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All CNC machining, by its nature, produces a lot of heat. As the cutter moves through a material at high speeds, it produces very high levels of friction. This frictional energy is ultimately converted into:

• The energy used to break through the material.

• Heat energy.

If the amount of heat energy produced by the cutting process gets too high around a combustible material (e.g. wood stock), there is a risk of fire. 

When you using SmartBench, you must:  Take necessary safety precautions to reduce the risk of fire as much as possible. Put emergency procedures in place which will be carried out in the event of fire.

Reducing risk of fire

The first tenet of fire safety is to reduce the risk of fire. 

There are three key elements needed to cause a fire: fuel, oxygen, and heat. These elements make up the fire triangle. 

During routing, all three elements of the fire triangle are present:

However, the operator can take precautions to reduce the likelihood of these factors (or the combination of them) reaching critically high levels.

Oxygen

Extraction pulls oxygen through the work area. If we want higher airflow for our extraction, it will naturally force more oxygen to travel through the work area. 

However, good airflow is completely necessary for removing waste from the cutting site, which in itself reduces fire risk. 

Therefore, we need to focus on minimising the other two elements of the fire triangle: fuel, and heat. 

A: Extraction hose

B: Working zone

C: Air flow direction

Fuel

For a fire to start there must be a material to burn, i.e. the “fuel”. Fuel can come in the form of any combustible material. 

The moisture content, size, shape and quantity of the material will all determine how easily and at what temperature it will ignite or burn. 

Cutting any combustible material is a fire risk

Cutting any combustible material (e.g. timber) means that there is a risk it can catch fire. 

The average ignition temperature of timber is 300°C.

 

Different woods will have different natural properties which can affect their combustibility.

• Resins, found in woods such as birch or pine plywood, can act as an accelerant (i.e., speeds the development or escalation of fire).

• Natural variations in the material can generate sudden and unexpected increases in heat as the cutter passes through it.

Trees pull in grit from the soil, which can become temperature-increasing particles once that tree has been turned into stock material.

 

Before cutting any combustible material, make sure you use the right cutter type and cut with the right feed and speed settings for that material.

Click here to learn about the right cutter type for specific material.

Click here to learn more about SmartBench feeds and speeds for specific materials.

Moisture content

When wood is in storage it acts as a sponge, absorbing moisture from the air. If the air is dry, the wood will become dry too. The drier the wood, the higher the risk of it catching fire. 

In the context of firewood, logs with higher than 30% moisture content would struggle to burn. A typical firewood moisture content would be 15 - 20%. Firewood with less moisture content is not even recommended, since it will burn so hot that it can actually damage a stove! 

If you are cutting wood with anything less than 30% moisture content, there is a much higher risk of fire. Be extremely vigilant.

What do I need to do?

Monitoring and controlling air humidity is a good way to control your wood stock conditions. Note that moisture content in the air drops with a drop in ambient temperature.

You can use a moisture meter to directly test your materials, or you can get a humidity meter, or hygrometer, for your workshop.  There are many affordable clocks that come with built in thermometers and hygrometers and can be purchased online.

 

Here is an example of historic data on air humidity in our factory in the UK:

From the graph, we can see that the humidity level never drops below 30%, so during this period we knew that our wood was low risk. 

Make chips, not dust

When routing, always aim to generate chips, not dust.

Dust has a high air surface to fuel ratio: this means that for each piece of dust, the combustible molecules that make it up are more likely to be in contact with an oxygen molecule in the air, which means better conditions for starting a fire. 

Due to their shape and volume, chips of material have a much lower air surface to fuel ratio: so each fuel molecule is far less likely to be in contact with an oxygen molecule. 

Creating chips instead of dust means that there is a lower proportion of the available fuel (i.e. the cut material) mixing with oxygen in the air, so the fire risk is reduced. 

Chips also hold on to excess heat more effectively than dust, so when they are extracted, they take that excess heat with them - away from the cutting site. 

D: Chips

E: Dust

To create chips instead of dust, make sure you: 

• Choose a cutter designed specifically for the stock material that you are cutting. Click here to learn more about choosing the right cutter. 

• Make sure the feed rate is fast enough for your job. Click here to learn more about choosing the right feeds and speeds for your application. 

Cutting too slowly may reduce the size of the chips to dust, or worst case cause what we refer to as rubbing (where the cutter is literally smearing the material out of the way, causing maximum dust generation). Since wood is a natural product, densities can vary and so every job may be different. In the case of hardwoods, different areas of the stock may vary to a point where the speeds and feeds may require adjustments throughout the course of the job.

