Safeguarding multi-blade roof truss saws
Roof truss saws are used to cut timber to size and shape ready for assembly and fabrication of wooden roof trusses. The work piece is fed to the saws under power, usually by a pair of chain conveyors. There is normally a fixed and moveable saw unit that allows timber of different lengths to be produced. Each sawing unit can be fitted with several circular saw blades on independent spindles. Each blade can be positioned at different angles to produce the desired angle of cut at each end of the work piece.
Legal requirements covering the use of these machines are contained in Safe use of woodworking machinery. Provision and Use of Work Equipment Regulations 1998 (PUWER) as applied to woodworking machinery. This document gives practical advice on the safe use of woodworking machinery and covers the provision of information and training as well as aspects of guarding and maintenance.
The main hazards associated with roof truss saws are:
- contact with the rotating saw blades, either when the machine is operating or running down;
- entanglement with drive shafts and chain and sprocket drives;
- shear hazards where dogs on the work piece conveyor pass fixed parts.
Additionally, there may be a hazard from off-cuts produced during the cutting process being ejected. This is a particular problem if the incorrect blade size or angle of cut has been used. Incidents have also been known to occur when operators reach into the machine to clear an off-cut or due to a failure to isolate the machine correctly before maintenance.
To reduce the risk of an incident occurring when the machine is running down braking should stop the saw spindles of all multi-blade roof truss saws within ten seconds.
Safeguarding roof truss saws
When buying a new roof truss saw, it should be supplied with a declaration of conformity and have a CE Mark. It should be designed and constructed to meet BS EN 1870 -15:2012 Safety of woodworking machines — Circular sawing machines Part 15: Multi-blade cross-cut sawing machines with integrated feed of the workpiece and manual loading and/or unloading. However, there are many older machines still in use and the safeguarding requirements for these machines will also be considered. It is not always practical for older machines to be upgraded so that they are safeguarded to the same standard as a new machine but this should be justified by risk assessment. The assessment should include details of what has been achieved to mitigate any risk on the machine.
The safeguarding requirements detailed in BS EN 1870-15 2012 include the following measures:
Prevention of material being ejected
Deflectors should be fitted at the front of each sawblade to prevent off-cuts being ejected. If the saw has feed chains fitted without dogs, to prevent any work-piece kick back the work-piece should be retained during its passage through the cutting area by means of a top pressure device such as belts or rollers at each saw unit. Pressure should be applied before any cutting starts. For feed chains with dogs then either the top guard can be used as an anti-lift device or a separate anti-lift device can be fitted. However, in each case they should be able to be adjusted down to the workpiece support, locked into position and extend 100mm either side of the saw line.
Safeguarding the non-cutting area of the saw blades
Use fixed guards, such as part of the frame, to prevent access to the non-cutting area of the saw-blades below the work-piece supports (feed chains). If these can be de-mounted for maintenance etc. then their fixing system should remain attached to the guard or machine ('un-losable' screws). There should also be no access to the saw blade through any chip outlet, see safety distances in EN ISO 13857:2008 Safety of machinery. Safety distances to prevent hazard zones being reached by upper and lower limbs for further details. If there is a movable guard fitted to allow saw blade changing then it should be interlocked with guard locking to the saw spindle. Interlocks should meet the requirements of BS EN ISO 14119:2013 Safety of machinery. Interlocking devices associated with guards. Principles for design and selection.
Access to the non-cutting area of each saw-blade above the work-piece support (feed chain) should be prevented by an adjustable top guard designed to cover the maximum saw-blade for which the machine is designed when the saw-blade is in its highest cutting position. If there are feed dogs fitted then there should be a 5mm clearance between the lower edge of the guard and the top of the highest dog. If there are no dogs fitted then there should be 5mm clearance between the guard lower edge and the surface of the feed chain.
Safeguarding the cutting area of the saw blades
To prevent access to the cutting area a combination of fixed guards on both sides of the machine and deterring/impeding devices at the in-feed end, Figure 1 and out-feed end, Figure 2 of the machine can be used. If the fixed guards can be demounted for maintenance etc. then their fixing
systems should also remain attached to the guard or to the machine when the guard is removed with ('un-losable' screws). To prevent access to the blade the fixed guards should be at least 1400 mm high from floor level with its vertical edges located at least 850 mm from the front and back of the largest saw blade the machine can use.
Figure 1 Safeguarding arrangements at the in-feed end of four-blade saw. Note also the trip wire. (Photograph courtesy MiTek Industries Ltd)
Figure 2 Safeguarding arrangements at out-feed end of four-blade saw. Note the trip wire attached to the lower edge of the guard. (Photograph courtesy MiTek Industries Ltd) The deterring/impeding devices should consist of a rigid structure such as 50mm diameter bar situated at a height between 800 mm from floor level and the height of the work-piece support. There should also be a horizontal distance of at least 900 mm to the front and rear of the largest saw blade that can be fitted when it is at its maximum cutting height. If there are any movable sections then they should be should be interlocked with guard locking to the saw spindle drive and feed drive motors. Interlocks should meet the requirements of BS EN ISO 14119:2013.
