CNC TURRET BASICS FOR SHEET METAL DESIGN

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CNC TURRET BASICS


             CNC Turret presses are used for low to medium quantity production of sheet metal parts. By utilizing standard tool, turrets are suitable for both highly flexible production of prototypes and inexpensive production quantities.

  1.     Tolerances:-         

               Feature to feature tolerance = +/- .004”
               Punched Hole tolerance = +/- .002”
         For dimensioning purpose, datum is preferred for the possible hole centers, since edges or corners can be tapered or misaligned.

  2.     Special Forms:-

       Many special features are created in flat sheet through turret punch press. Some of them are counter sinks, offsets, embosses, draws, lettering and weld projections. Any form can be made, if the form follows the essential constraints:-
           a)      Form depth < 5/16”
           b)      Fits in 3.5” dia circle

  3.     Hole to Edge clearance:-

       The hole should be at least one material thickness away from any edge. A bulge is formed if the hole gets too close to an edge. Extra web is required, if the hole is used for fastening two pieces together to compensate stress.

  4.     Hole Taper:-

           A punched hole has a tapered radius through the entire thickness of part. The taper of the hole is related to die clearance.
Die clearance = Difference b/w punch dia and die dia
           It is usually 10% of material thickness. A drilled hole has constant radius through entire thickness, which is a costlier operation. 

  5.     Hole dia to Thickness ratio:-

            Hole diameter should be greater than material thickness. As tensile strength increases, punch diameter must also increase.

  6.     Feature Placement restrictions:-


             Clearance should be allowed while placing formed features next to one another. The form will be flattened for inadequate clearance between features.

  7.     Nibbling large radii:-

            The large radius is made by making several hits with a flat tool. The angle and position of tool changes as it moves around edge simulating a large radius. The radius edge will have a scalloped look with tiny points formed. It reduces after deburring, but still remains. 

  8.     Counter Sinks:-

            A counter sink can be put in sheet metal by both machining and/or punching. The cross section of features is listed from least to most expensive.

i)                    Formed: -                      Low cost, used for thin gauge, 18 gauge and higher.
ii)                  Punched: -                     Low cost, most common, used for anything heavier than 18
                                      gauge.
iii)                Punched & Machined: -Medium cost, used for harder materials that cannot be
                                       formed with a punch. Eg: - heavy gauge stainless
iv)                Machined Complete: -   High cost, rarely used, only for high tolerance applications or

                                       materials too thick to be punched.

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