Comparison between sheet punching and coil punching
Fig. 7
Productivity-fl exibility
diagram and
placement of
the technologies
in question.
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.
Fig. 9
Examples of
products made
by punching from
coil (door panels).
Fig. 10
Products
punched from
coil (metal furniture
panels and shelves).
Fig. 11
Products
produced by
punching from
coil (safety
door frames).
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). The 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.
Raw material
The 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.
The coil punching machine instead works strips from coils that come from service centres with the right width. The 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.
Tools
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.
Hourly cost
The 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.
The labour cost depends on the type of system.
The sheet-fed punching machine requires the constant presence of at least one operator for the sheet loading and workpiece unloading operations. This 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 and unloads pallets. Th e cost of amortization for coil punching machines and sheet-fed punching lines depends on the type of system that has been purchased. In this analysis we are considering the cost of amortization of a sheet-fed punching machine with manual loading and unloading at an equivalent cost of amortization of a coil punching machine.
If the sheet-fed punch press has automatic loading and unloading, the hourly labour cost will be reduced, but the machine amortization cost will be higher.
There are also compact coil punching machines available that allow even less investment (fi gure 8) and therefore an even lower hourly amortization cost.
For the above reasons, in the various operating conditions, the coil punching machine has shown from 20% up to 30% lower hourly costs compared to the sheet-fed system.
Fig. 8
Coil punching
machine for
strips up to
1000 mm wide.
Production time
The value of the hourly cost should be multiplied by the hours of production. In the sheet-fed punching machine, as described in the paragraph on production speed, the time required for loading the sheet and unloading the piece must be added to the punch time.
This time goes from 30 to 50 hundredths of a minute, whether the operation is carried out manually or automatically. In the coil punching machine, however, the machine time is equal to the product cycle time, which means that the coil punching machine is normally faster than the sheet-fed machine and therefore requires fewer hours to complete the production. Generally speaking, the simpler the workpiece the more productive the coil punching machine compared to the sheet-fed punching machine.
Conclusion
Raw material and hourly costs are therefore lower for the coil punching machine, which also takes less time to complete the production.
The first and third addend of the sum
[raw material] + [tools] + [hourly cost] x [hours of production] = [production cost]
are less in the coil compared to the sheet-fed punching machine. It can therefore be concluded that for medium and small production runs and with suitable workpieces for production from coil, the latter is the technology that accomplishes the production at the lowest cost.
"Punching and stamping: how to compare production cost."
Article taken from "Lamiera" Magazine: October 2005