Fred Hou of Universal Tool and Engineering confirms the carriage position (Y axis) by measuring from the face of the pressure die to the face of the collet.

One of the most common concerns we hear from our draw-bending customers is that the straight lengths on their parts are inconsistent. This is almost always caused by either poor tooling setup or excessive tooling wear.

Tooling that slips on the part can cause short or inconsistent leg lengths in parts that otherwise look perfect. Too much slipping will cause quality defects in the bend, such as wrinkles, excessive wall thinning, and breakage.

So, how do you know if leg length variations are caused by inconsistency in the machine positions or by tube slippage in the bend tools? Understanding how CNC tube bender axes are measured and set may help to determine if part inconsistency issues are caused by tooling setup or possibly tool wear, allowing the tube to slip through the tools.

In a part formed on a rotary draw tube bender, the bend is a segment of a circle. Assuming a round tube, the circle’s radius is measured from its center point to a point in space at the very center of the tube. Before bending, while the tube is still straight, imagine a string floating in space at the center of the tube, forming a straight line for its entire length. When the tube is nested into the bend die, this centerline is theoretically equal to the circle’s radius at the point of bending, or tangent line of the bend die.

While any circle has an infinite number of tangent lines, only one tangent line defines the point of bending. In tube bending, this straight line from the center of the circle being formed perpendicular to the centerline of the straight path of the tube is the tangent.

If you measure the length of that tangent line from the center of the circle to the centerline of the tube, you will get the theoretical radius of the partial circle being formed during bending, or centerline radius (CLR). This tangent line is the measuring point for linear axes that move in the same direction as the straight tube on most tube benders. Carriage position, mandrel position, and pressure die assist positions are all determined by this tangent line, and pressure die length, clamp length, and wiper length are measured from it. These movements are said to be in the Y plane.

A machine that can bend the tube on different tooling stacks has to be able to move the tube horizontally from the bend die. This horizontal movement is referred to as the X plane and is measured from another tangent line—one that starts at the center point of the circle and runs parallel with the tube. On a machine with servo clamp and pressure dies, their distance is measured from that tangent line.

Depending on the machine design, the tube can be moved in the horizontal (or X) plane by moving either the entire bend head or the carriage. If the carriage is moving in the X plane, the mandrel assembly must move simultaneously in the X plane to prevent the mandrel rod from binding. The mandrel horizontal position is measured from the same tangent line that is parallel with the tube to the center of the mandrel rod, which should be the same theoretical center of the tube.

On a machine that shifts the bend head, the carriage and mandrel assembly (hence the tube) stay stationary, and the Y tangent line moves. However, the measuring points stay the same.

Also, on a machine that can bend a tube on different stacks, the tube must also be moved vertically. This movement is said to be in the Z plane. The measuring point for vertical movement is the lowest point of the bottom bend die to the center of the tube, referred to as centerline height (CLH). Similar to movement in the X plane, either the entire bend head can be shifted vertically, or the carriage and mandrel assemblies can be simultaneously shifted. Some machines also may shift the pressure die and clamp die vertically. Their vertical position also is measured from the same plane created at the lowest part of the bottom bend die.

Fred Hou confirms the carriage horizontal position (X axis) by measuring from the bend die to the tube while the tube is being held in the collet.

On a side note, it is a common practice to have adapter plates manufactured to allow bend tooling designed for one brand machine to be used on a different brand of machine. When this is done, the CLH of the adapted tooling has effectively changed, and the machine must be set up appropriately. For example, tooling designed to fit an Eaton Leonard VB300 has a CLH of 2.0 in. If an adapter plate is manufactured to allow that tooling to fit a SOCO SB80, the thickness of the plate must be added to the CLH when setting up the machine. If the plate is 0.75 in. thick, the new CLH of the tooling is 2.75 in.

But, understanding what is being measured is only half of the battle. How do you actually measure it? We have determined that the center of the circle is the point from which both horizontal (X) and linear (Y) axes are measured and the top of the bend die platen (bottom of the bend die) creates the plane for measurement of vertical axes, so we will call those “zero.” Imagine a plane that extends in all directions at the level of the top of the bend die platen; vertical measurements are taken from that plane. Usually, a bend die has a post running through its center—typically a large bolt that pulls the bend die down into place—so you may hear a technician refer to measuring from the “center of the post.” This just means measuring from the center of the circle being formed by bending.

