Press Tools
8.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 workpiece 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.
8.Progressive Dies
The linear travel of the strip stock at each press stroke
is called the progression, advance, or pitch and is equal to the interstation
distance.
The unwanted parts of the strip are cut out as it advances
through the die, and one or more ribbons or tabs are left connected to each
partially completed part to carry it though the stations of the die. Sometimes
parts are made from individual blanks, neither a part of, nor connected to a
strip; in such cases, mechanical fingers or other devices are employed for the
station-to-station movement of the workpiece.
The operations performed in a progressive die could be done
in individual dies as separate operations but would require individual feeding
and positioning. In a progressive die, the part remains connected to the stock
strip, which is fed through the die with automatic feeds and positioned by
pilots with speed and accuracy.
8.1 SELECTION OF
PROGRESSIVE DIES
The
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.
8.2 STRIP DEVELOPMENT FOR PROGRESSIVE
DIES
Individual operations performed in a progressive die are often
relatively simple, but when they are combined in several stations, the most
practical and economical strip design for optimum operation of the die often
becomes difficult to devise.
The
sequence of operations on a strip and the details of each operation must be
carefully developed to assist in the design of a die to produce good parts.
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.
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 pres slides.
12. Design the strip so that scrap and part can be ejected without interference.
Figure 8-1 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. 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.
Figure 8-2 shows the use of one die station instead of two
stations to maintain a close-toleranced 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 ± 0.02 mm. tolerance.
The
strip development for shallow and deep drawing in progressive dies must allow
for movement of the metal without affecting the positioning of the part in each
successive station.
Figure 8-3 shows various types of cutouts and typical distortions to the
carrier strips as the cup-shaped parts are formed and then blanked out of the
strip.
Piercing and lancing of the strip around the periphery of the part as
shown at A, leaving one or two tabs connected to the carrier strip, is a
commonly used method.
The semi-circular lancing as shown at B is used for shallow draws. The
use of this type of relief for deeper draws places an extra strain on the metal
in the tab and causes it to tear. The carrier strip is distorted to provide
stock for the draw. A popular cutout for fairly deep draws is shown at C. This
double-lanced relief suspends the blank on narrow ribbons, and no distortion
takes place in the carrier strips. Two sets of single rounded lanced reliefs of
slightly different diameters are placed diametrically opposite each other to
produce the ribbon suspension. The hourglass cutout in D is an economical method
of making the blank for shallow draws. The connection to the carrier strips is wide,
and a deep draw would cause considerable distortion. An hourglass cutout for
deep draws is shown in E, which provides a narrow tab connecting the carrier
strip to the blank.
The cupping operations narrow the width of the strip as the metal is
drawn into the cup shape.
The
hourglass cutout may be made in two stations by piercing two separated
triangular-shaped cutouts in one station, and lancing or notching the material
between them in a second station. The cutouts shown at F and G provide an
expansion-type carrier ribbon that tends to straighten out when the draw is
performed.
Fig. 8-3 Cutout reliefs for progressive
draws:
(A) Lanced outline; (B) Circular lance; (C) Double lance
suspension; (D) Hourglass cutout
Often
tool drawings are used only once, when the tool is constructed. They are brought
back into use only when changes become necessary, such as those caused by
product redesign or changes made to improve tooling performance. They are used
only by highly skilled toolmakers, tool room personnel, and tooling buyers. For
this reason, many shortcuts can be used in tool drawings that would cause
problems on product drawings. Product drawings have a greater circulation and
are used more frequently and usually over a greater period of time; therefore,
the shortcuts used in tool drawings are not permitted on product drawings.
2.1 DRAFTING PRACTICE
The
following list of drafting rules generally applies to tool drawings and is
intended as a guide to help maintain uniformity.
All
lines must be dark enough to produce a clear and sharp print
All
drawings should be on standard size that will allow the resulting prints to fold
to standard A4 size.
All
drawings should have a border line drawn 5 or 10 mm from each side of the paper,
depending upon the size of the drawing.
The
material and title block should be located in the lower right-hand corner of the
drawing.
All
dimensions should be expressed in mm, with the mm sign omitted.
Full-scale drawings should be used whenever possible. Otherwise,
use half or as per IS: 696 standard.
Drawing and dimensioning must help the person who will use the
drawing to make the item in the tool room. The toolmaker should not have to make
calculations before he can begin producing the tool.
Only
as many views as necessary to show all required detail should be given.
Use
uppercase engineering lettering (3 mm high) throughout the drawing.
A
name is always assigned to each tool and placed in the title block. The name
usually is the tool name plus the name of the part as noted on the part drawing.
For example, if the name in the title block of a part drawing is ‘Horizontal
actuating rod’ the correct title of the drill jig is ‘Drill jig-horizontal
actuating rod’.
Only
critical dimensions, overall dimensions, and location dimensions should be shown
on tool drawings. Dimensions of individual pieces can be indicated in the bill
of materials and need not appear on the drawing.
Standard purchased tool components need not be dimensioned. These
include die sets, screws, dowels, springs, knobs, and tooling specialty items.
Dimensions are not necessary because the components come ready-made and are
identified in the material list by number.
Standard purchased tool components that are to be altered by the
toolmaker should have the altered portion dimensioned.
Special tooling components that have been standardized by a
particular company do not need dimensions.
Dimensions that can be determined by or calculated from,
dimensions on the part print need not be shown on the tool drawing. Examples
would be the center of the nest, cutting edges on a punch, die clearance etc.
2.2 DRAWING LAYOUT
There are two different
methods of preparing tool-design drawings. One is to show all information,
including the details, on one sheet. The tool is shown assembled with only the
necessary views to give pertinent information. Detail drawings are included when
necessary. The method is generally adopted by companies whose tool-making
department is such that one toolmaker builds the entire tool or die and does
most of the work on it. This method will be explained in detail in
the following section.
The other method of preparing tool-design drawings is similar to the method of preparing product-design drawings. The assembly is drawn on one sheet, and each component is detailed completely on a separate sheet. In this case the tool is generally built by several people, each doing one operation on each component. Another person may complete the assembly. This allows the company to utilize different skill levels in the tool room. This method of drawing also ensures interchangeable components, which may be a real asset when repairing tools used on continuous production.