PUNCHING AND STAMPING - DALCOS - #4

the paragraph describing the technologies, that the sheet-fed punching machine is the most flexible system, followed by the coil punching machine and the stamping press line. At this point it is interesting to construct a productivity-flexibility diagram, which is given in fig. 7.

COMPARISON OF TECHNOLOGIES

With a production N to be manufactured, it is necessary to find the technology to accomplish it at the lowest cost.

In the following paragraphs the numerical value of the cost of the raw material or the hourly cost will no longer be calculated. he costs will instead be assessed in relation to other technologies to provide the required cost comparison. he concepts will be supported by numerical references; in this way the formulas given up to this point may then be applied to each individual working situation.

COMPARISON BETWEEN SHEET PUNCHING AND COIL PUNCHING

At the current level of knowledge, punching from coil (fig. 8) is particularly indicated for making products with two sides corresponding to the sides of the coil (figs 9-11) and for medium and small production runs (from 200 pieces to 500,000 pieces per year in one shift). Furthermore, coil punching is ideal for the mass production of pieces that are "different from each other" if they belong to the same family (lot 1). For example, window and door frames are mass produced as single pieces: the punching is similar, but the sizes (height and width of the door or window) differ from one product to the next.

Punching from sheet is used for manufacturing rectangular pieces (like the coil punching machine), but it may also produce highly complex shapes, nesting them in the sheet. Sheet-fed punching may also produce a single piece that is totally different from the others obtained from the same sheet. It is therefore also used for very small production runs (from 1 to 50 pieces). he two technologies will be compared for an average production run of pieces that can be made with both machines. Each of the factors making up the production cost will be analysed and the conclusions drawn, which will be valid in respect of the hypotheses already expressed.

he sheet-fed punching machine works from commercial standard sized metal sheets or sheared to size. In the first case from 10% to

20 % waste must be considered due to the impossibility of fully using the surface of the sheet.

Working with blanks sheared to size, the percentage waste is minimum but the cost of the sheared blanks is higher than for the standard size.

he coil punching machine instead works strips from coils that come from service centres with the right width. he percentage waste in a coil is 1% at the most. Whereas the coil punching machine basically uses all the material of the coil, the sheet-fed punching machine generates a higher raw material cost due to greater waste or the higher cost of blanks sheared to size.

Regarding tool cost, it is estimated that both punching machines, using standard punches (e.g. standard thick turret tools), incur the same expense for tools.

he last factor to be assessed is the hourly cost, which will be multiplied by the production time.

For calculation purposes the maintenance and

electricity costs are considered equal in the

two types of machines.

he labour cost depends on the type of

system.

he sheet-fed punching machine requires the constant presence of at least one operator for the sheet loading and workpiece unloading operations. his cost may be eliminated by introducing a robotised loading and unloading system on the sheet-fed punching machine; in this case the system obviously requires a greater investment with consequent increase in the cost of amortization of the sheet-fed punching machine, as will be analysed. A simple coil punching machine instead produces automatically without the constant presence of an operator, who just loads coils

Fig. 6

Composition of the production cost in relation to the production

hours.

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LAMIERA • OCTOBER 2005