Get the right balance

Design engineers have thankless jobs. They never stop spending energy to constrain tolerances and improve accuracy to fight failures and downtime. They have improved the design accuracy to about 1 micron year after year. They are perfect watchers.

But their thorough conscientiousness and close attention to detail is wasted when the knives are not properly balanced. Machining a part with an unbalanced tool is akin to shooting yourself in the foot. The tool will experience normal wear after performing the design task. However, a tool designed to perform that task is assumed to be well balanced. If you do this job with an unbalanced tool, you are introducing a new level of wear, not only on the tool and spindle but also on the part being executed. Unbalance can have several effects: it can introduce additional vibration to the spindle and its components, it can wear tools irregularly, it can reduce tool life and reduce the quality of the finished product.

Correction Imbalance

A properly balanced tool significantly reduces noise and vibration, which results in increased tool life and better part accuracy consistency. Centrifugal force amplifies vibration caused by unbalance in a relationship proportional to the square of the velocity. The resulting increased vibration minimizes bearing, pad, shaft, spindle and gear life. Also, if you don't balance the tool, you risk voiding the spindle manufacturer's warranty. Many warranties specifically state that the warranty is only valid if there is sufficient evidence that the tools used on the machine are properly balanced. In this respect, tool balancing can lead to huge savings.

Before balancing the tool, you need to measure the magnitude of the unbalance and the angular position of each selected correction plane. These variables are measured on two common types of balancing machines: non-rotating or gravimetric machines for measuring single-plane (stationary) unbalance, and rotating or centrifuges for measuring single-plane and/or two-plane (dynamic) unbalance balance.

After determining the magnitude and angle of the unbalance in the correct plane, you can correct it by adding or removing material from the workpiece. For components that are not tools, the most widely used method of material addition is to weld counterweights to the components. Other options for slightly unbalanced components are adding solder to the body of the component or adding weight to the pre-drilled holes.

For a tool, when you determine the unbalance and determine that material must be removed to get the correct balance, the easiest and most efficient way is to drill. This is a quick adjustment and the amount of material removed can be precisely controlled. Another option is milling, which is most effective when balancing thin-walled tools or where shallow cuts are forced.

Theoretically, perfect balance can be achieved when balancing the tool. In practical applications, due to cost considerations and tool limitations, the perfect balance is only achieved when very lucky. Therefore, the level of accuracy must be set to allow a certain amount of residual unbalance to control the deleterious effects to an acceptable level. The accuracy given in ISO 1940 usually yields satisfactory results, but make sure the standard you implement is appropriate for the tool to be balanced. For example, a machine tool will use significantly different values ​​than a rigidly loaded propeller.

Tool selection and maintenance

Tool balancing is more than just measuring unbalance and adding or removing weight. Tool selection is very important. Short, lightweight tools are easily balanced to good precision, while large, heavy tools are much more difficult and tend to vibrate a lot. You can also save time and money by choosing a holder that has been pre-balanced or pre-machined to minimize unbalance.

Furthermore, you can reduce the number of balances that must be done with regular maintenance and careful handling. Any surface damage to the shank will affect balance and concentricity. Why? The effect of shank defects is magnified as the rotational speed climbs. If your instrument measures negligible force at 1,000 rpm, the force increases by a factor of 100 at 10,000 rpm and 400 times at 20,000 rpm.

Excellent concentricity is also more important with high speed spindles, because if the tool does not rotate on the spindle centerline, it becomes the primary factor for additional unbalance. But the effect of an unbalanced shank is also noticeable at lower speeds. Small imbalances can cause very high forces to damage your machining center spindle bearings, and continuous large radial forces can lead to early bearing failure and expensive machine tool repairs.

Also, keep in mind that any adjustment (installing or removing a tool assembly, tightening a nut or any minor twist or shim) requires some degree of balance. Even if adjustment interferes with the balance of the tool by only a few grams x millimeters, this unbalance translates into an increase in vibration, causing faster tool wear, worse surface finish, and reduced part geometry (eg, loss of roundness or straightness when boring a hole). ).

Appropriate precision = better balance

In addition to proper maintenance and handling of high-quality tool holders, it is important that the tool assembly is properly loaded into the machine tool spindle. In order to obtain a firm and stable connection, the tool holder should be matched with the spindle taper hole as accurately as possible. The difference between a well-fitted and a poorly-fitted handle is especially noticeable at high speeds. You may have the best balanced tool in the world, but if it's not properly attached to the spindle, you're asking for trouble.

When you think that many machining centers sold today are equipped with spindles that can spin up to 10,000 rpm or more, you have to deduce that the quality of the tool holder must be on par with the performance of the spindle. They must be firm, centered, properly balanced, and free from surface damage and contamination. If not, vibration is sure to occur, which will create chatter and reduce tool life and surface finish.

Not all tools need to be balanced correctly, especially when the process incurs increased costs and extra steps. Whether to do tool balancing depends on the specific situation. Balancing is most effective at high speeds, but balancing the tool at any speed yields better geometry accuracy, improved surface finish and longer tool life.

Balanced tools yield the best parts

While it does require some extra time and care, the right balance will extend the life of your tools and spindles and will increase the available time, as well as produce accurate, high-quality parts for your customers.