A New Concept: Composite Forging Dies
A new design technique in forging dies has been proposed in which a tessellating field of stacking or interlocking steel shapes comprises the working faces of a forging die. A U.S. patent application has been filed for this die system design, which is outlined here in conceptual form.
This idea for a new method of forging parts matured from an intuition to a patent application. My involvement with forging, although indirect, was sufficiently close to the real thing because it dealt with the procedures used to approve forging processes for demanding items like turbine disks.
One of the main time and cost drivers of the industry is the exceedingly long lead time from order to supply of tool-steel blocks for forging dies. A substantial reduction of this wasted time will shorten manufacturing times and significantly reduce costs.
As a retired engineer, I found myself musing and playing with the challenge of finding ways to shorten that wasted time. This kind of pondering brought about the idea of trying a composite die, assembled with standard repetitive elements, to be enclosed in a fixture strong enough to withstand the high cyclical stresses of forging work.
Die materials could be selected from a large range of suitable tool-steel types. Bar stumps, taken as a bunch of contiguous units axially aligned in the vertical direction, could be a good element choice. The requirement to meet is that of tessellation, defined by Wikipedia as “the tiling of a plane using one or more geometric shapes, called tiles, with no overlaps and no gaps.” Consequently, round bars are excluded, but square, rectangular or hexagonal shapes could be considered. In a given die, all the bars could be identical. In special cases, some could be different (an example follows).
The bar size to be selected in any specific case is open. This feature gives designers remarkable freedom. Bars, in general, can be purchased already heat treated to their optimal hardness. The smaller the size, the higher the definition available when providing impression precursors of approximate shape to be finish-machined (details to follow).
The purpose of this composite die concept is to obtain the following beneficial outcomes:
- Eliminate the lead time for procurement of a raw-material block
- Eliminate heat treatment of the machined block
- Reduce machining time by assembling shorter bar stumps at selected emplacements to create a precursor impression (Fig. 3). At least two different procedures to that end can be described.
- Eliminate or reduce the need of electro-discharge machining (EDM)
- Use simpler and less-expensive tool steel, or place more wear-resistant steel at selected locations in the composite die
- Assemble a wide range of different sizes of dies, all with a standard supply of raw materials
- Significantly shorten the time from drawings release to die supply
- Combine economic benefits relative to mono-block dies costs
It is difficult to quantify in abstract the amount of potential savings. One can expect that substantial gains can be realized, however, at least in special classes of composite dies to be determined in different industries.
Anatomy of a Composite Die
A simple design for the working fixture was developed. The central bar block is enclosed in a set of four jaws constrained by three concentric rings. The first two have corresponding conical surfaces so that circumferential constraining stresses are generated by axially pressing the external collet on the internal one. A third cylindrical element added for stability is hot-mounted to develop an interference fit upon cooling.
In special or extreme cases in which the die assembly might be insufficient to withstand the shattering forces developing in the die, two different solutions could be explored. The first refers to the special shapes of the tessellating bars to provide an interlocking action. The second refers to external means to be employed to contain such forces.
In the composite die example, the impression consists of half a sphere attached to two cylindrical axial sections (Fig. 1). In this figure, some of the outermost bars were sectioned to present a continuous surface to the jaws – either between two opposite edges or between the middle of two opposite faces. Figure 2 shows an exploded view of the die assembly.
Calculation and Stress Verification
For any specific die design, preliminary calculations should be conducted to ensure that the material condition (tool-steel types and mechanical properties) of the constraining elements will withstand the extreme forces while forging is performed. In other words, it is essential to make sure that the most stressed parts of the dies are capable of bearing the working action of hammers or presses.
For the purpose of this exercise, the additional hot-mounted constraining ring designed to provide redundant safety and stability was excluded completely from the calculations. These calculations were performed by stress engineer Dr. David Barlam, who applied finite element analysis (FEA) using MSC.Nastran and MSC.Patran software.
As a typical illustration, a specific case was considered in a series of schematic diagrams (Figs. 4-8).
In the case of a typical forging, the stresses that develop in the constraining external ring must be determined in order to adequately design its dimensions and to properly select the type of steel required. These stress calculations were also performed by Dr. Barlam.
The impression of the part to be forged, selected to simplify calculations, was a concentric cylindrical hole of 40 mm diameter and 20 mm depth. The radius at its base was meant to reduce local stress concentration. The forging pressure required was assumed to be 30 tons/inch2 of the area of the item to be forged, roughly equivalent to 42 kg/mm2. (From ASM Metals Handbook, Vol. 5 – Forging and Casting – 1970, page 14, for a metal such as 4340.)
The horizontal axis in Figure 4 corresponds to the die axis. The area colored red depicts the central part of the composite die. For this example there is a simplifying assumption that the proposed part is axisymmetric. Half of the impression cavity section (appearing as white background on the bottom right part of the slide) is limited by the horizontal axis, the border of the red area and a virtual vertical line drawn from the red edge toward the bottom.
The diameters (shown as vertical arrows in Fig. 4) identify the assumptions selected to perform the calculation: 40 mm is the diameter of the circular impression; 90 mm is the inner diameter (ID) of the internal ring; 100 mm is the outer diameter (OD) of the internal ring coinciding with the ID of the external ring; and 120 mm is the OD of the external ring. Figures 4 and 5 show the die configuration and boundary considerations respectively.
Figures 6-8 show the stress calculations pertaining to this specific forging shape example.
This calculation demonstrates that both internal and external rings can easily bear the maximum stress under actual forging conditions in this example. They can therefore be made of any suitable construction steel. The outermost hot-mounted ring, excluded totally from this calculation, should provide ample margin of extra stability for a long production run.
Similar calculations should be conducted for any new composite die design.
In February 2016, a patent application was filed in the U.S. for this composite die system. Such a die system has yet to be constructed and physically tested. The concept is presented here to encourage thought in alternative die designs. In order to fully evaluate the economic advantages of this method of die construction, one would have to break down all operations and costs and compare them to the traditional fabrication practice.