess, and if you don’t like having to endlessly clean your workshop, then it’s in your interests to have good extraction. If you’re working in a domestic environment, this is particularly important, since customers/wives have a real aversion to mess in general.
Questions sometimes come up about workshop air cleaners. These are great for reducing the levels of airbourne dust in your workshop. However, they are only removing dust that has already got into the air (and which you are also now breathing). The best approach for dust should always be to remove it at source. Once you have done what you can do to this, by all means consider a workshop air cleaner to reduce particle counts further.Types of dust extraction
Dust extraction equipment is broadly categorised into two types – “high pressure, low volume” (HPLV) and “low pressure, high volume” (LPHV). A HPLV extractor is akin to a household vacuum cleaner, sometimes referred to as a “shop vac”. A LPHV extractor is also sometimes called a “chip extractor” edit - this term generally applies if the filtration is coarse (see later)
. The principle of both types – suck air and separate out dust and chips – is the same, but how they perform differs. Broadly speaking, HPLV extractors are good with power tools, while LPHV extractors are good with stationary machines.
HPLV extractors use high-speed brush motors (typically 20,000RPM) to spin a small fan. Some sort of filter is necessary – often a paper or cloth bag like in a traditional vacuum cleaner. Indeed, if you have a traditional bagged vacuum cleaner, such as a Henry, then this makes a reasonable extractor for occasional use. Under no circumstances use a bag-less domestic vacuum cleaner, such as a Dyson. The cyclone separators in these aren’t designed for very fine dust, and you will quickly clog the small filters or wreck the motor with fine dust. Some HPLV extractors also have small filter cartridges which require periodic replacement or cleaning.
Typically, a HPLV extractor sucks between 40-70 litres a second of air. This is enough for power tools, but not enough for a lot of stationary machines. They make a lot of noise (because of the high-speed brush motor), and draw around 1,000-1,300W of power. The stall-pressure of a HPLV extractor is high (typically around 20kPa) – i.e. they have good “suck” – which makes them very well suited to power tools. Normally the hose diameter is 30-40mm.
A few manufacturers (Cam Vac, Record, Numatic) make multi-motor HPLV extractors. These use 2 or 3 motors to increase the air flow, at the cost of increased power draw (2-3kW). These work ok on power tools and smaller machines, so can be a reasonable compromise if you want one extractor to cover all bases. Generally though, using one of these with power tools will restrict the air flow too much through the extractor for prolonged use (they use the air flow to cool the motors), whilst the air flow still isn’t really enough to get really good extraction on table saws and planer thicknessers. They are a neat, low hassle solution for some, but be aware of these compromises. The hose diameter on these machines is typically 63mm or 100mm, although they can be used with smaller diameter hoses via the use of step-down adaptors.
LPHV extractors use induction motors directly connected to a large centrifugal fan (blower), typically about 350-450mm in diameter. Because of the (2 pole) induction motor and direct connection, the fan speed is 3,000rpm. Typically, this is horizontally mounted, with a collection bag beneath the fan and a cloth filter bag or pleated cartridge filter above the fan. Because the extracted air passes through the fan before the dust and chips are separated, the fan is usually of metal construction to afford impact protection. Machines designed for the smaller workshop use single phase induction motors from 500-2,200W. Most use 100mm diameter hoses.
Because of the large fan, LPHV extractors move a lot more air than the vacuum type. They are ideally suited to extraction from stationary machines, which require this large airflow. Because of the low fan speed however, the stall pressure is very low (typically around 1.5-3kPa). If you have ever been around one of these machines, you have probably felt that there is very little “suck” if you cover the hose inlet with your hand. This means that for small diameter hoses and openings (such as those present on power tools), the air flow is much more compromised than when a vacuum type extractor is used. Therefore, they are unsuited to use with hand-held tools.
Confused about which to buy? For most people, a vacuum type extractor is the right place to start as even those of us who work with stationary machines also probably use hand-held routers and other power tools. They are also useful for general cleaning duties. They are generally more flexible and achieving good filtration of fine dust is straightforward. Just don't expect them to do well with stationary machines, if you have them.Filtration
Once you have air and dust entrained in the hose, the dust and chips need to be separated from the air stream. The air is then returned to the workshop environment. Some extractors do have the capability to vent the exhaust air outside; however this just makes the outside environment more dusty (not very neighbour friendly) and in the winter time, you are wasting all the energy you may have put into keeping the workshop warm. Edit
- as a simple rule of thumb, because HPLV extractors have loads of "suck" to work with they are generally supplied with some method of fine filtration that is good for separation of fine dust. Because the flow is small and there is loads of "suck", the filters can be small and cheap. For LPHV extractors, because the flow is much higher and the system performance is much more affected by moderate flow restrictions, fine dust filters have to have a very large surface area (i.e. big and expensive, hence not generally supplied as standard). Most LPHV extractors are supplied with cloth filter bags, which doesn't do a good job of capturing the finest dust produced mainly by saws and particularly when working with man-made products like MDF. These are strictly what we refer to as "chip extractors"; they still do a good job on saws capturing a lot of the dust but not
the health-damaging fine stuff. You can fit big filter cartridges to most of these extractors to also capture fine dust; however they need to be suitably sized to avoid restricting air flow (the pleated filters sold for this type of machine have several square metres of filter area). Specified large enough, performance should not be adversely affected. For any filter on either type of machine, look for claims about the particle size filtered (in microns) or an industry rating such as class L or class M.
