Today, a great array of internal precision machining operations is now facilitated via diamond or hard-metal tipped cutting tools. In addition, tool holders are available in various forms to suit the specific machining requirements. Now, factors such as the service lifespan of the cutting tool, the dimensional accuracy of the workpiece, and surface quality are affected by the material properties of the toolholder. If you do not get them right and need to facilitate a deep-hole machining procedure, you may encounter excessive vibration issues in the tool shaft. This can happen if there is a high degree of overhang; you will get that undesirable chattering. Let’s find out more about vibrations and how you can minimize them.
What are Vibrations?
With the increase in quality requirements of workpieces, the cutting edge of a cutting tool is subjected to intense development work. From there, the workpiece, cutting tool, and machine form a structural system that often features complex dynamic characteristics. Under certain conditions, various areas of that structural system start to vibrate. With that, vibrations can be broken down into three basic types, including:
- Self-excited vibrations: This type of vibration is caused by dynamic instability within a cutting process. Also known as machine tool chatter, self-excited vibrations can effectively limit metal removal rates. Here’s a typical scenario: the manufacturing plant chooses to cut with large tool-work engagements. Oscillations then start to buildup in the structure, without warning.
- Forced vibrations: These vibrations are a result of periodic forces within the system. For example, the multitooth cutters become intermittently engaged or the presence of unbalanced rotating masses. In these cases, the machine tool may oscillate at the forcing frequency. If that frequency corresponds to one of the structure’s natural frequencies, the machine will end up resonating in the corresponding natural mode of vibration.
- Transient or free vibrations: A free vibration or transient vibration occurs when impulses are transferred to the structure via its foundation. The source of the vibration can come from rapid reversals of reciprocating masses, e.g. the initial engagement of cutting tools or from machining tables.
It is pertinent to minimize vibrations as they can contribute to noise pollution around the immediate work area, cause cutting edge damage, and lead to poor surface finish.
How to Increase Stability and Decrease Vibration Tendencies
Fortunately, technicians have access to a greater range of techniques useful for enhancing stability and dynamic stiffness, apart from reducing mechanical weight. By increasing the chatter-resistance of long cutting tools and allowable overhang, vibrations can be kept to a minimal. Currently, the four most widely utilized approaches include:
- Active vibration control methods
- Passive dynamic vibration absorbers (DVA)
- High damping materials and/or Young’s modulus
- Anisotropic bars (make sure they feature specifically assigned orientations in terms of stiffness axes)
Today, the most common method of enhancing the dynamic stability of long bars is with the use of DVAs. Here’s an interesting note. High Young’s modulus materials usually contain sintered tungsten carbide. Alternatively, the materials may be machinable sintered tungsten alloy. The latter can have an additional 2-4% nickel and copper. Although expensive, both materials can be used to produce solid bars that enable cutting with ratios L/D < 7.
As for active vibration control means, e.g. active vibration dampers, they are effective but need vibration actuators and sensors to generate forces that oppose the deflections of the tool, during the vibratory process. In these situations, active systems featuring cavities in the mandrel body are widely used. They work by producing dynamic deformations that are designed to cancel chatter vibrations.
Summing Things Up
You have to keep in mind that vibrations can limit machining regimes, reduce overall tool life due to extensive wear, and limit geometric accuracy. If you are looking for a cost-effective solution to minimize vibrations, a smart way is to employ the use of passive vibration absorbers. They offer exceptionally high-performance characteristics and dynamic stability as well. The wonderful combo produced a performance comprising length-to diameter ratios up to L/D = 13.
