Accidents happen ... 14" cutterhead at 7200rpm

[purduephotog]

Afternoon all-

I thought I'd relay a story from my coworker about his son. This man came within inches of losing his life thru a sequence of very logical and extremely reasonable steps.

A 4" thick piece of mahogany was being machined for tenons. The piece was part of a huge 20' door and, as an oversized, required special cautions, including that the safeties be removed from the machine. Since mahogany is carcinogenic, they took the trouble to increase the airflow to prevent the workers from being exposed to the dust. The guides that were normally in place were too small to accommodate the large piece of wood.

Apparently the door stile would be cut in two parts- once would make the horizontal parts of the tenon and then it would be hand sawn to make the verticals. The piece was so large that it was bigger than even what the CNC machine could handle.

The gentleman's son had already completed this task on several other pieces. While removing the this piece, however, he clipped the edge of the rubber shield that was protecting him from the carcinogenic dust. The 14" cutter head was instantly wrapped in a thick blanket of rubber and, under the force of a 20hp motor, shattered.

The sensation, as described, was little more than a small 'kick' to the stomach and chest. For a moment he looked down and tried to understand what exactly had happened- obviously the 20' piece of mahogany hadn't kicked back otherwise he wouldn't have been standing. In seconds, however, he began to 'spurt blood'- rapidly, too. Three pieces of the carbide cutterhead had punched thru his stomach and chest. At the speed of 7200 rpm they left at over 400 fps and the equivelent energy of being shot with a .357.

Surgery was touch and go for several days- multiple surgeries were required to repair all the damage and stop the bleeding. At least for two of the days the doctors kept repeating they had no odds to provide on whether or not he would live.

Even the most reasonable intentions and safety analysis can show drastic flaws in hindsight. I quipped that another machine should be bought. Another coworker pointed out that a simple kevlar vest would have prevented the torso shots. You can imagine the suggestions and ideas to prevent this disaster that were all offered up... in hindsight. Machines are expensive. Bullet resistant vests are expensive and (considered) by all to be overkill. A lexan face shield scratches and is expensive to replace.

While I (obviously) don't have all of the details right (and I wasn't going to push for much more) it just amazed me that this happeend in a professional woodworking business that knew how to perform these tasks safely (and has an outstanding record of this). The entire logical arrangment was completely safe, up until they added the protection from the carcinogenic dust generated by the mahogany wood.

Hope everyone stays safe and has a sane and productive year.

Jason

[David Merrill]

Yep. Bought one of those Chevy Vega station wagons back in the 70's. It had heavy, reinforced doors, advertised to protect against side collisions. Damn near crushed my fingers in that bear trap of a door. When the aluminum engine started burning oil at 40,000 miles and the GM dealership service manager obnoxiously insisted that I wasn't changing oil (I was - mechanical engineer) and caring for the car (I was), I drove it to the Toyota dealer and never looked back.

David Merrill

[Pop`]

"Safeties be removed from the machine": A modification for "carcinogenic" dust:

Each one the largest NO-NO of every business and hobbyist alike. I hope they have good insurance because if they send out an investigator to look that over ... .

I'm not unsympathetic, really, but ... those two opening salvos pretty much set it up for a disaster which was not an "if" but a "when". Murphy never sleeps! and "right tool for the job" prevails here. It was a worthwile lesson, I hope; maybe not.

Glad he's still got his guts and chest at least!

Pop`

[CW]

The point of your story would have gotten across just fine without this serious exaggeration.

[Anonymous]

.357 projectiles are genreally approx. 1/3 oz. and travel at about

1200-1300 fps

CW wrote:

[Anonymous]

.357 projectiles are generally approx. 1/3 oz. and travel at about

1200-1300 fps

CW wrote:

[Max]

We don't know the size and *weight* of the pieces that struck the victim. The energy in pounds foot *could* have been equal to a .357 slug.

