Threading Large: General Industry Coverage

Courtesy of JBO-USA

A GFM thread mill from JBO-USA threads a large-diameter component.

With the right tools, thread milling large bore diameters doesn’t have to be daunting.

When a hole requires threads, part manufacturers basically can perform conventional tapping, cold-form tapping or thread milling. When it’s a large hole, meaning 1 ” to 1½ ” in diameter and larger or a taper pipe, thread milling is preferred. Partly, that’s because the total cost for applying taps for larger threadmaking applications can be higher than thread milling. For example, larger taps can cost $150 or more, according to Joe Mazzenga, owner of JBO-USA, Troy, Mich. He added that although a thread mill costs more initially—$250 to $350 apiece—it’s able to achieve higher productivity, making the thread mill more economical.

He added that a multiple-flute thread mill that creates all the threads in one pass is the quickest method for 1½ “-dia. and larger holes. “A solid-carbide or a multiple-insert thread mill would be your most economical way,” Mazzenga said.

In addition, tap costs can increase if a special tap is needed. “You might have larger diameters that don’t fall within the standard tap specification,” said Jeff Major, director of sales and marketing for Vargus USA, Janesville, Wis.

In addition to cost, there are several other advantages of thread milling large bore diameters compared to tapping them. This article examines those benefits and also explores coolant application, selection of the proper thread mill and toolholder for the job, issues related to a hole’s depth-to-diameter ratio and software to assist the thread milling process.

This video from Allied Machine & Engineering Corp. offers a brief threadmilling demonstration courtesy of Allied Machine & Engineering Corp.

Tapping vs. Thread Milling

Tapping is a common way to produce an internal thread, but that doesn’t necessarily make it the best method. “Taps become less of an advantage even in small diameters,” said Duane Drape, national sales manager for HORN USA Inc., Franklin, Tenn.

Because a thread mill cuts a thread using helical interpolation on a CNC machine with three simultaneous axes, the tool is smaller than the diameter of the hole being threaded. Therefore, if a thread mill does happen to break when threading, it doesn’t get stuck in the part and can be easily removed. “It definitely reduces rework costs,” said Rob Brown, product manager for Allied Machine & Engineering Corp., Dover, Ohio. “With a broken tap, you would have to burn the tap out and, a lot of the time, you have a significant amount of rework or you scrap the part.”

Not engaging all sides of a hole like a tap allows freer machining when thread milling. This reduces machine tool horsepower requirements while boosting productivity.

“It’s a freer-cutting tool, so you can run at a higher surface footage, resulting in shorter cycle times,” Major said.

Another thread milling advantage is that a standard tool can produce a nonstandard pitch diameter before operations like heat treating or plating to achieve the required thread specifications. This is performed by knowing how the workpiece material is going to respond when treated, such as shrinking or expanding a specific amount, and compensating for that movement when cutting the threads, Brown explained. “With a tap, it would require a special with a nonstandard pitch diameter,” he said.

As a result of its advantages when the application is appropriate, thread milling has become more prevalent. “People prefer thread milling because of the reduction in cycle times and increase in tool life. It also provides a cleaner, stronger thread than what you would get with tapping,” Major said.

Those advantages are only enhanced when thread milling large holes. In those applications, for example, there’s more room for chip evacuation and for flood coolant to effectively access the tool/workpiece interface, Drape noted, adding that through-coolant tools are more important for smaller applications. “Thread milling large parts is relatively easy and has been accepted for years now,” he said. “Versus tapping, you don’t have to convince people very much. That battle has been won.”

Courtesy of Vargus

An end user initially applied taps to manufacture an internal thread for a 59.6 “×11 “×65 ” wind turbine body, but average tap life was 250 threads and the surface finish was poor. In addition, tools often broke inside the workpiece material. After switching to MiTM25 thread mills from Vargus, tool life reportedly increased to 1,500 threads per cutting edge with a fine surface finish, and machining time was reduced by 25 percent.

Cool Down

Flood coolant can be effective in most thread milling applications, according to Brown, because there’s only a single point of contact between the tool and workpiece and chips are not being trapped. Allied offers through-coolant indexable thread mills, but flood coolant is applied for solid tools, Brown noted.

In contrast, all of JBO’s standard thread mills are through-coolant and direct coolant out the front of the tool. For through-hole applications, the toolmaker plugs the front end and EDMs radial coolant holes. “It’s not a major shift to run a made-for-application tool,” Mazzenga said.

In addition, Vargus offers its MiTM (multiflute thread mill) and TMSD (thread mill for deep holes) tools for thread milling larger bore diameters with 1,000-psi, through-coolant capability, Major noted. He added that MiTM products have straight rather than helical flute geometry to thread more aggressively.

Minimum-quantity lubrication is less common when thread milling, but can be effective depending on the application, such as when threading aluminum with a solid-carbide tool. In addition to the application, Drape noted that use of MQL depends on geographic location. “In Europe, they are amazed we get to use coolant everywhere,” he said.

Courtesy of JBO-USA

A JBO-USA thread mill is applied for producing a large taper pipe thread.

Tool Type