How did this kickback happen?

[charlese]

I had a scary kickback incident the other day...
... But I don't understand how the kickback happened. The cut off wasn't wedged between the blade and fence. The cut off piece was the only thing on the left side of the blade. How did it kick back?

Let me see if I can help - It's all mccabinetmakers fault!:eek: After all it was his wood!:rolleyes: Just kidding!

Now, seriously - there were a couple (maybe 4) things in your operation that caused the kickback.

1) (Although this item, by itself didn't cause the kickback -) - It must be said that it not a totally safe operation to trap a piece of wood between the rip fence and the saw! I know you had that piece held down to the best of your ability so it would pass by the saw blade without incident. Even so, the only time this should be done is with non-thru cuts.
2) You didn't have a splitter, or riving knife in place to separate the cutoff from the 4" piece you were holding.
3) The cutoff piece was 7" long by 4" wide - very thin and light weight. It would not take much of a hit to propel this light weight cutoff.
a) This cutoff could have easily vibrated into the blade (mentioned earlier)]b) [/B]Drag on a non-slick saw table could have put a drag on the cutoff so when freed from the 4X4 piece could have twisted enough so the corner got caught by the blade.
4) There may be one or more teeth on that saw blade that are bent a bit to the left. It would only take one to cause the kickback - - Or it could have been caused by a dull sawblade where instead of slicing the last bit of the cut, it caught that sliver and threw it.

Any combination of the above could have caused the kickback.

Here's how to fix - If you are going to crosscut - use a miter gauge! This way you can stand farther away from the front of the blade. If you could not safely hold the 4" piece safely on the miter gauge, then put the remaining 7" piece against the miter gauge. You could always use an extension on the miter gauge. Also use a splitter or riving knife. An upper blade guard is also a good idea!

Also, using the miter gauge would allow you to use the rip fence equipped with a spacer block to set the cutoff length. In this way, the workpiece must pass the spacer block before it cuts any more than a sliver into the workpiece.

In every crosscut I've ever made the cuttoff piece is always of concern to me. All cuttoffs need to be imediately removed from the table. There are several different actions the cuttoffs take when freed from the workpiece;

-Most of them seem to twist, with the cut end toward the back of the saw.
-Some make a hop away from the blade.
-Some make a short journey back toward the middle of the blade, but only slightly away from the blade. these are the most scary, and I think that's what happened to your flying piece. But at the same time, drag or vibration, or a tooth, caught the piece, causing it to twist into the blade and get thrown.

[pennview]

Remember, I was using my Craftsman, not the SS. So no safety grip.

You can get a knock-off version of the safety grip for a Craftsman or other saw with a 3/4" miter slot from Grizzly or others. Grizzly calls it a "clamping miter gauge" -- http://www.grizzly.com/products/G1822

[Ron309753]

Using the rip fence as the controlling surface for crosscuts is asking for kickback. If the workpiece gets just slightly out of square with the blade it will get pinched between the fence and blade and will kickback. The attached is a simple solution for making repetitive cuts on the table saw. The block clamped to the fence is well forward of the blade and acts as a depth stop for identical pieces.

Sincerely,

Ron309753

[heathicus]

Let me see if I can help - It's all mccabinetmakers fault!:eek: After all it was his wood!

That's right, it's his fault because he didn't include anti-kickback technology in the wood he gave me! Well, he'll be hearing from my lawyers soon!



But seriously, I have learned a lesson. Again.

[Ron309753]

I'm going with freak accident. Good on ya for standing in a safe spot. I am a little concerned that you are using the fence at 4" to make a 4" cutoff piece.
I would have held the piece that flew with a miter gauge and set the repeatable 4" with a block at the front of the fence. Butt the piece against the block, squeeze the fine SS miter gauge clamp, and Slide the cut through. The 4" piece must pass free of the block before it contacts the blade. One of the things I like the most about my unifence is that you can slide it back so it only sticks out a short way onto the table. That allows me to use the fence scale on the saw to set my 4" without a spacer block.

Paul M

I second Paul's recommendation (see attached).

Sincerely,

Ron309753

[heathicus]

I second Paul's recommendation (see attached).

Sincerely,

Ron309753

But when it really comes down to it, how is that much different than what I did? I've always heard of this technique used to keep the cut piece from wedging between the fence and the blade. This was a situation I was careful to avoid. I just did it using a shoe to keep the piece straight and in control until it fully cleared the blade and was removed from the table.

In that picture, there are two cut off pieces to the right of the blade (from the operator's perspective). In that situation, where there is plenty of clearance so the piece can not be wedged against the blade, how would one of them be thrown by the blade? Because that would be more analogous to my situation. In both the picture and in my incident, there was nothing to cause the cut off to be wedged against the blade. Whether a miter gauge was used or not doesn't change the end result of a cut off piece of wood sitting beside the blade with plenty of clearance to turn and move around without being wedged or forced into the blade. That part was the same in my real life experience and in that picture. So the cut off pieces in that picture have as much potential to be grabbed by the blade and thrown back as my cut off piece did. So we're either back to "freak accident" or the operation as illustrated in that picture isn't very safe either. After my incident, I'm inclined to think it's the latter and the safest method is one that controls both pieces until they are completely removed from the vicinity of the blade.

