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Runout Versus Tool Life

Dec 2, 2013   //   by Bob Warfield   //   Beginner, Blog, FeedsSpeeds, Techniques  //  4 Comments

We’ve all heard that runout is very bad for tool life, but here’s a quick and easy to understand chart that explains how runout affects your tool life:

Runout vs Tool Life

Runout (TIR) as a percentage of chipload versus Tool Life…

In this chart, runout is given as a percentage of chipload, which is the thickness of a single chip being sliced off by your cutter.  This is how I like to think of runout–it increases chipload beyond what you intended.  As you can see, tool life goes down dramatically when we get to TIR’s (Total Indicated Runout) beyond about 20% of your cutter’s targeted chipload.  This also explains why runout is so hard on tiny cutters–they have small chiploads and can tolerate very little runout as a result.

Let’s work through a couple of examples.  Let’s say we’ve got a 1/2″ endmill in a Side Lock (Weldon Shank) Toolholder.  Further, let’s suppose the spindle has maybe 0.0003″ of runout, so it’s not real tight, and our Toolholder adds another 0.0005″ since its set screws will tend to push the tool off center.  That means we’ve got 0.0008″ of runout.  So, if we want that to be less than 20% of chipload, chipload must be greater than 0.0008/0.2 = 0.004.  How realistic is that?

If we set up an aluminum cutting scenario in G-Wizard Calculator, it might look like this (the Cut Width and Cut Depth were optimized by CADCAM Wizard):

GWRunoutScenario

Roughing scenario for our 1/2″ endmill…

This scenario wants a chipload of 0.004, so we are right at that limit.  We’d be better off with an ER Collet Chuck (less runout) to give a safety margin, but we’ll probably live to tell the tale.

Now let’s try the same setup with a 3/8″ endmill.  Changing endmill size won’t make our spindle have any less runout and it probably won’t affect the Side Lock holder much either so our max chipload must be greater than 0.004 again.  G-Wizard suggests we need a chipload of 0.0028 in this case, which is less than our limit by a lot.  Looking at the Tool Life chart above, 0.0008″ of runout against 0.0028 is 29* TIR.  Not terrible, but we’re definitely going to reduce tool life to 80 odd percent of normal.  We would definitely prefer an ER Collet Chuck at this endmill diameter.

Where it gets really crazy is small endmills.  Running the same kinds of numbers against a 1/8″ endmill suggests we run 0.0006″ of chipload.  Now we have over 100% TIR since 0.0008″ is larger than our allowable chipload.  That endmill will definitely be broken very quickly in that scenario.

Now you can understand why there is so much harping about runout versus tool life.

 

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Runout Versus Tool Life
5 (100%) 1 vote

4 Comments

  • what do you do to compinsate for this. I`m new to milling.

  • Ah ha! What I’ve been searching and finding on the forums was that the acceptable tolerance for runout was relative to the size of the bit (which a close, but this article goes further). It’s nice to finally know (and be able to calculate) if a system will give acceptable performance based on the TIR relative the percentage of CHIPLOAD!

    Now if only I had an accessible way to measure the dynamic TIR under normal speed. The best I can do in real life is {dial test indicator + hand}

  • […]  The spindle and toolholders on the lightweight machine may not be as precisely made as on the industrial VMC.  The chief measure of this is called runout, and it is basically a measure of how far from a true circle the cutter is spinning.  Whenever the axis of the cutter isn’t exactly on the axis of the spindle, you get runout.  Excessive runout is very hard on tool life. […]

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

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