I like end mills holders, why shouldn't I use them?

by Bernard Martin

We often run end mill "tests" to determine which tool performs best. Obviously, our goal is to "win' the test and get more business for our manufacturer's. This is article is about one our "tricks" and it's also why we represent both cutting tool manufacturers and rotary tool manufacturers.  We want to make sure that the products work together.

As a general rule most cutting tool & tolholder manufacturers prefer to use single angle (ER/DR style) collet chucks for general purpose cutting tool applications under 1/2" (12mm).

The rules are a bit different in High Speed Machining, as there are many more things to consider, but the problems of TIR at high speeds, where you can hear and feel the chatter, are still there in general end mill cutting operations at lower RPM.

It's all boils down to runout and uneven chip load.

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End Mill Holders are Prone to Runout Problems

• The error accumulation on ID (H) tolerance of the end mill holder and the OD shank (h) tolerance of the end mill or drill causes radial runout. The smaller the diameter the more potential for problems...
• The End Mill can have a tendency to "rock" on the set screw fulcrum point when it comes under load causing axial runout.
• The imbalance created in the toolholder by the setscrew used to clamp the tool. When you tighten down the setscrew you not only offset the tool but also create an imbalance condition. At higher speeds this is where you get chatter.  
  

Interestingly, it's not always easy to replicate a problem.  If you are using less expensive end mills, that are not made holding tight shank tolerances, you can see variations on the runout. We sometimes see this when we are asked to run and end mill test: The competitors that where in use suddenly started to perform poorly. Although the end mills conformed to the ANSI shank (h) tolerance on all of their end mills, the lot number had changed the the grind, although still in spec, had changed.

Depending on the application, end mill holders can be used for holding larger insert style end mills, spade drills, etc.  But somewhere between 1/2” and 3/4” there is a line that only you can determine when you need to move from a collet chuck to end mill holder. Generally we recommend using end mill holders only for very specific applications

Using small diameter end mills (1/4" and below) in end mill holders with set screws will have a adverse affect of both surface finish and tool life .