CuttingEdge
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Figure 1: Modified flank infeed - for CNC and conventional machines
Figure 2: Flank infeed - for CNC and conventional machines. Choose flank infeed when modified flank infeed cannot be used
In cases where the gauge just would not go on properly at the end of the operation, and insert wear appeared to be normal, application experts began looking at the holder. They found that over time the holder could become compromised enough that the insert would begin to rock slightly in the pocket and cause the threadform to go out of tolerance. This was especially true in coarser pitch operations where cutting forces were relatively high. To combat this, a new style holder has been engineered that features two key advancements, a carbide pin in the back of the pocket and a strong D-style clamp. The resulting combination is that the insert is held more tightly in the pocket. Together, this design ensures rigid insert holding, minimised vibration and predictable thread turning performance. Understanding The Variables Perhaps the biggest of all challenges
for repeatable, quality thread turning, is not tooling-related, but programming. Although many machine tool controllers have canned threading cycles built in, they are based on machining for the masses and oftentimes do not represent the best methodology for the numerous threadforms and pitches. Every detail, including material and treatment, has to be considered when establishing the best machining parameters. Having said that, there are four variables to understand and determine in order to get the most from your operations. They are helix angle, anvil, infeed method and number of passes. The helix is basically (making) a spiral on the workpiece. The proportion between diameter and pitch gives the helix angle. The anvil is a separate seat underneath the insert that protects it and sets the angle (side-to-side) from zero to five degrees. This is what helps tilt the insert to chase Threading becomes a critical operation when you consider that more often than not it takes place at the end of the machining process.
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metalworking equipment news October 2012
the thread and prevents rubbing on one side and cutting on the other. A one-degree anvil is the most common choice and represents as much as 80 percent of all threading applications. But, coarser pitches may require up to three degrees or more to achieve the correct (constant) cutting pressure. When the proper chip load is achieved, users can expect 30 percent increases in tool life and better thread quality. Conversely, incorrect anvil selection will result in the improper cutting forces, leading to more rapid insert wear and threads that may not match the gauge at the end of the day. Infeed method is basically how the insert addresses the workpiece as it enters the cut. Depending on whether your machine is a CNC or conventional, there are four infeed methods: radial, flank, alternate flank and modified flank (see figures 1 - 4). The most consistently productive and reliable choice is modified infeed as it does the best job of maintaining constant chipload on both sides of the thread during the cut. Determining the proper infeed method can improve tool life by 30 to 50 percent especially when threading high-temperature alloys. The last of the four big issues is number of passes. It speaks for itself, but if improperly calculated, will result in the rubbing of insert cutting edge and compromises thread quality. There are numerous tables of information regarding the proper selection of number of passes based on pitch, threadform