Mitre saw stop block safety

[Windows]

I’m not a safety expert. Use your own judgement. This is just how I think about using a mitre saw:

Problems come when wood touches the side or rear of the moving saw blade. The problems are worse when wood touches both sides of the moving blade at the same time. Our job is to ensure that wood never touches the side of the blade while it’s moving.

Here are some points to consider:

1. When you advance the blade to make a cut, you exert a force on the wood in that direction. If there is not sufficient support (fence and/or table) behind the wood in the direction you’re advancing, that force can cause the wood to pivot and hit the side of the blade.

You can counteract this by using a zero clearance fence or table. Alternatively or in addition, you can clamp either or both sides of the workpiece to stop it pivoting.

Note that small pieces are more likely to experience this problem than larger pieces because of mass, friction, and the principle of moments.

1. When the saw has separated the two sides of the workpiece from each other, gravity (and air currents!) act on each piece individually. If either side of the workpiece considered alone could move or fall towards the blade, you’ve got a problem to address.

You can counteract this potential problem by being conscious of the two sides of the workpiece as individual pieces. Each must be fully supported. Make sure that your workpiece is supported from below at 4 positions with 2 on each side of the blade to counteract the force of gravity for that part of the workpiece.

Make sure when considering each side, that the support positions are on both sides of the centre of gravity of that part of the workpiece.

Clamping can be used to make up for instability, usually if the shape of the separated piece makes it intrinsically unstable. Try not to use clamping as a substitute for 4-point support from below.

1. Tension can be released during the cut and cause the wood to move into the blade.

Tension can come from two sources. It can be intrinsic to the wood, developed during growth. It can also be put into the wood (or put into equipment that is in contact with the wood) by the saw operator during positioning of the workpiece or during sawing.

In general, not much consideration is given to stresses within the wood that are released by cutting when using a mitre saw (as evidenced by the fact that mitre saws don’t have riving knives). This is because the mitre saw is mainly used for short cross cuts rather than long rip cuts.

Tension put into the system by the operator is a more pressing concern (literally). Pressing the workpiece against a stop block or a clamp can bend or compress the workpiece or the stop block or clamp. The gap opened by cutting the workpiece allows decompression or spring back, releasing the tension and potentially pushing the wood against the side of the blade.

The countermeasures for this issue are things like flip-up or two-piece stop blocks, but also awareness. You need to know that the issue is not specific to stop blocks, you can also put tension into the system by pressing against a clamp put in place for legitimate reasons as described above, or just by pushing the workpiece towards the blade with your hand or a stick.

If your stop block and a single clamp are on the same side of the blade and there’s no work holding (whether clamp, stop block, or hand) on the other side of the blade, you won’t have a problem. Tension could be locked in between the clamp and stop block, but it will never be released during sawing. (Obviously, don’t release the clamp before the saw blade is stationary or out of the way).

But as soon as you put any pressure on the workpiece that acts towards the blade, you could have a problem. It’s important to understand that the pressure can be locked in by clamps or stop blocks or applied during sawing by careless manual holding.

 

[guineafowl21]

Let the blade stop before lifting it, as mentioned.

Lifting while spinning is quite a hard habit to break, but if you do, it makes stop block work safer, as well as general use.

 

[Chubber]

Use a flip up stop. Press against the stop with left hand, when in place continue to hold against the back fence with the left hand, flip up the stop with the right hand, make the cut.
A strip of abrasive stuck to the left hand fence helps keeping it stationary.

 

[Mike Fletcher]

Along with the stop block spacer / flip stop, never raise the blade while it's still spinning. Only clamp one side of the piece you are cutting. The objective is to allow the unclamped piece to be able to move away from the blade once the cut is complete. If there is any play in the saws movable parts, you may (will) experience the dreaded kick back as the play can catch the unclamped piece.

 

[Croolis]

I’m not a safety expert. Use your own judgement. This is just how I think about using a mitre saw....

Nice - good answer explaining it all. Thanks.

Only one thing to further the discussion: So there's me with 2m of 2x4, I'm cutting off 300mm on one side with a flip up stop block. I flip up the block as suggested, holding the keeper piece in place (hands away from the blade now ), not holding the 1700mm long side then make my cut.

Heavy long pieces like that - heavy enough (and I appreciate "heavy" is how long's a piece of string) that it would act like it was clamped? If so, then the keeper piece I am holding with my hand is in danger of kickback, no? My hand being a weaker clamp than the weight of the board on the other side.

I know I may be getting into overthinking it now, but the logic of it piques me.

 

[Sgian Dubh]

I’m not a safety expert. Use your own judgement. This is just how I think about using a mitre saw:

Tension can come from two sources. It can be intrinsic to the wood, developed during growth. It can also be put into the wood (or put into equipment that is in contact with the wood) by the saw operator during positioning of the workpiece or during sawing.

You have evidently given some consideration to safer usage of a chop saw.

However, you should really not talk about tension in isolation. Tension is just one form of stress that can be found in materials, wood in this case. Stress is really the word you were looking for which encompasses, in various forms in a material, compression, tension, shear and torsional stress. Each of these forms of stress can cause problems for woodworkers.

It's the case that stress, including tension and other stresses, can be intrinsic to the wood, and that operator error during processing or the way machines are operated can put wood under stress as you say, but it perhaps slipped your mind to mention another potential and significant source of stress, i.e., drying faults that may be caused by, but not always, improper seasoning induced stress.

It always catches my eye when forum contributors discuss in isolation tension in wood when what they really ought to talk about is stress including tension, of course. You just happened to be the latest my sad and pedantic eye spotted, sorry. Slainte.

 

[Sgian Dubh]

I know I may be getting into overthinking it now, but the logic of it piques me.

Most people are overthinking this. Assuming you have plenty of support on both sides of the chop saw, enough to support a 2m length to the right of the saw blade anyway, all you really need to do is firmly clamp a suitable block 300 mm to the left of the saw blade, or use a good stop supplied with the machine if such a thing exists.

Slide the 2 m piece under the saw blade from right to left until the end touches the stop. Firmly hold down the piece up against the stop and to the rear fence and chop down. Lift and repeat, there should be no need to let the blade stop between cuts. If the length of wood is bowed, even slightly, make sure the concave face or edge faces up and/or away from the rear fence because you're much less likely to get saw bind which could lead to kickback. A similar judgement needs to made when dealing with cupped wood, i.e., concave face generally facing up, if possible. Slainte.