Workshop Machine Extraction archive

Tuesday 28 September 2010

Alan Holtham explains how chip collectors can be adapted to deal with dangerous dust too


Typical single-stage extractors are universally called dust extractors but in reality are only chip collectors. These all operate on the high volume-low pressure principle, shifting vast amounts of air by drawing the dust and chips directly through the fan and collecting them into a bag or drum.

Their main advantage is simplicity, therefore relative cheapness because they are little more than a motor with a big impeller on the end.

They do, however, have some equally distinct disadvantages, the main one being that everything is drawn through this arrangement whether it be dust, chips, small offcuts, screws or nails.

Consequently the impeller itself is liable to damage if it gets something large crashing into it at high velocity. Even a slight amount of damage to one of the fins is enough to set up an imbalance which manifests itself as severe vibration, so if your extractor suddenly starts shaking like mad, check that there is no damage to the fan.

You may also get bits of cloth or sandpaper sticking to the fins causing a similar problem, or they may actually block the outlet of the fan, so do keep an eye on what goes up the pipe.

Most of the impellers will be plastic, the theory being that a stray screw or nail whizzing through a metal impeller can generate a big enough spark to ignite the fine dust and air mixture.

Obviously this cannot happen with a plastic fan, but there is a potential risk of this same screw then bouncing off the metal housing. Fortunately this type of event is rare, but do be aware of the potential dangers.

Air dissipation

The high volume of air shifted by these machines has to be dissipated somehow once it has passed through the impeller, the standard method being through a top filter bag.

This generates a whole new set of problems. Obviously for maximum efficiency the high volume airflow generated by the fan must not be impeded by the filtration medium, so this must therefore be quite porous.

If it is porous, then fine dust will pass through, straight back into the atmosphere where you are working.

This is why they are rated as chip and not dust collectors, but you can have some control over the amount of fine dust being generated by using different materials for the bag. Standard cheap filter bags are made from an open weave cotton material, more expensive ones from finer Terylene needlefelt material, which is better at collecting up the fine particles.

I have seen figures of 25 micron quoted as the finest dust particle size you can effectively filter with this material, but this is obviously still nowhere near adequate enough to be safe. We need to catch the sub 5-micron size particles as these are the ones that will damage your health, but the finer the filter the harder you make it for the dust to pass through and the less efficient will be the airflow.

Filter area

Most of these small extractors have a less than adequate filter area anyway, but with a lot of use fine dust tends to clog and blind the filter.

Initially this actually improves the fine particle filtration efficiency, but as it develops further the airflow becomes more and more restricted, which builds up as significant back-pressure in the bag, putting strain on the fan unit. The overall performance then falls off, and in actual fact the higher bag pressure then forces fine particles right through the bag material.

This illustrates the importance of keeping the filter bags clean, but do not use them ad infinitum. Industrial filter systems use a variety of mechanical shakers and air jets to keep the bags clean. For our small-scale extractors just give the filter bag a good shake regularly, and renew it occasionally.

Washing them is rarely successful and you will just end up ruining the washing machine.

Siting system

A compromise solution to this problem of fine dust escaping into the workshop is to house the extractor outside somewhere and pipe through the wall or a convenient window. The snag with this is that you then extract all your heat as well.

They will also cause a wood-burning stove to smoke badly as well unless you make provision for a fresh-air inlet.

A more recent development, fuelled perhaps by the increasing use of materials like MDF, has been the introduction of cartridge filters to replace the filter bag. These collect much smaller particles than a cloth bag and offer much the same level of protection as a high-pressure vacuum system.

The loss of porosity needed to generate this increased performance is made up for by the pleating of the filter to increase its surface area hugely.

Although initially quite expensive these cartridges are well worth the investment, though cleaning them is rather more fiddly and time consuming.

However it must be said that any of these collectors will still allow through the harmful fine particles, so if you are siting one within the workshop you need to take additional precautions such as an ambient air filter and an air-fed helmet. Relying on the chip collector is not enough.

Big machine extraction

The planer thicknesser is probably the one machine that benefits most from a simple chip extractor. Without one it spews masses of dust and shavings into the air, even a light cut seeming to generate a disproportionate amount of waste.

