Endforming Stainless Steels
Success depends on analyzing the workpiece, tooling, and machine design
By Herb Friedrich

Charger®. Mustang®. Camaro®. Challenger®. Barracuda®. Regardless of which muscle car is your favourite, they all share a few characteristics. Masculine designs. Distinctive paint jobs. Sporty, racing-style interiors and accessories. And much more horsepower than you need to go get some groceries or run the kids to soccer practice.

The unsung heroes then, as always, were the exhaust systems. A well-designed exhaust system adds to the car's aesthetic appeal, enhances performance, and gives it a distinctive, signature sound. Forget about manifolds, pipes, and mufflers. The key to developing a deep, throaty roar is designing and manufacturing a good resonator. As the word resonator implies, this component’s job is to develop frequencies that resonate—that is, strengthen and boost the sound developed by the engine and other exhaust components. A muscle car should do more than go fast and look good. A muscle car should sound great, too.

Single-piece tubular resonators made from 409 stainless steel, which are increasingly popular, can present a unique challenge. Complicating matters is when the resonator design requires them to be stuffed with internal components before they undergo any end forming operations.

Analyzing every factor—the workpiece alloy, the ram tool material, the tool design and the machine design—can go a long way in ensuring that the processes turns raw materials into successfully formed resonators.

Reducing Challenges
Reducing stainless steel tube can be difficult especially when reducing the tube ends a large amount from the parent diameter. The characteristics of the alloy, the quality of the weld, and the properties of the heat-affected zone (HAZ) all factor into how much the tube can be reduced under production conditions.

The most common failure in the reduction process relates to the weld seam. Either the weld seam folds into the inside diameter of the tube during the reduction process or the weld seam shows signs of cracking. This is because, during the reduction process, the parent material and the weld seam elongate at different rates.

Fabricators might find that 409’s characteristics vary significantly from one supplier to the next, often enough that the tube from one source reduces significantly better than tube from another source. The lesson? Don’t be afraid to experiment with materials from different suppliers.

The material is only one factor in the process. The specific ram reduction tools used are equally important. Careful consideration must be given to the design of these tools, including the transition angle (the angle between the starting tube diameter and the target diameter for the particular operation), the material the tool is made from, heat treatment, surface finish, and the tool surface coating to make the reduction process a success.

Other factors that affect the forming process are the equipment’s rigidity and lubrication.

Ram Tool Design
The design of a ram tool for a specific application is relatively straightforward. For stainless steel, at least 70 percent of the total reduction has to be formed in the first operation. The transition angle must not be too steep or too shallow. Too steep an angle results in either the column of the tube failing (the tube collapses) or the forming operation stalling because the machine does not have enough force to complete the stroke. If the angle is too shallow it can lead to difficulty in stripping the reduced tube from the ram. The tube and the ram tool can become locked together.

The transition angles of the resonator design are the starting point for the tool design. If the initial design doesn’t form the specified or supplied tube successfully, the tooling designer must develop a design that does work.

A recommended material for an application like this is D2 hardened to 59/60 Rc. Applying a thermal diffusion (TD) coating and polishing the tooling provides a low coefficient of friction that improves material flow, increases tool life and reduces galling on the tool from the ram forming process.

Manufacturing a ram from tool steel is acceptable for part trials and preproduction part runs to ensure the finished sizes and forms produced from the tooling are acceptable. For high-volume production, parts that require high forming pressures, or for parts that cannot use a flood-type lubricant, carbide tooling is recommended because it will hold up much better under difficult forming conditions. Carbide tools are made only after the tool steel rams had been developed and proven.

A recent case study for this exhaust application, the resonators had an internal tube wrapped with fiber-type insulation, so a flood-type lubricant could not be used and the production requirement was very high. The tool geometry was developed with tool steel made from carbide recommended for the final production tools. An alternate method, swabbing the outsides of the tube ends with a lubricant, was used to aid the forming process.

Machine Design
The type and size of the forming machine recommended for an application depends on the production volume, tube size, the number of operations required to form the part, the forces required to hold the part while forming, and the force required to form the part.

Typical ram forming machines such as the Eagle model E, EF or F machine were not selected because they did not offer the flexibility to have a high tonnage large vise and multi shuttle forming head.

For this application, the recommendation was an Eagle model CFR 40 high tonnage C frame machine. This machine offers an opening large enough to accommodate a two-cavity clamp so that two tube ends could be formed simultaneously. This machine also has easy access for manual loading/unloading and provides the tonnage required for clamping the tube and the ease to fit a multi station shuttle head.

This application required three reduction operations to form the part to the point where the inner tube (with the insulation) and the reduced outer tube could be positioned and staked (locked) together in a secondary process. The machine had a four-station tool shuttle (one station to position the tubes in the vise cavities before clamping and three stations for the reduction operations). This tool shuttle was large enough to hold the four tools for each of the two tubes being formed. An additional feature in the dual-cavity vise jaw was a mechanism to free the clamped tube from the vise cavity to allow the operator to remove the formed tubes easily.

Herb Friedrich is the tooling manager for Eagle Precision Technologies Ltd.  Email hfriedrich@eaglept.com

An Eagle C-Frame machine.

The vise section of a C-Frame machine

Eagle multi operation ram tooling