Chapter 7

PROGRESSIVE DIES

PROGRESSIVE DIES

A progressive die performs a series of fundamental sheet-metal operations at two or more stations during each press stroke in order to develop a work piece as the strip stock moves through the die. This type of die is sometimes called cut-and-carry, follow, or gang die. Each working station performs one or more distinct die operations, but the strip must move from the first through each succeeding station to produce a complete part. One or more idle stations may be incorporated in the die, not to perform work on the metal but to locate the strip, to facilitate interstation strip travel, to provide maximum-size die sections, or to simplify their construction.

SELECTION OF PROGRESSIVE DIESThe selection of any multi operation tool, such as a progressive die, is

justified by the principle that the number of operations achieved with one handling of the stock and the produced part is more economical than production by a series of single-operation dies and a number of handlings for each single die.

Where total production requirements are high, particularly if production rates are large, total handling costs (man-hours) saved by progressive fabrication compared with a series of single operations are frequently greater than the costs of the progressive die.

The fabrication of parts with a progressive die under the above-mentioned production conditions is further indicated when

1)Stock material is not so thin that it cannot be piloted or so thick that there are stock-straightening problems.

2)Overall size of die (functions of part size and strip length) is not too large for available presses.

3)Total press capacity required is available

SELECTION OF PROGRESSIVE DIESStrip Development for Progressive Dies A tentative sequence of operations should be established and the following items considered as the final sequence of operations is developed:

1) Pierce piloting holes and piloting notches in the first station. Other holes may be pierced that will not be affected by subsequent non-cutting operations

2) Develop blank for drawing or forming operations for free movement of metal.

3) Distribute pierced areas over several stations if they are close together or are close to the edge of die opening.

4) Analyze the shape of blanked areas in the strip for division into simple Shapes so that punches of simple contours may partially cut an area at one station and cut out remaining areas in later stations. This may suggest the use of commercially available punch shapes.

5) Use idle stations to strengthen die blocks, stripper plates, and punch retainers and to facilitate strip movement.

SELECTION OF PROGRESSIVE DIES

6) Determine whether strip grain direction will adversely affect or facilitate an operation.

7) Plan the forming or drawing operations either in an upward or a downward direction, whichever will assure the best die design and strip movement.

8) The shape of the finished part may dictate that the cutoff operation should precede the last non-cutting operation.

9) Design adequate carrier strips or tabs.10) Check strip layout for minimum scrap; use a multiple layout if feasible.11) Locate cutting and forming areas to provide uniform loading of the press

slides.12) Design the strip so that scrap and part can be ejected without

interference.

PROGRESSIVE DIES

Use of three-stage die to avoid weak die block: (A) Pierced hole close to edge of part; (B) Pierced hole closed together

At B, the pierced holes are centered on the strip but close together. In this case the holes should be pierced in two stations to avoid thin sections in the die block between the holes. The adding of stations also provides better support for the piercing punches.

Illustrates the use of a three-station die to avoid weak die blocks. At A the pierced hole is near the edge of the part where it is cut off, thereby weakening the die block at this point. If an idle station is added so that the piercing operation is moved ahead one station, the die block is stronger and there is less chance of cracking in operation or fabrication

PROGRESSIVE DIES

Use of one station verses two to hold a close tolerance.

Figure shows the use of one die station instead of two stations to maintain a close-tolerance dimension. If two stations were used, the variation in the location of the stock guides and cutting punches could make it difficult to hold the ± 0.025 mm. tolerance.

PROGRESSIVE DIES

Cutout reliefs forprogressive draws:

(A) Lanced outline

(B) Circular lance

(C) Double lance suspension

(D) Hourglass cutout

PROGRESSIVE DIES

Cutout reliefs for progressive draws:(E) Cutout providing expansion-type carrier ribbon for circular draws

(G) Cutout providing expansion-type carrier ribbon for rectangular draws

(H) I shaped relief for rectangular draws

SELECTION OF PROGRESSIVE DIESStock Positioning

Of prime importance in the strip development is the

positioning of the stock in each station. The stock must be positioned accurately

in each station so that the operation can be done in the proper location. A

commonly used method of stock positioning is the incorporation of pilots in the

die.

There are two methods of piloting in dies: direct and indirect.

Direct piloting consists of piloting in holes punched in the part at a previous

station. Indirect piloting consists of piercing holes in the scrap-strip and

locating these holes with pilots at later operations. Direct piloting is the ideal

method for locating the part in subsequent die operations. Unfortunately, ideal

conditions may not exist, and in such cases indirect piloting must be used to

achieve the desired results of part accuracy and high production speeds.

SELECTION OF PROGRESSIVE DIES

Stock Positioning

The advantages of locating pilots in the scrap material area are:

1) Not readily affects by work piece change.

2) Size and location not as limited.

Disadvantages of locating pilots in the scrap section of strip are:

1) Material width and lead may increase.

2) Scrap-strip carriers distort on certain types of operations and make

subsequent station use impossible.

SELECTION OF PROGRESSIVE DIES

Stock Positioning

Parts conditions that require indirect pilots:(A) Small holes; (B) Close-tolerance holes; (C) Holes too near edge;(D) Holes in fragile areas of the part; (E) Holes too close together; (F) Slots in parts

SELECTION OF PROGRESSIVE DIES

DISPOSITION OF SCRAP STRIP

Alternate strip development for a work piece

SELECTION OF PROGRESSIVE DIES

Scrap less strip development

The strip development of Fig. illustrates how the strip is pierced, trimmed, and the part cut off and formed. A slug-type cutoff punch is used and the flange is formed downward. The part is then ejected by an air jet or by gravity. This arrangement is often referred to as a scrap less development since no carrier strips remain after the part is cut from the strip.

SELECTION OF PROGRESSIVE DIES

Scrap less strip development

Figure shows another development in which the stock is the same as the developed width of the work piece. The strip is pierced in station 1; piloted and notched in station 2; piloted, pierced, and formed in station 3.

SELECTION OF PROGRESSIVE DIES

Progressive Die Element

The die elements used in progressive dies such as

punches, stops, pilots, strippers, die buttons, punch

guide bushings, die sets guide posts, and guide post

bushings are of similar design to those used in

other types of die.

SELECTION OF PROGRESSIVE DIES

General Die DesignA progressive die should be heavily constructed to withstand the repeated shock and continuous runs to which it is subjected. Precision or antifriction guideposts and bushings should be used to maintain accuracy. The stripper plates (if spring-loaded and movable), when also serving as guides for the punches, should engage guide pins before contacting the strip stock. Lifters should be provided in die cavities to lift up or eject the formed parts, and carrier rails or pins should be provided to support and guide the strip when it is being moved to the next station. A positive ejector should be provided at the last station. Where practical, punches should contain shedder or oil-seal-breaker pins to aid in the disposal of the slug. Adequate piloting should be provided to ensure proper location of the strip as it advances through the die.