Don't Cut Threads -- Form 'Em
In the right application, thread forming can boost quality and throughput
By Jim Destefani
The vast majority of threaded holes--more than 90%, according to one supplier of taps and other cutting tools--are produced using cutting taps.
But many of those threads could be produced using forming taps, which can provide multiple advantages. For ductile work materials, thread forming can provide better size control and stronger threads while improving tool life and productivity.
Unlike thread cutting, no material is removed during thread forming. Rather, the process displaces the material to generate the thread form. Since the metal's structure is cold worked along the thread profile, the threads produced are generally stronger and have a smooth, burnished surface finish.
Cold forming technology has been used to produce threads for more than 100 years. Tap designs have changed considerably in that time, and speeds up to 260 sfm (79 m/min) are possible with coated taps, according to Emuge Corp. (Northborough, MA).
Forming taps must be applied in materials that cold form well. This includes light metals and light metal alloys as well as steels and other materials with tensile strength to 1200 N/mm2 and hardness below about RC 35 - 40. Generally, materials that produce a continuous chip when drilling are good candidates for thread forming. This list is longer than you might think, and includes wrought and cast aluminum and aluminum alloys, copper, brass, stainless steels, carbon steels, and zinc diecasting alloys.
| Photomicrographs from Emuge Corp. show grain structures of cut and formed threads. Displacement of metal during the forming process results in threads that are generally stronger and have a smooth, burnished surface finish.
Thread forming is useful for both blind and through holes, and can produce threads of any length. But because thread forming produces no chips, it is especially well suited to tapping blind holes. Thread forming can also eliminate the need for special taps to produce threads interrupted by longitudinal slots or cross-holes.
Cold forming can provide multiple processing advantages versus thread cutting. According to Precision Twist Drill (Crystal Lake, IL) potential benefits of thread forming include:
- Elimination of chip-removal problems and relatively easy threading all the way to the bottom of blind holes.
- Accurate thread size that can be held close to gage limits, improving tap life and reducing the chance of producing oversized threads.
- Stronger geometries than those of cutting taps, which require flutes for chip removal. This is especially important in smaller thread sizes.
- Tool life 3 - 20 X longer than cutting taps.
- Spindle speeds at least double those used for cutting taps.
The result of higher speeds and longer tool life is higher production rates and lower costs. Cost per threaded hole can be further reduced by a reduction in downtime normally created by chip problems, tap breakage, and frequent tool replacement.
Emuge offers a computer model service that compares expected performance of forming taps versus thread cutting. By inputting the work material, thread parameters such as size and tolerances, the type of machine being used, and hole type, users can get a direct economic comparison between thread forming and cutting.
In one example, the software was used to compare cost of producing a 15 mm deep, M10 thread in B226 aluminum casting alloy. The cutting tap could produce an estimated 2281 threads at surface speed of 10 m/min, spindle speed of 322 rpm, and feed of 483 mm/min for a cost per hole of $0.02. The forming tap, running at the same surface speed, spindle speed of 307 rpm, and feed of 460 mm/min, produced 6240 threads at a cost per hole of $0.01.
Forming taps work with standard tapping heads, CNC machines, automatic screw machines, or leadscrew tappers. They also work with tapping heads--either self-reversing or non-reversing--such as those manufactured by Tapmatic Corp. (Post Falls, ID). The company explains that thread forming requires more torque than cutting an equivalent thread in the same work material. Torque increases as the tap dulls, and the tap may continue to produce acceptable threads even after the torque required has doubled.
Because of the increased torque requirements, Tapmatic recommends reducing the acceptable tap capacity range for its tapping attachments by 25% when using forming taps. The company says this is particularly important for self-reversing heads, because the torque output of a self-reversing head is limited by the size of its components, and cannot match the output of the machine itself.
Formed threads are created by material displacement. The cold forming process redirects material grain orientation, resulting in thread flanks with higher surface tensile, yield, and shear strengths. Thread surface finish is also good, improving resistance to corrosion and abrasion. Thread flank quality is uniform and size is very consistent.
The minor diameter of a formed thread is created by material displacement. The material in the wall of the hole "flows" into the thread depressions of the tap when displaced by the crest, and the thread minor diameter will actually be smaller than the diameter of the starter hole. Thus drill diameter affects the minor diameter of the internal thread, and forming taps require a larger hole than cutting taps. Drill diameter also can have a significant impact on tool life, so selecting the correct drill is a key to good thread forming performance.
