I. Press Brake Condition
Accuracy and repeatability of formed parts are dependent on the press brake structural condition, the ram and bed die mounting surface condition, and the operating controls.
Die mounting surfaces of press brakes that have operated under heavy forming loads, particularly with bottom bending or coining, will require remachining to recover the load bearing integrity and positive crown.
II. Tooling Selection
Tooling is a key factor in determining the quality of the formed parts. Selecting the press brake die in accordance with accepted die opening and punch radius practices will generally result in parts formed to the required geometry.
A die opening that is too small requires greater press brake tonnage, which could result in cracking of the formed part.
A die opening that is too large can result in incorrect definition of the formed part, particularly with dies that have 80 – 85 degree included angle.
III. Tooling Installation
Press brake tooling installation and alignment will affect the repeat part definition.
Install and visually align the die holder and lower die to the punch. (Visual alignment is adequate for many general die set ups.)
A more exact alignment procedure is to equalize the clearance between the angled sides of the punch and the sides of the die, using feeler gauges. This is recommended for die set ups where formed part accuracy is a process requirement.
IV. Tooling Condition
Press brake tooling should be maintained in good condition and stored in a tool rack when not in use.
Any clearance between the press bed, die holder and die, due to worn/damaged surfaces or bent tooling, can cause the die assembly to flex under load. This will result in variances in the formed parts.
Tooling should be straightened so that the assembly lies flat on the press bed with no air gap between the press bed and die holder or the die holder and die.
Check for clearances with a feeler gauge between each component along the length of the press bed.
In localized areas that exhibit gaps due to flexure of the die set, shimming is recommended to correct the condition.
Continuous production of small parts in a localized area of the die set will cause non-uniform wear. Formed full length parts may eventually exhibit an inconsistent bend due to this wear. To correct the situation, either shim under the die holder in the area or remachine the die.
Frequently inspect the dies and have them remachined to correct for wear or damage before repeatability, accuracy and productivity are affected.
V. Material Specification
Material specification is the most overlooked variable that affects the definition and repeatability of the formed part.
Material specifications for any grade of material vary constantly. Typically, lower grade materials have undefined physical and chemical properties, and in the case of steel may be known as “mild steel.” Variations in physical properties, thickness and hardness within a material batch will affect the consistency of the bend angle-from end to end and from part to part.
Ideally, material for a production job should have Mill Certificates to identify that it is from one batch and is consistent in all other regards.
Prior to forming, part blanks should be sheared, punched or plasma cut with the batch material grain oriented in a consistent direction.
VI. Qualified Press Brake Operator
Most new press brakes are equipped with CNC controls. However, the quality of the formed parts and the overall productivity of the press brake will be further enhanced by a well trained and qualified press brake operator, who understands the principles of press braking, die selection, die set up, and part blank material properties.
With traditional press brake controls, setting up a job relied on teach mode or trial bend procedures. The operator learned the setting of the press ram bend position for a particular material thickness and die set, using mechanical indicators.
The operator would then offset the press ram bend position for subsequent jobs, to account for variability of material properties and for die wear.
A skilled operator continually monitors the formed parts and offsets the press ram and backgauge positions, to ensure that tolerances are maintained.
A training program to update operators on press brake safety, changes in material specifications, press brake technology and current manufacturing plant production layout will upgrade personal skill levels. A training program will also promote continuity of knowledge transfer to new workers.