Ask any machinist, whether they’re new or experienced, what annoys them when the shop is crowded. Runout is always brought up. That tiny wobbling can make a precise part useless in the blink of an eye. But let’s get to the point: there is one certain ultra precision ER collet, the ER40, that has a bit of a cult following. Why? Easy. Some people remark that it crushes runout so far down that it’s as near to zero as they’ve ever seen without a microscope.

You might be wondering, “A collet is a collet, right?” Not really. One thing is very important: concentricity comes first. When machinists start talking quietly on the work floor, everyone knows about high-precision ground ER collets. These aren’t something you can get for cheap. You’ll see holds as firm as a vise grasp when you spend time with one. But they don’t bite down so hard that they hurt your endmills.

Let’s take this apart. Average collets can promise a runout of about.0008″, plus or minus. But the best ones are the ones with .0002″ or less. When you’re trying to meet tolerances, sweating over surface finishes, or using picky cutters that get upset at the first indication of wobble, that tiny difference makes a big impact. Your tool stays sharper with less runout. Your parts start to fit together just like the blueprint says they should. The containers for scrap don’t overflow anymore. The new guy in the corner even gets a break.

This is when machinists get really poetic. “She just holds,” says Mike, who has worked on the floor for 30 years and has hands so rough he could sand wood with a handshake. He’ll tell you to spin the collet and the dial will stay perfectly still.

Want to know? A lot of people didn’t believe it at first either. Some people even thought the improvement would be so small that it wasn’t worth it. But it’s nice to see a test indicator scarcely move. It feels like winning a lottery in a small town. Of course, it won’t remedy faulty spindle bearings or keep tool holders from becoming sloppy. But why settle for okay when you can cut your mistake down by another half-thousandth?

Don’t expect these collets to roll off the loading dock for a few cents each. Costs of precision. But if your job depends on everything fitting together perfectly, making that last little bit of runout better might save you a lot of time and money in a month.

Some people say it’s snake oil. Give it a shot. If you need to, ask a neighbor for help. Give it the dial test. It might not make you a fortune, but it will get you closer to zero runout, which is what everyone who cuts metal wants.

That’s the main point. One minor change makes a big difference for shop sanity. This folklore has some teeth, so to speak.