1. GENERAL
The thread-rolling die cost per thread produced is directly related to number of parts obtained in the useful life of the dies. Each time a pair of dies is used, there is a loss encountered in the life of the dies due directly to variations in setup. Die life and die cost per part depend greatly on the care and attention given to setup of the dies and control of blank diameters and material.
Proper application and setup of the dies is of prime importance and contributes more to lower die cost than is failures of the threaded surfaces of the dies are progressive in nature and are a result of repeated loading of the die threads. The rate at which these failures progress is greatly increased because of improper setup of the dies, the presence of abrasive material in the coolant and on the surface of the blanks. Careful attention to the control of operating conditions will greatly extend the useful life of thread rolling dies.
Large quantity runs are more conducive to longer die life, and therefore contribute to lower die cost.
Machines should be maintained in good operating condition. The machine rams should be tight and unnecessary looseness eliminated. Die pockets should be kept in good condition and be free from burrs and chips that could affect the proper seating of the dies. Stationary die blocks must seat firmly on the base and against the adjusting screws.
2. INSERT DIES IN DIE POCKETS
The dies should be placed and clamped in the die pockets with trademarks in the down position. Make sure the die pockets are clean and the dies rest solidly in the die pockets. Dies must not move in the die pockets during the rolling operation. Filler blocks may be used under the dies, when depths of die faces are less than the depths of the die packets, and must be parallel and free of any burrs.
3. CHECK POSITION OF DIES FOR UNIFORM HEIGHT AND LENGTHWISE PARALLELISM
The dies should be checked with a straight edge, as shown in illustration, to insure dies are setting at the same height. The top edges of the dies must be collinear to the axis of the machine stroke and also parallel with each other. The accurate positioning of the dies eliminates unnecessary adjustment when the dies are later checked for match.
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4. POSITION STRAIGHT PORTION OF DIE FACES FOR APPROXIMATE DISTANCE APART
The threaded faces of the dies must also be in a parallel position with each other in order for the dies to finish roll the thread to size during the last few revolutions of the blank. With the dies in parallel position, adjust the spacing of the dies until they are approximately 0.005 to 0.010 apart at the point section at the finish end of the dies. Be sure the rises on the dies do not contact each other.
5. CHECK BLANKS FOR CORRECT DIAMETER, LENGTH AND MATERIAL CONDITIONS
Blank diameters must contain the correct volume of material to form the threads to proper profile and diameter. If the blank contains too much material, the dies become overloaded; and if too little material, the threads will not be completely formed. Blank diameters must therefore, be held within prescribed limits of size and tolerances, commensurate with the thread specifications and for most economical die life. Recommended blank diameters for Types A and AB screws are given in Sterling Bulletin 2-610.
Different thread rolling die settings may be required to cover the full diameter limit range produced by new and worn blank header dies. The blank diameter limits for a given setting should not vary more than the permitted blank tolerance, otherwise overloading of the thread rolling dies can result. By keeping different lots of formed blanks segregated into individual blank diameter limit groupings, the thread rolling dies may be readily set up for rolling the individual groupings, with little or no die adjustment for thread size.
It is important that the lengths of the blanks are not less than the finished length of the thread. Blank lengths equal to or in excess of the thread lengths are preferred. Blanks that are too short will produce threaded points with stringers and in some cases, no threaded points at all. Blanks should be checked to make sure they are not bent or out of round since these conditions will tend to make the blanks slip in the dies when rolling and result in premature die failure.
If the hardness of the blanks is not uniform, variation in threaded points may occur and result in good points, short or no points, and points with stringers. Hard wire may cause the slugs from the self-pointing operation to break off prematurely with resulting threads having short or broken points. Very soft wire tends to stretch during the self-pointing operation and prevents the slug from separating from the blank, thereby rolling points with stringers. Process annealed wire of Rockwell B60-80 hardness will generally give best results.
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