Wednesday, January 30, 2013

End Mill Holder and Runout

As a general rule most cutting tool & end mill manufacturers prefer to use single angle (ER/DR style) collet chucks for most cutting tool holding applications under 1/2". Why? Runout and uneven chip load. This is because of two reasons:
  • The error accumulation on ID tolerance of the end mill holder and the OD shank tolerance of the end mill or drill. The smaller the diameter the more potential for problems...
  • 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 that is not repeatable due to the ANSI shank tolerance on all end mills.
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.  
"The most important thing to say about toolholders in high speed milling of aluminum,” said Dr. Sinan Badrawy, formerly of Cincinnati Machine in a 2001 article “is not to use a set-screw holder.” Because the set screw moves the end mill off center to hold it against the wall opposite the set screw, it creates run-out.  “At high speeds, at set-screw holder will chatter no matter what,” he said.  The better choice are collet holders and shrink fit holders. Both do clamp the tool from all directions around the shank.”

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