The Bolt Creation Process
Step One
Northbridge Motorsport is able to offer both hot and cold forged products. It is the head of the bolt that is forged to shape. Forging gives a much greater strength over a bolt that is only machined to shape. Here we will follow the hot forge process. The cut metal slug has been heated within a very narrow tolerance band.
Step Two
The “heated slug” is immediately transferred to the press for forging. The forging tools within the press shape the head of the bolt with either one or two blows. The forces involved in the forging process are massive and the setting of the press is a very skilled job. During the process a forging is slit at regular intervals, etched with acid and the grain flow checked by a Level 3 Technician.
Step Three
After forging most materials are heat treated to release the stresses introduced in the process. Northbridge Motorsport has its own Heat Treatment Facility.
Step Four
Bolts and studs are turned to shape using CNC machining centres. The final surface finish, for which Northbridge Motorsport is well known, is achieved through grinding. The photograph shows a part being ground in a specialist dry process.
Step Five
This is the fillet roll process. The under head to shank area of the bolt is sitting on a set of very hard rollers. The bolt is rotated and a carefully calibrated force is applied through the rollers. This process introduces cold work into this critical area of the bolt which prevents a failure mode through the head.
Step Six
All threads are formed through rolling. Thread rolling imparts much greater strength into a bolt or stud than thread cutting. Sometimes heat is applied to assist the thread forming process.