I don't expect a definitive answer now, but I'm still curious... how did it happen? Not the safety (or lack thereof) or operational reasons, but the physics. With the cut off piece free to move away from the blade just as in that picture, why was it thrown by the blade instead? A slightly bent tooth grabbing the wood seems possible, but wouldn't that also leave tell-tale lines on the wood? I have no such lines with that blade (Freud Avanti 50T thin kerf). The blade being slightly out of vertical (tilted toward the cutoff so it would be "pulled" into the blade and would be wedged between the blade and the table) also seems very possible. I'll check my alignment again, but my square and my inclinometer (a digital Wixey is on my wish list) both said the blade was right on 90 degrees last I checked.

[shipwright]

You're worrying too much about this. I'm sticking with "freak accident". There is no reason for this to happen but as they say "....happens, then you die" or more acceptably "that's life".

If you are determined to delve into the physics and future danger of this incident:
1) I'd say the size of the offcut (very small and light) was the primary cause.
2) I wouldn't worry about a three foot piece of 4x4 doing this and seriously hurting you.
3) Ron's attachment shows a time honored, safe operation that many of us have been using for years without incident.
4) Just maybe the blade was set too low. I'm A fan of not too much blade showing because I never use an upper guard, but if the teeth were not completely clearing the wood they would be able to "catch" over a much larger area ie: the whole cut surface.

Try standing clear and trying to repeat the kickback (but use a miter gauge on the right side and no fence). This should indicate any abnormality, burr, or whatever in your blade, table, insert, etc. that may have contributed.

Paul M

[Ed in Tampa]

This wasn't a freak accident it was caused by operator error.

You said there was no blade guard and knowing Craftsman tools it also probably means no splitter. First operator error cutting anything on a table saw without a splitter opens the door for problems.

Second you didn't maintain control of both pieces, watch David Marks cut wood. Even when using a sliding cutoff table David always maintains control on both pieces of wood. Second operator error. He usually has a the eraser tip of his pencil on the cutoff and his good piece clamped to either the Miter fence or crosscut sled.

Third because there was no blade guard the piece could not only touch the blade but fly up. A blade guard would have diverted the wood down. Third operator error.

Fourth you didn't read you operators manual, it would have warned you never to make a cut without the splitter/blade guard and to always maintain control of the wood. Fourth operator error.

This is a fairly common happening one of reasons people are taught to stay out of the directly in front of or directly behind the blade. Over the years I have had more than one piece of wood fly past me but thankfully I did remember to stand to the right or left. The other four errors I sometimes duplicate much to my horror when something does happen. I always end up wondering how could I be so stupid I knew better.

[heathicus]

You're worrying too much about this. I'm sticking with "freak accident". There is no reason for this to happen but as they say "....happens, then you die" or more acceptably "that's life".

I'm not really worried about it. More interested and curious. I know what I did wrong and how to prevent it happening again. But, by everything I've heard and read about the causes of kickback, it shouldn't have happened. So now I'm just really curious about the WHY it happened (beyond the list of operator errors that Ed listed).

Just maybe the blade was set too low. I'm A fan of not too much blade showing because I never use an upper guard, but if the teeth were not completely clearing the wood they would be able to "catch" over a much larger area ie: the whole cut surface.

Maybe you're on to something there! (But I'm not sure how willing I am to try to replicate the kickback!!)

This wasn't a freak accident it was caused by operator error.

I hear you. I agree. I take full personal responsibility. And I've learned from it. *I* created the unsafe situation. But... going beyond that... my cutoff piece was no different than those in the picture Ron provided. A picture which, as shipwright said, demonstrated a time-honored, safe operation that people have been using for years without incident.

You said there was no blade guard and knowing Craftsman tools it also probably means no splitter.

My blade guard was shattered in my last kickback incident several months ago when a small cutoff got wedged between the blade and the blade guard (at least I think that's what happened). I usually have the splitter installed, but I had removed it recently to set up my assembly table (a half sheet of 3/4" MDF sitting on top of the table saw). You are correct that I did not install it before this cut. But I don't know that it would have made a difference because the wood was completely cut into two pieces before it would have ever reached the splitter or anti-kickback prawls.

First operator error cutting anything on a table saw without a splitter opens the door for problems.

Even if the wood is completely cut before it reaches the back of the blade? Maybe this is something else I need to learn a lot more about, but I thought the purpose of the splitter was to keep the kerf from closing in and pinching the blade in the back which would cause kickback. But what if, by the time the wood reaches the back of the blade, there is no "kerf" but two pieces of wood?

Second you didn't maintain control of both pieces, watch David Marks cut wood. Even when using a sliding cutoff table David always maintains control on both pieces of wood. Second operator error. He usually has a the eraser tip of his pencil on the cutoff and his good piece clamped to either the Miter fence or crosscut sled.

This is something I absolutely and totally agree with and will put into practice every time I use the table saw from now on. Maintaining control of both pieces is now a primary rule.

Third because there was no blade guard the piece could not only touch the blade but fly up. A blade guard would have diverted the wood down. Third operator error.

I'm not sold yet on this one. I had a blade guard. It caused kickback (I think) by allowing a small cutoff piece to get wedged between it and the blade, it exploded in the resulting kickback and I'm lucky I didn't end up with a 3" long shard of plastic in me. Faulty blade guard? Poor material and design? Operator error again in using a blade guard with too small of a cut off?

[charlese]

Heath - Here's another possible explanation for what happened.

Perhaps your downward pressure on the 4" piece caused the table insert to bend slightly toward the blade. This would cause a downward dip of the cutoff also. Such an action would cause the cutoff to rub on the blade, where it could be caught and thrown.