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.
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