Getting the Best Performance from ER Collet Chucks

ER Collets are actually an excellent workholding system. Consider how the different toolholders ranked ranked in terms of stiffness (see my Milling Surface Finish page for more on this):

Surprising that the lowly collet chuck performed nearly as well as finicky shrink fit tooling and quite a bit better than more expensive hydraulic and power chuck-style holders!

For best surface finish, we want to maximize the modal or static stiffness. Here, the collet chuck is second only to the shrink fit holders. Where roughing is concerned, the Dynamic Stiffness is important for suppressing chatter. For maximum material removal rates, we want to maximize Dynamic Stiffness. Here the Collet Chuck also performs pretty darned well, and it is in hogging out lots of material that the hydraulic and milling chuck style holders start to come into their own. The shrink-fit performs poorly because the shrink fit doesn’t dampen the vibrations, it just holds the tool very very tightly. Makes you wonder if it doesn’t make it more likely to ring like a bell when held so tight?

Here is another interesting factoid gleaned from my research: ball bearing nuts for ER Collets can improve their performance even further. A ball bearing nut has about 14% more static stiffness for surface finish, but a whopping 50% more damping, which helps keep the chatter out when you have to get aggressive with a cut. I can tell you from experience that the ball bearing nuts are much nicer to user too–applying enough torque with one is much smoother.

Also from that last link are tests indicating that a standard ER collet chuck is 15-58% statically stiffer than a Weldon shank end mill holder. That and the much reduced runout (Weldon end mill holders grip the cutter from one side which adds runout) make them preferable for surface finish. If you use a Weldon-shank cutter in your ER collet chuck, expect holding power to be reduced about 30%, which is the amount of surface area that can’t be gripped because of the Weldon cutout.

Which brings me to my next point: most machinsist don’t realize just how much torque is needed to properly tighten an ER collet nut. When you hear a machinist talking about having an endmill slip in an ER collet chuck, you have to wonder two things:

- How clean was the chuck and cutter shank when they were tightened together? The slightest residue of oil or cutting fluid can rapidly reduce the collet’s holding power.

- How much torque was used to tighten the collet nut?

Consider the following torque specs from Technik USA:

Are you torquing your ER32 collets to 100 ft/lbs? That’s a bunch more than a lot of folks expect going on feel alone.

Whether you use a torque wrench or not, and Technik as well as others make appropriate adapters, make sure you’ve at least got a nice wrench for your chuck. The single pin spanners are the worst for me when I have to apply a lot of torque. In addition, you’re going to need a way to solidly mount your collet chuck while you tighten the nut. Bench fixtures are the most helpful for this task.

Now aren’t collet chucks kind of expensive compared to end mill holders? Actually, they’re not too bad, especially if you buy from sources like Glacern or MariTool. Be sure to get good quality collets too. I’ve had the occasional problem with collets and traced it back in both cases to a bad collet rather than a bad chuck.

Another way to think of saving is to use endmill holders for roughing with larger cutters and collet chucks for smaller cutters. In my shop, 1/2″ and up shanks go in endmill holders and everything smaller goes into the collet chucks.

Related posts:

1. What Are the Best Toolholders?
2. Bodysnatcher’s 5C Collet Indexer

12 Comments

• Toolholding in Collets: How to Avoid Tool Pullout « CNCCookbook 2011/02/05 at 2:11 am

  [...] for ER collets, and these are often much higher than machinists assume. See my article, “Getting the Best Performance from ER Collet Chucks” for some more [...]

  Reply
• Toolholding in Collets: How to Avoid Tool Pullout « CNCCookbook 2011/02/05 at 2:11 am

  [...] for ER collets, and these are often much higher than machinists assume. See my article, “Getting the Best Performance from ER Collet Chucks” for some more [...]

  Reply
• 
  Peter 2011/03/03 at 4:49 am

  I’d love to see a side by side size comparison chart too. We often use an ER-16 chuck with 3/8″ endmills. The ER-20 chucks are so much more stiffer though…… ER 25-32 gets in the way.

  Reply
• 
  Peter 2011/03/03 at 4:49 am

  I’d love to see a side by side size comparison chart too. We often use an ER-16 chuck with 3/8″ endmills. The ER-20 chucks are so much more stiffer though…… ER 25-32 gets in the way.

  Reply
• Glacern Indexable Endmills - CNCzone.com-The Largest Machinist Community on the net! 2011/06/05 at 8:52 pm

  [...] [...]

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• Glacern Indexable Endmills - CNCzone.com-The Largest Machinist Community on the net! 2011/06/05 at 8:52 pm

  [...] [...]

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• We Are Devoted to Fanatically Loyal Customers « CNCCookbook 2011/09/21 at 2:47 am

  [...] be boogers if you don’t torque the heck out of them. We ran an article on this that’s worth a link. GA_googleAddAttr("AdOpt", "1"); GA_googleAddAttr("Origin", "other"); [...]

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• We Are Devoted to Fanatically Loyal Customers « CNCCookbook 2011/09/21 at 2:47 am

  [...] be boogers if you don’t torque the heck out of them. We ran an article on this that’s worth a link. GA_googleAddAttr("AdOpt", "1"); GA_googleAddAttr("Origin", "other"); [...]

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• If the TTS is so awesome, why do I get horrible chatter with a 3/8 end mill? - Page 3 - CNCzone.com-The Largest Machinist Community on the net! 2011/10/30 at 6:28 pm

  [...] [...]

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• If the TTS is so awesome, why do I get horrible chatter with a 3/8 end mill? - Page 3 - CNCzone.com-The Largest Machinist Community on the net! 2011/10/30 at 6:28 pm

  [...] [...]

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• End Mills and Feeds n Speeds - Page 3 - CNCzone.com-The Largest Machinist Community on the net! 2012/02/18 at 12:32 pm

  [...] [...]

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• Newbie School me on ER collets... - CNCzone.com-The Largest Machinist Community on the net! 2012/03/24 at 8:16 pm

  [...] [...]

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