Why is a trapped bevel rip unsafe?

[cgallery]

I don't want to hijack the other thread, so I'll ask here.

Lots of users point out what I've heard as conventional wisdom, which is that you want the blade to till AWAY from the fence when doing a rip w/ a bevel.

On the BT3K line of saws, this would require moving the fence to the left side of the blade.

But my question is, why is it unsafe to rip with the blade tilted towards the fence?

Yes, the wood you're cutting would be trapped between the blade and the fence. But, if you were to take a 1x4 that is maybe 2 feet long, and start to rip an edge off (stopping halfway, so 12"), what happens if you start twisting the piece you're cutting so the bevel starts to contact the blade? Wouldn't the teeth cut a bit of the bevel, but then the blade's body stop you from lifting the wood any further?

PLEASE THIS IS THEORETICAL ONLY, I sure don't want anyone trying it and reporting back about impaling SWMBO (and NO, I'm not trying to give anyone any crazy ideas).

But I do notice that Nahm (you KNEW I was going to bring this up, didn't you) never moves his fence, he always does a trapped bevel rip. Has he just been lucky? Does he experience kickback and not care? I mean he seems like a pretty bright guy. It only takes one (max. two) kickbacks for someone to say, "I'm never gonna do that again."

Kickback I've experienced and seen pictures of seems to get its power from the wood contacting the TOP of the blade. There is a typical circular gouge pattern on the wood that is thrown. It would seem with a trapped bevel that this sort of problem would be unlikely?

So I'm wondering whether there is something I'm missing here.

[leehljp]

1. When it is trapped like that, if a tooth gets caught as happens with a kickback, it will push the board away and up just a smidgen and then the force and momentum of the blade will impale itself more into the wood, the blade will more than likely deflect, warp and ruin itself, but that is a secondary problem to the real problem as the trapped wood flies back.

2. Nahm has been lucky somewhat, but also his skill and experience is on his side. There are lots of dangerous things that can be done with saws and other tools - done relatively safe - by those with considerable experience and who are self-aware at the point of doing the procedure. However, most people don't follow these "self aware" moments or think things through. Even experienced people will have mistakes but for them - the risk is evaluated in the process.

The point is, many many people have a "high regard" for themselves and figure if Nahm can do it, then they can do it - and - blam! Then those who regard themselves "highly" - they will quickly shift the blame to Nahm or the manufacturer.

In a leadership training session this past week, we discussed this very issue of "blame". Many people cannot accept for themselves "blame" for mistakes or accidents. A. Some people analyze the situation to determine the problem or fault; B. While Others analyze the possibilities that put them in that position in order to relieve the blame. This is a fact.

We have to deal with this issue often - When something doesn't go right, the quick reaction to shift blame and aversion to accepting blame instead of objectively analyzing the problem.

Low Challenge people can't handle blame or failure in a project; High challenge people accept responsibility and failure - as a stepping stone to success. (Low challenge does not mean passive - as low challenge people can be quite agressive; it relates to how well they analyze and handle "challenges". This is part of the Birkman leadership/assessment styles.)

Low challenge people will see a Nahm make a good cut, and yet blame him when they get impaled. This is the reason for all of the warning labels, warning pages in manuals and redundant safety devices on simple as well as complex tools. It is also the reason people have to have "caveats" and warnings in posts.

Back to the saw question: It can be done, the risk is much higher IMO.

[Uncle Cracker]

2. Nahm has been lucky somewhat, but also his skill and experience is on his side.

Not to mention video editing...

[RAFlorida]

Hank, that's the best explanation I've seen to date.

Many people just don't get it, that they themselves causes the problem/accident and not except the blame. Thanks for your post. Very enlightning.

[SARGE..g-47]

To add to what Lee said CG.. not only can it warp and ruin the blade.. the work-piece (the least scenrio is scortch and burnish0.. I have seen one instance that it was trapped so badly it twisted to bend the fence mechanism before it flew out the rear.

And that's not to say you can't get away with it all your life as Nahm appears to do (I have met Nahm but never seen his show) but.. it could also happen the very first time someone tries it.

[cgallery]

The conventional wisdom says it is totally and completely unsafe.

Yet Norm does it all the time.

Now, Norm also works without guards and he obviously takes other liberties.

But I do wonder why he wouldn't have mentioned it at some point during the show or elsewhere.

If you look at the Q&A section for the NYW, three different people asked the different between LT and RT saws, and the only benefit to the LT saw (as far as Norm is concerned I guess) is that you get better cut quality cutting laminates (money side up). Never once do they mention safety as an issue.

Norm seems like a pretty decent fella. I don't think he'd want any fans to hurt themselves. So if he thought it was unsafe, or risky for a newby, wouldn't he mention it at some point?

