Tiptop tapping: Turning Performance

When it comes to tapping — the process of cutting a thread inside a hole — there’s much to be said for choosing a synchronous approach, as well as tapping hardware tailored to the special characteristics and challenges of that technique.

Synchronous tapping features a rigid tapholder rather than one that’s flexible, or floating. The floating type also is known as a tension-compression tapholder because it allows axial extension and compression to compensate for some mismatch between the machine’s spindle speed and the infeed of the tap. The idea is to prevent damage to the tap and workpiece caused by pushing or pulling during the tapping process.

In rigid tapping, however, spindle rotation and feed rate are synchronized, and the tap can be held rigidly in a chuck. Today, most CNC machines come with synchronized or rigid tap cycles that are easy to program and capable of very accurate tapping depth control, said Mark Johnson, president of Tapmatic Corp. in Post Falls, Idaho.

Other rigid tapping advantages are the result of reduced runout compared with floating tapping.

Synchro Taps reportedly run much faster than conventional taps. Image courtesy of YG-1 Tool (USA)

When rigid tapping, “you are not allowing the tool to follow whatever path it wants to follow,” said Mark Ford, director of global product management for threading tools at YG-1 Tool (USA) Co. in Vernon Hills, Illinois. “You are controlling it very tightly by holding it rigidly, which helps you on runout.”

He said runout is the main cause of tapping problems with thread finish, breakage and sizing consistency. In addition, he said not permitting the tap to cut a much bigger path than necessary increases tool life. What’s more, rigid tapping typically lets users run tools faster.

“With threading, you cannot adjust the feed rate because it has to match the pitch of the tap,” Ford said. “So the only option to increase productivity is to increase spindle speed, something rigid tapping allows.”

Considering all this, it’s not surprising he recommends tension-compression holders only to people using older nonsynchronous machines.

“If someone is using a tension-compression holder with a modern CNC machine,” Ford said, “they are automatically giving up tool life, finish and consistency in gauging for no reason other than that’s the way they have always done it.”

Free-Cutting Taps

YG-1 Tool (USA) recently came out with a line of taps for high-volume, repetitive manufacturing operations looking for productivity gains. Called Synchro Taps, they can run roughly two to three times faster than conventional taps, Ford said. This is possible, he said, because the new taps are extremely free cutting — that is, designed for minimal contact with the part, which reduces cutting forces and heat. Additionally, the company controls runout features very tightly during the tap manufacturing process.

“We hold pitch diameter, OD and chamfer runout to within a few microns, whereas (applicable) standards allow tolerances of as much as 0.0015″ (0.0381 mm) when grinding these features,” he said. “By doing that, we can run the tap at extreme speeds.”

YG-1 Tool (USA) also holds a tolerance for the tap’s shank diameter that’s much tighter than normal — the same shank tolerance that commonly would be used for a carbide drill or endmill.

“That allows us to increase the pitch diameter of the tap ever so slightly,” Ford said, “but enough to add more tool life.”

The STM holder allows users to quickly adjust the projection length of the tap by turning the guide ring on the holder. Image courtesy of NT USA

Some tap cutting tools include a drill that permits the pilot hole and threads to be cut in one operation. Those who choose this option eliminate a step from the process and don’t have to worry about a separate drill and tap lining up 100% correctly, said Kiwamu Stewart, general manager of NT USA Corp. in Franklin, Tennessee.

On the other hand, using one tool to drill a hole and cut threads puts a considerable amount of stress on the tap.

“A company like Toyota that makes thousands of parts a day wants less stress on each cutting tool,” Stewart said.