March 8, 2005

Understanding the relationship between the elevation of the entrance guide and the shape of the roll formed strip is crucial to satisfactory roll forming. The entance guide elevation, if improperly set, can cause the strip to bow and twist as it leaves the first forming pass. Setting the elevation properly can help to eliminate additional stress and strain at the strip edges that cause bow and twist.

Fig. 1shows elevation of the entrance guide affects the sectional curvature. Fig. 2shows an Edge Bent Down Model. |

In a roll forming line, the entrance guide serves to guide the metal strip into the roll former at the optimum angle and elevation.

To achieve the correct feeding angle, the entrance guide should be parallel to the metal forming direction. Any horizontal rotation will push the strip off-center horizontally. The correct elevation can eliminate additional stress and strain at the edges of the metal strip and can reduce the distance between the entrance guide and the first pass.

**Figure 1**illustrates the relationship between the elevation of the entrance guide and the sectional curvature before and after the first pass. In the top drawing, the entrance guide is too high. The metal may curve up after coming out of the first pass. In the bottom drawing, the entrance guide is too low, which could cause the metal to curve down after coming out of the first pass. When the entrance guide is set at the correct elevation, as shown in the middle drawing, no additional elongation occurs at the edge section, and the metal will be flat out of the first pass, without bow or twist.

A calculation method is available to estimate the best elevation of the entrance guide. This method is based on both the bending corner

AD and the side edge BC having the same traveling distance from the entrance guide to the first pass—in other words, AD and BC are the same length (see**Figure 2**).

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Figure 2 shows the edge bent down model. The equation when AD and BC are the same length is:

For example, assume the width of channel side L is 0.800 inch, and the channel side is formed 17 degrees at the first pass. The elevation of the entrance guide H should be calculated by Equation 1 as the edge bent down model.

Thus, the elevation of the entrance guide H should be 0.114 in.

**Figure 3**is a table of coefficient (H/L) for the edge bent down model. For example, assume the width of channel side L is 0.800 in. and the channel side is formed 25 degrees at the first pass. From Figure 3, when the forming angle a is 25 degrees, the coefficient (H/L) is 0.201.

The elevation of the entrance guide H, then, should be set to 0.161 in.

From Figure 3, when the forming angle a at the first pass is 90 degrees, the elevation of the entrance guide should be zero in height.

**Figure 4**shows the edge bent up model. The equation when AD and BC are the same length is:

Fig. 4shows Edge Bent Up Model. Fig. 6shows an Arc Edge Bent Up Model. Fig. 8shows an Arc edge Bent Down Model. |

For example, assume the width of channel side L is 0.800 in., and the channel side is formed 17 degrees at the first pass. The elevation of the entrance guide H should be calculated by Equation 2 as the edge bent up model.

The elevation of the entrance guide H, then, should be 0.120 in.

**Figure 5**is a table of coefficient (H/L) for the edge bent up model. For example, assume the width of channel side L is 0.800 in., and the channel side is formed 25 degrees at the first pass. From Figure 5, when the forming angle a is 25 degrees, the coefficient (H/L) is 0.222.

The elevation of the entrance guide H, then, should be set to 0.178 in.

From Figure 5, when the forming angle a at the first pass is 90 degrees, the coefficient (H/L) is 1.000. The elevation of the entrance guide H should be L (H = L) in. in height.

**Figure 6**shows the arc edge bent up model. The equation when AD and BC are the same length is:

For example, assume the forming angle is 17 degrees, with a 0.500-in. radius at the first pass. The elevation of the entrance guide H should be calculated by Equation 3 as the arc edge bent up model.

The elevation of the entrance guide H, then, should be 0.011 in.

**Figure 7**is a table of coefficient (H/R) for an arc edge bent up model. For example, assume the formed angle is 45 degrees, with radius 0.8 in. at the first pass. From Figure 7, when forming angle a is 45 degrees, the coefficient (H/R) is 0.157. The elevation of the entrance guide H is:

The elevation of the entrance guide H, then, should be set to 0.126 in.

**Figure 8**shows the arc edge bent down model. The equation when AD and BC are the same length is:

For example, the forming angle is 17 degrees, with a 0.500-in. radius at the first pass. The elevation of the entrance guide H should be calculated by Equation 4 as the arc edge bent down model.

The elevation of the entrance guide H, then, should be 0.011 in.

**Figure 9**is a table of coefficient (H/R) for arc edge bent down model. For example, assume the forming angle is 45 degrees, with radius 0.8 in. at the first pass. From Figure 9, when the forming angle a is 45, the coefficient (H/R) is 0.136. The elevation of the entrance guide H is:

By ensuring your entrance guide is set to the correct elevation, you can reduce the stress and strain on formed parts and reduce the longitudinal elongation differential in the cross section. The correct elevation of the entrance guide will form metal flat out of the first pass. The straight metal can easily self-thread, getting into the second pass without any assistance.

The entrance guide isn't a direct forming tool, but as the starting point of the forming process, it is a very important accessory. A good start will make the whole forming process run more smoothly.

*Hanhui Li is senior engineer with Worthington Armstrong Venture (WAVE), 9 Old Lincoln Highway, Suite 200, Malvern, PA 19355, 610-722-1232, fax 610-722-1246, hanhui_ li@armstrong.com.*

*Kuang-Xu Li is senior physicist at Wuhan High School, Liang-Dao St., Wu-Chang District, Wuhan, Wubei Province, P.R. China, 86-27-8884-5737.*

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