Engineering‎ > ‎

Balanced Tool Holders

The largest re-seller of balancing machines in North America says on the tool holders they sell
"Chuck body fine balanced G2.5 at 25.000 rpm or U < 1 gmm".
U < 1 gmm is the maximum allowable displacement the machine can sense accurately.  We can calculate what tool weight is required for the machine to accurately balance the tool holder.
  • 2.8 lbs tool weight to balance for G2.5 @ 30,000 rpm
  • 2.4 lbs tool weight to balance for G2.5 @ 25,000 rpm
  • 1.9 lbs tool weight to balance for G2.5 @ 20,000 rpm
  • 1.4 lbs tool weight to balance for G2.5 @ 15,000 rpm
For reference here is the weight of the tool holder without cutting tool:
  • 1.1 lbs for BT30-ER16H-070
  • 2.1 lbs for CAT40-ER16H-0250
  • 2.0 lbs for HSK63A-ER16H-100
  • 7.3 lbs for CAT50-ER16H-0400
If you have an operation at 20,000 rpm in your BT30 machine with the BT30 ER16 chuck above, the best the balancing machine can do is G2.5 @ 12,000 rpm to achieve 1 gmm of displacement.  It is physically impossible to balance any finer with a tool holder balancing machine.  Any manufacturer of tool holders stating the tools are balanced for a finer balance cannot maintain that balance.  Another way of saying this is if you check the balance 3 times you will get 3 different answers.  The assembly is to light to measure properly and guarantee.  Adding the cutter weight to the assembly will make it easier for the balancing machine to sense the tool and balance for higher operating speeds.  

A west coast American machine builder that starts with an H updated their balance requirements in 2015 to recommend G2.5 balance grade for speeds over 10,000 RPM's on BT30, BT40 & CAT40, 7,000 RPM's on CAT50.  They designate "tooling" not "tool holder".  To meet this requirement the assembly must be balanced to G2.5 at operating speed not the tool holder alone.

To maintain balance the tool must be re-balanced on every tool change because cutting tools are not balanced.  Variable pitch and helix cutting tools are designed to be out of balance on purpose to reduce harmonics.  Remember we are discussing grams of weight, it does not take much to take a tool out of balance.

Unbalance is a condition that  exists when the Mass Axis
of a rotor does not coincide with the Rotational Axis.
Click on images to enlarge...                        
What Causes Unbalance?
  • Flaws in the base material 
    • Voids, seams, and porosity 
    • All result in unbalance and structural weakness
  • Tolerances during fabrication of Tool Holders & Cutters
    • Standard Production Tolerances of Drive Slots, Preset Screw Holes, Coolant Ports,
      & DIN AD/B Flange Ports
    • Out of roundness from turning or heat treat
    • 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 with a weight of 2.75 lbs 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
  • Unbalance for G 2.5 at 20,000 rpm gives an allowable unbalance of 3.3
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 at 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.

2 different types of Dynamic Balancing Machines:
  1. Hard Bearing Balancing Machines:  Tool Holder Balancing Machines are Hard Bearing, the spindle sits in bearings and the sensor measure the difference in displacement.  The tool holder is measured, indexed 180 degrees and then measuring again. The difference in readings in the imbalance, correction is made and the process is repeated to verify. These machines have a sensitivity down to +/- 0.25 or 0.50 total.  Depending on tool weight, these machines can balance and repeat G2.5 @ 30,000 RPM.
  2. Soft Bearing Balancing Machines:  Soft Bearing employs a tray hanging from music wires.  The tool holder mounted in an arbor is rotated in the tray, the imbalance creates movement on the music wires which is measured.  Soft Bearing Balancing is the most sensitive and G0.4 @ any RPM can be achieved on these machines.  Due to the cost of this type of balancing it is not longer used for tool holders.
All PIONEER tool holders are 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.