Altan T. Metal Forming Handbook

Подождите немного. Документ загружается.

When producing small sheet metal components such as drawer

slides, disc carriers for automatic transmissions, wishbones, bicycle

brake levers etc., the coil stock can be fed directly to the press by a coil

line (cf. Fig. 4.3.1).The sheet metal forming process takes place in a com-

pact piece of equipment, generally a high-speed press (cf.Fig.4.6.6),a

universal (cf. Fig. 4.4.1)or transfer press (cf. Fig. 4.4.23).

The finished parts are palletized either manually or by using a stack-

ing device (cf. Fig. 4.4.41)and made ready for downstream processing,

for example welding, painting or hemming. At the same time, scrap dis-

posal takes place.

For the introduction of new dies for large parts and transfer presses,

try-out presses (cf.Fig.4.1.19and4.1.20)and simulators (cf.Fig.4.1.24)

are employed. These are used by the die shop to prepare tooling as well

as try-out of the dies before the final try-out on the actual production

press.

This shortens the die try-out time on the production presses sub-

stantially for obtaining the first OK parts. Consequently, this procedure

shortens the time necessary to produce the first OK parts and increases

the machine utilization rate.

4.9.2Layout

The following provides a detailed description of a typical example stamp-

ing plant layout. To simplify matters somewhat, our explanation is based

on the assumption of ideal conditions and new installation of the com-

plete equipment (Greenfield plant). This is a typical task to establish

manufacturing facilities for the production of medium-sized and large

car body panels such as doors, roofs, hoods, floor panels, side panels,

fenders etc. for a particular car type. There are three press systems avail-

able: tri-axis transfer presses (cf. Fig. 4.4.27),crossbar transfer presses (cf.

Fig. 4.4.38)and press lines (cf. Fig. 4.4.19)of various bed sizes.

Before preparing the plant layout, it is first necessary to determine

which parts are to be outsourced and which are produced in-house. The

following parts are generally produced in-house:

–outer panels whose surface could be damaged during transport,

–joining parts with narrow gap tolerances for precise matching,

–parts of large volume resp. size which are difficult to stack or transport.

391

Organization of stamping plants

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

392

Sheet metal forming and blanking

These so-called A-parts – there are generally between 30 and 40 of

them per vehicle type – must be grouped together to families of parts.

These are then allocated to the different press systems on which they

can be economically manufactured.

Figure 4.9.2and Table 4.9.1 illustrate the correlation between part size

and press system. When processing medium-sized panels, a press line with

a medium bed size or a tri-axis transfer press will be employed. Press lines

with large bed sizes or crossbar transfer presses offer the ideal conditions

for processing large panels with low inherent stiffness or double panels.

Figure 4.9.3offers a more precise allocation of parts to both available

transfer press systems. Some parts can be handled by crossbars with

suction cups as well as by gripper rail systems. They represent the

overlap between two families of parts. Some of these parts, such as

doors, are produced as single parts using gripper rail transfer systems

(cf.Fig.4.4.29)or as double parts using a crossbar transfer system

(cf.Fig.4.4.34).

Fig. 4.9.2 Press systems for the manufacture of car body parts

medium dimension:

tri-axis transfer press

crossbar transfer press

press line

typical family of parts

large dimension:

typical family of parts

up to 3,000 mm

up to 5,000 mm

time

up to 5,000 mm

up to 3,000 mm

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

393

Organization of stamping plants

The selection and loading of presses depend primarily on the num-

ber of vehicles to be produced. The number of vehicles determines the

production requirement of all parts per day. If the A-parts of a vehicle

type are manufactured on large-panel transfer presses, the stamping

plant must have a capacity of between 3 and 4 million parts per year

(basis of planning: 1 vehicle type = 100,000 vehicles per year).