The simplest way to achieve filtration in an HPLV extractor is by a filter bag in the extractor. The bag is permeable (fine fabric or paper); the air passes through the walls of the bag and the dust and chips stay inside. You can then throw the bag away, or in some cases empty it out for re-use. The down-side to just using bags is the quantity of waste you will generate with some tools means you will be changing and buying filter bags very frequently. For fine and hazardous dust, it’s an attractive option though.
Lots of HPLV extractors have the capability to run bagless. In these extractors, the dust hose is connected to a collection bucket that forms the body of the extractor. The motors in the extractor draw air from the collection bucket through cloth and/or paper filters, which remove dust from the air stream. These filters require periodic cleaning or replacing. Some vacuum type extractors have automatic or semi-automatic filter cleaning which is activated during operation.
On LPHV extractors, the air is drawn through the fan before being forced through a cloth filter bag or pleated filter cartridge. A collection bag sits beneath the filter which all the waste falls into.Separation
When working with machines, the large volumes of waste generated means frequent bag changing. Particularly if you’ve ever used a planer or thicknesser, you’ll likely be very familiar with having to empty the collector several times an hour. For this reason, some people use a separator. The machine is connected to the separator inlet, and the dust collector to the separator outlet. All of these separators work using inertial separation – by changing the flow direction in the separator, the denser dust and chips don’t change direction as readily as air and so are essentially flung out of the airstream and then collected in a bin. Three types of separators are commonly used – “bin lid”, cyclones and Thien baffles. Some of these types also capture finer dust quite well, meaning you need to clean filters and empty the dust extractor’s bags less often. They can all be used with any type of dust extractor, although they should be sized appropriately for the airflow.“Bin lid”
These are very simple devices that are made as the lid of the collection bin. There are two hose ports that penetrate the lid vertically. The incoming air and dust flows down into the bin, and the air then has to do a 180 degree turn to exit again. The heavier dust struggles to turn the corner and falls into the bin. These simple devices work very well for catching bulky chips; if your main objective is to reduce the frequency of emptying the collector when using planers and thicknessers then using one of this with a big collection bin is a great low-cost option. They don’t achieve good separation efficiency for finer dust however, because the inertial separation is very crude. You will still get a lot of dust on the extractor’s filters.Cyclone
Cyclones work by creating a swirling path of air down the inside surface of an upside-down cone. As the air is constantly turning, denser material is flung out onto the wall of the cone. Because of the “boundary layer” flow effect, the air is very slow moving next to the cone wall, so the separated material is then free to fall downwards and out of the cone at its base (the tip of the cone is cut off which forms the cyclone exit into the drop box). The air enters the cyclone at a tangential angle, and exits via a lid placed on top of the cone.
There are a few manufacturers of cyclones, or you can build your own. Bill Pentz’s website (http://billpentz.com/woodworking/cyclone/
) is an excellent and very detailed online resource for information on cyclone design and construction. In order to optimise the pressure drop and small particle collection efficiency of the cyclone, the design and sizing needs to be carefully thought out. Essentially, if you make the cyclone too big then the air speed in the cyclone isn’t fast enough so you basically have a “settling chamber” that doesn’t work much better than the bin lid type for extracting fine dust. If you make the cyclone too small then the air speed becomes so fast that a high pressure drop is created, which is particularly a problem for the LPHV type extractors and you get a significant flow reduction as a result.Thien baffle
Cyclones are quite tall so take up a lot of space. A more compact alternative somewhere between a cyclone and bin lid type separator is the Thien baffle. This is basically a thick lid (75-100mm or so) for your collection bin. The principle is similar to a cyclone – spin the air around the lid to separate dust out – but these are smaller and easier to build. The pressure drops generated are also lower. Some people get on really well with their Thien baffles, and they do a much better job than the bin lid type at capturing smaller dust. Like a cyclone, they should be sized according to air-flow. Their separation efficiency for very small dust isn’t as good as a cyclone however, so you will still get more build-up of fine dust on the extractor filters over time than you would with a suitably designed cyclone. More information can be found on Thien’s site here: http://www.jpthien.com/cy.htm