The Cold Forming Value Chain in Micro/Miniature Component Design - Part 2

Madan Mathevan, Sussex Wire Inc. | 3/18/2016

The four participants in a critical micro/miniature cold value chain include the component manufacturer, the material supplier, the tooling provider, and the machinery manufacturer. Each can provide significant input to a successful execution.

Here, we are going to show how the four work together.

Component manufacturer. Since the component manufacturer is the primary contact with the customer, we’ll start there. The component manufacturer naturally takes the lead in coordinating input from the other participants and presenting options and alternatives to the customer to improve component value.

In our case, Sussex Wire serves as the component manufacturer. A designer and manufacturer of specialty metal parts, Sussex uses advanced engineering solutions for tool design, stress and strain analysis, and document control. The tools allow the engineering team to develop prototypes for their customers’ NPD staff to help them predict the ultimate qualities of the product.

The cold forming process starts with analysis of part geometry, including optimal material and dimensional qualities and limitations, as well as analysis of tool design, to allow development of tooling that can withstand repetitive impact to yield exceptionally long life with complementary precision.

This illustration shows the thought process that is needed and the information required for the tool designer to work around. (Source: Sussex Wire)

In-house tool design is accomplished by 3D using parametric modeling to reduce lead times. Stresses and strains induced during cold forming can be predicted using finite element analysis (FEA) software. Information from these analyses can be provided to the customer to assist their designers to predict the strength of the product. FEA software is also used to predict final shape using part forming analysis.

Concurrent with component and tool design, one needs to start a discussion on materials, a principal and fundamental factor related to manufacturability. Certain alloys that work for machining simply don’t do as well in a cold forming environment. On the other hand, a material perfectly suited to cold forming may require an exception by the customer to accommodate the roll forming process and its associated benefits. These considerations are best worked out early to avoid engineering delays in the later stages of product development.

Materials supplier. Carpenter Technology Corp. develops proprietary iron-, nickel-, cobalt- and titanium-based materials ideally suited to cold forming applications, and is a frequent source for Sussex Wire’s custom component projects. Numerous specialty materials are very well-suited to cold forming, and so expert materials counsel during component design can be rewarding in terms of selection for ultimate strength, geometry, manufacturability, and other critical properties.

According to Carpenter’s white paper, Heading Hints, A Guide to Cold Forming Specialty Alloys, heading is a metalworking process that goes back before the turn of the century and for many years was used only to produce simple fasteners. Today heading is a high-speed, automated, and multi-station operation that is capable of producing not only increasingly complex metal fasteners economically, but a growing variety of other components, including some that are asymmetrical. Combined with this dramatic improvement in heading equipment is the ability to successfully cold form parts from tougher metals, including stainless steels and high-temperature alloys. In other cases, analyses which have been modified for cold forming provide a means for the economical production of certain fastener designs. Tougher tool steels extend the life of heading dies.

Producers like Carpenter also make alloys that are versatile enough to meet fabrication operations that call for both heading and machining. For example, Carpenter’s 302HQ-FM stainless combines the advantages of a popular cold heading grade and a free machining grade.

Tool maker. With part design and material selection resolved, the next contributor to the value chain is the tool maker. A frequent collaborator on Sussex Wire’s projects is Rol-Flo Engineering Inc. According to Frank Locke, a senior tooling engineer with Rol-Flo, “Mechanical designers often engineer parts with impractical shapes and tolerances for cold forming, roll forming, and/or threading due to a lack of knowledge of the strengths and limitations of the technology. The tooling designer can provide invaluable assistance to the NPD engineer who is amenable to some degree of part geometry modification to permit cost-effective manufacturing alternatives without compromising core functionality or critical dimensions.”

As an example, the thought process when designing a fastener with the most cost effective means possible (i.e with cold heading and cold (form) rolling in mind) is: Where is the material for the shape to be formed coming from?” Also: “Where is the material to be displaced going to?” Keeping in mind that, in general, there is no “cutting away” of material in form rolling or cold-heading but rather material is being displaced and re-directed either radially or axially.

This illustration shows a typical captive groove and 'where the material is going', i.e. axial growth. The length of part prior to rolling must take into account this displaced material growth. Also note that the material is being forced down away from the underside of head and must therefore be allowed to flow unimpeded. In other words a thread form or any other shape below the groove must be done in a separate rolling pass or engaged the captive groove is completed. (Sussex Wire)

As one might imagine, the forces required to force the displaced material axially are much greater than when displacing material radially. Additional pressures need to be exerted on the rolling equipment to keep the material from “swelling” excessively around the groove area. Allowing for some diameter build-up in the proximity of the groove aids in the production and longevity of tool life.

Roll form tooling can create many types of profiles and shapes, including grooves, points, special threads, knurls, annular rings, and more, but some of the limiting factors to be considered are: total amount of volume displacement and reduction in area. There is only so much volume that can be displaced for each revolution before the stresses and strains overcome the core strength of the material being rolled. Thread rolling technicians are accustomed to calculating five to six revolutions for most thread forms. However, with form rolling there are times where 10 or more revolutions are required when forming wide captive grooves or multiple shapes in one rolling pass. This necessitates a larger flat-die and all becomes part of the cost analysis and feasibility.

As in cold-heading, there are limitations to how much of a reduction in area can be achieved by cold-rolling or how little. A small reduction in area of 5% to 15% results in very poor core material flow. Also, a very high reduction of 80% to 90% can result in fracturing, as the stresses supercede the core strength of the material. The ideal range for rolling wide grooves is 30% to 35%, which produces a very predictable flow rate. Captive groove widths of two to three times the blank diameter are achievable in this reduction range, whereas when approaching 40% to 50% a groove width of one blank diameter may be the limit. Finding this balance; volume, reduction, width, axial or radial displacement are all part of the collaborative effort that can transform many high volume fasteners into a cost-effective high-speed cold-rolling success.

Machinery manufacturer. Sussex Wire employs more than 30 micro machines from Nakashimada Engineering Works Ltd. The micro machines are also in use by other major manufacturers worldwide to produce components in a variety of sizes from 0.5 mm to 0.7mm diameter and from 1D2B to 4D5B in configuration. Every machine is hand scraped to ensure these extremely close tolerances.

Nakashimada makes a line of cold heading and forming machinery, from single blow through seven die, and their machines are being used in every facet of industry to produce parts for aerospace, automotive, medical, electronic, and commercial applications. Since material waste is close to zero except when a trimming and piercing operation is required, many environmentally conscious manufacturers are now looking at ways to replace machining processes with cold forming technology. This can only be achieved with a forming machine that yields the tightest tolerance.


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