Dormer Tools Thread forming reduces costly machine downtimes
Thread forming as an alternative method to thread cutting, although not new, remains a relatively unexploited resource that can offer significant benefits over traditional methods.
Although there are many ways of creating threads, probably the most commonly accepted is by cutting the workpiece material with a thread cutting tap (internal) or die (external).
Threading with fluteless taps – also known as cold forming or roll tapping – involves the creation of a thread by forming the material rather than cutting it like other taps.
There are several significant advantages of using this method compared with cutting taps. These include speed, as cold forming is faster than ordinary thread cutting, longer tool life, stronger threads (higher stripping strength) compared to threads obtained by cutting, no chips and a lower surface roughness on the thread.
Fundamentally a cutting tap without the flutes, various designs of fluteless taps exist, but all essentially follow the same basic concept: A thread is ground onto a blank with a specific geometry which introduces forming lobes at the same time, or, conversely, the cylindrical thread is relieved at various positions around the circumference to leave the forming lobes. These lobes spaced periodically around the tap carry out the forming process, creating the thread as the tool is advanced into the material.
Chip-free process avoids jam and tool breakage
One of the key differences between this process and thread cutting is that it does not produce chips. Subsequently, there is no need to periodically back out the tap to clear away swarf, which, with a cutting tap, can jam and break the tap, resulting in costly machine downtime and component rework as the broken tap will need to be removed – often using an EDM drill.
Tests have shown the process of thread forming increases not only the yield strength of the thread, but also its surface finish, hardness and wear resistance. As a result, higher cutting speeds are recommended for forming taps than for the corresponding cutting taps in the same material.
To enable companies to take full advantage of thread forming, Dormer has a comprehensive program of forming taps. Manufactured from premium grade High Speed Cobalt (HSS-E), all are designed to support high performance thread production in a wide range of materials, and can be specified with an optional oil groove to ensure lubrication supply to the working edge.
The one important characteristic recommended for fluteless tapping is the use or addition of an extreme pressure additive within the lubricant used. Lubricants maintain the lubricity required in this type of machining application. A lack of lubrication during tapping will allow metal-to-metal contact with possible breakage.
For a thread cutting tap, the drilled hole is sized to the peak of the finished thread, with the tap removing the material between the pitch peaks to create the thread profile. No parent material is removed during thread forming, so the drilled hole size for an equivalent thread needs to be a slightly larger diameter. The raw material is then "pushed" by the tap to form the finished thread profile.
As the material flows to create the thread it becomes stronger and the surface finish generated is smoother, which means that the finished thread will offer an extended application life. Finally, it could also offer design engineers the opportunity to evaluate the use of smaller diameter fixings.
By Simon Winstanley, Dormer Tools.