Not only does an extractor make it more pleasant to work, but it also has real benefits on the quality of the finish.

Without any form of extraction, many of the shavings produced circle around the cutterblock in the airflow and are pressed back into the surface by the outfeed roller. This results in a very poor surface quality, particularly if the timber is soft, so extraction at source is the only answer. You may have similar problems with a sawbench where the dust binds and clogs the blade unless it is sucked away.

Actually getting at the dust so that it can be extracted is sometimes quite a task, bearing in mind that it is surrounded by blades and cutters all spinning at high speed and generating their own peculiar air flows.

The dust may not always go where you think it will, but most modern machines are fitted with purpose-made hoods to maximise extraction. Radial arm and chop saws are notoriously difficult to extract from and some form of general collector hood is the best you can do.

Bigger machines may require multiple extraction points. Bandsaws in particular are quite easy to collect from beneath the table, but much of the finer dust comes down on top of the work, so consider building in an additional movable but rigid Stayput nozzle.

If you are running several machines from a single portable extractor it is just a question of sliding the pipe onto the hood and away you go. Whether it will actually stay there is another matter, and just a blast of a few seconds from a thicknesser with the pipe off is enough to have you Hoovering up your previously pristine workshop for several hours afterwards.

A safer bet is to attach the tube with a Jubilee clip, but this makes changeover a little tedious. A more elegant solution is to fit a quick-change flange, which is permanently attached to the pipe but locks positively onto each different hood.

Going for pro approach

Having got your extractor and seen the benefits of it, you may be tempted to expand its operation in an attempt to mirror the setup of a professional workshop. These usually have a central, single-stage collector unit working on the same principle as our small workshop extractors, but each machine is permanently piped back into it.

If you think there is sufficient airflow in your unit you might be able to rig up something similar to serve several machines at once, but if you do try this remember a few basic principles.

1. This type of extractor relies on moving high volumes of air so the extractor must be powerful enough, with ductwork diameter as large as possible. Reducing the pipe diameter does not increase efficiency

2. Keep pipe runs short and where possible make any branches as gradual as possible. Right-angle bends cause a significant drop in airflow. A large number of standard fittings is available to help build your system

3. If you are going to use a plastic pipe system do remember the hazards of static build-up on the pipework and make appropriate earthing provision. However, dedicated metal ducting is actually as cheap as plastic and has a much better range of suitable fittings

4. If you are connecting to several machines at once, but do not use them simultaneously, build in some blastgates to allow you to shut off unused connections. These are available in metal or plastic and are very cheap, but make a difference to the overall efficiency of your system

Also bear in mind...

I have already stressed that low-pressure extractors need vast airflow for maximum efficiency, so do not be tempted to buy the reducer fittings designed to connect to power tools. Contrary to popular belief these drastically reduce rather than increase the suck. If you do a lot with power tools buy a proper high-pressure vacuum extractor.

Look closely at the hose that comes with the extractor. 100mm seems to be the standard diameter, but the quality of manufacture varies enormously. Cheaper hoses are stiff and brittle. Clear ones are usually much more flexible, far less cumbersome and you can also see any blockages. You will also find that the rigid hoses wear quickly, both where they drag on the floor and on any bends as the action of the fast-moving shavings is quite abrasive.

The collection bags vary too. Standard bags are about the same quality as bin liners, and you only have to pick up something vaguely sharp for it to be quickly punctured. The best bags are made from a rip-proof material that deforms rather than tears, handy when you have to move a full bag.

The action of the swirling airflow tends to pack the dust down very firmly. A full bag is very heavy and unless you are very careful it can rip apart under the weight, depositing the carefully collected dust all over the workshop floor. Good-quality bags are well worth the small extra cost.

This weight problem is why I also prefer extractors that support the bag. Some just hang in mid air and as they gradually fill up are prone to suddenly drop off. If the extractor is in use at the time the results are spectacular to say the least.

Woodworkers Institute

Tagged In:

Alan Holtham , Shop Talk , chip collectors , dust extractors