Getting the Best Performance from ER Collet Chucks

4 months by cncdivi

Getting the Best Performance from ER Collet Chucks

Short of much fancier and more expensive solutions, ER Collets are one of the best toolholding systems available. Consider this comparison from our Ultimate Guide to Selecting Toolholders for Milling:

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ER Collet Chucks are inexpensive and can really do the job as long as we’re not talking super heavy duty or very high rpm applications.  For most endmills, they’re perfect.

What makes ER Collet Chucks perform so well?

We start with runout, which is much better for ER Collet Chucks than setscrew holders.  Low runout is critical to tool life.

BTW, making runout repeatable is one of the jobs of the drive dogs found on many spindle tapers.

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, so 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?

How do we get the most out of an ER Collet Chuck?

Having seen that ER Collet Chucks can perform well, it’s also important to realize that some care is required to ensure maximum performance from an ER Collet Chuck. Maximum performance means:

  • The shank of the tool can’t slip during machining, nor can it be pulled out or pushed in.
  • Runout should be minimized.

Those are really the two issues we want to focus on, and here are the Tips you need to Maximize Performance in these areas:

Tip #1: Collets and even the collet chuck nut are consumables.

If you’ve taken care of all the other factors and still can’t get the performance you expect, replace the collet.  If the problem traces to a collet chuck, try replacing the nut as well.  Both are readily available separately–you don’t have to buy collets in a set and you can purchase collet chuck nuts separately too.  Treat them as consumables because they can wear out.

Inspect collets for undue wear before each use.

Look for fretting and scoring on the collet that indicates it spun inside the collet chuck.  Similarly, check the inside bore for the same kind of wear, indicating a tool spun inside the collet.  If you see such marks, burrs on the collet, or gouges of any kind, its probably time to replace the collet.

Tip #2:  Keep it clean!

A tiny chip stuck in the collet, chuck nut, or the chuck’s interior collet pocket can introduce a huge amount of runout and may also prevent the collet chuck from gripping the tool shank properly.  Make sure everything is squeaky clean when you assemble a tool and a collet chuck before putting them into the machine.

Also, a little residue of oil or cutting fluid can rapidly reduce the collet’s holding power.  Hit all the components with some degreaser before you assemble them so they’re dry and oil free.

How much does degreasing help?

Tormach ran some tests on their R8 collet system in order to come up with guidelines to minimize pullout.  The difference on a clean and dry shank based on drawbar torque from 20 ft lb to 40 ft lb was an increase in holding force from 1850 lb to 3600 lb.  That’s significant!

When the collet chuck is ready to be returned to toolroom storage, give all components a light wipedown with a rust preventative.

Give your tooling a “check-in” procedure that involves thorough degreasing followed by application of a rust preventative…

Never use a collet chuck or collet straight as it is shipped to you.  It will have cosmolene or some other protective grease that needs to be removed before use.  I give all my tooling a “Check-In” procedure that involves degreasing with kerosine (use WD40 in a pinch) to remove the cosmolene and then I apply a rust preventative.

I like Break-Free as my preferred rust preventative, which is used by the US military and others as a firearm protectant.  Break-free is available as an aerosol, but I prefer the pump bottle as is shown.  I’ve taken to using the pump style with WD-40 too–I still use WD-40 for lots of things, just not as a rust preventative.  You can order a bottle of Break-Free from Amazon for a little over $20.

I live on Monterey Bay, and there’s often a sea breeze.  The breeze is nice, but it’s full of moisture and salt.  Having a good rust preventative like Break-Free helps me sleep better.  I’ve never noticed any rust on my tools.  On tools I use a lot, I may give them an extra wipe down before they go back in the tool box once a year or so.

After you remove the heavy shipping lubricant, and your tools are in use, you’ll want something lighter than kerosene for degreasing.  A quick shot of brake cleaner works or there are many other products that will do.

Lastly, with enough use, collets pick up a residue on the tapered areas.  A lightweight brass brush is a good cleaning tool to remove these deposits.

Tip #3:  Torque the Collet Chuck Nut to Proper Spec.

Most machinists 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?

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

Consider the following torque specs from Technik USA:

Collet Nut Type Collets I.D. Collets I.D.
OVER 1/16″ (2mm) UNDER 1/16″ (2mm)
ER 16 slotted 42 ft/lbs 30 ft/lbs
ER 16 hex 42 ft/lbs 30 ft/lbs
ER 20 slotted 59 ft/lbs 24 ft/lbs
ER 20 hex 59 ft/lbs 24 ft/lbs
ER 25 slotted 77 ft/lbs 77 ft/lbs
ER 25 mini nut 25 ft/lbs 25 ft/lbs
ER 32 slotted 100 ft/lbs 100 ft/lbs
ER 40 slotted 130 ft/lbs 130 ft/lbs

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.

To hit the higher specs comfortably and reliably, you’ll need a fixture to hold the toolholder while you tighten the nut.  Don’t rely on tightening it in the spindle of the machine.

Tool tightening fixture

Here’s a nice tightening fixture from Maritool.  Tosa Tool also makes a nifty fixture for TTS Collet Chucks

A torque wrench is ideal for tightening the nut if you want to make sure you’re hitting the torque spec.  This adjustable wrench from Techniks along with the adapters for various collet nuts would be just the ticket for a well-equipped toolroom:

er collet chuck torque wrench

Adjustable torque wrench and adapters for ER Collet Nuts from Techknics

Whether you use a torque wrench or not, 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.

Over-tightening is just as bad as under-tightening because it can deform the collet in a way that leads to increased runout.

Tip #4: Consider ball bearing nuts for your ER Collet Chucks

Ball bearing collet nut

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 use too–applying enough torque with one is much smoother.

The disadvantage of ball bearing nuts is they have moving parts, and so are difficult to keep well balanced for high rpm applications.

Tip #5:  Worried about the cost of all those collet chucks?  Use a mix of collet chucks and set screw holders.

ER Collet Chucks are pretty cheap as toolholding solutions go, but sometimes we want an even cheaper solution.  Many machinists worry about collet chucks slipping too, so between that and the cost, they use more setscrew (Weldon shank) type holders.

Here’s the problem with those setscrew holders.  Tests indicate 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.

The thing is, larger diameter endmills can tolerate more runout and generate higher cutting forces, so they’re better candidates for set screw holders.  Consider using endmill holders for roughing with larger cutters and collet chucks for smaller cutters. In my shop, shanks over 1/2″ go in endmill holders and everything smaller goes into the collet chucks.

If you can afford them and take proper care as described in this article, you’ll wind up putting the setscrew holders aside over time, though.

Tip #6:  Avoid Weldon shanks on endmills–they’re more likely to slip in a Collet Chuck.


Weldon shanks reduce the holding power of ER Collet Chucks by 30%…

If you use a Weldon-shank cutter in your ER collet chuck, expect holding power to be reduced by up to 30%, which is the amount of surface area that can’t be gripped because of the Weldon cutout.

Tip #7:  Assemble your collet chuck properly

There’s a right way and a wrong way.  The right way is to snap the ER collet into the collet nut before threading the nut onto the chuck and tightening.

Tip #8: Make sure you insert tools deep enough in the collet

Collets have a rated minimum tool depth.  If you don’t insert them deep enough, they will have increased runout.  Typically, you will want to use at least 2/3’s of the collet’s length.


ER Collet Chucks are an excellent workholding solution, and with a little extra care, you can ensure you’re getting the best possible performance form your ER Collet Chucks at all times.



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