It just seems (and I don't know for sure) that with the blade tilted towards the fence that it would be more difficult to achieve the dangerous type of kickback where the wood comes in contact with the top of the blade.

Does anyone have any personal experience or pictures of kickback that occurred with a fence-tilted towards the fence?

[cabinetman]

It just seems (and I don't know for sure) that with the blade tilted towards the fence that it would be more difficult to achieve the dangerous type of kickback where the wood comes in contact with the top of the blade.

Does anyone have any personal experience or pictures of kickback that occurred with a fence-tilted towards the fence?

Being trapped, there is no free place for the piece to go. It can contact the blade, and shoot out at you, or can just get all caught up and mangled, by the time you shut down.

Pictures? No, no pictures. But, I've had all kinds of kickbacks. That happens when using the saw on a daily basis. Fortunately, a lot of the problem can be detected early by being sensitized to the clues.
.

[cgallery]

Being trapped, there is no free place for the piece to go. It can contact the blade, and shoot out at you, or can just get all caught up and mangled, by the time you shut down.

Pictures? No, no pictures. But, I've had all kinds of kickbacks. That happens when using the saw on a daily basis. Fortunately, a lot of the problem can be detected early by being sensitized to the clues.
.

But isn't kickback where the piece being cut lifts and rides the TOP of the blade?

How does that happen with the blade tilted towards the fence? Doesn't the blade plate act as a sort of riving knife and prevent that from happening?

[SARGE..g-47]

But isn't kickback where the piece being cut lifts and rides the TOP of the blade?

How does that happen with the blade tilted towards the fence? Doesn't the blade plate act as a sort of riving knife and prevent that from happening?

Kick-back is caused by the rear rising teeth. It doesn't have to happen at the top of rear teeth.. it can happen on the lowest rear tooth rising out of the insert. But.. when those teeth in the rear catch the stock they are "rising" so the stock is going to rise and God only knows where it goes from there is there is no time to do a scientific analyzes. Kick-back is violent.. if the blade traps the stock left.. it has to go right or up. It it goes right or up.. it is going to twist and violently impact the fence. I doubt you would want to stand around and take pictures of kick-back just to see what is happening as it is over in an instant and hard to analyze just what did take place. I have had 4 gut shots and I basically know why but.. could not tell you what happened step by step as by the time it happens.. it is over. The higher the HP the quicker.

The stock often goes up but it can also twist whike it does. As far as the blade plate acting as a riving knife.. a riving knife or splitter is there to keep the wood from touching the rear teeth. But.. if it flex.. etc. it still can as thre is a gap between the rear teeth and the riving knife or splitter. The gap is larger with a splitter because it cannot be set as close to the blade to close that gap. The splitter is permanently fixed in one position and when the blade is raised.. it is closer to the rear teeth. When lowered the splitter is farther away and that is the advantage of a RK over a splitter.

As far as what Norm does on his show.. that does not make Norm correct. If you want to follow Norm's example you can as you are free to do so. If Norm balances on a tight wire over a spinning blade and makes a cut... that doesn't mean you should. You can if you wish but common sense based on logic tells me to ask Norm himself Why in lieu of just following Norm's example as Common Sense trumps "just because I do it that way" each and every time.

It has been a long day....

[IBBugsy]

Just a quick note on safety risks.....

I was involved with chemical safety for 9 years (fires/ explosions/spills/etc. at chemical factories) and one (of many) interesting rule-of-thumb I learned: Assume a human will make a mistake every 1000 times he does something that he knows how to do.

We do complicated calculations to estimate the frequency of accidents at our factories. Even if the activity is simply pushing a button, we will assume the button is not pushed (i.e. either forgotten or the wrong button is pushed) once every 1000 times. Then we look at the consequences of that mistake and try to have systems that will counter it.

And the 1000 times is not a guess. Several companies (including mine) keep track of this data (and other data like it) and report it broadly for others to use. So it's based on industry experience. (I rounded the number to 1000 - It's my rule-of-thumb).

So, regardless of the experience of the person, we assume a mistake will be made.

Food for thought...

[LCHIEN]

i will not have claimed to see every accident or experience most of them.
I think that a classic kick back involves as CG says, the piece riding up and over the top and carries that classic curved tooth bite in the wood afterwards (and yes, I have done one of these), I think the normal conditions that makes this likely is a smallish, almost squarish piece of wood in the range of 2"x2" to 5x5". The fact that the wood gets jammed sidewaysbetween the fence and the spinning blade gives it a lot of traction getting up and out.

Now there are other instances where wood is sent backwards and those may or may not correctly be called kickback. I don't think they are quite as violent as the classic kickback since they don't get set off the tips of the blade where the velocity is fastest.

