Overarm guard caused TS kickback?

[Van Huskey]

Thanks for sharing! Very important information. Now the question is how to set up a test to determine if suction needs to be reduced on an overhead guard and if so by how much. An initial guess would be if it can lift a piece large (heavy) enough to cause a significant injury then it needs to be reduced...

[Ryan Brucks]

can the excalibur not be set to ride up higher or anything? does it have an ability to fix the height at a certain level, or does it always have to be lifting up by the piece?

I ask since raising the guard is more effective at stopping pieces lifting than reducing airflow. even at low airflow, you'll get some suction with a smaller gap. increasing the gap lets you have the same airflow without picking up the piece.

[Tim Morton]

So too much dust collection CAN be a bad thing huh?!? Who would've thought!

So how will you address this? Open another blast gate in the system to reduce suction and/or choke the blade guard blast gate down?

I suggested that we not use the over head dust collection on small work like this, and he gave me "the look"...like that was not an option. But with a ZCI and dust collection at the saw i think thats worth looking at.

As for raising the guard itself up , and in effect lessening the suction...to me that seems risky. It for sure would help, but i think you want to eliminate or minimize the risk, rather than just reducing it.

[Mike Goetzke]

UPDATE: It was easy to reproduce the cause of the kick back this morning, and i will let bruce explain further, but the suction of the overhead dust collection hose was strong enough to lift the off cut off the table an into the blade. This was compounded by the fact that the off cut was slightly smaller than the space between the Plexiglas of the blade guard, and the blade. So a teaching moment for sure. There are some unanswered questions, and he will put some pictures up.

Here is the video
View My Video

This is why when people ask which Shark Guard to get I recommend the one with the 2.5" port versus the 4". With the 4" I have had small cut-offs get sucked in just enough to hit the top of the blade and bounce around like you were making popcorn. I added a gate to the 4" port and solved the problem. I also always use a splitter.

Mike

[mreza Salav]

Here is the no so old thread in which I reported a similar incident (but I wasn't hurt). I was cutting some acrylic 1/4" sheets:

http://www.sawmillcreek.org/showthre...ing&highlight=

I haven't had the time to fix the problem but my intended solution has been to add a couple of strips running parallel to the blade at the bottom of the guard to the left of the blade;
this way when the guard is on the piece the off-cut pieces will be held down by those strips and yet you don't sacrifice air-flow.
BTW my homemade guard has a 3" hose that merges with the 5" hose of the base to a 6" duct going into a 5HP cyclone.

[Bruce Darrow]

This AM's experimentation and reconstruction of the accident made the whole thing quite clear (or nearly so....) It is certain that it was DC airflow/suction that lifted the offcut up off the table, allowing it to fall onto the spinning blade. POW! Be sure to check out the short video link in Tim Morton's second post.

Neither a riving knife or splitter would have prevented this. Flesh sensing technology would not have prevented this. I do not say either is unnecessary or undesirable. Just saying, doncha know.

Chosen pics are posted below:

Immediate aftermath.jpg This is a reconstruction of the immediate aftermath

the culprit.jpg The culprit. Note the bloody dent in the right end. Also note that the piece has somehow flipped, with what was the leading edge when it contacted the blade now trailing. It had to have been cartwheeling when it hit me (!)

Guard alignment.jpg Note the offcut entirely contained within the guard housing

Guard alignment 2.jpg Another view of same

So the jury is in. Overhead dust collection can be quite hazardous if precautions aren't taken to prevent small offcuts from lifting off the table. In this instance, a right lateral shift of the guard housing so that its left side rested on the offcut would have prevented the accident. The use of a sled, with the whole arm/guard assembly pivoted out of the way, would have prevented the accident. Making the cuts with no DC airflow at all would have prevented the accident. I think each individual operator/shop needs to make the call, depending on the circumstances surrounding the task at hand. The important thing here is that a lot of us have been unaware of this hazard, until now, at least in the case of those who have followed this post.

