A CNC Close Call
Dull tooling leads to more adventure than we bargained for
Fire is a specter that haunts all of us who operate a woodshop. Most of the materials we work with are flammable, so fire safety is a top priority. Recently, we had a close call with a workpiece on our CNC router. Here’s what happened.
One of our CNC operators was mid-cut when the engineer pulled him aside to go over project details. He stepped away without stopping the machine.
Most days that's a non-event. The machine will run the program to the end whether or not anyone is standing in front of it, which is why it is tempting to step away.
That day the cutter in the spindle was well past its service life. And a dull tool doesn't cut; it rubs. Instead of slicing chips thick enough to carry the heat out of the cut, it burnished the material into fine dust and left the heat behind in the part, the tool, and the spoilboard. The program feed rate didn't help. Although it was written for a sharp cutter, it was still a little slow, which added to the problem.
The dust packed into the kerf. The vacuum table, doing precisely what it was built to do, pulled a continuous column of air down through the spoilboard. Fuel, oxygen, heat, and no one present.
By the time anyone smelled smoke, the MDF spoilboard and the part we were cutting were smoldering.
We smelled smoke in the shop and initially assumed it was coming from someone’s BBQ or a burn pile. When we shut the shop door, we realized the smoke was coming from inside the shop, so we quickly isolated it and put it out. Nobody was hurt, and the damage was limited to one part and a section of spoilboard.
It was a good outcome. We were also fortunate, and we knew it.
Looking back, several things stood out.
Keep your tooling sharp, and know when it stopped being sharp
Programs are written for sharp tooling. A dull edge rubs instead of cuts, and rubbing causes friction, which causes heat.
But the harder problem isn't sharpness. It's knowing. A dull cutter doesn't announce itself from across the shop, and ours didn't. We needed a better way to answer a simple question: how long has this tool been in the spindle? Sharpness is a habit. Knowing is a system. As a result, we made some improvements to our tool tracking system.
Don't leave a running machine unattended
The dull tool started it. Nothing caught it, because nobody was there.
Operator presence is the number one way to catch a machine problem while it's still a machine problem. A change in sound, a wisp of smoke, a smell; all of it is early, all of it is free, and none of it helps if no one is present. This is the least sophisticated control on the list and the most effective one.
Double-check your feeds and speeds
Ours were part of it. The feed rate was set a little slow, but not wrong enough to cause trouble on its own. Chipload is how much material each cutting edge takes per revolution, and the chip is what carries the heat out of the cut. Feed too slow for your RPM, and the chip gets thin; thin enough, and the tool stops cutting and starts rubbing. A dull edge does exactly the same thing. Ours did both at once, and they compounded.
That's also why the instinct to slow down when something sounds off is usually backward. Dropping feed without dropping RPM makes a thinner chip, more rubbing, and more heat, not less.
Pulling feeds and speeds from a maintained tool library instead of entering them by hand removes both this issue and its louder cousin, a feed typed 10x too slow because a digit got dropped.
Decide how you'll reach the fire before you need to
Standing at a smoking machine is the wrong time to think this through.
Two things are worth settling in advance. First, the spindle and dust hood will likely be sitting right on top of whatever is burning. Move the head clear with the controller and jog it away. Second, understand that e-stop kills motion. If you want the head moved, move it first, then hit e-stop. If you get that order backward, and the head stays parked over the fire.
Turn off anything that is feeding the fire
The vacuum table's downdraft feeds a fire in a very real way. So does dust collection, which carries the added risk of pulling an ember into your ductwork and collector. Be sure to shut down the vacuum pump and the dust collection.
Use the right extinguisher
A smoldering MDF fire is a Class A blaze: ordinary combustibles. Two things put it out: an ABC dry chemical extinguisher, or water once power is off. Keep an ABC near the machine, keep it inspected and charged, and know where it hangs without having to look.
In Conclusion
Hopefully a router fire never happens to you. But mistakes are part of running machines, and good machines will still find whatever gap you left open. We found our gaps and made some changes to help close them.
The best time to find yours is on an ordinary afternoon, before anything is on fire.