Extract waste (a.k.a., excess fuel)

If waste builds up around the cutting site:

• It can prevent excess heat from the cutter from dissipating, as the waste (already warm from being cut) is acting as an insulator around the cutter.

• It increases the amount of readily available fuel at the cutting site, should anything ignite. 

Therefore it is important to maintain good extraction throughout the job. 

Use an extractor with an airflow rating of at least 70 l/s. Click here to learn more about choosing the right extraction system for your application.

 

It is easy for extraction to fail, due to full vacuum bags or blockages, but a few simple checks can significantly improve reliability:

• Always empty the extraction bag before a job, regardless of level, to ensure that the highest possible draw is achieved. This reduces risk of failure due to reaching vacuum bag capacity. 

• Pause the job if you need to empty the vacuum bag. Once the bag is emptied, you can resume the job.

• Always do a manual check for any blockages prior to starting a job: pulse the extractor on/off with a hand over the dust shoe aperture, to check the airflow. If there are any issues, do a thorough visual inspection of all hoses to check for blockages. 

• Do regular visual checks of the cutting site; if there is a build up of waste, pause the job and do a thorough check of the extraction. 

Heat sources

A heat source causes the initial ignition of a fire. Reducing the friction between a cutter and material is the main way to reduce the amount of heat the friction produces. 

Blunt cutters get hot

As a cutter loses its sharpness, it generates more friction, which makes the cutting tool hotter. 

To overcome the extra friction, the spindle will demand more electrical energy to maintain speed, and this extra energy will continue to get dissipated into the material through friction, creating more heat.

Note that a dull edge will tear the material fibre rather than cut clean, thus generating more dust.

 

F: Cutter in original good condition.

G: Cutter has been exposed to high temperatures and overheated.

Pick the right cutter type for the job

The best cutter to use is a cutter that is designed for a specific material. Using the wrong type of cutter will make an inefficient cut, which will increase the amount of heat and dust.

Click here to learn more about cutter types.

A 2-flute spiral upcut end mill is a great go-to cutter for routing most woods, but not necessarily efficient for all of them.

Down cutters and compression cutters increase friction and heat

Downcut flutes produce a better finish on the top surface by forcing waste downwards, but they achieve this by compacting dust into the channel of the cut. This will make it harder to evacuate cutting waste, and may contribute to increased friction and heat.

Never use down cutters or compression cutters for drilling holes (vertical movement without any side movement), since cutting waste does not have any way of escaping the down-facing flutes and may get very hot.

Earth your extraction equipment

High airflow extraction is essential for any CNC job, but through a plastic or rubber tube it also naturally generates static electricity. 

Extraction equipment, including the length of the hose, needs to be earthed. If the extraction system is not earthed, static electricity will build up, and if it reaches a critical level it will discharge to the nearest earth in the form of a spark. 

If the nearest earth is a big metal SmartBench (which is very likely), this spark is enough to ignite chips and dust particles around the cutting site. 

Click here to learn more about earthing, and how to earth your extraction system. 

Containing an ignition

Do not leave SmartBench unattended

Never leave the job unattended.

It’s tempting to let SmartBench do it’s own thing - after all, isn’t that the point? But the reality is that the machine won’t know if it’s run into trouble, and you need to be there to deal with any issues if necessary click here to learn more.

Extinguishing a fire

Not every fire is the same, and using the wrong type of fire extinguisher could make things worse. 

Fire extinguishers are developed to eliminate one of the three elements of the fire triangle: 

• If we remove fuel, the fire will smoulder out.

• If you can cool a fire down it will lose heat and go out.

• If the oxygen is removed it will suffocate. 

For example, a water-type fire extinguisher will cool the fire down and remove heat.

It is therefore important to match the right type of fire extinguisher to the correct class of fire. 

Fire classification

Whilst there are 6 different types of fire class, for SmartBench we have shortlisted 3 relevant classes:

• Class A: fires caused by ordinary solid combustible materials such as wood, paper, cardboard and textiles.

• Class D: fires caused by combustible metals such as potassium, aluminium or magnesium.

• Electrical fires: Caused by electrical equipment. Note that this fire will change class once the electrical item is shut off or removed (and will then be classed based on the combustible material).

Note that electrical fires have different classification in different parts of the world:  US: Class C. Australia: Class E. EU: Unclassified - Electrical.

Keep a suitable fire extinguisher on site

Keep a dry powder fire extinguisher in a visible and accessible location on your jobsite, as this can be used for Class A, Class D and electrical fires.

 

The following table shows you which type of extinguisher is suitable for each class of fire. 

To identify fire extinguisher type, look for the label on the front: 

Ensure that you have a suitable fire extinguisher for all possible classes of fire, and do not use the wrong type of extinguisher on the wrong class of fire.

 

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