The production scope per year, i.e. the number of parts which can be

produced on the press system per year, must be co-ordinated with the

production requirement. This calls for an output analysis of the indi-

vidual press system. The basis for calculation is the production stroking

rate. In transfer presses, this is equivalent to the stroking rate set at the

press. In the case of press lines, the production stroking rate is reduced

to some 50% to 60% of the set press stroking rate due to standstill peri-

ods caused by the single stroke operation. The number of parts which

can be produced during press operation also depends on the forming

requirements of the individual parts. A complex part requires a longer

processing time. In addition, the time necessary for transportation of

Table 4.9.1: Criteria for press selection

large-panel transfer presses

press lines

with differing

bed sizes

part spectrum

medium-sized parts all parts

++ + 0

0+++

+++ 0

10 10 30

crossbar

transfer

tri-axis

transfer

large, unstable parts

and double parts

complete part

production

low

investment

low space

requirement

die change

time [min.]

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

394

Sheet metal forming and blanking

large, unstable parts increases. Because of its range of parts and longer

feed pitch, a crossbar transfer press is not able to run at the same high

stroking rates as a tri-axis transfer press. In general, it is true to say that

the output of a press system must always be considered in the light of

the part family planned for production on the press.

In case of double part production in a crossbar press the processing

time per part is reduced. When double dies are used, the average output

of a crossbar transfer press is larger than the output of a tri-axis transfer

press using single dies, despite of the fact that the average production

stroking rate is lower. Double dies are preferred for the manufacture of

outer and inner doors, front and rear, left and right. Where 10 doors per

minute can be produced on a tri-axis transfer press, for instance, the

output of a crossbar transfer press can be increased by 50% to 15 parts

per minute.

The number of hours worked also plays an important role in calculating

the production level, based for example on the number of days worked per

week, the length of the seasonal shut downs and shut downs due to pub-

Fig. 4.9.3 Families of parts for large-panel transfer presses with different transfer systems

tri-axis transfer

crossbar transfer

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

lic holidays which vary from country to country. Let us assume between

200 and 300 effective days’ work per year and daily working periods of

between 840 and 1,260 min depending on two or three-shift operation.

Then the annual production capacity fluctuates between 100% at the low-

er end and 225% at the higher end. Major influencing factors which can-

not be freely selected by the manufacturer, such as statutory labor legisla-

tion, collective agreements etc. may determine the possible framework.

This means that production capacity must always be calculated relative to

personnel requirements.

Other influencing factors include for example necessary production

system maintenance intervals. If the stamping plant operates at full

capacity, these periods may have to be deducted from the production

capacity. In case a five day week is worked, maintenance can be done

on Saturdays and Sundays without loss of production.

If we reduce the working time by the periods in which no production

takes place, we arrive at the press availability or uptime. Particular factors

which must be planned for during working hours include for example die

change, die setting or die cleaning times. In addition to scheduled press

stops, unscheduled downtime must also be allowed for. This can be caused,

for example, by failure of the blank supply, press or dies. Overall, the

uptime of modern large-panel transfer presses is generally in the range of

70% of the total available work time (Fig. 4.9.4).The actual average line

output is thus calculated from the production stroking rate, the press oper-

ation time and the proportion of press running time during which double

parts are produced. Extrapolated over a whole year, values of up to 4.3 mil-

lion parts may result, depending on the press system and working hours.

Table 4.9.2 illustrates an example of output for a production system com-

prising a blanking line and two large-panel transfer presses. Providing that

double parts are produced for 50% of the time on the crossbar transfer

press, with a total of around 3.9 million production strokes of the transfer

presses per year, we arrive at an output of 4.8 million sheet metal parts per

year.

When generating a press load schedule, batch sizes and the produc-

tion sequence are defined. The sequence is repeated within each com-

plete cycle, whereby a part can be produced several times during any

one cycle. Figure 4.9.5illustrates an example of a press load schedule

and the relevant inventory in the intermediate storage of finished parts

for the production of four different sheet metal parts. Parts A and B are

395

Organization of stamping plants

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

396

Sheet metal forming and blanking

each manufactured twice in one cycle with batch sizes of 5,000 and

3,000, while type C and D parts are only produced once in a cycle with

batch sizes of 4,800 and 3,300 respectively. During production, the

inventory of the relevant part increases in the intermediate storage.