Now how do we, the enlightened, spread the word to the rest of the woodworking community, before another incident triggers the dreaded lawsuit, with all its undesirable aftermath?

Thanks to all who responded. There are some good questions and suggestions above that I, at least, haven't responded to yet. I'll log on again tonight, when rates-by-the-minute (dial-up orphan - poor me!) are cheaper and try to respond individually.

Be safe!

Bruce

[glenn bradley]

The use of a sled, with the whole arm/guard assembly pivoted out of the way,

That one gets my vote. Thanks for taking the time to go through this so thoroughly. We can try to forsee what is going to occur during an operation but, we can't always see every possibility. The more we share info like this, the more thorough our safety thinking will be. Kudos and heal soon.

[Bruce Darrow]

Salem,

With regards to body placement: All "tutorials" or safety articles I have ever seen recommend placing the body to the left of the blade, keeping it out of the path of kickback from between the fence and blade, which is where most feedback occurs.

[Bruce Darrow]

Keith,

Thanks for the recommendation. I'll look into these and, as I usually use a ZCI, their splitters as well. I've also seen (somewhere/when) where folks make their own removable splitters.

[Bruce Darrow]

Mike,

I'm with you here. I bought the Shark guard for my own saw, after buying the Excalibur for the saw at work, and went for the 2.5 as well. Even with the small port and a 6" port on the cabinet, I have gotten some veneer thin rip offcuts sucked up into it. After yesterday, I'll be rethinking my use of it as well.

[Bruce Darrow]

Mreza,

Very similar story. Sorry now that I missed it the first time around.

[mreza Salav]

I hope your injury heals soon.

I somehow think the danger caused by this situation isn't publicized enough and many people (like you and me) might get caught off-guard.
I think that posting them on these forums helps but the threads may get lost soon and not everybody sees it.

[J.R. Rutter]

I have had this happen to me, fortunately with less pain, but I still have the scar.

I will just throw out here that I have done some cuts in the past where I taped a zero clearance filler into the bottom flat section of the overarm guard. In my case a Biesemeyer. IIRC, it was a piece of MDF. The front of the guard was still open for air flow, but the filler kept little offcuts from getting sucked up.

[Ryan Brucks]

Interesting note on the linked thread for people making their own guard.

Someone mentioned the position of the dust port on the gaurd in relation to the blade could matter. if the port is directly over the blade, or (worse) centered on the opposite side of the blade as the offcut, that will cause a tendency to pull pieces into the blade more. If you can adjust the port so that its centered over the same side as the thin offcut, any "lifting" action would me more straight up or away from the blade. Of course not sure how feasible this is for different guards. For my guard its hard to follow that advice since my guard gets narrower at the bottom than the top... so I don't get much wiggle room either way.

[Ryan Brucks]

some pics of my blade guard. has pieces to surround the riving knife which keeps the guard safely away from the blade should it get bumped. Also reduced the volume of the guard some.




With the design of my guard, those little "baffles" prevent pieces from lifting up very high (probably doesnt help much). More important, they also mean that once a piece has been cut, it's already mostly out of the airstream, with the exception of very short pieces.

Now I cant say how much of a factor it was, but I notice with the excalibur, there's quite a bit of open space behind the blade, with nothing to stop the pieces from lifting up.

I also think its a poor design that the excalibur guard has a large opening at the rear and front only. that means that strong airflow is always going to be channeling right in through the back, especially when cutting a thin piece (ie, sides close to table). Cutting a thicker piece would raise the guard more, and then the opening at the rear would no longe be a large source of intake air, meaning pieces aren't as likely to get sucked up.

Another way to "fix" this (instead of blast gates) would be to put a hole in the guard somewhere with some kind of closeable flap. The idea would be to open this little port when making cuts in thin material, and it would allow the air to come in from other places besides the front and back. I think this would be more effective than simply lowering the airflow with a blastgate. Simple way would be to just make a circle hole, and get a larger circle piece and just use one screw to pivot the flap to open or close it. Looks like the side has lots of room.

or you could just open up the front more and close in the back more.