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Balanced Tool Holders

Unbalance is a condition that  exists when the Mass Axis of a rotor does not coincide with the Rotational Axis.

What Causes Unbalance?
  • Flaws in the base material 
    • Voids, seams, and porosity 
    • All result in unbalance and structural weakness
  • Poor tolerances during fabrication
    • Un-machined portions of the forging
    • Out of roundness
    • Improper placement of through holes
    • Any machining performed on the tool holder that diminishes the absolute concentricity about the rotational axis contributes to unbalance
  • Asymmetrical tool holder design
  • Improper balance practice (Mass Symmetry / Par Symmetry)
  • Cutting Tool Weight
  • Unbalanced Cutting Tools
  • Axial movement of preset screws and cutting tools
  • Unbalanced Components of the Tool Holder

Unbalance Effect

The result of the conditions listed above create a new center of gravity in the tool holder.  This center of gravity can be corrected by adding or removing material from the assembly.  To make this correction you need to know the following:
  • W = Weight of the Assembly
  • G = Required “G” rating
    (see chart to the right, click to enlarge)
  • RPM = The operating speed for this assembly

Applied with a 9549 constant you use the following formula to
calculate your allowable unbalance.

(U = G x 9549 x W) / RPM

Calculation of “G” 2.5 for a #40 Taper Tool Holder Assembly at 8,000 and 20,000 rpm a general 
expression for G 2.5 works out to the following:

(U = 2.5 x 9549 x 2.75) /  RPM

Solving these expressions for 8,000 rpm and 20,000 rpm range shows:
  • Unbalance for G 2.5 at 8,000 rpm gives an allowable unbalance of 8.2 g.mm
  • Unbalance for G 2.5 at 20,000 rpm gives an allowable unbalance of 3.3 g.mm
This shows that as spindle speeds increase the tolerance for allowable unbalance decreases.

There is 2 different types of balancing:
  1. Static: also know as production balance is when you calculate the imbalance of the holder pre-production to make the holder a symmetrical design.
  2. Dynamic: is when you use a machine to physically spin the holder to measure the imbalance and correct by adding or removing material.
Some manufacturer’s believe a symmetrical design will provide sufficient balance correction.  In today’s market with spindle speeds averaging 15,000 RPM’s and higher, Dynamic Balance is required on all tool holders and components to reduce machine tool wear and improve cutting tool performance.

All PIONEER tool holders are symmetrically designed and Dynamically Balanced for optimal performance.  Some components like ER Collet Nuts are individually Dynamically Balanced to help eliminate any unbalance effect.  This balance will exceed required balance for most machining applications.  
For high speed applications, it is recommended to balance the assembly with the retention knob locked in place and re-balance after every tool change.  Ultimately this is the only way to guarantee the required balance.