n most cases, unless otherwise specified, the radius of a bend refers to the inside radius, which is the radius of the curvature on the inside surface of the material being bent. This is a standard reference in many bending operations.

Here’s why the other options are not typically correct:

• Option 2: The inside radius plus 1/2 the thickness of the metal would describe the location of the neutral axis, not the bend radius itself.

• Option 3: The outside radius is generally not used as the standard reference for bend radius unless specifically noted.

• Option 4: The neutral axis is an imaginary line in the material that doesn’t stretch or compress during bending, and the radius of the neutral axis plus the thickness is not a typical way to specify bend radius.

In aviation and metalworking, the inside radius is a common measurement for describing the bend in a material.

In bending operations, the neutral axis is the location within the material where there is neither tension nor compression during the bend. It’s typically located a bit away from the inner radius and closer to the inside surface of the bend. For practical purposes, when precise bend allowance tables are unavailable, it’s often estimated that the neutral axis is located about halfway through the material thickness from the inside of the bend. This is why adding the stock thickness (or a fraction of it, depending on the specific material properties and bend angle) to the bend radius provides a useful approximation for locating the neutral axis.

Here’s why the other options are not correct:

• Option 1: Subtracting the stock thickness from the bend radius wouldn’t give the location of the neutral axis; it would place it incorrectly towards the inner surface.

• Option 2: The point where the bent and unbent sections meet is simply the bend tangent, not the neutral axis.

• Option 4: The neutral axis doesn’t represent the actual length of material required for the bend but is more about where the material experiences no stretching or compression during bending.

Therefore, adding approximately the stock thickness to the bend radius is the best estimate for the neutral axis when computing

Reason:

When bending aluminum alloy, it is important to consider the grain direction of the material. Aluminum, like many metals, has a grain structure resulting from the rolling process used to form sheets. If the bend is made parallel to the grain, the material is more likely to crack, especially when bent to a minimum radius. Bending 90 degrees to the grain reduces the likelihood of cracking, as the material will have more uniform strength in this orientation.

Here’s why the other options are not the best practices:

• Option 1: Bending slowly does not necessarily prevent cracking, especially if the bend is made along the grain of the material. The key factor is grain orientation rather than speed.

• Option 2: Less pressure during bending is not advisable, as insufficient pressure may not form the bend properly. The focus should be on proper pressure for the specific bend.

• Option 3: Moving the clamping bar back from the working face could lead to an inaccurate bend and is not related to preventing cracking.

Thus, aligning the bend 90 degrees to the grain of the sheet is the most effective method for preventing cracks in aluminum alloy during bending.