July 16, 2012
Bad bends and abnormal tooling wear can result from a number of factors, two of which are worn machine components and work durable tooling. Check these areas first to identify whether your operation is overlooking a hidden problem.
Bending pipe and tube often is described as a black art, which characterizes the precarious nature of quality and repeatability. The myriad factors that can affect bending do so by impeding efficient production, compromising setups, shortening the time required between machine adjustments, or compromising the aesthetic appearance or strength of the finished product.
Tube bending isn’t a black art, however; it’s a science, and troubleshooting is a matter of understanding the underlying mechanical principles. It’s important to start troubleshooting where trouble is most likely to start, searching for simple causes. Overlooking simple areas before evaluating more complex factors can be costly.
Three main areas of focus are the consumable tooling (mandrels and wipers), machine precision, and durable tooling. Bending machine operators often adjust the positions of the consumable tooling to compensate for wear and tear on the machine and durable tooling. Adjustments to the consumable tooling are short-term fixes but often mask underlying problems.
As anyone who has been bending for any length of time can attest, consumable tooling quality is the easiest place to identify a problem and normally the cheapest short-term fix. However, over time other factors can play an increasingly larger role, such as machine wear and durable tooling quality. Most operators compensate by making adjustments to the consumable tooling, which then has to shoulder a larger share of the burden to get good bends.
Simple steps that can improve bend quality include moving the wiper tip closer to tangent; adjusting the rake of the wiper to provide more tube support; moving the mandrel forward to allow the nose to form the outside radius of the bend; and reducing or eliminating the clearance between the tube ID and mandrel (or mandrel balls) to provide more support. All of these solutions can get you through the end of a run and usually are the most economical way to do so.
In the long run, relying on these shortcuts to produce good bends consistently can result in a much higher cost of operation. This isn’t much different from driving a car or truck that wears out a set of tires every 10,000 to 15,000 miles, rather than at 40,000 to 50,000 miles as you’d expect. You could ask a mechanic to look for the root cause, such as poor wheel alignment or a badly worn suspension, or you could just keep buying more sets of tires. Just as the car’s poor mechanical condition can lead to premature tire wear, a loose machine head or oversized groove on an old pressure die can lead to poor bend quality. Making adjustments to the mandrel or wiper puts additional stress on these components, causing them to wear faster and possibly break.
Identifying when you’ve reached that point at which the durable tooling or machine is at fault can be a challenge, especially because this problem usually starts small and develops slowly over time. In addition, some bender operators fear exposing problems that may not be repairable easily. This is similar to replacing the vehicle’s tires; do you really want to find out that you need an expensive suspension rebuild? Confronting the issue head-on takes courage and perseverance.
While it is impossible to go through all the different factors in an article, following are some simple, commonly overlooked areas to review.
Solid, concentric operation is the foundation of quality bending. Because of this, a sloppy machine can override any other factor. Often new tooling can compensate somewhat for subpar machine condition, but as the bends get more difficult (tighter radii, thinner walls, or shorter grips) this problem is much tougher to overcome. Checking your machine with a simple dial indicator can identify most of the issues that affect bend quality.
Concentricity (Runout) Without Load. Runout is a term used to describe a lack of concentricity as the bend head rotates. This is caused as the machine is cycled under load repeatedly, wearing the bend head bearings and the bend head shaft. As the bend head rotates, if the bend head is moving away from the original grip position or pressure die, the bend die can separate or lose its ability to capture the tube to bend at tangent. This can also lead to issues with constant pressure die force and wiper die separation.
To check runout, set a dial indicator on a stationary part of the machine, such as the pressure die mount, and set the indicator tip on the counterbore of the bender at the point of tangency. Rotate the bend die and watch the dial indicator through the rotation. Ideally, the total indicated runout (TIR) should be less than 0.003 in. on a new or rebuilt 3-in.-OD machine, and up to 0.005 in. on a 5-in.-OD machine.
Concentricity (Runout) Under Load. Sometimes concentricity will be within specification on a machine while dry cycling (no load), but show too much runout when bending. To check for this, perform a bend under heavy clamp and drive force. Locating a spot to mount the dial indicator is a little more difficult because you’ll have machine parts and tooling in motion to avoid, and you will be unable to use the counterbore as your indicator point. You’ll have to locate your indicator tip on the toolpost or possibly the far side of the bend die groove.