However, at the same time parts are removed (illustrated in Fig.4.9.5

for part A). The steeper the production curve, the shorter the processing

Fig. 4.9.4 Availability and downtimes of large-panel transfer presses

availability

70%

organization

part-specific

tooling

blank logistics

die cleaning

dies 4%

transfer

blankloader

press

die change

blanks

Table 4.9.2: Actual part output of a blanking line and two large-panel transfer presses

blanking line 20 ... 55 27 65 17.5 80 3,916,000 3,916,000

12 ... 16 14 70 9.8 80 2,193,000 2,193,000

10 ... 12 11 70 80 2,584,500 1,723,000

crossbar transfer press

50 % single dies,

50 % double dies

tri-axis transfer press

set stroking

rate

[1/min]

average

production

stroking

rate

[1/min]

availability

[%]

net output

[parts/min]

planned

utilization

rate

[%]

output with

1,260 min

working period

on 222 days/

year

strokes/year

resp.

blanks/year

single dies 7.7

double dies 15.4

average 11.55

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

397

Organization of stamping plants

time per part. Die changes and setting of new dies takes place within

press stops between production, whereby the die changing time plays

an important role (cf. Sect. 3.4).

Production must be resumed once the storage drops to the defined

safety inventory level, generally one day’s production. If we take type A,

for example, this level would be reached at 6,200 pieces, while for type

B the corresponding figure is 2,800 pieces. The 5,000 or 3,000 parts pro-

duced, less the parts used during manufacture, must last until the next

planned production period – around 14 resp. 22 hours later. When com-

piling the load schedule, a fine balance must be drawn: On the one hand

the production period for a batch size has to guarantee sufficient time

for set-up of the die set for the next part. At the same time, however,

excessively large batch sizes violate the principle of just-in-time produc-

tion in terms of capital costs for work-in-process and costs for interme-

diate storage of finished parts.

Fig. 4.9.5 Load schedule and parts inventory for production on a large-panel transfer press

inventory of finished parts

in the intermediate storage

for part A

part A: 7 p/min

part B: 10 p/min

part C: 8 p/min

part A: 11 p/min

daily working hours:

15 hours

time [hours]

10 min die change after each batch

batch size [pcs.]

production cycle: ABCABD

10,000

8,000

6,000

4,000

2,000

61218243036424856

production cycle of 50 hours

6,400

9,000

6,200

8,800

6,400

4,800

3,300

part cycle A

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

398

Sheet metal forming and blanking

Example:

A production unit has to be configured in which 44 pressed parts – primarily A

parts – have to be produced every year for 100,000 vehicles. The unit comprises

a blanking line, a crossbar transfer press and a tri-axis transfer press. A possible

configuration of this stamping plant with optimized material flow and periph-

eral devices is illustrated in Fig. 4.9.6.The material is supplied in the form of coil

stock and stored in the coil storage, from where it is fed to the blanking line.

This supplies through the blank storage both large-panel transfer presses with a

total of 4 million blanks a year. Following production on the transfer presses,

the parts are transported in part-specific racks to the intermediate storage for

finished parts.

Table 4.9.3 lists the part spectrum for both forming presses. The crossbar trans-

fer press is run using eight individual and eight double die sets, whereby the

double dies in this example always produce two different parts. Their die

change time amounts to 10 min. The tri-axis transfer press manufactures 20 dif-

ferent parts using 20 die sets. The die change time is also 10 min. Overall, the 44

parts are produced using 36 sets of dies – 20 on the tri-axis transfer press, and 24

on the crossbar transfer press.

If 222 working days are available per year, parts for 450 vehicles must be pro-

duced every day. In three-shift operation with working periods of 1,260 min per

day, the total of approx. 4.8 million parts indicated in Table 4.9.2 (achieved

with 3.9 million strokes) can be produced every year on the transfer presses.

