HIGH SPEED STEEL DIES: WHY?

Before analyzing the different die-versions IMU offers to its customers, it is worthwhile to touch on several reasons why IMU has committed itself to exclusively manufacturing high speed steel dies (as opposed to popular tool steels, such as D2).

In a word, high speed steel dies yield far better die-life than "common" tool steels. Enhanced die-life implies:

1) Greater consistency of the quality of the rolled parts.

On "common" (D2) dies, wear leads to a gradual and progressive modification of the geometry of the thread profile. This modification is observed on the thread crests which typically appear flat, as opposed to their radiused form. As a result, the internal thread radii of the rolled parts increase and thread depth decreases. Thus, parts rolled when the dies are still new normally carry a different thread profile when compared to parts rolled just before replacing the dies. At the same time, wear on "common dies" brings to a pronounced coarseness of the surface of the thread crests. This coarseness primarily affects the surface finish of the bottom portion of the thread of the rolled parts.

Parts rolled with IMU dies maintain consistency throughout nearly all of the dies' useful life. Dimensional inconsistency and poor surface finish appear only towards the very end.

The figure below illustrates a typical die-life / wear curve of IMU dies, as opposed to that of "common" dies. Quality is considered here in terms of dimensional consistency and good surface finish.

2) Less frequency in die change.

The longer life of IMU dies reduces the number of die changes taking place every time worn dies have to be replaced. Reduction in down-time is the result.

3) "Re-grindability".

Die-users sometimes remark that "dies just don't hold up after re-grinding" and proceed to tell of 40% to 50% reductions in die-life as compared to figures achieved by new dies. This may hold true for certain types of "common dies". By the time any dies reach the end of their useful life they have accumulated stress tensions which the re-grinding operation by itself will not eliminate. Moreover, some brands of "common dies" are not core-hardened: the hardened structure below the threaded surface may not have undergone a complete structural transformation during heat treatment.

High speed steel lends itself very well to stress-relieving operations and various coatings. In addition, the heat treatment process applied to IMU dies guarantees a consistent structure throughout the whole die section. These factors guarantee consistently identical performance between new & re-ground IMU dies.

4) Lower percentage of die cost on the cost of the rolled screw.

Even if a set of IMU dies is more expensive than a set of "common dies", the greater quantity of screws rolled with the set of IMU dies more than makes up the difference with a reduced cost per part.

The main reason die-users give for resisting the move to high speed steel dies is brittleness: D2 dies are considered tougher, and thus more forgiving when operator set-up & threadroller conditions are not under full control.

While there is a kernel of truth in this, IMU has designed its dies by dedicating special attention to the heat treatment process in order to achieve greater toughness without sacrificing wear resistance. Therefore, IMU dies can be safely used in any application (obviously, a proper die set-up procedure is always required - see our section on set-up) .