Solutions Come in Many Forms
Tapping and thread milling head the list
By Jim Lorincz
Technological advancements in thread mills, taps and tapholders, and CNC machine tools are enabling manufacturers to meet many of their production objectives for quality threading. These include reducing cycle time, increasing production, and eliminating costly scrap and associated downtime that result from broken tools.
Modern CNC machine tools feature synchronous rigid tapping and helical interpolation capabilities that are needed to control processes, which by their very nature are regarded as more complicated than traditional milling and drilling.
The most dog-eared pages in your shop copy of Machinery's Handbook (Industrial Press, New York) are likely to be those dealing with the complexity of threading, including such technical aspects as ANSI and ISO standards, types and sizes of thread forms, and how and to what purpose threads are produced.
"Tapping is a process that goes back a long way, but the reality today is that it is much faster than ever before," says Alan Shepherd, technical director, Emuge Corp. (West Boylston, MA). "CNCs that have synchronous tapping cycles, coolant through, and high-pressure coolant capability have provided significant advantages to the cutting process," he adds.
"The basic perishable tool, the tap itself, has been developed to a point where tapping can be done at speeds formerly not practical, in large measure due to improvements in CNC controllers and machine tools, as well as new coatings, like our GLT-1, which enables us to run 20–30% faster in stainless and alloyed steels.
"What is high speed? If you were tapping cast iron at 70–80 fpm, or even 100 fpm, we can now bring that up to 200–250 fpm. We can double speed and sometimes quadruple it. Stainless steel that would normally be run at 15–20 fpm can be run at 60–90 fpm," says Shepherd.
When tapping, the productivity of the operation is governed by the cutting speed (sfm) and feed per revolution is fixed to the pitch (thread per inch) of the thread being produced. Unlike drilling and milling tools, tapping feed rates cannot be increased unless the rpm is increased accordingly, to match the required thread pitch.
For high-speed tapping, Shepherd recommends quality tool holders and application-specific coated tools. "Years ago, when I conducted training seminars on tapping, the first thing I talked about was tap breakage. It always topped my list. Today it's on the bottom of my list of topics. At the top is cutting oversized threads because modern taps have high rates of relief and require rigid collet-type holders to be able to run at high speeds," he says.
The ability of the tap to follow the same cutting path as closely as possible is essential to extending tool life by minimizing tool wear. Coatings on taps and coolant-through capabilities are important to reduce the heat-generating friction that limits tap life. Tap holding and machine feed control are critical to minimize the effects of backlash and thrust that all modern machine tools will have to some degree, says Shepherd.
Emuge, which is a manufacturer of both taps and thread mills, as well as end mills, thrillers, and holders, recently moved into a new HQ and technology center in West Boylston, MA, where its customers can see machining demonstrations, receive training, and benefit from applications engineering.
At IMTS, Tapmatic Corp. (Post Falls, ID) demonstrated on a Haas VF1 machining center how its selfreversing RDT and RCT tapping attachments produced a steady rpm, compared with fluctuating rpms when running a rigid tap driver. Tapmatic has designed its tapping attachments to compensate both axially and radially for the unavoidable discrepancies between the machine's programmed rpm, feed, and traverse to produce exact thread pitch and precise hole locations.
In the demonstration, a machine load monitor showed spiking during machine reversal for rigid tapping. However, there was very little load—about one-fourth as much—when the Tapmatic self-reversing tapping attachment was used. Less load translates into reduced machine wear and energy costs. In addition to CNC machining centers, Tapmatic self-reversing tapping attachments are also available for conventional drill presses and milling machines, manual or automated equipment, as well as for CNC machining centers or CNC lathes with nonsynchronous tap cycles.
An increasingly popular alternative to tapping is thread milling, which traces its use back to aerospace applications in the Gemini Space Program. The drawback at the time was that engineers had to write programming manually, in the absence of helical interpolation routines now commonly available on CNC machining centers.
The basic distinction between tapping and thread milling suggests applications where each is likely to be preferred. As holes get larger and deeper, tapping takes more spindle power. There is always the danger of broken taps that lead to scrapped workpieces, downtime to remove broken taps from high value workpieces, and the recutting of chips.
Thread mills can be of a singlepoint (tooth) design or a multi-point design. Thread mills generate the thread profile by helical interpolation. To generate the thread, a single-point cutter requires the same number of interpolations as there are pitches, i.e. an 8-pitch thread, 1" (25.4-mm) long would require eight circular interpolations around the workpiece threaded diam. A multipoint cutter, which is essentially a series of single cutters on one body/flute, can normally complete a screw thread in one revolution of the work.
Advent Tool and Mfg (Lake Bluff, IL), a supplier of thread and form milling products in solid carbide, carbide-tipped, and indexable tools, describes helical interpolation: "Thread milling requires the use of a machining center capable of helical interpolation. This means that the machine must be capable of three-axis simultaneous movement. Two of the axes perform a circular movement around the center of a plane while the third axis moves perpendicular (axially) to the circle’s plane the equivalent of one pitch in a 360° circle. For the most part this is achieved by using standard G-code commands."