This is based on a planned capacity utilization of 80%. Three days are selected

as a production cycle. If we take the simplified assumption that every part is

only run once per cycle, three times 450 parts must be produced per cycle. The

intermediate storage for finished parts has a theoretical maximum inventory of

1,800 pressed parts of each part type: 1,350 parts from running production plus

450 parts as a minimum safety inventory. On average, however, as body-in-

white production runs on a parallel basis, only around half of the parts are

located in the intermediate storage. At any point in time, the inventory com-

prises 900 344, i.e. approx. 40,000 parts. As a result, the part storage and the

capital investment this involves are considerably lower than when producing

on conventional press lines.

4.9.3Quality assurance through quality control

The aim of achieving zero-defect production calls for continuous qual-

ity control. This involves the detection of any changes in the produc-

tion process which could precede a defect, and the segregation of indi-

vidual defective parts. Conventional methods of quality assurance in

stamping plants include visual inspections – in the case of deep drawn

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

Crossbar transfer press

Parts/ Part require-

Part name

stroke ment/day

body side, outer, right

1 450 1350

body side, outer, left

1 450 1350

door, outer, front, right + left

2 450 1350

trunk lid, inner

1 450 1350

roof

1 450 1350

trunk lid, outer

1 450 1350

door, inner, front, right + left

2 450 1350

body side, inner, rear, right + left

2 450 1350

door, outer, rear, right + left

2 450 1350

door, inner, rear,right + left

2 450 1350

hood, outer

1 450 1350

hood, inner

1 450 1350

fender, outer, front, right + left

2 450 1350

wheel house, inner, front, right + left

2 450 1350

floor, mid

1 450 1350

floor, front

2 450 1350

Part batch

parts/cycle

1350

2700

1350

2700

1350

2700

size

Part batch

size

2700

Tri-axis transfer press

Parts/ Part require- Strokes/

Strokes/

Part name

Stroke

ment/day parts/cycle

floor, rear

1 450

1350

lock reinforcement, front, right

1 450 1350

lock reinforcement, rear, right

1 450 1350

rib, reinforcement, rear, right

1 450 1350

hinge pillar, front, right

1 450 1350

hinge reinforcement, front, right

1 450 1350

rib, reinforcement, front, right

1 450 1350

lock reinforcement, front, left

1 450 1350

lock reinforcement, rear, left

1 450 1350

rib, reinforcement, rear, left

1 450 1350

hinge pillar, front, left

1 450 1350

hinge reinforcement, front, left

1 450 1350

rib, reinforcement, front, left

1 450 1350

dash panel

1 450 1350

shelf, rear

1 450 1350

seat, reinforcement, rear

1 450 1350

wheel house inner, rear, left

1 450 1350

wheel house inner, rear, right

1 450 1350

1350

sunroof

1 450 1350

1350

sunroof aperture panel

1 450 1350

1350

Production cycle:

3 days

Availability:

70%

Die change time (min):

Die sets:

Planned utilization rate:

80%

Production cycle:

3 days

Availability:

70%

Die change time (min):

Die sets:

16 (24)

Planned utilization rate:

80%

399

Organization of stamping plants

Table 4.9.3: Production data for crossbar and tri-axis transfer presses

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

400

Sheet metal forming and blanking

parts, for example, for wrinkles and cracks (cf. Table 4.2.2) – and also

the testing of workpieces for dimensional accuracy using measuring

devices or gauges. However, this type of control process is both time-

consuming and labor intensive, and is generally only used in the case

of large, complex workpieces such as body panels or for random sam-

pling of smaller parts.

Where small punched parts are involved, the preferred system is one

of on-line controls in which quality assurance is integrated in the man-

ufacturing process itself, i.e. in the press. This permits monitoring of

each individual part and of process parameters such as die wear.

Visual inspections or workpiece gauging are increasingly being

replaced by optoelectronic and image processing systems. Simple opto-

Fig. 4.9.6 Layout of a stamping plant with material flow

1 coil storage; 2 blanking line; 3 intermediate pallet storage;

4 intermediate blank storage; 5 tri-axis transfer press; 6 die storage for 5;

7 racks for finished parts for 5; 8 crossbar transfer press; 9 die storage for 8;

10 racks for finished parts for 8; 11 to intermediate finished part storage;

12 offices and recreation facilities; 13 die repair and cleaning

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998