Bend Die Deflection. To maintain adequate containment of the bend in the tangent area, the bend die must not deflect away from the pressure die during bending. If the bend die does not remain flat and seated on the bend head and perfectly opposed from the pressure die groove, its grips will lose their effectiveness and the groove itself may not track true to the rotation of the bend, potentially wrinkling the side of the tube.
To check bend die deflection, put an indicator on the top face of the die in an unclamped position, then clamp up on a tube with heavy force to see if the bend die rises up from the bend head or if the bend head deflects. An overhead tie can help to reduce bend head deflection. This check can also help to measure flatness or perpendicularity, which should have no more than 0.003- in. variance.
Pressure Die Travel. During the bend process it is important that the pressure die tracks perpendicular to tangent. Just as the bend die must not move away from the pressure die, the pressure die must not move away from the bend die or toward it. If the machine has a force-controlled pressure die control, this normally is not an issue, because the pressure die will maintain a steady amount of force on the tube as it is being pulled through the bend.
If the pressure die is position-controlled, it is put into a set position relative to the tube and has no force control to maintain a preset pressure against the tube through the bend. This is where misalignment can affect the bend quality greatly.
The pressure die groove should remain a constant distance from the center of the bend. To measure this, put a dial indicator at the bottom of the pressure die groove at the wiper die mount or another static point on the bender, and dry-cycle the machine with only the pressure die. The TIR should not exceed 0.010 in. Some old machines are not able to achieve this; the linear bearing guides typically used on machines these days are more precise.
Bowing can also occur at the pressure die follower as well.
Only after you are sure that the machine is in good working condition should you check the condition of your durable tooling (bend die, clamp die, and pressure die).
Bend Die Groove. A worn bend die groove can fail to contain the tube properly in the bend. Verify that the bend die groove is sized to its specification all the way around the bend area, including the area where the wiper die would be at the termination of the bend. The best way to do this is to have a ring gauge turned to allow you to see not just the size of the groove, but also the shape of the groove.
It is usually impossible to measure the groove accurately using a pair of calipers because of the radius that exists to prevent tube marking. Also, high-volume bending can create wear only in certain areas of the groove that calipers alone cannot detect. While it is possible to recut the groove on some bend dies, this normally shrinks the CLR and often costs close to the original price of the die. Grooves should normally be within 0.002 in. overall of the tube OD for most bending applications, though wall factor and D of bend can warrant different tolerances.
Grip Groove. In many circumstances, high-strength material wears down the grips and they lose their effectiveness because of out-of-roundness, friction loss, or both. In some situations, fixing a worn grip can be as simple as returning it to a tooling manufacturer for regripping (by tungsten carbide application or thermal spray), or by gritblasting the surface to return it to its original state. For bending soft material, gritblasting also can remove some of the tube material that tends to remain in the grip area, which also reduces the grip’s effectiveness. If grips are serrated or knurled, replacement is normally the best option.
Bend Die Concentricity. Just as an out-of-round bend head can affect bend quality, an out-of-round bend die can have exactly the same effect. To check this, put the indicator at the lowest point of the groove and rotate the bend die. Another check is to place the indicator tip at the median point of the groove (CLR) or the interlock step, although this can be compromised by machine collisions and is also not typically a critically held dimension in the tooling process. Bend die TIR should not exceed 0.003 in. for difficult bend applications.
Pressure Die Groove. To check the pressure die groove, use a ring gauge as you did for the bend die groove check. An oversized pressure die groove can allow wrinkles to form, especially when the worn area is over tangent. Check the entire length of the pressure die.
If the machine and tooling are in good working order, other pitfalls include variances in the tube material and the lubricant. It is common for a new batch of material to lead to sudden changes in the bending process. Material variances can include hardness, wall thickness, and OD, for example. Each of these can have significant impacts on the bending process. The lubricant is also a factor, especially when bending new materials. A lube that may work for many applications may not be suitable for a new material.
These tolerances are a guide for performing quality bends in a normal commercial bending application. Specialized bends, such as thin-walled tube, aesthetic applications, or tight bend radii, will make these factors even more sensitive to variances. It is important to note that this article is a guide to some easily identifiable factors that are often overlooked. A trained tooling consultant or machine specialist can provide more guidance in dealing with tooling and machine issues.
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