An Expensive Lesson

I violated a rule a couple of weeks ago. I didn't even know it was a rule until after the fact, when Phil Oles acquainted me with it. It's called the "Three Tooth Rule," and it basically says, "when cutting, make sure you have at least three saw teeth in the material at any time."

I found out that if you violate the rule, you are liable to have two simultaneous outcomes:

1. A loud bang.
2. Broken parts.

Sadly, the broken parts were on one of my most prized tools, my Bosch compound miter saw:

This is a Bosch GCM12SD, with a 12" blade, and an amazing mechanism called the "Axial-Glide System" that allows it to expand and cross-cut boards up to 14" wide without needing any clearance behind the saw. It is one of the few major woodworking tools I've bought brand new, and it has been a real workhorse.

On the day I violated the Three Tooth Rule, I was cutting aluminum. Specifically, a piece of aluminum angle 1" X 1", 1/8" thick. All the woodworking books say it's perfectly OK to cut aluminum with woodworking tools, and indeed I had done it before. But this time, the work snagged and stopped the saw with a huge bang and a great deal of force. It broke two teeth off the blade, and bent and cracked the left-hand saw fence. Indeed, it even bent the bolt holding the fence in place:

That was seriously annoying, but not fatal. The fence is bolted on, and Bosch has a detailed parts list for the saw. Worse, however, was what I found when I mounted a new blade and carefully tested the saw. There was now a significant vibration, and the saw didn't cut cleanly. Something else was bent!

At first, I thought I'd just take it to a Bosch service center to get it thoroughly checked out, but there is not one within driving distance, and I wasn't prepared to ship a 65 pound saw both ways. So I started by testing the runout at the flange where the blade mounts. It seemed bad, but was hard to test with it on the saw. I disassembled the saw far enough to get that part off, and tested it in a fixture I made to hold it. I found runout of over 0.015", which is terrible. This brief video shows that test:

If your browser or email won't show the video, the link is here.

This seemed conclusive, but I wasn't quite sure. I talked it over with Phil, and we were both dubious that the heavy flange could have been bent that much. To further test, I made an arbor to hold it on my lathe. The shiny turned nose holds the flange, and the tapped hole in the end lets me secure it.

Mounting the flange there, I found identical runout to the measurement in the video - proof that the shaft wasn't bent, just the flange. I decided that it was already broken, so I could experiment with it if I wanted to. I faced off 0.020" to make the flange flat again:

I put it all back together, and went through some very careful tests. First, I used an extension cord and a clamp to turn on the saw with me standing several feet away. Nothing exploded, so I made a test cut and was rewarded with a very clean cut. I had found the problem!

After that, I unplugged the saw and removed the blade. I wasn't willing to use the saw with that compromised flange. I needed to order the fence parts anyway. That led to the next shock: Bosch wouldn't sell me those parts on their website!

Fortunately, I discovered ereplacementparts.com, which had the same parts diagram as Bosch, and had all the parts but one in stock. About 90 bucks to buy it all. As I said, an expensive lesson! But the parts came today, and after checking the new flange on my lathe arbor (it was fine) I put it all together and it works great. Quite a relief.

EDIT: A week later, I finally found time to re-mount the saw on its stand and hook up dust collection. More test cuts, and this shot of a very thin slice shows that the saw is cutting clean and true once again!

In retrospect, I now know what to look for in evaluating cuts like this. The two pictures below (mock-ups with the power unplugged) show the two ways to orient aluminum angle when cutting. Both are unacceptably dangerous on a chop saw. I was using the left hand orientation, with the angle like a tent. I felt that was a safe way to cut, but the blade was still able to catch the front part of the angle in a gullet between two teeth, and I got a quick lesson on just how much force a spinning 12" blade can generate.

Interestingly, Troy Nace told me that in the metal shop where he used to work, they mounted the saw blades backward when cutting aluminum like this. That would aim the gullets "away" from the stock, minimizing catches. Of course, it also means they were gnawing the metal with the back edge of the saw tooth! But he said it worked fine.

I 'm not interested in trying that. Indeed, aluminum will never come anywhere near my chop saw or table saw again. Now that I just finished my band saw cutting vise, I'm set for cutting metals with the tool intended for the job. 

There's no need for me to get the Three Tooth Rule tattooed on my arm - I'll never forget that one!