Is switching to thread rolling on CNC a good choice?
BY TEAM MANUFACTURING TODAY
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For many CNC applications, thread
rolling is the best choice for creating higher quality external threads in a single pass, chip-less operation. Thread rolling technology has been resorted to on multi-spindle machines, engine lathes and dedicated rolling machines for decades.
However, rolling threads on machines that are not dedicated to a single part/operation is a relatively new and unfamiliar concept. It offers particular advantages on NC and CNC machines as it eliminates the costly multiple passes required in single point threading. Producing the thread in only one pass can reduce threading time by as much as 90%!
Today’s CNC machines are very versatile and can be changed over quickly to help meet demands for JIT manufacturing. A more striking benefit of CNC machines is its ability to produce higher quality parts more quickly and efficiently. Tool changes are faster and adjustments can be made without stopping the machine.
Like the CNC, single point threading is also very versatile. Most of today’s single point threading is performed by indexable insert tools as part of a very rapid CNC process. A typical part that requires a thread is routinely accommodated through fixed cycles of numerical control and a variety of other machine mechanisms. This can be a very economical method of producing smaller quantities of parts.
However, when parts are produced in larger quantities, single point threading’s versatility is less beneficial. This is because, in order to maximise
effective tool life, single point inserts are processed with a limited depth of cut with multiple passes needed to cut to the thread’s full depth. The
time needed to take these multiple passes can
create a bottleneck. Additional passes may also
be taken to deburr the thread, requiring more machine tool time or a secondary operation outside the machine.
Although CNC is bringing down the total threading cycle times by making non cutting functions more efficient, these time savings are negated by the added time it takes to single point the thread. Thread rolling on the other hand, produces threads in one pass, reducing expensive CNC machining time. In addition, when a thread must be rolled, the use of a thread rolling head–attachment on CNC equipment can complete the work piece in one single set up, thereby eliminating a secondary operation on a separate thread rolling machine.
• Technical advantages: In addition to doing more work on one machine in less time, thread rolling has many technical advantages over single point threading. Instead of cutting or shearing the material as is the case of single point threading, thread rolling cold forms the profile to be produced. In this process, the component material is stressed beyond its yield point, being deformed plastically, and thus permanently. A hardened die made from tool steel or HSS displaces the material along the contours of the thread profile, plastically deforming the material into the final form. The work piece material is stressed beyond its yield point, which causes it to flow and conform to the mirror image of the die’s profile (refer to figure).
Stronger threads, improved wear resistance, burnished thread flanks and profile accuracy are some of the benefits derived.
• Economic advantages: These range from material savings and extremely short machining time to longer tool life and full utilisation of the machine.