Catching the Vibration

Here’s to know ways to achieve vibration-free internal turning, and how TaeguTec’s HUSH BORE can help in the process.....

Internal turning or enlarging a pre-drilled hole is one of the most common operations in machining cylindrical parts. The tool used for this purpose is called boring bar. The length of the bore relative to its diameter has an influence on the quality of the machined hole. This term is generally referred to as L/D ratio. This ratio influences the choice of boring bar selection as machining systems are inherently low in dampening and susceptible to different forms of vibration. In internal turning operation with long boring bar, the vibrations are more pronounced due to poor static and dynamic stiffness of the boring bar material. The chatter due to vibration has a negative effect on the surface finish, tool life and productivity.

Different vibrations

Vibrations are generally classified as free vibrations, forced vibrations and self-excited vibrations.

Free vibrations are due to external impact on the machine structure or internal impact on the boring bar when the insert passes over a hard spot in the part being produced.

Forced vibrations are a result of periodic forces that arise while turning interrupted parts like splines, keyways and with uneven depth of cut. Unbalanced masses also transmit the forces to the tool. The vibration frequency of the tool is very close to that of the frequency of the periodic forces.

Self-excited vibrations usually are a result of the dynamic instability of the machining system. If large tool-work engagement is attempted, sudden oscillations build up in the structure and the structure vibrates in one of its natural modes of vibration.

Some directions

The key to eliminate or reduce vibration from the cutting tool is to increase static stiffness, adjust cutting parameters, increase dynamic stiffness, use positive geometry and positive rake insert to reduce cutting forces. In addition, an entering angle close to 90^o and smaller corner radius will direct the feed force in the axial direction of the boring bar thereby reducing deflection.

To increase static stiffness, select the largest possible diameter for the boring bar and minimum length required to do the operation. The maximum overhang of solid steel boring bars is up to 3x diameter. Solid carbide boring bars are recommended up to 5x diameter, as carbide has three times more co-efficient of elasticity than steel. However, when machining with high overhangs of more than 5x diameter, even the use of a solid carbide shank can cause vibrations. Machining high depth to diameter bores requires the boring bar to have higher dynamic stiffness. Use of passive dampers in the boring bar close to the cutting area increases the dynamic stiffness.

Knowing HUSH BORE

TaeguTec, a leading cutting tool company, offers vibration-free boring bar line with exchangeable heads for deep boring known as HUSH BORE.

The HUSH BORE’s damping system, which is located inside the shank close to the cutting area, dramatically damps vibrations and enables boring up to L/D = 10 with good surface finish and longer tool life. It also ensures increased feed and cutting speed, making it more productive.

Shanks of the HUSH BORE line are available as standard items in seven different diameters (Ø16, 20, 25, 32, 40, 50, 60 mm) and 2 lengths (7XBD, 10XBD). The various exchangeable heads can be securely fastened with serrated couplings located in the boring bars. The corrosion resistant stainless-steel boring bars are with through coolant facility directly to the cutting tip for better tool life.

The exchangeable heads give the flexibility to choose from a wide range of insert shapes from the ISO range (CCMT 09T3, DCMT 11T3) and economical RHINO series of double-sided inserts. The RHINO-X-TURN double-sided insert XNMG 090404 FLP with high positive rake can be used in rough boring applications.

Setting instructions

The below recommended setting instructions are to be followed for effective and efficient use of the HUSH BORE system.

The mounted shank requires a minimum length of 4XBD
The holding system should be a slit type so that the shank cylinder will have full surface contact
Set the shank to the holder so that the head’s upper surface is horizontal to the gauge. This ensures center height.

Leading cutting tool company, TaeguTec offers vibration-free boring bar line with exchangeable heads for deep boring known as HUSH BORE.