Altan T. Metal Forming Handbook

Подождите немного. Документ загружается.

modules are also indicated; in this case of the blank loader. With the aid

of the so-called zoom function, it is possible to access a detailed synop-

tic for each of the listed conditions. This can be actuated manually by

selecting with the cursor, or in the “Automatic” synoptic mode. Here, a

detailed synoptic is displayed automatically with the first unfulfilled

condition. As synoptic displays are so clear and easy to understand,

they are also used for the die changing sequence and for operator

prompting. The die change synopticdisplays all the sequential steps and

their current execution status.

Operator prompt synopticsprovide the user with an indication of all

the conditions for initiating individual functions in the “set-up” and

“die change” modes (Fig. 4.4.46). The visualization system makes this

type of synoptic available for each set-up function.

The malfunction diagnosticssystem detects malfunctions occurring

anywhere throughout the press. The visualization system indicates the

existence of a malfunction in each image by highlighting the menu

261

Sheet metal forming lines

Fig. 4.4.46 User prompting for slide adjustments in a crossbar transfer press

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

point “Error” in the menu line. Error messages contain an error number,

plain language, reference to the affected assembly (operating resources

identification) and reference to the point in the PLC program. This

information, in conjunction with the status displays, supports the

detection and localization of the source and cause of the malfunction.

Die-dependent data related to all dies used in the line can be entered

and edited at the visualization system. They are stored and managed in

the visualization system. During die change, this data is automatically

transmitted to the relevant control units.

Alongside the visualization system, which is imperative for line oper-

ation and monitoring, PC-based systems are also frequently used for

the acquisition and processing of machine and operational data as well

as measured values.

Machine and production data acquisition and evaluation

The machine and production data acquisition system (MDA/PDA) – or

PMS (Press Management System) for short – logs error and standstill

data as well as production data accumulated in the press line. As such,

it plays an important role in detecting and analysing weak points and

in registering and monitoring press line productivity.

The hardware basis of the PMS is an industrial PC which is directly

linked to the line’s control system. The central PLC is responsible for

making available the necessary data, and so functions as the communi-

cation partner for the PMS. Table 4.4.1 indicates the spheres of opera-

tion of a PMS for large-panel presses.

The field of machine data acquisition is subdivided according to the

acquisition and evaluation of errors and standstill periods.

According to the definition used here, errors are constituted by

machine statuses which are the direct result of faulty or damaged line

elements or components and which generally necessitate repair work.

Errors are automatically classified by the PMS. In addition, a freely con-

figured event-related comment is possible for each error message.

Errorsdo not, however, always cause a standstill; Standstills, in turn,

do not necessarily result in errors. As a result, alongside the detection of

errors, separate acquisition of all press standstill statuses is required.

Standstill,in accordance with the definition used here, consists of all

times during which no parts are produced (cf.Sect.4.9.2). Depending

on the line in question, a difference is made between approx. 30 differ-

262

Sheet metal forming and blanking

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

Machine/production data acquisition and evaluation

automatic errors standstill job data,

acquisition – during production production data

– during die change

manual commentaries commentaries number of reject parts,

acquisition (plain language) (plain language), number of rework parts

classification

evaluations chronological list chronological list chronological list,

individual printout

(job data table)

sorting according to: sorting according to: sorting according to:

error frequency, standstill frequency, die number,

total duration, total duration, workpiece number,

error category frequency error category frequency batch size

and total duration and total duration

selection according to: selection according to: selection according to:

period, period, period,

shift, shift, shift,

job number, job number, job number,

die number, die number, die number

error number, standstill number,

error categories standstill categories

Table 4.4.1: Functional areas of machine and production data acquisition systems in large-panel

presses

ent causes of standstill. The conditions of the ready-to-run status syn-

optic form the basis for monitoring standstills.

An important function in monitoring standstill periods is the deter-

mination of the actual cause of the standstill. The control system fre-

quently determines only the so-called secondary causes, in particular in

the case of manual operator intervention. The actual cause of the prob-

lem, termed the primary cause, can be determined by the user by means

of event-related modification of the standard standstill classification

and by the addition of event-related comments.