Zero is center of the post. Some manufacturers use the furthest point from the center of the bend post as their zero position. Knowing the point of measurement used by the manufacturer is essential in determining accurate axis position.

Most of the axes on a machine cannot actually move to the zero point, because with tooling on the machine, they would crash. Also, finding the actual center of a circle to measure from is not very easy.

Fortunately, bend tooling provides a great reference point for measurement. Bend dies are designed and manufactured to a very tight tolerance, so once they are mounted to the machine, the CLR and CLH of the bend die provide a precise measuring reference for determining the horizontal position of the carriage.

With a tube clamped in the collet, after shifting horizontally and vertically until the tube is fully nested in the bend die, the displayed value of those axes should match the CLR and CLH of the bend die. When checking these positions, be sure to use a short tube and bring the carriage as far forward as safely possible to minimize measuring errors. The clamp and pressure die positions can be measured from the CLR to their mounting surfaces (bolsters).

To measure the position of the carriage, take a measurement to the tangent line. usually marked on the bend die to the face of the collets. In theory, when fully returned, the face of the pressure die should close on the tangent line, so this can be an easier point of reference.

Most manufacturers determine that an axis movement is positive in its normal direction away from the zero point of the center of the post. The distance displayed gets larger:

Because of this, if an axis can move past the zero point, it is displayed as a negative number. Some machines are designed to allow the carriage to move past zero to ease loading/unloading, and that position in front of zero would be displayed as a negative number. On a machine with a servo mandrel, when the nose of the mandrel is at the tangent point, it is at zero. As the mandrel is retracted, the displayed number increases. If the nose is moved past the tangent point, which is very common, the display will show a negative value.

But what happens if the bend arm is not at perfect zero? The apparent tangent line marked on the bend die then is no longer the tangent line starting the point of bend, and the linear measurements taken from the marked line are no longer accurate.

Fred Hou of Universal Tool and Engineering confirms the carriage position (Y axis) by measuring from the face of the pressure die to the face of the collet.

So, before measuring any linear axes, make sure the bend arm is at true zero. At zero, the face of the pressure die and the back of the clamp die are parallel to each other. To determine if they are parallel, take two measurements between these dies at different distances from the center of the post. If they are the same, the bend arm is at zero. If the measurement taken farthest from the post is less than the measurement taken closest to the post, the bend arm is said to be behind zero, and the display should indicate a negative number. If the measurement taken furthest from the post is larger than the measurement taken closest to the post, the bend arm is in front of zero, and the display should indicate a positive number.

Let’s return to the original question about inaccurate or changing leg lengths on bent parts.

We know the position of the carriage at loading, and we can determine the correct position of the carriage at the point where the first bend should start. Because of varying elongation during bending, the carriage position may change from one bent part to the next. However, the length between bends—the tangent point from the end of one bend to the tangent point at the beginning of the next bend—should stay consistent.

To test carriage accuracy and consistency, measure the carriage from tangent at the programmed load position. If this is correct, make a scribe mark on a fixed surface on the machine bed against a moving surface at the front of the carriage—for instance, use a marker to trace the face of one of the linear bearings onto the bearing rail. Then let the carriage move to its first bending point and make a scribe mark on a fixed surface on the machine bed against a moving surface on the back of the carriage.

Once you’ve made the marks, have the machine move from load to the first bend about 10 to 15 times, and vary the velocity of the carriage. If the machine consistently hits the marks you made, then the length consistency problems are likely being caused by tooling setup or tool wear.

If it does not consistently hit the marks, you can find the cause of inconsistent length by asking a few questions:

If there is a little movement, you can modify the test to use dial indicators at the front and back of the carriage movement rather than just a scribe line. If the dial indicators show that the carriage is within the servo axis tolerance provided by the bender manufacturer, no further testing is needed.

See More by Jay Robinson

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