Fill in the gaps: Drilling Performance

Tapered-pipe tapping is a way of threading pipes that, when assembled, do not leak, even under pressure. Because important safety and performance issues exist when assembling pipes tapped via this method, tapered tapping processes are governed in the U.S. by two key standards.

National Pipe Thread Taper (NPT) is the American National Standards Institute (ANSI) standard for tapered threads on threaded pipes and fittings. NPT threads require a sealing agent, such as Teflon tape, to complete the assembly. At some point in the service life of the assembly, parts with NPT threads will be taken apart and reassembled.

Courtesy of Emuge

Emuge says its NTP tapered-pipe taps feature a newly developed flute form with a variable skip-tooth geometry and are recommended for materials that produce long, stringy chips, such as aluminum, carbon steels, stainless steels and mold steels.

The ANSI dry-seal pipe thread standard, National Pipe Taper Fuel (NPTF), does not require a sealant and is for applications where the part assembly will most likely never be taken apart.

NPTF threads have the same basic shape as NPT threads but with asymmetrical crest and root truncation, according to a video report by cutting tool manufacturer Emuge Corp., West Boylston, Mass. The crest and root heights are adjusted for an interference fit, allowing a dry seal. Truncation is the bottom of the root or tip of the crest that is not produced during the tapping process.

Courtesy of KOMET of America

KOMET offers Vario HZ interrupted-thread pipe taps in North America with its partner BASS GmbH.

“The difference is where the male and female threads engage. On the NPTF thread, both sides of the flanks and the root and crest are all engaged, so it creates a complete seal,” said Tim Holmer, application engineer at toolmaker OSG Tap & Die Inc., Glendale Heights, Ill. “The NPT thread is not engaged on the full crest or root. The NPTF thread has 100 percent engagement.” He added that an NPT tap cannot be used for NPTF applications, as it can produce a thread that could leak, but an NPTF tap is acceptable for NPT applications.

The difference when machining NPT and NPTF threads is in the details. “NPTF threads are not necessarily more difficult to machine, as it takes roughly the same amount of torque and load on the tool,” said Cullen Morrison, threading business development manager for toolmaker KOMET of America Inc., Schaumburg, Ill. “But they are more difficult to produce accurately because they have a much tighter tolerance. With a tighter tolerance, you have less wear time on the tools before they need to be replaced, and be- cause tool consumption is higher, the quality of the tool and gaging is critical.” Because thread quality is crucial, NPTF threads typically require various gaging processes.

Creating the Taper

The taper on the hole to be threaded is created by drilling a straight hole followed by a tapered-pipe tap or by reaming to create the taper, followed by the tap.

A straight-drilled hole that has not been taper-reamed has a much higher volume of chip removal during tapping. “That tap has to not only cut the thread but shape the hole to the correct taper, so more material is removed in the process,” Morrison said. “This creates a higher load and more torque on the tool, which greatly reduces tool life and can lead to tool breakage. While using a reamer is not necessarily required, shaping the hole to the correct taper first will always provide better tool life and thread quality.”

Courtesy of OSG Tap & Die

OSG’s Hypro List 12053 interrupted-thread pipe tap is available from 1/8″ to 1″. The tap is recommended for carbon steels, alloyed steels and die steels.

Courtesy of OSG Tap & Die

The face of an interrupted-thread tap shows the rake angle and enlarged size of the flutes for enhanced chip evacuation.

The downside to reaming first is the additional tool cost, setup and cycle time. “And it can be a slow operation, depending on the quality of the reamer,” Morrison added.