VTube Reverse Calc from MIL-D-9898C Absolute Bender Data to Centerline XYZ Data

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This military spec bender chart data chart can be converted by VTube-STEP and VTube-LASER in the LRA grid section of the Part Data menu.

This specification was created decades ago when CONRAC benders were popular with the US Air Force. It was made inactive according to NOTICE 1 on January 21, 1986. No new designs are created with this spec.

CONRAC benders have been obsolete for decades, but legacy tube shape data is still stored with this specification.

Bendxyz militaryspec.jpg

Vtube mil-d conversion.png

General Principals for the Specification


This bender data spec uses ABSOLUTE LENGTHS between bends as if they were derived from a tape-measure attached to the Feed axis of a bender. CONRAC benders sometimes had tape measures riveted along the rail of the carriage that moves along the length of the bender. The lengths indicate where each bend begins.

Tape measure.png


According to the spec, the bender data uses ABSOLUTE ROTATION data (twist angle between the planes of the bends). It is absolute because it always depends on the rotations in all preceding bends accumulated. The dial on an absolute rotation spindle reads from 0 to 360.

If a rotation does not change from one bend to the next, then the rotation value remains at the degree position that it was at in the previous bend. For example, if a rotation is at 270 degrees, and the next bend has no rotation relative to this bend, then the rotation dial stays at 270 for the next bend. Every rotation value depends on the proceeding rotation values. This is why it is referred to as "absolute".

Conrac rotation dial.png

Mil-d-9898c rotation dial illustration.png


Although it seems to be ambiguous in the specification, most of the part data from this specification assumes DRAW bending versus COMPRESSION bending - which means the LENGTH values move the carriage so that the tube is positioned at the start of each bend (for clamping and drawing around the bend die that rotates with the bend arm) rather than the end of each bend (for clamping with rollers or wipers and compressing around the bend die that does not rotate with the bend arm).

Conrac bender horizontal.png

VTube Absolute LRA Data Rules

  • The Absolute LRA grids are in both STEP and LASER modes of VTube. They are automatically calculated when you import a model or enter centerline XYZ data.

  • You can Reverse Calc to XYZ data from Absolute LRA data by entering data in the Absolute LRA grids in either VTube-STEP and VTube-LASER. When entering data to reverse calc, be sure to keep the mouse in the grid while you are entering new data to prevent the XYZ to LRA calculator from overwriting your entries..

  • The LAST length value is always the actual length of the last straight of the tube. So this value has a different meaning than the rest of the L values in this grid.

  • All other lengths should progress from larger values at the top of the grid, and then decrease as you move down the grid.

  • None of the lengths should be larger than the Cut Length - or negative incremental length values may be calculated.

  • The rotation values are always within a range of 0 to 360 degrees. If you enter a negative rotation value, then it will automatically be converted to its equivalent in the 360 range when you reverse calc. For example, -45 degree will be converted to +315 degrees (360 + -45)

Vtube v2.7 absolute lra.png

Vtube v2.7 absolute lra tubeshape.png

Reverse Calc with Absolute LRA Data Requires an Accurate Cut Length

Reverse calculation requires an accurate cut length, so this dialog will display to allow you to enter a correct Cut Length before VTube finishes the calculation. This Cut Length value will overwrite the value in the Part Setup menu.

Remember that the Cut Length value should be a larger value than the largest LENGTH value in the Absolute LRA grid.

Vtube v2.7 absolute lra cut length.png

Calculating the Last Length "LL" value

Notice in the MIL-D-9898C bender chart that there is no LAST LENGTH value. However, VTube includes a LL (Last Length) value in the last row.

You can calculate the LL value by using a popup menu for the Absolute LRA grid only.

Rules for calculation:

  • 1 - The LENGTH value in the preceding row must already be entered.

  • 2 - The BEND ANGLE in the preceding row must already be entered.

  • 3 - The RADIUS value in the preceding row must already be entered.

If these three conditions are true, then you can use this feature to accurately calculate the LL value in this chart.

Mil-d-9898c bend chart with data.png

Vtube v2.7 calc lastlength absolute.png

Mil-d-9898c vtube absolute bend data sample.png

Example of Switching from CW to CCW Bend Angles

This bender protocol assumes CLOCKWISE as the default bending direction. However, sometimes the part designer creates a tube shape where the bend hand changes from CW to CCW in the middle of the part. See the changing of bend hands in the two images on the right.

- The image with the blue arrow shows the standard CW bending.
- The image with the yellow arrow shows CCW bending.

Changing from left to right or right to left bending changes the rotation plane by exactly 180 degrees inside VTube.

So here are some rules to follow in converting the rotation data to be compatible with VTube:

Rule Idea Rule Explanation
Add or Subtract 180 to the Rotation if the Bend Hand Changes to CCW The rotation at that bend must change by 180 degrees to take the bend plane direction change into account.
Valid Rotation Value Range The rotations for this protocol are based on a 0 to 359.99-degree dial. Rotations are always a positive number in that range. Whether you add or subtract 180 depends on which method (adding or subtracting) gets you a positive number inside the valid positive range. (The only time rotations exceed 360 is for coiled parts - which is a special case for this protocol.)
Propagate Rotation Changes When the bend hand changes to CCW, it is necessary to also change all following CCW bend rotations by 180 degrees.
Propagation Stops When However, the propagation of changes must stop when you come to a bend that switches back to CW. So, in the example case below, the last rotation is never changed.

Mil-spec cwbend angle.png Mil-spec ccwbend angle.png

Apply the rules above to an example.

Case Handling
Bending switches to CCW at bend 3. Adjust the bend 3 rotation of 216.9-degree rotation by 180 degrees.
Determine +180 or -180 216.9 + 180 = 396.9, and 216.9 - 180 = 36.9
The answer is 36.9 because it fits within the 0 to 359.99 range..
Bend 4 is still CCW, so change the rotation by 180 Change the rotation from 306 to 126.
Bend 5 is still CCW, so change the rotation by 180 Change the rotation from 359.7 to 179.7.
Bend 6 is CW, so stop changing rotations The bend hand switches back to CW, so leave the rotation unchanged.

Mil-spec cw ccw cw bend angle.png

Mil-spec cw ccw cw bend angle in VTube.png

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