BEND TOOLING INC.:  Rotary-Draw Tube-Bending Tools ~ Die Sets ~ Mandrels ~ Wipers ~ Mandrel-Bending Tools

Troubleshooting

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Step #1 | Step #2 | Step #3 | Step #4 | Troubleshooting


Troubleshooting

 

One of the advantages of the four-step set-up procedure just described is that it facilitates troubleshooting.  Most defects in a tube bend can be traced to the set-up, and most set-up defects correlate with problem in one of the four steps.  Therefore, once you have identified the nature of the defect, you will have a specific set-up parameter you should examine first.

 

Mandrel Nose Problems

 

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Ovality (i.e., general deformation of the tube's cross-section) is excessive.  Check if the mandrel nose is undersized or not placed deep enough into the bend according to the instructions under Step #1.  If undersized, a temporary fix may be to advance it deeper into the bend.  However, optimal bending will require a new mandrel made to the correct nose diameter. 

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The inside radius buckles.  Check if the mandrel nose is placed behind the line of tangency.  If so, advance past tangency according to the directions for Step #1

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The outside radius collapses.  Check if the mandrel nose is placed behind the line of tangency.  Advance past the line of tangency according to the directions for Step #1

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A hump or humps form on the outside radius.  This is usually not because the mandrel nose is too deep into the bend, but because there is excessive drag or insufficient assist from the pressure die.  See below for details.  However, if you do suspect the mandrel nose is the problem, check the depth of its placement and compare it to the first formula for Step #1.  If the mandrel nose placement is too deep according to this formula, then retract it. 

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Drag is excessive.  This is not a defect but an immediate cause of many defects.  Too much direct pressure-die pressure is usually the culprit, however, an oversized mandrel nose can be the problem.  Check if the mandrel nose diameter is too large by using the second formula for Step #1.  If so, the mandrel will have to be replaced.

 

 

Direct Pressure Problems 

 

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Continuous wrinkling of the inside radius.  If the entire arc of the inside radius is wrinkled, this indicates that the direct pressure-die pressure is too low.  Use the guidelines under Step #2 to determine the proper pressure.  Note that this defect is distinct from a single hump or a small series of humps forming on the inside radius at the end of the bend.  This type of wrinkling is associated with the wiper die.  (See below.) 

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Excessive flattening of the outside radius.  A very common problem that results from too much direct pressure die pressure.  In effect, the pressure die is clamping on the tube at the point of bend causing the outside radius to stretch and flattening between the pressure die and the clamp die.  Reduce the pressure according to the directions under Step #2.  If the mandrel nose is properly placed and the direct pressure is correct and flattening is still too much, then the assist pressure should be increased.  (See below.)

 

Wiper Tip Problems 

 

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A hump or humps form on the inside radius at the end of the bend.  As described in Step #3, the role of the wiper is limited.  These humps are the only problem the wiper is designed to solved, and these humps only occur if the wiper is not raked correctly or is worn out.  Decreasing the rake will eliminate this probem.  See Step #3 for the best way to set the wiper tip rake. 

 

 

Assist Pressure Problems

 

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Excessive flattening of the outside radius.  If excess direct pressure has been eliminated as a source of this defect (see Step #2), then increase the assist pressure according to Step #4

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A hump or humps on the outside radius.  Respond to this in the same way as to excessive flattening if mandrel nose placement is correct.  (See Step #1 for mandrel nose placement.)

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Excessive wall thinning.  If ovality and flattening are under control, then increase the assist pressure according to Step #4

 

Other Sources of Difficulty

 

While the set-up is most often the source of a bending problem, other factors can cause trouble.  If you have double-checked yourself and have not discovered the problem in the set-up, consider these possibilities:

 

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The machine is not applying pressure consistently.

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The machine is not lubricating the tooling properly.

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The tools are worn out.

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The working surfaces of the tools are mismatched or dimensionally incorrect for the bending application.

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The tubing material is undersized, oversized, or the wrong wall thickness.

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The tubing material is too hard or too soft.


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