Specifying materials, coatings for tube bending tools
September 30, 2008
The best material for a tube bending tool is the most cost-effective in terms of the ratio of tool life to tool cost. A cost-effective tool tends to wear out rather than break at the end of its service life. This article addresses choosing the optimal material for a rotary die tube bending machine's full toolkit.
During the past half-century, many ideas have come and gone as to what the best materials are for rotary draw bending tools. This is not surprising because of the many advances in materials, heat treatments, and coatings during this period. What may be surprising is that the materials that were the right stuff in the middle of the 20th century remain, for the most part, the best choices for tooling at the beginning of the 21st century.
What does best means? The best material for a tube bending tool is the most cost-effective one. This refers to the ratio of tool life to tool cost, which usually is expressed as the per-bend cost of the tool. Cost-effectiveness must take into account process control, or the stability of the tube-bending machine's operation which in turn determines productivity. In terms of tool life, this translates into exhausting that life through wear rather than failure. A tool failure often causes a process failure, which can shut down a tube bending machine.
Stated simply, a cost-effective tube bending tool tends to wear out rather than break at the end of its service life. The first step in manufacturing cost-effective tooling is specifying the optimal material. The full toolkit for a rotary draw tube bending machine comprises a die set, a mandrel, and a wiper, and each of these has its own material requirements.
A rotary draw die set has three components: a bend die, a clamp die, and a pressure die. The key material requirement for these dies is that it must be tough. A tough die has hard working surfaces that can absorb a shock without breaking. A tough die has some give—it is not brittle. For the same reason carpenters do not use glass hammers, fabricators should avoid materials and heat treatments for bending dies that make them brittle instead of tough and break instead of wear out.
Consequently, the tool steels that sometimes are specified for rotary draw die sets, such as A2, D2, or even a shock-resistant material such as S7, are not the most cost-effective materials. While they can be heat-treated to high hardness levels, they are designed to hold a sharp edge in metal cutting operations. The brittleness that comes with this hardness is suitable for that use, but is unnecessary in tube bending.
Tube bending requires durable work surfaces, specifically in the tube cavities of the bending dies, which brittleness degrades. This is why die sets made of tool steel must be drawn back (softened, or tempered) during heat treatment to eliminate their inherent brittleness. The tempering process also degrades their hardness. Therefore, the extra cost of tool steel for die sets does not deliver the superior hardness that is the rationale for it.
For almost all tube bending jobs, the best material choice is a steel that can be heat-treated to a high hardness and then case-hardened, which further hardens the working surfaces. The challenges lie in heat-treating without inducing brittleness and case-hardening without causing significant dimensional distortion. Nitriding and carburizing are effective methods of case-hardening die sets. The result is a durable and shock-absorbent material, which is why heat-treated alloy steels are used for NASCAR® engine components. Likewise, heat-treated alloy steel bending dies will tend to wear out over a long life rather than fail from breakage.
Since the introduction of the modern ball mandrel in the 1950s, aluminum bronze and hard-chrome-plated steel have been the most common materials specified. The former has been preferred for ferrous and titanium tubing and the latter for aluminum, copper-based, and in some cases mild steel tubing. Even with the advent of remarkable, life-extending thermal diffusion and titanium coatings, many good reasons remain for using aluminum bronze and hard-chrome-plated steel for mandrel tooling.
For example, the primary cause of shortened mandrel life is not the material, but the setup. The mandrel's purpose is to maintain the original tube shape throughout the arc of the bend. It does this only when it is set at the proper location; its nose should support the point of bend, the location at which the tubing material becomes plastic as it is drawn into the bend. Too often setup personnel or operators position the mandrel nose behind the point of bend so that the balls and links, which are weaker than the nose, must do the work of the nose, which significantly shortens the service lives of these components.
To compensate for this defective setup, manufacturing engineers often specify a coated tool steel for the mandrel to make the balls last longer. This typically works, although the solution comes with extra costs and a risk—the extra costs of expensive materials and surface treatments and a significantly increased risk that the tool will break before it wears out. Therefore, if the mandrel is set up as designed, with the nose advanced into the point of bend, the proven aluminum bronze and hard-chrome-plated steel material specifications are the most cost-effective for almost all tube bending applications.
Some of the dissatisfaction with these well-established materials arises from the use of improper grades. It is important that the aluminum bronze specified for a mandrel is alloyed with nickel, which can double the tool life compared to a non-nickel grade. Ampco® 45 and similar grades are best; avoid Ampco 18 and other non-nickel aluminum bronzes. (Note that this is not always possible for large-diameter and nonround mandrels for which nickel aluminum bronze is not readily available.) Similarly, be sure that a hard-chrome-plated steel mandrel is made of a through-hardened alloy steel and not a case-hardened low-carbon steel. Also, the plating must be an industrial hard chrome and not a buildup of decorative chrome, which quickly chips and flakes off.
Like mandrels, aluminum bronze and steel have been the most commonly specified materials for wipers. Aluminum bronze is best for ferrous and titanium tubing, and steel for nonferrous tubing. Soft grades of these materials are preferred because otherwise the finely tapered feathered edge of the wiper will be brittle and likely to break prematurely. Good choices are Ampco 18 or similar grade of aluminum bronze or an unhardened steel. The most significant exception to this rule involves bending aluminized or galvanized steel tubing. For mitigating the buildup of debris from these coatings on the bore of the wiper, a hard-chrome-plated alloy steel is a good alternative to heavy lubrication or routine manual brushing of a soft bronze or steel wiper.
As with mandrels, the most important factor in extending wiper tool life is setup, not material specification. The key is whether the wiper can be raked or must be set at zero rake. Raking the wiper usually lengthens its life, because less of the wiper bore is used to prevent the terminal hump from forming at the end of the bend. However, raking is not always possible. Some rigid materials, like many stainless steels and hard coppers, require full tube containment at the point of bend and so the wiper must be set at zero rake. In such cases, a wiper with an offset feathered edge is necessary.