The field of production data acquisitionis subdivided into job data

acquisition and, based on this, production data determination. Job data

refers to the individual production orders. Its acquisition stops on com-

pletion of the order, and the relevant data is stored (Fig. 4.4.47).

Job data is automatically acquired, supplemented by optional manu-

al inputs in the press control system. At the end of a shift, and at the

end of an order, the current data table is transmitted to the PMS. The

263

Sheet metal forming lines

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

production datais ascertained on the basis of this information for an

evaluation period which can be freely defined by the operator.

The production status illustrated in Fig.4.4.48offers a survey of the

most important data from all the shifts of a production day. In addition,

the pre-processed data can be exported in a standard data format in

order to give the user the chance to execute his or her own evaluation.

Data acquisition and processing for draw cushions

The acquisition and processing system for measured values for the draw

cushion, known as the Cushion Monitoring System (CMS), permits the

observation of the drawing process. In addition, this provides support

for repair personnel in terms of malfunction diagnostics and also when

resuming production after a component failure. All the relevant set val-

ues, control signals and process variables (actual values) for pre-acceler-

ation and for the actual drawing process are monitored and stored, for

example

264

Sheet metal forming and blanking

Fig. 4.4.47 Job data

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

–set drawing pressure values

–actual drawing pressure values

–actual system pressure values

–press angle

–slide position and

–draw cushion position.

The CMS comprises a unit for measured data acquisition and an industri-

al PC with software capability for acquisition, evaluation and display of

the above signals and data. The system permits the graphic display of the

progress of pre-acceleration, the drawing process and upstroke both in the

form of an overview (Fig.4.4.49and cf. Sect.3.1.4) and detail displays.

The measurement can be depicted in real time for every individual stroke.

Zoom functions and the optional representation of measured values rela-

tive to the time or the press crank angle permit detailed analysis of mea-

sured values. The measurements are stored in the PC and can be accessed

at a later date if required. In addition, the CMS provides diagnostic images

for troubleshooting and settings during start-up of the press line.

265

Sheet metal forming lines

Fig. 4.4.48 Overview of one day’s production data

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

Outlook

Further developments in the field of control engineering in large-panel

presses will be determined by technical and economic factors and also

by organizational requirements. Initially, the innnovation in automa-

tion components is relevant.

Nowadays, low-cost small control systems with communication

capability, fast field bus systems and decentral input/output modules

are available. This opens up the possibility for further decentralization

of control systems, and consequently for a substantial reduction not

only of costs and system complexity but also of input for installation,

start-up and troubleshooting.

Analogue technology, in particular in controllers for electrical and

hydraulic drive systems, is being replaced by digital technology. This

permits the parameterization of controllers by means of data transmis-

266

Sheet metal forming and blanking

Fig. 4.4.49 Overview of process signals for the drawing process using a four-point draw

cushion: slide displacement (red), draw depth (blue), set pressure value (red right-

angled curve), actual pressure value (green right-angle curve), cursor (yellow)

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

sion following a change of requirements or after exchanging defective

components. The benefits here include shorter repair times and thus an

increase of press up-time.

Highly-dynamic electrical feed systems in conjunction with efficient

control and regulation systems for movement synchronization are

increasingly replacing mechanically coupled drive systems. This devel-

opment not only allows mechanical mechanisms to be simplified and

flexibility to be further increased, but in certain cases also enhances

productivity.

The changes taking place in the field of work organization in many

stamping plants are imposing new demands, in particular on the line

control center. Production lines are being managed increasingly by

independently acting teams of employees on their own responsibility.

These teams require information on site related to production and

machine data, to production quality, waiting production jobs and

maintenance work to be carried out. Only with this information at

their fingertips are team members in a position to optimize the produc-

tivity of their line and to assume control functions on the production

level. As a result, machine and production data acquisition systems

such as those described above will continue to be more extensively used

and the functional scope of their application extended to cover ever

wider fields.

Multi media-based production line information systems which per-

mit fast, selective access to stored line documentation and additional

information on the elimination of faults and routine maintenance will

be integrated in the future into the press control center. Using these

systems, it will also be possible for operating personnel to enter their

own findings into the information data base and so develop their own

experience data base.

In general terms, future developments in the field of control engi-

neering will concentrate on reducing complexity and on the develop-

ment of more efficient systems for repair and maintenance support to

reduce the costs of press control and increase production line uptime.

