Tips/Tricks for Tool Balancing (ISO and Other Standards)
A well-balanced tool is critical in a machine as it helps avoid major problems in manufacturing. Learn more about tool balancing here.
Tool balancing is a critically important capability that is often performed less than perfectly. The need arises when the center of rotation of a machine component and its center of gravity is not sufficiently well aligned.
Unbalanced tools cause both major and minor problems, including:
- Machine oscillation and wear.
- User and potentially neighborhood-wide discomfort and irritation.
- Malfunction at the point of application.
- Higher-speed equipment suffers much greater impacts from unbalance.
The balancing process ensures the concentricity of the centers of mass and rotation. Discord between them results in vibration and can have dramatic consequences. The addition of or movement of masses, adjustment of axis positions, and improvement of bearing performance are all features in the balancing process.
Unbalance can result from wear, damage, poor design, and “bedding in.” Whatever the cause, it is best corrected to allow machine cyclic loading forces to be within the specified limits.
International standards such as the ISO (the International Organization for Standardization), ANSI (American National Standards Institute), BSI (British Standards Institute), and DIN (German Industrial Standards Organization) have created several standards that pertain to tool balancing. Here are a few of the most relevant ones:
- ISO 1940-1: Specifies the methods, tolerances, and conditions for the balancing of rigid rotating bodies (flywheels, prop shafts, cranks, etc.). It only applies to rotors spinning at up to 40,000 rpm, covering static and dynamic balancing requirements and methods.
- ISO 19464: Specifies the methods and requirements for the balance of flexible rotors, up to a maximum speed of 15,000 rpm. It covers both the static and dynamic balancing of rotors.
- ISO 9751: Applies to the balancing of tools used in woodworking machines (i.e., saw blades and router cutters). It specifies the procedures for checking the balance of these tools and determining the permissible unbalance.
- ANSI S2.19: This standard specifies the procedures for balancing rotating machinery and guides the measurement of unbalanced and balanced correction.
- BS 7888: This standard provides guidelines for the balancing of high-speed rotors (greater than 7,500 rpm). It covers both static and dynamic balancing and the determination of balance quality.
- DIN 4150-3: This standard provides guidelines for the balancing of any rotor with speeds up to 15,000 rpm. It covers both the static and dynamic balancing of rotors.
The overlaps between standards are significant. In general, it can be assumed that a balancing approach that is compliant with any appropriate standard will be compliant with other national and international standards that apply.
Machine and tool balancing covers a range of methods, equipment, and skills. It can be very precise, or “good enough” depending on the application, spindle speeds, system damping, and technical limitations in the balancing process. The requirements vary widely between differing applications, but the methods and equipment are very similar.
For example, machine tool balance is often specified by a G value (measured in mm/s). A grinding machine may require balancing to a very high quality, because of the higher precision required in its output. The machine-induced unbalancing forces are large, as grindstones are quite heavy and often large in diameter. Grinders are often balanced to a G value of 1.0 mm/s. However, the cutting-driven “unbalancing” forces for a cylindrical grinder are quite low, as each cutting event (for a single grinding node) is very small. The opposite is true for most tipped cutting tools, as their rotation speeds and diameters are smaller but the individual cutting-event forces are larger. Milling machine spindles are often balanced to around 6.0 mm/s, whereas cutters and collets are generally balanced to 2.5 mm/s or better.
In contrast, the balance of a gas turbine engine is very much more complex and cannot be boiled down to a single G number. The multiple turbine rings are generally attached to a single shaft, and the rings and shaft must all undergo localized balancing processes. Sorting blades by weight and distributing them in a uniform array to achieve static balance is the first step. But spindle speeds are very high, so precision dynamic balance is needed. Compared with a gas turbine, an air movement fan is a simple device and might only require static balancing to achieve acceptable performance in noise and vibration.
Acceleration, peak-to-peak displacement, and velocity are other ways to describe vibration in a part spinning at some rotational speed.
Listed below are some tips for balancing machine tools:
Make sure that the machine surfaces and tools are clean and free of any debris or contaminants. Run the machine at full RPM until its motor and spindle temperature stabilizes. Use an infrared thermometer to assess temperature status and in particular to map hot spots, as these may indicate condition issues. Perform a balance evaluation using appropriate tools, both cold and when warmed up. This process allows you to evaluate the current balance state, bearing and motor condition, and lubrication status.
The right equipment for tool balancing includes an appropriate balance assessment tool (particularly important for cutter/collet combinations), weights/screws, and sometimes shims, to make the process efficient. Gain experience, if any of the processes are unfamiliar. Perform each test several times to ensure results correlate. If they don’t, look for the variable.
Overall machine balancing needs to be done less often, as the rate of change of balance is lower. Many CNC machines include an integrated balance mode that allows assessment and adjustment to be performed without additional tools. Machines very often have radially spaced screw points, so that setscrews can be strategically added, to reduce shaft/pulley vibrations. Onboard software will plot the oscillation frequency of the machine and “advise” where balance weights are needed—and then the effect of the weights can be assessed by re-testing.
Measuring the out-of-balance helps in planning the correction. Static balancing may be useful in certain classes of equipment with large rotating masses and large diameters. Machine tools are virtually impossible to static balance and require precision accelerometer systems. Sometimes the issue can be gross displacement, rather than fine balance. Look for opportunities to re-center drives that may have moved, and understand motor clamping arrangements to look for opportunities to adjust. Correcting the unbalance may involve adding or removing adhesive-mounted or bolted weights, repositioning the tool in the machine, or repositioning elements of the machine itself. Repeat the process of measuring and correcting until the machine or tool is balanced to within the desired tolerance. Check for balance regularly, as the balance of the machine and tool is important in ensuring that performance is good and prevents excessive wear and tear.
Calibrate your balance equipment regularly and improve your skills, to ensure that the process is as accurate and reliable as you can achieve. Faulty equipment and unfamiliar processes will result in poor balancing and consequent quality and machine wear issues.