A few practical tips for balancing knives

Tool balancing is an extremely important function that is often performed imperfectly. When the centre of rotation of a machine component is not adequately aligned with its centre of gravity it invariably leads to a number of problems including:

· Machine vibration and wear.

· Potential thermal irritation.

· Application point failure.

· The unbalancing effects of high-speed equipment.

The balancing process ensures the concentricity of the centre of mass and centre of rotation, and disharmony between them can lead to vibration with potentially serious consequences. The addition or movement of masses, the adjustment of shaft positions and the improvement of bearing performance are all features of the balancing process. Unbalance can be caused by wear, damage, poor design and ‘break-in’. Whatever the cause, it is best to correct it so that the machine cyclic load forces are within specified limits.

1. National and international standards

International standards such as ISO (International Organization for Standardization), ANSI (American National Standards Institute), BSI (British Standards Institution) and DIN (Deutsche Industrie Standards) have developed several standards related to tool balancing. Here are some of the most relevant ones:

ISO 1940-1: Specifies methods, tolerances and conditions for balancing rigid rotating bodies (flywheels, drive shafts, cranks, etc.). It applies only to rotors rotating up to 40,000 rpm and covers both static and dynamic balancing requirements and methods.

ISO 19464: Specifies balancing methods and requirements for flexible rotors up to 15,000 rpm. It covers static and dynamic balancing of the rotor.

ISO 9751: Applies to the balancing of cutting tools (i.e. saw blades and milling cutters) used in woodworking machinery. It specifies procedures for checking the balance of these tools and determining allowable imbalances.

ANSI S2.19: This standard specifies balancing procedures for rotating machinery and guides the measurement of unbalance and balance correction.

BS 7888: This standard provides guidance for balancing high-speed rotors (greater than 7,500 rpm). It covers static and dynamic equilibrium and the determination of equilibrium mass.

DIN 4150-3: This standard provides guidelines for balancing any rotor with speeds up to 15,000 rpm. It covers static and dynamic balancing of the rotor.

There is considerable overlap between standards. In general, it can be assumed that a balancing approach that meets any appropriate standard will also meet other applicable national and international standards.

2. Example of balancing method

Machine and tool balancing covers a range of methods, equipment and skills. It depends specifically on the application, spindle speed, system damping and technical limitations in the balancing process. The requirements vary greatly from application to application, but the methods and equipment are very similar.

For example, machine tool balancing is often specified in terms of G values (measured in mm/s). A grinder may require a very high quality balance because its output requires greater precision. The unbalanced forces caused by the machine are large because the grinding stones are very heavy and often large in diameter. Grinding machines are typically balanced to a G value of 1.0 mm/sec. However, the cutting drive "unbalanced" force of a cylindrical grinder is very low because each cutting event (for a single grinding node) is very small. The opposite is true for most tip cutting tools, as they have smaller rotational speeds and smaller diameters, but larger individual cutting forces. Milling machine spindles are usually balanced to around 6.0 mm/sec, while cutters and chucks are usually balanced to better than 2.5 mm/sec.

In comparison, gas turbine engine balancing is much more complex and cannot be reduced to a single G value. Multiple turbine rings are often attached to a single shaft, and both the rings and shaft must be partially balanced. The first step is to sort the blades by weight and distribute them in a uniform array to achieve static balance. However, the spindle speed is very high, so precision dynamic balancing is required. Compared to gas turbines, air movement fans are simple devices that may only require static balancing to achieve acceptable noise and vibration performance. Acceleration, peak-to-peak displacement, and velocity are other ways of describing the vibration of a part rotating at a certain speed.

3. Tips for balancing machine tools

Listed below are some tips for balancing machine tools:

1. Assess machine condition

Make sure machine surfaces and knives are clean and free of any debris or contaminants. Run the machine at full speed until the motor and spindle temperatures stabilize. Use an infrared thermometer to assess temperature status, especially mapping heat spots, as these may indicate condition problems. Use the appropriate tool (both cold and hot) to perform a balance assessment. This process allows you to evaluate the current balance status, bearing and motor condition, and lubrication status.

2. Use the right equipment

Proper equipment for tool balancing includes a properly balanced assessment tool (especially important for tool/chuck combinations), weights/screws, and sometimes shims to make the process more efficient. If any process is unfamiliar, gain experience. Perform each test multiple times to ensure results correlate. If not, look for the variable.

Because the rate of balance change is lower, less overall machine balancing is required. Many CNC machines include integrated balancing modes that allow evaluation and adjustment without the need for additional tools. Machines often have radially spaced screw points so set screws can be added strategically to reduce shaft/pulley vibration. Onboard software will map the machine's oscillation frequency and suggest where balancing weights are needed, whose effectiveness can then be assessed through retesting.

3. Measure imbalances

Measuring imbalances helps plan corrections. Static balancing may be useful for certain equipment with large rotating masses and large diameters. Static balancing of machine tools is virtually impossible and requires a sophisticated accelerometer system. Sometimes the problem may be gross displacement rather than fine balance. Look for opportunities to re-center the drive that may have moved, and understand the motor clamping arrangement to look for opportunities to adjust. Correcting an imbalance may involve adding or removing adhesive-mounted or bolt-on counterweights, repositioning the cutting tools in the machine, or repositioning elements of the machine itself. Repeat the measurement and correction process until the machine or tool is balanced within the desired tolerance range. Check the balance regularly as the balance of the machine and cutting tools is very important to ensure good performance and prevent excessive wear.

4. Calibrate balancing equipment

Calibrate your balancing equipment regularly and improve your skills to ensure the process is as accurate and reliable as possible. Defective equipment and unfamiliar processes will result in poor balance and lead to quality and machine wear issues.