267

Sheet metal forming lines

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

4Sheet metal forming and blanking

4.5Blanking processes

In blanking operations, a difference is made between open and closed

contours (cf. Fig. 2.1.29). Open contour blanking techniques are used

mainly for blanking strips made of sheet plates. This technique is simi-

lar to the action of a pair of scissors (cf. Fig. 2.1.31). The blanking ele-

ments used for open blanking can be either longitudinal or circular

knives. In closed blanking processes, the sheared contour is closed, for

example when piercing. Here, the process is applied using blanking

dies: The relative movement of the blanking punch to the female

blanking die seperates the metal (Fig. 4.5.1).The punch makes contact

with the sheet metal, initially causing elastic deformation. The plastic

deformation stage then follows, leaving the sheet metal with a perma-

nent camber. The upper edge of the sheet metal then bends and draws

in, followed by a shearing action which leaves a visible, smooth area on

the cut surface. If the shearing strength is exceeded, cracks are formed.

These generally run from the edges of the female blanking die and lead

Fig. 4.5.1 Phases of the blanking process

contact of the

punch

elastic and plastic

deformation

shearing and

crack formation

breakthrough stripping

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

to complete breakthrough of the metal as the movement of the punch

progresses. A force-time diagram of a typical blanking process is provid-

ed in Fig.4.6.7.A rough fracture zone then forms on the cut surface

underneath the shearing zone (cf.Fig.4.7.8, left): The sheet metal mate-

rial springs back after the blanking process, causing it to clamp onto the

lateral surface of the punch. The sheet metal has to be separated from

the punch by means of a stripper during the return stroke.

Due to the wide variety of applications in stamping plants, only

processes used in the production of single parts by closed dies will be

described here.

Positioning of blanks in the strip and material savings

Guidelines on the economical layout of blanked, punched and drawn

parts:

Close cooperation between component developers and die designers

can help to substantially reduce material waste.

Workpieces should be designed with the smallest possible surface

area; their shape should be such that they can be lined up or nested in

each other in the sheet metal strip so as to ensure minimum waste. The

arrangement of parts illustrated in Fig.4.5.2b,for instance, is more

favorable than that in 4.5.2a.The best possible material utilization is

achieved where the surface shapes are completely interlocking, i.e. can

be blanked of the strip without leaving any scrap whatsoever (Fig.4.5.3).

The drawback of scrap-free blanking is that burr occur on both sides of

the workpiece and the design of the blanking tools needed in this case is

more complicated.

In cases where it is impossible to design workpiece shapes so that opti-

mum in-line arrangement is possible in the strip, it may be possible to

269

Blanking processes

Fig. 4.5.2 Arrangement of parts in the strip: boffers greater material savings than a

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

utilize the remaining scrap for other workpieces (Fig.4.5.4).If scrap

skeletons or webs have become deformed during the initial blanking

process, they must go through a straightening machine before further

processing (cf. Sect. 4.8.3).

T, L and U-shaped workpieces are ideally produced in alternatively

reversed manner (top to tail) (Fig.4.5.3 e, fand Fig.4.5.5). It is impor-

tant to remember when blanking top to tail that the straight strip bends

within the strip plane if subjected to unilateral stress and stretched dur-

ing the blanking process, or if unilateral tensions are released. This

applies in particular to soft, thick sheet material. In these cases, it may

be more practical to blank using a multiple punch arrangement for the

simultaneous production of several workpieces rather than using the

top to tail arrangement(Fig. 4.5.4).

Occasionally, it is possible to improve material utilization by blank-

ing another usable component bfrom the main piece a, as demonstrat-

ed in Fig.4.5.6.Figure 4.5.6cillustrates the two parts assembled. The

multiple arrangement of workpieces is often economical (Fig.4.5.7).In

the interests of optimum material savings, web or skeleton widths

should be kept as narrow as possible (Table 4.5.1). The designations on

the blanking strip are indicated in Fig. 4.5.8.It is important to remem-

270

Sheet metal forming and blanking

Fig. 4.5.3 Examples of favorable part arrangement in the sheet metal strip

Fig. 4.5.4

Utilization of scrap pieces

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