Most suppliers of form taps offer guideline recommendations for hole size for various threads and materials. But because the flow of a given work material cannot always be predicted, OSG Tap & Die (Glendale Heights, IL) recommends testing various drill sizes if the percent of thread required is critical. OSG also recommends chamfering or countersinking the hole before tapping, because some work materials may extrude above the top of the hole during forming.
Starter hole diameter is so important in thread forming that Titex Tools (Crystal Lake, IL) etches the size of the corresponding drill directly onto its cold forming taps. For each tap, the Titex Plus program offers the corresponding drill.
Lubrication is also important in thread forming, and most suppliers recommend a tapping or extrusion fluid rather than a conventional metalcutting fluid. Some suppliers even call for neat cutting oil for all work materials; however, coated cold forming taps can provide good results with a little as 5% soluble oil or with mist lubrication.
Extreme-pressure additives in the lubricant minimize the chance of friction welding and reduce coefficients of friction considerably. Many forming taps are supplied with oil grooves to improve lubricant flow, and coolant through the tap can improve tool life, allow an increase of tap speed, and maintain thread quality in tapping of blind holes.
Forming taps differ considerably from cutting taps. According to Kennametal Industrial Products Group (Augusta, GA), thread forming takes place on high spots or "lobes" in the thread form on the circumference of the tap.
Another difference is the lack of flutes in forming taps. No chips are produced in thread forming, so the tools don't need any path to evacuate them. Forming taps thus have a larger cross section and are stronger than equivalent-size cutting taps. Most forming taps include oil grooves that provide lubrication to threads at the front of the tap.
Forming taps feature initial threads that are tapered instead of chamfered as in cutting taps. Either plug or bottoming tapers are available to accommodate through or blind hole tapping.
Products for Thread Forming
The design and materials of thread forming taps continue to evolve considerably. Here's a look at current technology from several manufacturers.
Hy-Pro NRT high-production forming taps from OSG Tap & Die Inc. (Glendale Heights, IL) are said to produce fine thread finishes and have longer tool life as a result of reduced friction. The tools use a reduced neck thread design for a variety of applications. They are available with three different types of chamfers and a variety of ground thread limits to match specific applications. The taps can be specified with TiN and other coatings.
Rekord Druck-PM taps from Emuge Corp. are said to extend the conventional application range for forming taps to materials with tensile strength to 1200 N/mm2 or hardness to RC 38. Available with a variety of coatings, the taps are said to provide improved process safety in material with microhardness to HB 360 - 370. They can be supplied with oil grooves to improve coolant and lubrication flow, especially in horizontal operations.
X-Press taps from Besly Products Corp. (S. Beloit, IL) are produced with two lubrication grooves to carry lubricant to the work zone. The grooves are said to help prevent seizing or galling in stringy materials, and to provide better performance in deep or cored holes. The tools are available in machine screw, fractional, and metric sizes.
PT170 and PT 178 series taps from Precision Twist Drill (Crystal Lake, IL) are general-purpose tools available in plug chamfer and bottoming chamfer configurations. They are produced for metric thread sizes from M3 to M12 and fractional UNS and UNF threads to 1/2" diam. The tools feature ground threads, straight lubrication grooves, and a bright finish.
Titex Tools (Crystal Lake, IL) produces several lines of forming taps. Type B.25 tools have no lubrication grooves and are said to offer optimal process stability in production of small thread sizes down to M1. They are available in bright, nitrided and steam oxidized, or TiN-coated finishes. At the high end of the Titex line, type B.67 taps are produced from cobalt HSS. The tools' geometry is said to uniformly deform the work material and improve wear resistance. Type B.67 taps are available with a multilayer TiCN coatings for metric thread sizes from M3 to M16.
EM-TLD Tru-Lede forming taps from Kennametal Industrial Products Group (Augusta, GA) are designed to allow dry tapping of soft steels and thin sheet steels. The cobalt vanadium HSS tools are said to run 1.5 - 2 X faster than the speeds recommended for cutting taps. They feature a TiCN coating and plug chamfer design, and are available in metric and fractional sizes.
This article was first published in the April 2004 edition of Manufacturing Engineering magazine.