Table saws, back tooth kickback and flying lumber

[Cabmaker]

Table saw kick-back is separated into two categories:

1) straight line kickback: when the material is pushed against the blade without perpendicular resistance applied to the material between the blade and the fence.

--materiel shoots like an arrow straight back.

2) back tooth kickback: when the material hits the back tooth of the blade and climbs up the blade, blade digs into material, flings the piece back, while rotating.

--- piece shoots up at you.

(More detailed info on kickback here: http://www.waterfrontwoods.com/Articles/Tablesaw/tablesaw.html#sthash.VSpQnDIR.dpuf )

Here is a video of what back-tooth kick-back looks like in slow motion:

Now here comes the big question... in back tooth kickback, your material rotates as it flies up/back. I'm wondering to what degree a piece rotates in order to establish danger zones. I've only ever seen smaller sizered material kickback--but what if you had kickback with ten foot lumber? How large/where in relation to the blade would the danger zone be for flying lumber injury potential?

The general assumption for woodworkers is that material always go back towards the operator no matter what, but having researched this I have doubts that it is based in fact.

-If you want to narrow down some parameters... a delta unisaw is a common cab shop standard-- 3 hp, 10 inch blade. 4000 rpm. (http://www.deltamachinery.com/products/table-saws/item/36-l336?category_id=1) We can assume a typical specimen for a lumber unit could be a 10 x 5 x 7/8 alder beam weighing 20 pounds if you want.

Please let me know if any other info is needed, or if my question isn't making sense.

Thanks very much!

 

[dragoneyes001]

large pieces don't tend to kick back because the weight of the piece tends to make the blade cut through it instead of skipping and letting the teeth do a "toss-the-piece"

wood will always shoot back because of the direction of the spinning blade which is turning towards the operator. another way to create kick back is have the piece being cut be wider than it is long when its pushed between the saw guide and the blade this can shoot pretty large pieces back at the operator.

one of the people working in the shop with me had a octagonal piece of plexi glass shoot back and shatter into him because the width between the guide was larger than the last length being cut off was.

the safest place to go when a piece is about to kick back is to the side of the guide this will usually help deflect the piece beside the operator as it goes by.

 

[Cabmaker]

Thanks for your response Dragon eyes. I know that that is the general consensus in the industry, that things shoot back only, but I am not sure if that's just passed on knowledge without actual factual basis, and if it extends to odder shaped stock. Some sort of parameters could be acheived. I do recognize that it is a complex question--I asked 2 engineer friends over dinner last week and it turned into a lot of scribbled napkins and two hours of debate/theories/calculations. It truly is a fun question, that I don't think has been answered yet.

The question originated from a real life situation. We have a moron in the shop that free-hand "joints" bent raw stock with the table saw because he is too lazy to use the jointer. (too many passes + physically holding the lumber + understanding of jointer use) And let me be specific--by free hand, I mean no fence. At all.

I know that there is a rotational force to back tooth kickback--hence the arc-shaped blade mark on the flung material. Now if your materiel is long (10 feet), could that lumber piece clock a poor bastard (me) that works 4 feet perpendicular to the blade? It's a worthy question, for the fun of theory, aside from not wanting to get a flying raw beam in the head.

I'm not convinced that a 20 pound piece would "sink" into the blade and get cut through/stall the blade on a saw that powerful. 20 pounds of stock long and narrow vs 3 hp 4000 rpm + wrong angle of attack begs the question. The rotation in back tooth kickback occurs once the back tooth grabs the stock--then it climbs up the blade, turning, till the top tooth (force is upwards in the back till the top tooth), then gets launched forward at the high tooth, where the force is parallel to the table (and towards the operator). Now if you have a long beam, no fence to stop the rotation, could an end of that long beam reach a target that is perpendicular? Or kitty corner? There are workers and things everywhere in a launch direction. I don't mind the idiot taking himself out, but another worker, or me, not so good.

(before anyone brings up safety and business practise, yes I know. But I still want theory on the matter. Just because it's interesting.)

I have these two engineers passing on the question to other science minded folk as no conclusions were reached, but I thought I would post it here to see if anyone else nerds out on this sort of stuff :)

 

[Baluncore]

Kickback at the start of the cut presses the material onto the table. Kick back at the end of the cut lifts the material off the table and then propels it back by tooth contact with the lower surface of the material. The rate of rotation about the horizontal axis parallel with the saw spindle axis will depend on the length and thickness of the material.

Kickback is only a problem when the mass of material is too low to oppose the cutting tooth impact with the material. A similar thing can happen with a milling machine or lathe. If the grip by the vice or chuck on the material has less section than the depth of the cut, then the material can be thrown out of the machine.

 

[dragoneyes001]

I've been a master carpenter for over 20 years so I've seen table saws do some pretty funky stuff when people ignoring common sense use them.
a 3 horse motor with a ten inch blade can toss a very large piece of lumber if the wood is allowed to move freely when in contact with the blade but the heavier the piece the less effort needed to keep the blade cutting the stock instead of setting a toss into motion. the only time I've seen anything get tossed forwards on a table saw is when a small piece starts to fall into the void around the blade and gets propelled out in some random direction.

the fastest projectiles are always smaller pieces where as larger ones can jam the blade or even force an unusual cut into the stock. a ten foot 2x4 freehand cut will rarely fling back when cross cut because there is no guide to increase the jam against the blade BUT if the person pushing it through tries to push it with their hands close to the blade the stock will bend into the blade causing a kick back before the cut can complete the same would be true if the far ends of the 2x4 were pushed. the ideal way of cutting is to make the stock move evenly both hands pushing on one side of the blade in an even motion allowing the cut off end to move on its own which will as the cut progresses turn the stock away from the blade.

industrial table saws with 5+ horse power and 12 inch blades have been known to toss ten foot 2x4's through plywood reinforced walls at the back of the shop. so there really is a danger with improper use of saws beyond cutting yourself.

 

[Cabmaker]

"The rate of rotation about the horizontal axis parallel with the saw spindle axis will depend on the length and thickness of the material."

Bingo. That.

Anyone know if anyone has ever dug into this further? It's all math really. The napkin scribbling night pondered some interesting equations on how to measure this, but no solid answer yet. But the engineer friends were both of the electrical sort, and digging hardcore into material from school, long unused. I know I need a mechanical engineer for this one.

Dragon eyes--I'm in the cabinet making industry and am constantly surprised at some of the things I see with machinery use. Free hand cuts with hand held flipped over routors, hands used to press stock inot dados right over the blade, you name it. But this free hand "jointing cut" ( a rip cut on a 10 foot beam btw) is by far the stupidest I've seen yet! We've been joking about telling the guy to remove the throat plate and rise the blade because sometimes we are aweful like that :)