General Information (part 1 of 3)
Introduction back to top
The purpose of this section is to provide the Design Engineer and Buyer with all the information they need when considering the use of thread rolled worms.
All of the design information necessary to take full advantage of the tremendous productivity and performance benefits of the thread rolling process over traditional thread production techniques can be found below. Information on basic material selection, design of thread lead and diameters, the relationship of thread depth and number of starts, the design of thread crest and root features and suggestions about overall part configuration to facilitate thread rolling, will help the design engineer avoid common pitfalls.
The end of this section contains a table of popular worm sizes with specifications and full scale thread profiles shown (see Table 37).
Advantages of Rolled Worms back to top
Traditional threading techniques for worms are metal removal or chip producing processes. Thread rolling, on the other hand, is a cold forging or chip-less process. A hardened cylindrical threading die rotates and presses into a blank workpiece, impressing the die’s thread shape into it. The process is similar to rotating a cylinder of modeling clay between clenched fists.
Advantages of thread rolling are material savings, improved surface finish, better fatigue and ultimate strength, and above all economy of manufacture. Savings of 75% of threading time are common when thread rolling replaces hobbing, milling, and die head threading. Even greater savings result when rolling replaces thread grinding and single point threading. Also, rolled parts are more efficient, and run cooler and quieter than cut-thread parts because of their superior surface finish.
Material Selection back to top
During thread rolling, the yield strength of the work piece is exceeded by the roll die pressure forcing material to cold flow into the shape of the die. Many materials cold form well, and only a few cautions should be observed. Materials with very high yield strengths and those that do not cold form well are poor candidates for rolling. In general, materials harder than HRC 35 and those with elongations less than 12% lack the ductility necessary for rolling. Materials harder than HRC 35 can be rolled in some cases, but die life is so short that the die cost/ productivity trade-off must be studied carefully.
In some cases, materials as hard as HRC 50 have been rolled even though the overall cost was actually higher than that of thread grinding. Most of these applications are related to aerospace or nuclear applications where increased fatigue resistance justifies the cost premium.
Some materials that seem to be ductile and soft enough still cannot be rolled because of their extraordinary work hardening properties. These are typically exotic stainless steels or temperature-resistant alloys on which cold forging data are limited. Cast materials such as gray iron, bushing bronze, and die cast zinc alloys are soft enough, but they lack the necessary ductility and should be avoided.
Materials that machine well usually do not cold form well and vice versa. Additives that promote machinability, such as lead and sulfur, cause the material to separate rather than flow during rolling. Leaded steels and steels with more than 0.15% sulfur content should be avoided for worms and power screws although they are sometimes used for fasteners.