COMPARISON BETWEEN PUNCHING FROM COIL AND STAMPING BY PRESS

  With these two technologies available, given the high cost of press tools, the aim is to determine the discriminant number of pieces N below which it is more economic to work with the coil punching machine. For production runs greater than N, it will be more economic make a special press die. Also in this case each of the factors making up the production cost will be analysed and the conclusions drawn.

Raw material

  Both machines work from the coil and the raw material cost is therefore identical.

Tools

  The cost of special press tools is higher than the cost of punch press tools, which are standard and may be reused in various productions. Th e cost of setup time is included in the tool cost; it is considered as being equal in the two machines and negligible compared to the cost of the special press tool.

Hourly cost

  Both machines need the constant presence of an operator and the labour cost is therefore negligible. The annual maintenance and electricity costs are similar for the two machines. Th e cost of amortization of the two machines also here depends on the type of system. Basically the coil punching machine, with a slightly higher investment, offers great medium and long-term savings in tools. That is why the cost of amortization and therefore the hourly cost of the stamping line will be considered as slightly lower compared to the coil punching machine. 

Production time 

  The stamping press, as has been seen, produces one piece at each press stroke. Productions of 40, 60, 80, 120 workpieces and more per minute are therefore normal. Th e coil punching machine requires from 0.5 to 1 second for each operation. Th e production of one workpiece may require, for example, from a few seconds to tens of seconds.

Conclusions

  To obtain the discriminant number of pieces N, which for the given definition indicates whether to produce with a coil punching machine or to make a special tool for stamping with the press, it is necessary to equalise the two production costs calculated from the equation: [production cost] = [raw material] + [tools] + [hourly cost] x [hours of production]. Th e raw material cost is the same and may therefore be eliminated from the calculation. Given the great difference between the cost for the punch press tools and the press die, the cost of the punch press tools are considered as practically nil compared to those of the special die. Also given the greater speed of the die press over the punch press, the necessary hours of production when manufacturing with press and die are considered at first analysis as zero.Th e necessary hours of production for completing the production with the coil punching machine are equal to the number of pieces N multiplied by the hours required to make one piece (time-cycle). After a few passes, it can be calculated that N = [Press tool cost] / ([coil punching hourly cost] x [hours per piece in coil punching machine]).For example, suppose that it costs 8000 euro to make the tool. The hourly cost of the coil punching machine is 40 euro and the machine takes 20 seconds (equal to 0.0056 hours) to make the actual piece.

  The discriminant number N will be equal to 35,700 pieces. For smaller production runs it will be more economic to produce with the coil punching machine. For larger production runs it will be more economic to make a die. It should be noted that the suggested formula, due to the approximations, is incomplete, as it lacks the cost of the punch press tools (which increases the formula denominator) and the cost of the hours of production with the press (which instead increases the numerator). Apart from the advantage of simplicity, this formula also offers a valid indication regarding the discriminant number N and leads to certain reflections. Consider the hourly cost of the punching machine and the cost of the press tool as constant. The graph of the trend of the discriminant N in relation to the punching machine time-cycle shows the trend as can be seen in fig. 12. The discriminant number N increases as the coil production time-cycle decreases; in other words they are inversely proportional. Lastly, if the coil punching machine hourly cost and the coil production time-cycle are considered as constant in the formula, then the discriminant number N increases as the die cost increases.

CONCLUSIONS

  Firstly the structure of the production costs and the hourly cost of punching from sheet, punching from coil and stamping with press and die has been analysed. Punching from sheet and from coil were then compared. The result is that punching from sheet is indicated for manufacturing pieces with complex shapes or for very small production runs (from 1 to 50 pieces).For simpler shaped pieces (rectangles, squares, notched and punched panels) and for medium and small productions (from 200 to 500,000 pieces), punching from coil is always preferable. Lastly punching from coil and stamping by press were compared.

  The result is that stamping by press is to be preferred for large and volume mass production (millions of pieces), while, again, punching from coil is more indicated for medium and small production runs. In more general terms, a simple formula has been obtained that, given the tool cost, the hourly cost of the coil punching machine and its production time, gives the number of pieces N below which it is more economic to punch from the coil than to stamp by press.