by Mike McNulty, Editor
Wire & Cable Technology International (WCTI)
Even though the process of using a machine to pull a wire through a die to reduce its diameter and increase its length is very old, and the fundamentals have not changed much since the 19th century, the machines, processes and tooling used to draw wire have and will continue to change. Keep in mind that these are not fast- paced innovations, but ones that evolve and improve.
In the area of wire drawing machines, we have long- term trends towards multi-wire drawing technology, higher speeds, ﬂexibility, automation, quick die-change systems, increased output levels and ranges, variable drafting, reduced costs and improved lubrication. On the wire drawing die side, a good portion of the news is focused on better service, and we have seen the introduction of complimentary engineering services, free trials, inventory management programs, die leasing and die matching. On the technology side of wire drawing dies, developments come in the area of tooling materials and sources, manufacturing techniques for making the tools and surface ﬁnishes. One example is tungsten carbide dies coated with a thin layer of nanocrystalline diamond particles to improve die performance and life.
A summary of developments that we have observed over the last few years include the following:
- Modern multi-wire drawing lines for the manufacture of wires made of copper, aluminum and other nonferrous metals.
- Flexible machine
- Compact drawing machine for various applications including materials research and development.
- Ultra-ﬁne, high-tensile steel wire drawing
- Individually driven and electronically controlled wire drawing capstans with minimized slip.
- Improved ﬁnal wire surface
- User-programmed back-pull setting with automatic measurement and adjustment.
- Nanocrystalline diamond wire drawing
- Die blanks made of new high-performance
- Improved delivery of lubrication to wire
- Material savings via maintenance of closer tolerances on diameter or conductivity.
- Modular machine constructions in all size ranges of machines: ﬁne, intermediate, breakdown.
- Drawing of special materials including aluminum, alloys, superconductors and bimetals.
- Reduced energy
- Miniaturization: superﬁne wire drawing of many types of materials at high production levels.
Advances in the wire drawing process as well as the development of new wire materials have resulted in the desire for more consistent and accurate die geometries, longer life materials and special die sets such as dies matched by elongation for use in multi-wire drawing.
In the area of die working equipment, some innovations include:
- Automated angle polishing and sizing
- Multi-station, ultrasonic die working
- High-powered inspection
In addition to keeping up-to-date on developments, a strong knowledge of the wire drawing fundamentals is necessary for successful selection, use and maintenance of wire drawing machines and dies.
Wire Drawing Fundamentals
Wire drawing is a steady-state, cold-forming process in which a rod or wire is pulled or drawn through a single die or a series of consecutive dies. Each die and the subsequent dies have diameters that are smaller than that of the input material, and the cross-sectional area of the wire is reduced as it passes through each die. Theoretically, wire drawing is a “chipless process” in which no material is intended to be removed. Because of this, the volume of the wire essentially remains the same as it is drawn, but length of the wire increases, or elongates, according to its new diameter. In practice, some metal ﬁne generation occurs, depending on the material being drawn and lubrication conditions.
The wire drawing process is said to be cold because heat is not applied to aid in the metal forming. During drawing, the input material properties change due to cold working, and the temperature does rise, often dramatically, as the wire is drawn down to smaller diameters.
To draw wire, you need a drawing machine, wire, lubricant and dies. The process can be a wet or dry one, which is deﬁned by the type of lubrication used: water or oil-based products for wet drawing and dry lubes for dry drawing.
In the wire drawing process, the machines used are single, but more often, multiple die (multiple draft or tandem) devices that are designed to draw one or more wire(s) at a time. As the rod or wire is pulled through each die, the diameter is reduced (area reduction) and the length is increased (elongation).
In wire drawing machines, pulling of the material to be drawn is done by driven capstans. On a single draft machine, only one capstan exists, and multiple draft machines will usually have the number of capstans equal to the number of drafts. The capstans can be arranged in-line (tandem) where each capstan is driven by a single shaft or in a cone-type arrangement where more than one capstan is on the same shaft. The key parameters in wire drawing are area reduction, wire elongation and slip:
Area reduction, AR, % AR = (A1 – A2)/A1 x 100 AR = (1 – A2/A1) x 100 AR = (1 – (D2/D1)2) x 100
where, A1 = input cross sectional area; A2 = output cross sectional area;
D1 = input diameter; D2 = output diameter
Wire Elongation, E, % E = (L2 – L1)/L1 x 100 E = (L2/L1 – 1) x 100
E = ((D1/D2)2 – 1) x 100
where, L1 = input length; L2 = output length; D1 = input diameter; D2 = output diameter The relationship between elongation and area reduction is: E = 100/(100 – AR) – 100
The % Slip is the diﬀerence in speed between the wire
and the drawing capstan, as follows:
% Slip, S
S = (Vc – V)/Vc x 100
where, V = wire speed; Vc = capstan speed
In a multiple draft, slip-type drawing machine, slip will accumulate throughout the machine until it reaches the ﬁnal drawing capstan where the % slip is theoretically zero. In a zero-slip machine, the rod or wire is drawn at the same speed of each drawing capstan. Other useful and interesting calculations can be made regarding speed and production of wire drawing machines. These are helpful when making capacity plans. When selecting the parameters of your wire drawing machine, many factors must be considered including the input wire material, number of wires, ﬁnal wire diameter, ﬁnal wire speed, operation style, lubrication conditions and surface quality requirements.
At the heart of the wire drawing process is the wire drawing die, which is the tool that is in direct contact with the wire as it is being processed. There are dozens of suppliers of wire drawing dies. In my opinion, the following areas are important in selecting a supplier and obtaining wire drawing dies: part consistency and quality, service, experience and know-how, design assistance, price and delivery time.
Wire drawing dies must have the correct geometry for the appropriate application if good results are to be expected. The die can be broken down into various zones: entrance zone where the wire and lubricant enter the die, reduction zone where the wire impacts the die and begins to be deformed, the sizing zone where the wire diameter is set and the exit zone where the wire leaves the die. Various terms are used to describe the parts of a die, but the most important attributes are die angle, bearing length and diameter. In addition, good performing dies should provide some type of blending from one zone to another so that the wire surface is not damaged or stressed.
Optimum die angle and bearing length are functions of material being drawn and area of reduction taken. There are many good technical papers available on the subject of die geometry. This knowledge coupled with practical experience, testing and possibly simulation software will lead you in the right direction to obtain optimum die geometry.
The materials used for wire drawing dies include tungsten carbide (TC or WC), single-crystal natural diamond (SCND), single-crystal synthetic diamond (SCSD) and polycrystalline synthetic diamond (PCD). For each material, there are diﬀerent “grades” available according to factors such as grain size, polish level, density, diamond content percent, etc. Each material and grade has its advantages and limitations, and there are published opinions on the subject.
Many factors inﬂuence the decision including processing material, surface quality requirements, product diameter and shape, temperatures, speed, % slip in drawing, cost, etc. The most important data on these factors will probably come from practical experience and collection and analysis of performance data in individual factories. Developments in materials have been along the lines of higher PCD strengths, wider diameter capacities in PCD and SCSD materials, surface treatments for lower coeﬃcients of friction, proprietary process tool steels and use of disposable inserts.
WCTI Spotlight Articles on Wire Drawing from 2020, 2018, 2016, 2014, 2012 and 2010.
This article can be found in the March/April 2022 Issue of WCTI.