Energetic discussion G


erman CNC specialist Dr Johannes Heidenhain GmbH (Heidenhain) has undertaken studies in its own factory


in Traunreut, Germany, to map where energy is consumed in metalcutting machines. In a first example, machining a pocket in an aluminium billet of 150 by 50 by 25 mm on a modern Hermle C30 vertical machining centre, having a working envelope of 850 by 700 by 500 mm, on its shopfloor was considered, with both roughing and finishing cycles analysed (see diagram, below). During roughing, a total of 13 kW was


drawn, the majority of which was taken by coolant and compressed air functions, 6.4 kW; the spindle and drives accounted for 3.5 kW and auxiliary components the remainder, 3.1 kW. The finishing cycle sees energy consumption reduced by almost half,


with cooling and compressed air now taking 2.8 kW, more than halving the first figure, with auxiliary components taking 2.8 kW and the spindle and drives accounting for just 1.8 kW. Heidenhain’s manager, marketing, application development, Dr Jens Kummetz, describes this machining centre as a production machine, able to support extended unmanned running and so requiring ancillary functions, such as loading and unloading, with the energy associated with these functions less influenced by the actual production cycle. Machines without such automatic production-related features would consume less energy, yes. “This can be a dangerous discussion, when you talk about energy per part. Of course, you can use a manually operated machine, without any hydraulic or lubricant, so energy per part


Milling process – power consumption breakdown


External processing Cooling lubricant: 5.1 kW Compressed air: 1.3 kW


Workpiece Dimensions: 150 mm by 50 mm by 25 mm Material: Aluminium


Auxiliary components of the machine: 3.1 kW


Feed drives: 0.25 kW


Electrical power of the machine


Roughing


Vertical machining centre Work envelope: 850 mm by 700 mm by 500 mm


Electrical power of the machine


Finishing


External processing Cooling lubricant: 5.1 kW Compressed air: 1.3 kW


Auxiliary components of the machine: 2.8 kW


Feed drives: 0.25 kW


CNC control


Spindle 1.55 kW


Where is energy consumed in a machine tool and where can savings be made? Heidenhain has undertaken some studies, which were explained to Andrew Allcock. While some things are clear cut, others are much less so, he reveals


would be lower, but the cost per part would be dramatically higher. There has to be some balance between the two.” In a further analysis, a more extreme production machine example, a Mikron Multistep XS with three 3-axis modules and used for six-side machining for the production of complex aluminium bracketry/location parts, was undertaken. In looking at the cutting energy consumption of this 250 second part production cycle in isolation of coolant and lubricating system, there are some steady consumers, such as dust extraction, two 24 v supplies and the hydraulic pump (which has some peaks). Together, these account for a pretty consistent 2 kW.


The feed drives for the separate modules CNC control


Spindle 3.25 kW


have a more spiky nature, but feed drives in machine unit one, which does most work, take more than those in the following machine units. So, machine unit one looks to draw between 5 and 6 kW on average; unit two is lower, at around 4 kW; with machine unit three probably around the 3 kW mark on average. Short duration peaks up to 15 kW are seen in machine unit one, however, with unit two peaking at just below 12 kW and unit three hitting 9 kW. When looking at cooling and lubrication system power consumption, the figure never drops below in excess of 8 kW, with the average level probably above 12, but below 14 kW. Peaks of 18 kW of several seconds’ duration exist across the cycle, however. And in analysing the standby and operation power consumption levels of the Hermle C30 vertical, a Heller MC16.2 horizontal production machine and the Mikron Multistep, Heidenhain offers figures for standby and operation of just under 5 kW and 8 kW; around 7 kW and 10 kW; and 16 kW and 24 kW, respectively. “To improve the energy per part, there is a clear understanding that idle power consumption


10 May 2013 www.machinery.co.uk


7.4 kW


13 kW


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