Thrown CNC Tool -- What Happened?

When a tool slips out of the collet while the machine's running, it's dangerous. Here, pros troubleshoot a case. November 26, 2007

We had an insert tool thrown from our CNC router today, and it hit the wall behind the machine with tremendous force. The tool was not under load when thrown. It was approximately 4" long, 1.75" in diameter, with .5" shank, and was running at 18,000 RPMs. I calculate that the tool was thrown at approximately 94 mph at a force of 275 ft/lbs. Does anyone agree with my calculations, and what is your feeling on the RPMs? I feel about 6,000 RPMs would have been more appropriate for a tool this size.

Forum Responses
(CNC Forum)
From contributor D:
I think you have more to think about than how fast, and at what force, the tool came flying out of your machine. *Why* did it come out? Did you torque the nut enough (130 in. lbs.)? Is the tool balanced? 18,000 is a bit too fast. I'd keep it around 15,500 to get started. Also, most well made tools of that size will have 3/4 inch shanks. That's not a large tool, but depending on your machine, there's some good force there. I spin some really big tools on my Northwood and never have any problems. Buy good tooling and pay attention to your setup, first off so no one gets killed, and also so as not to destroy your machine.

From contributor M:
That's too fast for a cutter that big. Had the same thing happen a long time ago with a 4" flycutter that someone had programmed at 18,000 RPMs. At 6000 RPMs, though, many spindles don't have a lot of horsepower. You would have to check the power curve on your make.

From contributor L:
Was that thing in the tool changer? You need to call that tool out in the table and give it a limited parameter of max RPM, and only let it come in on a tool change, or eventually you're going to have a Lifeflight chopper at your shop. You're lucky that thing did not disembowel someone.

From contributor S:
It sounds like the RPMS are way too high. We run 1 3/4 cope and stick insert sets in our CNC and keep the RPMs around 8,000. The bodies themselves are stamped max RPM 12,000. Did it sound like a helicopter when the tool came up to speed?

From contributor O:
I tend to agree with contributor D; you need to be concerned more with how the tool was being used than how fast it exited the machine. The maximum spindle speed for a 1.750" diameter bit should be 12,000 RPMs and that is with a 3/4" shank diameter. With a 1/2" shank you should probably be more in the range of 10,000 max. Proper machining speeds are usually about 50% to 75% of maximum for shape cutting tools.

Proper torque specification for tool holders will depend upon the collet system used. Recommended torque is 80 - 90% of maximum. Some of the more common max torque specifications are as follows.

ER25 - 95 ft/lbs
ER32 - 125 ft/lbs
ER40 - 140 ft/lbs
SYOZ25 / TG100 - 100 ft/lbs

At this point, now that you have thrown a tool, I recommend you replace the collet in your tool holder. You may also want to look into a tool with a larger shank, such as 3/4".

From the original questioner:
Thanks. I did not install the tool, set the parameters, or run it. At closer examination of the tool today, it was 2" diameter, not 1.75", and weighed 200 grams. I believe the tool was thrown because it was too large to be run at those RPMs and not balanced for that speed. Also, I would have preferred a tool that size with a 1" shank.

From contributor C:
I agree that is a little too fast, but not much. It would work just as good at 14,000 to 16,000 RPM. How is your collet and when was the last time it was changed? If you are running production, it should be changed 2-3 times a year. Also, are the collets tightened with a torque wrench, or a guy with an extension bar? One note - the collet is more important than the tool.