by Kevin Burton
Ensuring your inserts stay fixed during thread turning operations
Keeping an indexable insert fixed exactly in position in the toolholder has become one of the major challenges in today’s machining. Clamping inserts securely in tools has always been a priority but today’s machining demands and component quality have highlighted the need to eliminate micro-movements of the cutting edge. Shortened tool life, inconsistent performance and unsatisfactory results can often be traced back to the cutting edge moving very slightly during machining. This has particularly been the case in operations such as profile and thread turning.
The most common reason for quality failure in screw-thread machining is profile errors. This is due primarily to tolerance or surface finish deviations. Specifically, micro-movement of the indexable insert used for thread turning is the main cause of profile error. This leads to limited cutting tool data and premature insert tool life. Insert instability in turn, leads to cutting edge chipping, especially at the nose radius. Users can improve stability in conventional tools by selecting an alternative clamping screw but total insert fixation in the seat is an ongoing problem.
The start and end of cuts are the most sensitive moments in thread-turning operations. As the pointed cutting edge enters the cut and is withdrawn at the end of the pass, the insert is subjected to relatively large and sudden variations in magnitude and direction of cutting forces. It is then that the insert is most vulnerable to insert movement. The tip of the thread-profile on the insert forms a lever with the screw, which tends to force the cutting edge slightly out of position, deforming the support points in the toolholder seat and escalating the instability. Cutting forces act on the insert from different directions, tending to move the insert back and forth in the seat until it generates an unacceptable screw thread. In fact, thread turning inserts are changed more due to the consequences of movement than actual tool wear.
Fast and accurate positioning of the insert in the holder is important to avoid time consuming machine setup and to minimize or eliminate scrapped components. Repeatability is also an important aspect to insert location and fixation in the toolholder. Generally, insert indexing is avoided between passes because of the risk of step-formation on thread-flanks. Should insert indexing need to take place between passes, a precise location of the insert is critical to continuing and achieving acceptable results. Combined with this, repeatability in between passes is also important because of the impact on machine downtime.
The quality of the location and locking of the thread turning insert in its toolholder seat is critical to obtaining high productivity, a secure operation and consistent component quality levels. Keeping the insert securely fixed in the insert seat has, in many cases, been virtually impossible with past insert-locking and location solutions. The method of retention, shim clamping and ease of indexing has meant compromises. Limitations in insert manufacturing methods have also curbed possibilities in arriving at unconventional solutions.
New indexable insert technology that improves insert location and locking, even when the insert is exposed to varying cutting forces, means manufacturers have more security, longer tool life and the potential for higher productivity in turning and milling. An example is the CoroThread 266 thread turning program that uses the iLock interface. The insert shim provides the secure base in the toolholder. The system offers high index repeatability: the M-tolerance insert ensures a cutting edge position of +/- 0.05 mm axially (feed direction) and the E-tolerance insert is +/- 0.01 mm. A rail facilitates insert indexing speed and handling, eliminating the potential for misalignment.
Eliminating micro movements through the use of new indexable insert technologies will give manufacturers the opportunity to more efficiently handle higher temperatures and higher cutting speeds during thread turning operations. SMT
Kevin Burton is a product specialist with Sandvik Coromant in Canada.