It's a forming method, wherein the operation performed is on a grinding or CNC machine with the use of specially dressed wheels to match the shape of the threads. This type of forming process is complicated. And the process may vary depending on the specific requirements of the application.
These wheels of the CNC machine are abrasive and are used to produce accurate threads even in the hard materials. Screws manufactured under this technique can meet custom backlash requirements as well as tight tolerances.
Some alloys or metals are too hard or brittle to permit thread rolling. Therefore, in instances like these - manufacturers use this technique. This technique is well-favored amongst many manufacturers for several reasons. Even though it is not as cost-effective as thread rolling, this method offers accuracy to a high degree.
Those bolts or fasteners that entail an elaborate or intricate threads, this is the type of a forming technique that can adopt these patterns. This process likewise could be the sole type of forming arrangement feasible for the creation of threads in some alloy grades materials. For instance, if the matter employed to make these screws are very dense or if they are fragile to roll. Or if using heat-treating prepares the metal. A prominent application is ball screw mechanisms.
There are three methods of obtaining these types of threads. They are as follows –
center-type with axial feed - The most common technique amongst the three is similar to cutting threads with a single-point cutting tool on a lathe with the exception that a cutting tool will substitute a wheel that grinds. Typically, this operation makes use of a single rib wheel; Though, there is an option for the usage of multiple sets of ribbed wheels. The formulation of a thread using this technique commonly requires passes in multiples.
Center-type infeed threads - This technique uses a wheel with multiple ribs. This wheel is longer as compared to the length of the desired threads. First, the wheel reinforces the blank to the depth of a full thread. Following this, the blank is slowly rotated through approximately 1.5 rotations while axially proceeding ahead one pitch per revolution.
Centerless threads grinding - This type of process is used to make headless set screws, is similar to that of centerless type. Feeding blanks is via a hopper to the wheels. Wherein the formation of threads is complete. The rate of production for common centerless thread type ranges between 60 to 70 pieces per minute for a 0.5 inch or a 13 mm sized long set screw.