Truth in Balancing
Some suppliers provide incomplete information: "This tool is balanced for 30,000 RPM." But at what grade? Without both the grade and RPM, the claim lacks a point of comparison. It is like saying a car can go 100 without specifying miles or kilometers per hour.
The goal is to remove as much imbalance from the holder as cost-effectively as possible. That leaves less imbalance to correct when the cutting tool is added. Most machine companies recommend pre-balanced tool holders at G6.3 at maximum spindle RPM; after adding the cutter, compensation is then for the cutting tool itself.
A tool holder manufacturer cannot balance a complete assembly containing a collet chuck, collet nut, collet, preset screw, and cutting tool without knowing the cutting tool's size, length, and weight. Manufacturers balance the chuck body; Pioneer also balances the locking nut. The collet and cutter specifications are still unknown, and small weight changes can affect balance when carbide is added at the nose of a holder.
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".
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 its balance requirements in 2015 to recommend G2.5 balance grade for speeds over 10,000 RPM on BT30, BT40, and CAT40, and 7,000 RPM on CAT50. It specifies "tooling," not "tool holder." To meet this requirement, the assembly must be balanced to G2.5 at operating speed, not just the tool holder.
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.
The Variable Factor
Much high-performance tooling uses variable-pitch, variable-helix designs. The cutter flutes intentionally differ in thickness and depth, which reduces harmonics. These cutters are designed to be out of balance. If tooling must be balanced, it must be rebalanced at every tool change. Because cutter orientation is not fixed, the center of gravity can shift with each tool, even when using the same brand and part number. Variable cutters can therefore negate repeatable pre-balancing of the tool holder.
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
- These can cause unbalance and structural weakness
- Tolerances during fabrication of tool holders and cutters
- Production tolerances in drive slots, preset screw holes, coolant ports, and DIN AD/B flange ports
- Out-of-roundness from turning or heat treatment
- Improper placement of through-holes
- Machining that reduces concentricity around the rotational axis
- Asymmetrical tool holder design
- Improper balancing practice (mass symmetry / part symmetry)
- Unbalanced cutting tools
- Axial movement of preset screws and cutting tools
- Unbalanced tool holder components
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
- RPM = The operating speed for this assembly
Using the 9549 constant, calculate allowable unbalance with the following formula:
(U = G x 9549 x W) / RPM
For a #40 taper tool holder assembly weighing 2.75 lbs, the general expression for G 2.5 at 8,000 and 20,000 RPM is:
(U = 2.5 x 9549 x 2.75) / RPM
Solving these expressions for 8,000 and 20,000 RPM gives:
- 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.
Indexable Tooling
Many indexable cutters need to be rechecked after indexing. Most insert ICs are held within 0.002 in.; corner radius and other tolerances can also shift weight. When balancing within grams for G2.5 at a given RPM, insert tolerances and indexing can take the assembly out of balance.
There Are Two Types of Balancing
- Static: also known as production balance, this calculates holder imbalance before production to create a symmetrical design. This is also known as engineered balance.
- Dynamic: a machine spins the holder to measure imbalance and correct it by adding or removing material.
Some manufacturers believe a symmetrical design provides sufficient balance correction. In today's market, with spindle speeds of 15,000 RPM and higher, dynamic balancing is required on all tool holders and components to reduce machine tool wear and improve cutting tool performance. Engineered balancing, if done correctly, can provide balance for 5,000 RPM or less. Most tool holders have coolant holes through the center. Engineered balancing assumes the drilled hole is straight, but every drill can wander depending on depth; these differences cannot be compensated for by static or engineered balancing.
2 different types of Dynamic Balancing Machines:
- Hard Bearing Balancing Machines: Tool holder balancing machines use a spindle mounted in bearings and sensors to measure displacement. The holder is measured, indexed 180 degrees, and measured again. The difference between readings indicates imbalance; material is added or removed, then the process is repeated for verification. These machines have sensitivity down to +/- 0.25 g.mm or 0.50 g.mm total. Depending on tool weight, they can balance and repeat G2.5 at 30,000 RPM.
- Soft Bearing Balancing Machines: Soft bearing machines use a tray suspended by music wires. The tool holder, mounted on an arbor, rotates in the tray; imbalance moves the wires and is measured. Soft bearing balancing is highly sensitive, and G0.4 at any RPM can be achieved on these machines. Due to their cost, this type is no longer used for tool holders.