At first glance, die-life appears unpredictable given the number of variables involved.

While exact prediction still remains impossible, we can provide fairly accurate die-life figures for most applications in which our dies are employed thanks to an extensive die-life data bank. IMU stores any significant die-life data submitted by its customers; constantly updated information is crucial to the development of the proper dies for each specific application.

The aim of this chapter is not to provide hard statistical data for all examined cases, but to give general information on the most important variables which determine die-life on machine screw thread applications.

Die-life is the result of three factors, namely:

In this case, the term "fastener" stands for a general concept covering all the technical features of the part to be rolled. By the same token, "machine" stands for all aspects related to the employed threadroller. These aspects are twofold: on the one hand, the technical features of the machine and on the other, the way it is operated.

Below is a list of the most important classes of variables related to the first two factors, i.e. the fastener & the machine, both key determinants to die-life yielded by a set of thread-roll dies. The third factor - the dies - is treated in a separate section which lists die-life figures of IMU dies for user reference.

Each variable listed is considered as independent. All data referred to apply to ISO standard threads within the M1 - M24 range for metric threads and 0-80 - 1" UNC/UNF range for standard threads.

Thread Size.

Minor differences in thread profile bring major differences in die-life between two virtually identical screws. For example, dies rolling MJ6x1 thread screws will yield significantly more parts, as opposed to those rolling M6x1 screws. The greater radius on the crest of the dies carrying the J thread profile will endure wear significantly better than the same thread size with a standard profile.

Thread diameter & length are other important variables.

When two screws differ only in thread diameter, the one with a larger thread diameter will cause greater wear than the one with a smaller diameter. For example, dies rolling M14x1.5 parts will wear out faster than dies rolling M10x1.5 parts, as material displacement during the rolling operation is distributed within a lower number of revolutions. Should these two parts be rolled on a # 40 roller, the M14 blanks will revolve only 6.53 times vs. 8.16 revolutions for the M10s.

An analysis of the relationship between thread length and die-life is more complex. As a general rule, greater thread length reduces die-life, since greater thread length requires greater rolling pressure. There are exceptions: parts having extremely short thread length (as do some miniature screws) are prone to tilting during the thread-roll operation. The resulting uneven distribution of pressure on the threaded portion of the dies may cause premature chipping.

Part Size & Form.

When the thread starts directly underhead or on a shank chamfer, dies are liable to chip on their chamfered corner, thus greatly reducing die-life. A similar problem can occur with parts having points insufficiently chamfered. In this case, premature wear will be generated in that area of the die which rolls thread in proximity to the point of the part. Geometrical inconsistencies of the blank to be rolled cause a reduction in die-life.

Typical examples are cold headed blanks with an oval shape or inconsistent diameter. These conditions keep the operator from performing proper set-up. The resulting rolling pressure will prove inadequate to optimize die-life. The same problem occurs when blanks have an error in straightness originating either with the cold-heading operations or during heat treatment (for parts rolled after heat treatment).

This analysis of factors which influence die-life does not include a wide range of non-standard fasteners, whose geometry may influence die-life negatively. IMU die-life data provided in another section will always refer to standard hex or socket head screws with no underhead non-threaded shank.

Heat treatment.

Often un-noticed variations or inconsistencies in the heat treatment process of the blank part may affect die wear considerably.

For example, if too short a tempering cycle is performed, the parts will be less ductile when thread-rolled than a part which is properly tempered. Consequently, actual die-life can be considerably reduced.

Alternatively, when heat treatment is not performed under full control, the surface of the parts to roll may be oxidized. This increases friction during the rolling process between the surfaces of the rolled part and the dies, again reducing die-life.

Tensile Strength and Hardness.

When rolling any parts exceeding 110,000 psi or 20 HRC, minor variations in tensile strength or hardness greatly affect die-life.

This graph shows the reduction in die-life observed on Standard IMU dies on a # 10 roller rolling identical M6x1 parts heat treated to different hardness values.

Required grade of quality.

Of all variables, this is the single most significant.

Two fastener makers rolling the same parts to the same specifications will often decide to replace their worn thread-roll dies according to significantly different levels of quality approval. One manufacturer may decide to replace a pair of thread-roll dies after having rolled 30,000 parts, while a competitor rolling the same parts under very similar conditions may decide that 45,000 parts is the right figure. In both cases, threads may be well within geometrical specifications, but inconsistencies or even laps observed on the threads will betray the difference between the two parts. A careful inspection of worn dies normally indicates if all parts rolled with a set of dies conform to the same standard of quality.

Machine conditions & operator set-up.

Good machine condition must be presumed; regular maintenance of the threadroller is the key. Two areas of the threadroller deserving special attention are the ramslide and the feeding devices (pusher blades, etc.).

Operator set-up is just as important a pre-condition as a good machine. In the set-up section we suggest a number of steps to optimize the die-life of IMU dies and consequent fastener quality.

In addition to poor machine condition & improper set-up, frequent change-overs can adversely affect die-life. A set of dies which is repeatedly removed from a machine, undergoing different set-ups at different times, will typically yield less parts than a set used on one job till the end of its life.

Speed (pcs/min).

Speed is a variable whose influence on die-life is often underestimated. Experience shows that die-life of IMU dies proportionally increases with speed up to a point, beyond which die-life decreases precipitously. We are not in the position of suggesting the right speed for each application; rather, each customer should optimize speed according to the application. A few useful indications can be found in die-life charts which appear below.

Coolant.

Oil is always recommended as a coolant. Again, experience shows that for any application oil maximizes die-life over any type of water-soluble coolant (keeping in mind that running parts with water-soluble coolant - or even dry - usually stems from environmental reasons).

The die-life data below requires a few preliminary explanations.

All submitted data are valid provided that all the variables which adversely affect die-life (as previously discussed) are addressed.

IMU dies, whether Standard or Superdie versions, are generally not considered to be a variable as they are manufactured according to a very high standard of consistency.

The tensile strength of stainless steel parts always refers to the cold-headed blank (following a certain degree of upset during the cold-heading operation) and not to the wire prior to being cold-headed.

Oil is employed as coolant.

Single or split face dies with "boltmaker lead" are preferable over duplex dies in terms of the quality of the rolled part, as their longer and smoother roll-on guarantees a progressive formation of the thread which, in the case of duplex dies, occurs in a far lower number of revolutions of the blank. Moreover, single face dies - unlike duplex dies - remain stable in their die-pockets thanks to their flat-ground bottom face. We have included data on both types of dies below.

Die-life is always given per die-edge. Die-life data are recorded for one initial set-up which is not modified-save for an eventual (slight) reduction in die squeeze due to machine heat-up.

Full control of all variables is impossible. Submitted data represent average values obtained by comparing tests run by different companies under very similar conditions. These results have been achieved by replacing the dies just before the wear curve manifests itself.

We have divided IMU die-life data into 3 groups depending on rolled material:

        stainless steel AISI 300 series

        heat treated parts for automotive applications

        heat treated parts and alloys for aircraft applications

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