Safeguarding the feed chains
Fixed guards should be used to prevent access to the parts of the chains not required for feeding the work-piece. If they can be demounted for maintenance etc. then their fixing systems should also remain attached to the guard or to the machine when the guard is removed ('un-losable' screws). To prevent access to parts of the chains needed for feeding the work-piece use deterring/impeding devices that meet the requirements previously specified.
To prevent any access to any trapping point between the uprising and down going dogs of a chain and any fixed part of the machine use either:
- plain internal surfaces of the neighbouring fixed parts with a maximum distance of 8 mm between neighbouring fixed parts and the external surface of the dog;
- fixed guards.
Safeguarding older roof truss saws
Older roof truss machines can come in a variety of different forms, depending upon the manufacturer, some of whom are no longer in business. However, where possible to do so, the manufacturer should always be consulted on possible safeguarding upgrades for their older model machine. There is not a single safeguarding solution that suits or is applicable to all machines but the general aim is to enclose the two saw units on all sides with fixed guards to the greatest extent that the height of the work pieces being processed will allow. Residual risks can then be reduced by a combination of suitably interlocked adjustable guards and distance guarding techniques as used on the newer machines. Some machine controls may have to be extended or relocated, or moveable interlocked access points provided. All interlocks should meet the requirements of BS EN ISO 14119:2013.
If there are any viewing panels then they should be designed to contain ejected off-cuts. Either a transparent material of appropriate thickness or a suitable mesh material should be used and properly fixed into place.
In some circumstances construction of a partial enclosure or fixed perimeter fencing around the saw may be appropriate. This should be constructed around the ends and rear of the machine preventing access from these directions. Any perimeter fencing should be at least 1.8 metres high with letter box openings at the rear of the enclosure or in the fencing to allow removal of processed work pieces. These openings should be just bigger than the largest material processed by the machine.
Moveable interlocked guards or doors should be built into the enclosure where necessary for access to controls or to facilitate blade changes or maintenance. The machine should always be fully isolated before any person enters the enclosure.
At the front of the machine a combination of fixed and moveable guards as described below should be provided.
Guarding the front of the roof truss saw
At the in-feed position, both the front of the fixed saw unit and the front of the movable saw unit should be provided with adjustable interlocked guards. The best position is if they extend over the lower feed conveyor and across the front of the saw unit as shown in Figure 3 (Note: Figures 3 and 4 show different makes of machine). The gap beneath the guards should allow for the maximum height of work piece normally processed. The guards should either:
- extend away from the machine, following the slope of the feed conveyor, so that the reach distances in EN ISO 13857:2008are satisfied. It should not be possible to reach the largest blade normally in use at the front blade position from beneath the front and sides of the guard;
- be vertically mounted across the front of each saw unit and used in tandem with fixed distance guarding to achieve the reach distances in EN ISO 13857:2008. A triangular design mounted in the horizontal plane on either side of each lower feed conveyor at thigh/waist level has been seen to be effective. The guards should be mounted to the machine bed adjacent to the fixed saw unit and to the moving carriage of the moving saw unit. Mesh infill to these prevents the operator standing inside the distance guarding.
Normally, the adjustable guards hinge upwards to allow adjustments and blade changes and are best mounted to the moving frame of the top hold down conveyor. This means the gap beneath is automatically adjusted to the height of the wood being cut. Each adjustable guard should be interlocked with the power supply to the blades with interlock switches.
Where the use of an enclosure or perimeter fencing has not been chosen, the sides and take-off position should be provided with simple fixed guards of adequate construction. This is to prevent access to saw blades, short drive shafts, chain and sprocket drives and in-running nips on the machine and waste removal conveyors. Reach distances through any openings, or the frame of the machine, to any dangerous parts should be in accordance with EN ISO 13857:2008. Extension or relocation of manually adjustable controls may be necessary to bring them to the user side of the fixed guarding.
Where access from the rear of the machine is required for blade changes then interlocked moveable guards should be used in conjunction with the fixed guarding.
Where there is a risk of trapping at the returns of dogged conveyors between the chain and sprocket, or between dog and chain track flanges, additional flanges or mesh covers should be provided. Figures 5 and 6 show details of upgraded front guards fitted onto an older model multi-blade truss saw.
Figure 3 Partial enclosure and adjustable interlocked saw unit guards
Figure 4 Take-off position with fixed distance guarding and trip wires
Figures 5 and 6 Upgraded front guards fitted to an older model multi-blade truss saw.
(Photograph courtesy MiTek Industries Ltd)
Many machines have an exposed feed conveyor drive shaft, commonly seen at the out-feed side running the full length of the machine bed. Fitted along side this should be a trip wire linked to the emergency stop system designed in accordance with EN ISO 13850:2008, Safety of machinery — Emergency stop — Principles for design (ISO 13850:2006. Its purpose is to stop the machine should anyone become entangled or want to retrieve timber that may have slipped off the out feed conveyor (see Figure 4).
If necessary, a trip bar can be provided at the front edge of the interlocked adjustable guard closest to the machine operator's position. This should be positioned to stop the machine should the operator become entangled with the wood, or feed conveyor, and be drawn towards the blades.
Some users have also found it beneficial to have an additional trip wire fixed at shoulder height at the in-feed position. However, this option may not be practical for all machine configurations.
Find out more
For information on the design, manufacture, purchase, use and maintenance of work equipment, see Work equipment and machinery