Analyzing problems, tracking down solutions
April 1, 2009
Bending tube or pipe successfully is a matter of managing a handful of processes and variables and controlling the flow of the metal. The same principles apply to bending profiles, or rolled steel sections (RSS). However, because profiles aren't closed, they are more prone to distortion than tube or pipe. A close look at an RSS bending job provides a step-by-step approach for tackling this sort of project.
Bending tube or pipe successfully is a matter of managing a handful of processes and variables. For example, the bending process causes thickening where the metal is under compression and thinning where it is under tension. Too much thickening can result in wrinkles; too much thinning results in failure. A key variable is clamping pressure; too little allows the tube or pipe to slip during the bending process, leading to an unsatisfactory bend; too much causes the workpiece to collapse if a mandrel isn't used and wrinkle if a mandrel is used. Problems such as humps and wrinkles have a similar cause: Metal flows into areas where it isn't supposed to go.
Bending a rolled steel section (RSS) has these challenges and one more. Unlike tube or pipe, RSS dimensions can vary from one production run to the next. Successful bending then is a matter of understanding all of these factors, processes, and variables and designing tooling that contains the metal so it flows where it is meant to go.
A garage door track is one example of an RSS (see Figure 1). Garage door track typically is bent to a 12-inch or 15-in. inside radius (ISR). A new design, and a new challenge, called for a 6.25-in. ISR. Of course, the track manufacturer also required repeatable, high-quality, and aesthetically pleasing bends.
Six components comprise a complete bending tool set for this type of profile (Figure 2). This project started with four tools: a bend die, clamp die, pressure die, and mandrel.
The tooling must form the RSS to the specific angle while preventing it from moving freely within the bend die. The key to producing a quality bend is to maintain support on all exterior surfaces of the RSS. Unlike tooling for round tubing, in which the dies are designed for the exact tube diameter, tooling for RSS requires a little more tolerance (additional clearances). If the clearances aren't sufficient, the fit will be too tight and loading the track into the bender will be difficult; if they are too loose, the material will move into areas that are not part of the profile, become wedged, and prevent the workpiece from releasing after bending. Furthermore, material buildup can lead to slipping, humps on the inside radius, and more severe problems. Another key factor is the mandrel's design. Garage door track mandrels must have a special type of linkage that allows the balls to flex about the plane of bend without twisting.
Determining proper tooling dimensions requires two steps. First, measure track samples from several production runs; then compare the measurements with the print. This reveals the locations and extent of the dimensional variations. Second, machine the track's profile into test blocks; insert the track into the test blocks and check it two ways—visual inspection and dimensional measurement—to verify a good fit, neither too tight nor too loose. In this case, the fit was suitable, so the next step was to design and manufacture the tool set.
For the garage door track application, only the bend die and wiper die need to be designed around the track's profile. Typically, the bend die and wiper die have just 0.010-in. clearance. For this application, the dies were machined approximately 0.030 in. larger than the profile to accommodate dimensional variations.
The first bends weren't successful; the track slipped as soon as the mandrel exited the bend area. In other words, the slippage occurred because the RSS was not supported internally throughout the clamp section. The slipping caused wrinkles to form throughout the bend area and a hump to form at the end of the bend (see Figure 3). In addition to preventing quality bends, slippage can mask many other problems.
It's rare that a single troubleshooting effort resolves every bending problem. This project, mainly because the bend radius was so much tighter than normal, required three attempts.
Phase I: Add Tooling. A clamp plug was added to maintain the track profile during initial clamping and to prevent slippage during the bend cycle. Slippage is a result of too little clamping pressure; collapsing is a result of too much pressure. A clamp plug provides a solid surface to which an adequate clamping force can be applied. It allows you to increase the clamping pressure without crushing the workpiece. To be effective, the clamp plug and ball strand must be as long as the clamp section.
The clearance of the mandrel and clamping plug are the same whether working with round tubing or RSS: 0.003 in. to 0.010 in. This leaves just enough clearance to allow easy loading and provide enough support to prevent the piece from collapsing.
The clamp plug eliminated the slipping, but the bends still were not satisfactory. A hump formed behind the tangent point of the bend, and slight indentations appeared between the ball segments. This is a sign that the mandrel's pitch—a distance between the ball segments—was too great. The garage door track was also twisting on the mandrel, indicating that the track was not fully contained.
Phase II: Redesign the Mandrel. The mandrel was redesigned to have a smaller pitch and additional segments (to maintain the length). This eliminated the wrinkles, but did not eliminate the hump (see Figure 4).
Third Time's a Charm: Add a Wiper Die. Adding a wiper die was the obvious remedy for the hump. The wiper die for this project was the first one of its kind—a combination wiper die and material guide. It contained the profile and engaged into the bend die, offering full support of the garage door track during the bending cycle. The wiper die works with the bend die; as the name implies, it uses a wiping action to eliminate humps (see Figure 5).
The knowledge associated with RSS applies to many other specialty applications. These simple steps help in designing, developing, and implementing a bending process, and can lead to a successful bend, regardless of the challenges.
Dawn M. Whims is a design engineer and John C. Miller Jr. is a senior product engineer for Pines Technology, 30505 Clemens Road, Westlake, OH 44145, 440-835-5553, www.pinestechnology.com.