In the trapped bevel cut the wood can't actually rise up and get propelled by the tips. If its a longer piece it can't get sent sideways. I have to confess that I've made this kind of trapped bevel cut - but I used a sacrificial push piece that lay flat on the table and pushed it through at least until the cut was complete - the push stick has to be about 10 inches long and get 3-4 inches into the blade. The good news is for multiple similar cuts it can be reused. In any case, it prevents the wood from ever coming backwards. I don't recall having any problems with the end of the wood being chewed up by the blade as the blade came to a stop... of course you have to have an outfeed table.

So I'm sort of with CG - this can be done safely if you do it carefully.

[Carlos]

Interesting. I can't decide if I'm glad I read this, or wish I hadn't. I'm a trapped bevel kind of guy. Have been all my life. I knew that it was "better" not to do that, and is the reason for left-tilt, but really didn't know it was considered so dangerous. I've never experienced anything more than a burned edge when I let something get out of alignment. Maybe I'm just completely lucky, or I can attribute some of the success to fanatical alignment of the blade/fence and only using wood that is well milled, along with frequent waxing of the table and fence for smooth feeding..

I can't say that I'll kick the habit cold turkey, but I'll think of options next time I need to do it.

[cabinetman]

Interesting. I can't decide if I'm glad I read this, or wish I hadn't. I'm a trapped bevel kind of guy. Have been all my life. I knew that it was "better" not to do that, and is the reason for left-tilt, but really didn't know it was considered so dangerous. I've never experienced anything more than a burned edge when I let something get out of alignment. Maybe I'm just completely lucky, or I can attribute some of the success to fanatical alignment of the blade/fence and only using wood that is well milled, along with frequent waxing of the table and fence for smooth feeding..

I can't say that I'll kick the habit cold turkey, but I'll think of options next time I need to do it.

I don't know if I attribute success to doing chancy procedures. I've committed all of the TS sins. I admit to being foolish, but, the show must go on. So, is it luck? This is a quote from a poster to this thread:

Just a quick note on safety risks.....

I was involved with chemical safety for 9 years (fires/ explosions/spills/etc. at chemical factories) and one (of many) interesting rule-of-thumb I learned: Assume a human will make a mistake every 1000 times he does something that he knows how to do.

We do complicated calculations to estimate the frequency of accidents at our factories. Even if the activity is simply pushing a button, we will assume the button is not pushed (i.e. either forgotten or the wrong button is pushed) once every 1000 times. Then we look at the consequences of that mistake and try to have systems that will counter it.

And the 1000 times is not a guess. Several companies (including mine) keep track of this data (and other data like it) and report it broadly for others to use. So it's based on industry experience. (I rounded the number to 1000 - It's my rule-of-thumb).

So, regardless of the experience of the person, we assume a mistake will be made.

Food for thought...

Reading that I'm surprised that 1000 times was the number. It could have been 5 or 500. What I agree with is that we can get complacent to the point that we forget, or overlook the possibilities of the set up or procedure as a whole. It could be just a safely done procedure that was immensely repetitious, and a slight lack of concentration caused a problem. The two injuries I've received were really minimal, and I view them as a "heads up", or a warning from the woodworking gods to learn from. They were both a repetitious procedure that gets real boring, real fast.

So, because someone hasn't had a problem with "trapped" or "feather boarded" procedures doesn't mean they won't ever. It's just that their number hasn't come up yet.
.

[LCHIEN]

In rethinking this thread, I think there's a big difference between a rip bevel thats a few inches wide vs. a ripped bevel that's only 1" wide.

With the wide rip bevel, I can keep a push shoe on top of the workpiece and pressure twoards the fence the whole way and this almost assures me of no kickback since the material is held down against rising teeth in the back and from being trapped to the blade since I direct the force to the fence. I feel confident when I keep a shoe on top I can safely make an inside rip bevel if that's what we call it.

With a narrow rip bevel, the ability to keep a shoe on top is gone, I have to push from the rear as I mentioned before, this is more hazardous.

As for IBBugsy, I think the number is closer to 100 than 1000 for people in a routine job, from personal experience*. I try not to take my woodworking as a routine job, though.

* In one of our previous manufacturing experiences, we had many items we could put together one of two ways, the correct way was clearly marked with two distinctly colored wires and a dot on the positive terminal. One of my jobs was a measuring device to find reversals in the finished product. we found an average of one per product with 128 devices in it. That rate was similar to other products we made that could be assembled backwards if not paying attention to the markings.

[Carlos]

Agreed on the width point. Probably makes a big difference.

What I agree with is that we can get complacent to the point that we forget, or overlook the possibilities of the set up or procedure as a whole.

I guess since I know the trapped bevel can be problematic, I pay much more attention and take more care than for other cuts.