What is Thread Rolling?............How can I Maximize die life?
Thread rolling is the forming of a thread on a workpiece with a die by exceeding the yield point of the workpiece material and flowing the material into the thread roll to take on the mirror image of the form in the thread roll.
Thread rolling is done at room temperature.
Thread rolling is a chipless operation.
Thread rolling is typically run at speeds about 3 times that of thread cutting.
The minimum practical speed for rolling is 100 Surface Feet per Minute (31 Surface Meters per minute). Lower surface speeds may produce threaded parts but thread roll life will be extremely poor (the reason is the material will not easily flow into the rolls and can work harden to the point where material flows stops entirely). Higher surface speeds are feasible with mild, ductile materials.
To calculate the proper RPM for 100 SFM (surface feet per minute) use the following formula: RPM=382 divided by the finish diameter in inches.
To calculate the proper RPM for 31 SMM (surface meters per minute) use the following formula: RPM=9868 divided by the finish diameter in millimeters.
Thread rolling is possible if the workpiece material has an elongation factor of 12% or more.
Thread rolling normally increases the tensile strength of the workpiece because of the work-hardening that goes on during the rolling process.
Blank diameter is normally the pitch diameter when rolling a 60 degree thread.
Chamfers on the blank prior to rolling should be 30 degrees or flatter. A 30 degree chamfer will roll-up to approximately a 45 degree angle during the rolling process.
The greatest cause of thread roll failure is over-rolling. The thread roll can only be filled to 100% of capacity. When more material is pushed into the roll than the volume of the space between threads, the material flows axially and breaks the thread roll.
Thread roll dies are very strong under compressive load but not strong in resisting axial load (caused by over-rolling or improper chamfer on the blank).
Coolant should be used with extreme pressure additives to keep the lubricant from being displaced during rolling and having metal to metal contact and gauling.
Water soluble coolants are best for heat removal. Check concentration often. Make-up solutions should be weaker to keep the correct ratio in the tank (water evaporates while concentrate does not). Use a refractometer to check concentration regularly.
Oil will give the best finish but must have extreme pressure additive to work well.
Rerolling of parts with high carbon content will greatly shorten die life. the higher the carbon content, the greater the work hardening.
Pitch line is the line through the thread where the thickness of the thread and the space between the threads is equal.
Pitch diameter is the distance from the pitch line on one side of the workpiece to the pitch line on the opposite side.
Addendum is the distance from the pitch line to the crest of the thread.
Dedendum is the distance from the pitch line to the root of the thread.
The distance between the crests of the thread rolls produces the workpiece minor diameter.
The thread rolls create both the workpiece minor diameter and the workpiece pitch diameter. By closing the dies the workpiece minor diameter and pitch diameter are reduced.
The blank diameter will control the major diameter of the finished workpiece and nothing else. A larger blank will yield a larger major diameter and a smaller blank will yield and smaller major diameter after rolling. The minor and pitch diameters are unaffected by a change in blank diameter.
Central Rolled Thread Die Company 16 Research Park Drive Saint Charles, Missouri 63304 USA