302 Changes that Affect Part Quality
Lot Changes: Once a job is set up and running, material lot changes are the most common problem with part repeatability. There will usually be a difference in material hardness between lots. A change in material lot is noticable by a sudden and consistent difference in each bends' degree of bend (DOB). A hardness change will affect all bends by requiring each bend's DOB to be increased (material got harder) or decreased (softer).
Tool Wear: Changes to parts from tool wear are gradual. As a wiper die tip wears it will show up as an increase in the tendency to form wrinkles on the inside of the bend. As a mandrel wears it will show up as an increase in the tendency for the outside of the bend to flatten or oval. Tooling wear usually has little effect on DOB's.
Oil Warming: This will affect part quality during the first 30 to 60 minutes of operation after a machine is started up cold. Cold hydraulic fluid is thicker (more viscus) than oil at normal operating temperatures. Thicker oil doesn't flow as fast as oil at temperature and can cause DOB's to change while warming up. Many companies have maintenance power up machines 30 minutes before a shift starts.
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