Correcting Discrepancies in Rigid Tapping

There will always be a small discrepancy between the actual pitch of a specific tap and the machine’s synchronized movement. Using a tap holder with a small amount of axial compensation allows the tap to better follow its pitch and this reduces the axial forces for a dramatic improvement in tap life and thread quality.
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By Jim Lorincz Contributing Editor, SME Media

Successful tapping applications depend on several important factors. These can include axial compensation on CNC machines, internal coolant supply, and special advantages related to self-reversing tapping attachments, according to Mark Johnson, president of Tapmatic Corp., Post Falls, Idaho. “Rigid or synchronized tapping on CNC machines has become an industry standard,” he said. “Even though the synchronization of the machine’s feed advance to the spindle rotation is accurate, there will always be a small discrepancy between the actual pitch of a specific tap and the machine’s synchronized movement.”

Using a completely solid holder means that even a very small discrepancy will cause high axial forces on the flanks of the threads. “This has a negative effect on tap life and thread quality,” said Johnson. “Using a tap holder with a small amount of axial compensation allows the tap to better follow its pitch and this reduces the axial forces for a dramatic improvement in tap life and thread quality.” 

Tapmatic’s SynchroFlex tap holders include a machined flexure that provides a small amount of axial compensation. “The SynchroFlex flexures have a high spring rate, which is important for avoiding axial miscutting,” he said. “If the spring force is too soft, like in the case of traditional tension-compression tap holders with a large amount of axial compensation, axial miscutting can occur. This is especially true when using free-cutting, high-speed taps often recommended for synchronized or rigid tapping cycles. SynchroFlex is designed to provide just the right amount of compensation for synchronized tapping.”

A SynchroFlex II SFT50 performing synchronized tapping with M8 on a CNC lathe with live tooling uses a self-reversing tapping attachment at 2,500 rpm to tap ten M8 holes in 11 seconds. (Provided by Tapmatic)Lubrication is important for tapping in general and especially for roll form tapping. “Using a tap holder with internal coolant capability allows the use of taps with either an axial coolant hole for blind holes, or radial coolant holes for through hole applications,” said Johnson. “This gets the coolant where it is needed for the cutting or forming of the threads. It also greatly helps with clearing chips out of the hole when using cutting taps.”

Tapmatic’s SynchroFlex tap holders all include a balanced, high-pressure internal coolant system. Pressure up to 80 bar or 1,200 psi can be used and because the system is balanced, the coolant pressure does not influence the tap holders axial compensation.
Self-reversing tapping attachments offer special advantages compared to synchronized tapping. “With synchronized tapping, because the machine spindle needs to stop and reverse within a very limited number of revolutions, it is not possible for the machine to maintain the programmed spindle speed. Using a self-reversing tapping attachment allows the machine spindle to turn continuously in one direction at the actual program speed. The result is a shorter cycle time,” said Johnson.
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For example, in one test using rigid tapping to tap ten M8 holes at 2,500 rpm, cycle time was 18 seconds. Repeating the same test at 4,000 rpm saved just one second. Using a self-reversing tapping attachment at 2,500 rpm to tap the same ten M8 holes took just 11 seconds. “In addition to improving cycle time, using a self-reversing tapping attachment also allows the tap to run at the recommended speed, and this provides longer tap life as well,” said Johnson.

SynchroFlex
​The Unique Solution

Modern CNC machines have the capability of synchronizing the spindle rotation to match the feed advance for a specific tap pitch. The "Rigid" or synchronized tapping cycle is very accurate, but it is impossible to avoid small discrepancies between the machine synchronization and the actual pitch of the specific tap being used. Using a rigid tap holder means that any deviation at all increases the thrust forces acting on the tap and this dramatically reduces tap life.

At the heart of SynchroFlex® is a precisely machined flexure which provides axial and radial compensation for the unavoidable discrepancy between the machine feed advance and the actual tap pitch. By compensating for this error, the thrust forces acting on the tap are dramatically reduced.The result is the longest possible tap life, 100% improvement or more, and much better quality threads.
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SynchroFlex® II with increased flow rates for high pressure internal coolant.
Tapmatic's 80 bar, balanced, internal coolant system is included in all SynchroFlex tap holders and our system does not influence the axial compensation forces.The holders may be used with or without internal coolant.

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