Max

[CW]

[todd]

According to my sources, a .357 slug has a weight of about 125 gr. and a speed of 1450 fps. For an equivalent amount of energy at 438 fps, there would have to have been roughly 10 times the mass, which works out to about

80 grams combined.

todd

[Mark & Juanita]

OK, based upon previous posting that .357 shoots 1/3 oz bullet @1300 fps, total energy is 1/2 m v^2

1/2 * 1/3 * 1/16 * 1/32 * 1300^2 = 550.13

Now, to get that same equivalent energy, the mass required to have been ejected at 400 fps:

m = 2 / v^2 * E

m = 2 / 400^2 * 550.13 * 32 = 0.22 lb = 3.52 oz

For the size of cutter, that doesn't seem too implausible

Note: this assumes the 400 fps estimate is correct, it further asssumes that the .357 projectile mass and velocity estimates are correct. Finally, it assumes that I did my math correctly. I'm not sure that the 400 fps estimate is correct, 14" cutter head @7200 rpm would be moving at 440 fps (close enough)

+--------------------------------------------------------------------------------+ If you're gonna be dumb, you better be tough +--------------------------------------------------------------------------------+

[Andy Dingley]

Just _which_ serious exaggeration? 14" cutterheads simply aren't spun at 7200rpm. From that, the whole story starts to sound dubious. The machine spindle might well run at this speed (it's high, but not unheard of on modern kit) but it's not a safe speed to run on that size of cutter.

Of course people working around big powerful machinery are at risk of injury if things go wrong. Of course taking guards apart increases risk (and it's rarely necessary - if the guard doesn't work, you need a better guard, not no guard). The first rule of reporting such things though is to be accurate and impartial. If you let hyperbole creep in, then the chance of having a useful analysis of the accident and avoiding it in the future diminishes.

For some years I designed and built machine control systems (big power presses) and our office neighbours were a firm of forensic engineers. Lots of interesting and useful conversations were had between the two of us, on the topics of how to kill or maim workers and how to avoid doing so.

[somewildmonkey]

Well, call me prehistoric, but why even use a machine at all if all they're doing is cutting a tenon on a piece that large, especially if they're not mass producing it.

[Doug Miller]

Maybe I'm a little slow yet this morning (only one cup of coffee so far), but where does the 1/32 come from? The rest I see:

1/2 is from mv^2 / 2 1/3 is projectile weight in ounces 1/16 to convert projectile weight to pounds 1300^2 obviously is velocity squared

But what's the 1/32 ?

Similarly, where does the 32 come from here?

[todd]

Well, I don't drink coffee at all, but without putting a lot of thought into it, could it be the conversion from lbs-force to lbs-mass?

todd

[Doug Miller]

Yep, I think you nailed it. Thanks.

[purduephotog]

< snip>

Andy-

I'm just repeating what his fathe said. This was part of a large door and one of multiples being made. I had to take the speed of the cutterhead as fact; and a 14" tenon bit, while large, isn't unheard of. The linear angular velocity of the bit is pretty close to 440 fps at the outer edge of the cutter at that given speed. I can try quizzing him some more but I don't know what I'll get, and I doubt I'll get the shop (very prestigious one, at that) any information off the machine they were using.

In short, no exaggeration on my part, just relaying what I knew. If the facts as presented to me were exaggerated I did try to spot check as much as I could before presenting it. No malicious intent here... just shocked at how quickly something could go wrong.

I'll see what I can find out.

[Mark & Juanita]

As Todd pointed out -- Formula is for mass, so the 1/32 is needed to convert from lb-force to lb-mass

+--------------------------------------------------------------------------------+ If you're gonna be dumb, you better be tough +--------------------------------------------------------------------------------+

[bf]

Don't worry about it. I see the point of your story. Basically, good intentions (the cancer shield) can have disasterous unintentional results. Naturally, this being usenet, people focus on insignificant details such as whether the fragments were as powerful as a bullet.. whatever. I have a life, so I don't worry about it. I know you weren't trying to decieve anyone with your story. Although maybe I'm the only one :)

[Prometheus]

Go ahead if you like, but I think the point is clear enough without poking into what is likely to be a pretty touchy subject around that shop. Let's just say it was a few hunks of sharp metal going fast enough to impale a guy, and let those folks get on with getting past what sounds like a pretty nasty day at work. I'm sure we'll survive without diagrams and expert witnesses.

Doesn't really matter if the projectile was going as fast as a bullet or an arrow, or a speeding car- if it pops one of your organs, that's reason enough to avoid the situation.

[Edwin Pawlowski]

Well it certainly matters to the victim once healed and telling the story over a couple of beers. "it was a 36" wheel that disintegrated and just like a meteor . . . . . . . .