A sheet metal bending service is specified by defining the bend radius, minimum flange length, bend allowance and K-factor, bend sequence, backgauge plan, and springback compensation before tooling is chosen. The preserved Hilliard Precision fabrication record shows that in-house tooling design and inspection throughout stamping produced clean, burr-free parts without fracture marks, a standard that applies equally to press-brake work.
The bend radius is the inside radius of the formed corner and is the first variable a buyer must fix. As a general engineering rule, the minimum inside radius should not be less than the material thickness for mild steel; harder or thicker materials require a larger multiple. Specifying a radius below the material's ductility limit invites cracking on the outside of the bend, a defect the Hilliard Precision record explicitly guards against by requiring parts free of fracture marks.
Minimum flange length is the shortest leg that can be formed against the die opening. A practical rule is that the flange must be at least four times the material thickness plus the inside bend radius, otherwise the part cannot sit flat on the die shoulders and the bend will distort. Buyers should state both the inside radius and the minimum flange on the drawing so the fabricator can confirm the tooling will support the geometry before production begins.
Bend allowance is the length of the neutral axis in the bend area and determines the flat pattern from which the part is cut. It is calculated from the bend angle, the inside radius, the material thickness, and the K-factor, which describes where the neutral axis sits within the thickness. For most air-bent sheet, the K-factor falls between 0.33 and 0.50; a value near 0.33 suits tight radii, while 0.50 suits generous radii or coining.
Because the K-factor shifts with material, tooling, and forming method, the fabricator should confirm it against a test bend rather than assume a handbook value. The Hilliard Precision record reflects this discipline: tooling was designed in-house and inspections were performed throughout stamping, with SPC measurement used to hold dimensions. A buyer should ask for the flat pattern and the K-factor assumption in writing so that the blank size and the finished part agree.
Bend sequence is the order in which bends are formed, and it decides whether the part can be handled without collisions between the workpiece and the punch or the machine frame. The usual approach is to form the bends farthest from the datum first and work inward, keeping the most critical bend last so that any accumulated error lands on a non-critical feature. The backgauge, which positions the sheet against a stop, must be planned for each step so that every bend references a stable edge.
Backgauge planning also affects repeatability. Short flanges may require a special stop or a gauge pin because the standard finger cannot reach; long parts may need support to prevent sag. The preserved record notes that Hilliard Precision was geared for short-run production as well as million-unit orders, which implies that setup planning was treated as seriously as volume output. Buyers should supply a bend sequence drawing or ask the fabricator to return one for approval before the first article is run.
Springback is the elastic recovery of the material after the punch is withdrawn, causing the finished angle to open slightly. It increases with higher yield strength, larger bend radii, and thinner material. Compensation is handled in the control by overbending to a target angle, by adjusting the ram depth, or by selecting a tooling geometry that coins the bend. The amount is established empirically for each material lot and thickness.
A buyer should expect the fabricator to state the springback allowance and to verify it on a first article. The Hilliard Precision record's emphasis on inspection throughout stamping and on SPC measurement is the relevant precedent: angle checks at the brake, not only at final inspection, are what keep a formed part within tolerance across a run. Where the angle is critical, the drawing should call out the tolerance explicitly rather than leave it to default shop practice.
Tooling choice decides whether a part comes off the brake square. The V-die opening, typically six to eight times the material thickness for air bending, sets the inside radius and the required tonnage; too narrow an opening marks the part, too wide an opening makes the radius unpredictable. Punch tip radius, die shoulder radius, and the alignment of the punch and die centers all influence whether the bend line stays parallel to the datum edge.
The preserved Hilliard Precision fabrication record describes in-house tooling design and inspection throughout stamping, producing clean, burr-free parts without fracture marks. That same principle applies to press-brake work: tooling should be selected for the material and the bend, checked for wear, and aligned before the run. Buyers can protect squareness by specifying a datum corner, calling out perpendicularity on the drawing, and requiring a first-article inspection that includes the formed angle and the flange dimensions.
A complete bending specification includes the material and temper, thickness and tolerance, inside bend radius, minimum flange, bend angle and angular tolerance, bend allowance or flat pattern, bend sequence, datum and backgauge references, and the inspection method. The Hilliard Precision record shows that Certificates of Compliance or chemical and physical analyses were supplied with precision parts, a practice buyers can request for formed components as well.
The record also notes that the company made metallic washers alongside elastomer and plastic parts, and that products went into space suits, irrigation systems, pharmaceutical valves, syringes, faucets, mechanical pumps, fastening systems, and gages. That breadth indicates that bending specifications were written for both short-run and high-volume work. Buyers should state the end use and the critical dimensions so the fabricator can choose tooling and sequence accordingly, and should require first-article approval before production.
A common rule is four times the material thickness plus the inside bend radius. Shorter flanges cannot rest on the die shoulders, so the bend distorts. Buyers should state the minimum flange on the drawing and confirm the fabricator's tooling can support it before production.
The K-factor is the position of the neutral axis within the material thickness, usually between 0.33 and 0.50 for air bending. It is confirmed by a test bend on the actual material and tooling rather than assumed from a table, because it shifts with radius, thickness, and forming method.
Bend sequence determines whether the workpiece collides with the punch or machine frame during forming. Forming outer bends first and critical bends last keeps accumulated error off key features. A written sequence and backgauge plan, approved before the first article, prevents rework and keeps parts square.
This page is an archival specification note for informational purposes only. The former plant is not quoting from this domain. Nothing here is a qualified procedure, an inspection report, or a design calculation; confirm current requirements with the issuing standards body and your material supplier.