"When asked what we do," says Advent's Ross Wegryn-Jones, "my standard answer—my 'pitch,' if you will—is that at root we are a formmilling company that specializes in thread forms. Thread forms are predefined ANSI and ISO standards. We duplicate that form and put it on the shelf, ready to go. If someone orders a 3-mm pitch thread mill, we have it. We have a good milling platform that is a very good ground tool body and insert-locking and locating system."
Thread mills are selected based on the application, considering the number of parts or holes that are being produced. In the case of larger lot sizes, cycle time may be an issue along with tooling cost. This is where a single or multiple-flute replaceable insert thread mill would be the best choice.
The main advantage of an indexable thread mill is the ability to change out inserts quickly and inexpensively while utilizing the benefits of increased wear resistance and tool life inherent in carbide. In the case of smaller holes, where replaceable indexable tooling is not available, solid carbide or carbidetipped tooling should be considered.
For selecting the right thread mill, Advent advises having ready access to the following application parameters:
- Major and minor diam of the thread to be milled
- Length of the thread form
- Pitch (number of threads per mm or inch)
- Material to be thread milled and its inherent machining properties
- Relative quality of fixturing and rigidity of machining center
- Amount of tool extension; the shorter, the better
"Due to the cutting action of a thread mill, the forces acting on the tool and the workpiece differ greatly from those that occur with traditional tapping. The more rigidly the part is fastened to the fixture, the faster you can thread mill. The speeds and feeds are maximized when vibration of the part and fixtures is minimized," says Wegryn-Jones.
"There's a lot of interest in thread milling among customers and engineers as well as a lot of intimidation about it, but once they have seen it in action and try it, they love it," says Don Halas of Seco Tools Inc. (Warren, MI). "Thread milling works in many key areas, especially in high-temperature alloys like Inconel, Waspaloy, stainless steel, and titanium, where taps are more likely to break. Another common application is tapered NPT threads for pipe where you eliminate the need for taper reaming prior to tapping, getting rid of an entire step."
Halas points to the quality of thread produced by thread milling. "Thread milling produces a superior thread because thread milling is free cutting. Chips are very small and recutting chips as in tapping is not a problem. Thread milling can be done with the lightest duty machine in the shop, as long as it has helical interpolation," he adds.
"A rule of thumb for cycle times in typical material is that in 1/2" [12.7-mm] diam holes and smaller, tapping is quicker. In larger diam holes, thread milling is quicker," says Halas. "Although you also need to consider that if a tap breaks and you scrap the part, this can still blow your cycle time."
The consequences of breaking a tap when threading a small hole in high-temperature alloys, however, is far more serious. "In this instance, it'll take time to get that broken tap out, because these are intrinsically expensive parts that cannot be readily scrapped," says Halas. "So, with high-temp alloys, you should really look at thread milling as a first machining choice."
Halas says: "One of the primary applications for thread milling is aerospace, but we are starting to see it across the board, including big-valve manufacturing where you have 3 or 4" (76 or 102-mm) type pipe or couplings. It's both more cost-effective and easier to thread mill in these types of operations. You can rely upon one thread mill with different thread pitches to handle the various applications. Another common area for thread milling is on automotive engine blocks—both in cast iron and aluminum—where they are getting away from tapping on transfer lines and moving toward machining centers to thread mill," says Halas.
Software has taken a lot of the mystery out of the thread-milling process and made it more user friendly. Seco Tools has developed its Thread Milling Wizard software to simplify programming for users of its indexable thread mills and solid carbide thread mills.
The Thread Milling Wizard requires only that the operator enter the type of thread, diam, depth, and material group. The software generates the machine code, reducing setup time and creating the required thread from the first cut. The correct cutter body is chosen and all cutting data are downloadable to the CNC.
Flowdrill Processes Malleable Materials, Making Holes and Threading Them
What happens when the material to be drilled and tapped isn't thick enough to provide support for a threaded surface or a sleeve-bearing application when you require a welded or riveted nut or special insert?
One answer is the Flowdrill Inc. (St. Louis) process that uses relatively high axial pressure and rotational speed to heat the material to the stage where it is soft and malleable enough to be formed and perforated. As the Flowdrill pushes into the material, some of the displaced material forms a collar around the upper surface of the workpiece. The rest of the material forms a bushing in the lower surface of the workpiece. The bushing can be up to three times thicker than the original material and can be used as a bearing sleeve or threaded.
In a separate operation, a roll-formed Flowtap can produce a high-torque threaded surface without producing chips. Flowdrill tools can be used on malleable materials including mild steel, stainless, copper, brass, and aluminum. They can be used on standard drilling machines, NC, or CNC systems with 1.5–3.5-kW motors capable of rotational speeds from 1000 to 3500 rpm. They are available in sizes from 1.5 to 46-mm diam.
This article was first published in the November 2006 edition of Manufacturing Engineering magazine.