TECHNICAL TIP #45; THREADING; THREAD FORMING TAPS
Thread forming has become commonplace in the metalworking industry.  The ability to generate threads without creating chips, using a tool that is generally much stronger than cutting taps, and the ability to produce parts much faster, has made forming an attractive option.  While there are limitations to applying these tools, they have found their way into all types of manufacturing operations producing anything from simple components to advanced aerospace parts.

Unlike thread cutting where material is removed to create the thread, thread forming moves or displaces the material to generate the thread form.  Since the metal’s grain structure displaces along the thread profile rather than being severed by cutting, the threads produced are much stronger.  Generally, the threads have a smooth, burnished surface finish.

The taps used to produce formed threads have a special shape.  The forming takes place on the high spots or “lobes” in the thread form located around the circumference of the tap.  Oil or lubrication “V” grooves are located between the lobes to provide lubrication to the forming threads at the front of the tap.  The forming threads are taper threaded instead of chamfered as with conventional cutting taps.  Plug and bottoming tapers are available as standard to accommodate through and blind hole tapping.  The H limit is generally larger than for the equivalent cutting tap to compensate for the slight shrinkage or “closing in” that occurs in the hole after that tap is removed.

There are many applications that are suitable for thread forming.  However, there are a few requirements for them to work successfully.  First, the drilled hole size must be larger than for cutting taps.  When forming, the minor diameter will be generated by the forming process.  Therefore, the minor diameter will be smaller than the pre-drilled hole size.  If the part has been punched or drilled for cutting, the hole will have to be opened up for forming.  Second, displacement should take place very rapidly.  Normally, the RPM should be double the speed used for cutting taps.  Third, the material must be capable of being displaced.  Generally, materials over 30 Rc, cast materials, and non-metallics should not be formed.  Finally, since there is a lot of friction and heat created during this process, lubrication is essential.  Straight oils are preferred over soluble oils.  Thin film coatings such as TiN and TiCN are also beneficial.
Thanks and a gift to Tim Murray for suggesting this topic to cover.

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