December 2010 / Volume 62 / Issue 12|
By Alan Richter
Courtesy of JBO-USA 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 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 Tool Type
The vast majority of large bores are thread milled with indexable- or replaceable-insert tools because, similar to other operations, the cost of a large, solid-carbide tool becomes prohibitive. Brown noted that the largest standard, solid thread mill Allied Machine offers is a 1 "-8 tool.
The choice often depends on the application. “If the customer wants flexibility and the ability to use one tool for a number of pitches, then I’d say go with a replaceable-blade tool,” said JBO’s Mazzenga. “If it’s a dedicated production operation, then it’s more than likely a solid-carbide tool is going to win out.”
For indexable- or replaceable-insert tools at the smaller end of the large spectrum, Drape noted that the options are a single-flute, multiple-pitch insert or a multiple-flute, single-pitch insert.
Drape added that as the bore diameter to be thread milled increases, space permits applying multiple-insert indexable tools. Those can either have a single pitch or multiple pitches. A user can feed a single-pitch tool substantially faster than a multiple-pitch one because there’s less contact area between the tool and workpiece. However, a multiple-pitch tool has less distance to travel because it’s cutting multiple threads simultaneously, he explained. “If the total length of your component is 12 threads, you take one circular pass and that’s it,” Drape said. “Whereas a single-pitch tool has to travel around the component 12 times but it’s traveling faster.”
When applying a multiple-pitch tool, at some point the level of coarseness and depth of pitch have too much contact with the workpiece surface and the cutting pressure overwhelms the tool and machine, Drape noted.
The type of toolholder is also an important consideration when thread milling large holes. Similar to any milling operation, minimizing runout increases tool life and part quality, and Drape recommends milling chucks or hydraulic toolholders.
Mazzenga concurred that milling chucks, with their combination of vibration dampening and high gripping power, are ideal for thread milling, which generates a significant amount of side pressure on the tool. Shrink-fit toolholders are subject to “penciling” when thread milling, he added, where a high level of vibration maneuvers the tool out of the holder.
In one application “that should have been a slam dunk,” Mazzenga recalled that an engine manufacturer wanted to mill a 36mm thread in cast iron. The manufacturer used shrink-fit holders exclusively, but experienced problems when thread milling. Mazzenga first tried adjusting the speeds and feeds, but to no avail. “So we went to a milling chuck and it settled down immediately,” he said. “That’s happened on more than one occasion.”
Mazzenga also doesn’t recommend hydraulic holders because they don’t have the strength and grip to handle thread milling’s high radial forces.Not So Deep
Large-diameter bores are not necessarily deep, according to Drape, but because of the fixturing often required to hold large and heavy workpieces, a thread mill could have a long reach. “If your depth-to-diameter ratio is too great,” he said, “you may need to take multiple passes.”
In some cases, coarse-pitch thread milling requires a roughing and a finishing pass, and deep-hole threads require taking a pass at one depth into the hole and then another pass at a deeper depth, Mazzenga noted.
“One of our newest tools has only three rows of threads,” Mazzenga said. “We then take multiple passes all the way down the thread. Separate roughing and finishing tools are not needed and cycle times can still be acceptable with multiple passes.”
Nonetheless, too much side pressure when thread milling can negatively impact thread quality. “To compensate for that, generally you take a lighter feed, requiring more passes,” said Allied Machine’s Brown. “Sometimes, a standard tap is quicker.”
For example, machining 6 diameters deep might be beyond a thread mill’s capability. “Based on the deflection problems you’d have with a thread mill that long, thread milling probably isn’t the right way to do it,” Brown said.
When thread milling is suitable, software is readily available to aid the process. Brown noted that Allied Machine’s Web site has a program where an end user enters application information, such as the type of thread mill, workpiece material and thread specifications. The program generates NC code that can be dropped into the part program as a subroutine.
And Allied Machine’s program is not unique among toolmakers. “We provide customers with CNC programs for their specific applications,” said JBO’s Mazzenga.
In addition, Vargus offers technical software for identifying the proper thread mill based on the required thread form and workpiece material. The software provides a selection of different tool options while calculating machining parameters and estimated cycle time and generating the CNC program to complete the thread, Major noted.
“Thread milling is quite easy,” Mazzenga added. “Just becoming comfortable with the idea of thread milling is the only obstacle to overcome.” CTEAbout the Author: Alan Richter is editor of CTE, having joined the publication in 2000. Contact him at (847) 714-0175 or firstname.lastname@example.org.
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