6 DESCRIPTION OF EXPERIMENTS
6.5 Equations used
CNC machining
There were three electrical power driven units identified in CNC machining: namely Pspindle
,
Paxes and Plubtication. The average of individual process power PCNC, measured in each machined phase was determined by summing the average values from all the machining processes. These amount of power values of the spindles motors and carriage servos.
Since machining is aided by lubrication, power value for the lubricating motor
P
lubrication(400 W) as given in machine manual were considered during the actual cutting estimation.
This was however excluded from reference cut power calculation as lubrication was not activated during the phase. Equation 1 was used to determine the power in kW for the motors.
0.126 ∙ . (1)
where Pkw power in kW,
pv voltage.
The recorded voltage during the CNC machining were converted to power values as equation 1 shows (Ratava, 2014).
Equation 2 was used to define the combined power value in each phase.
(2)
where PCNC power value during CNC machining, Pspindle power to drive spindles, (S1 +S2), Paxes power for moving X, Z, and Y axis, Plubrication power for lubrication motor.
Equation 3 combined the different machining processes for the determination of total average power, Pavgper part.
∑ ∑ ∑ ∑
∑ ∑ (3)
where Pavg power average,
Poutside power during the outside turning process, Pdrilling power during the drilling process,
Pinside power during the internal turning process, Pmilling power during the milling process,
Pcutting power during to cut ready part off stock,
Ptool tool change power (power measured during tool change).
LAM
The four modes identified were 1) standby, 2) heating, 3) process during the LAM process;
and one ancillary mode 4) sawing. The total electrical power consumed in LAM, PLAM was determined using equation 4.
(4)
where PLAM total of average power in LAM,
Pstandby average of power for activities such as cleaning and removing, Pheating average of power to heat LAM chamber,
Pprocessing average of power values during recoating, laser scanning, platform travels, etc,
Psawing average of power values to saw work pieces from platform.
6.5.2 Energy analysis
The energy used in each of the modes of production were determined for both manufacturing processes. The power of each process mode was multiplied by time to get the energy, Emethod, consumed in producing each part according to equation 5. The subscript, method is replaced with the manufacturing process accordingly in both processes. Average values of recorded power were used.
.
(5)where Emethod energy consumed in each manufacturing method, Pprocess average power in each method,
tprocess time taken to perform each manufacturing phase.
CNC machining
Two approaches were used to calculate the total energy in CNC machining to ensure reliability of energy calculations. The energy consumption values were determined using summation of electrical energy consumption. Equation 5 was also used to find energy by multiplying averages of power values in each phase with time taken to complete machining task.
The total energy used in CNC machining was calculated as shown in equation 6.
∑ ∑ ∑ ∑
∑ ∑ (6)
where ECNC Total energyused to machine one test piece, Eoutside energy consumed during the outside turning, Edrilling energy consumed for drilling,
Einside energy consumed for inside turning, Emilling energy consumed during milling, Ecutting energy consumed to cut-off test piece, Etool energy consumed for tools change.
LAM
The energy consumption during the LAM process (ELAM or Etotal) was estimated by combining the energy used in all modes as equation 7 shows.
(7)
Where ELAM total energy consumption in LAM,
Estandby energy used for activities such as preparation, cleaning, cooling, Eheating energy used to heat and create inert atmosphere,
Eprocessing energy used for recoating, scanning, platform travels, etc., Esawing energy used to saw work pieces from platform.
An evaluation of the energy consumption per each of the test pieces were calculated using equation 8.
. (8)
where Econ estimated energy consumed as per part, ELAM total energy consumed to make all test pieces,
vi volume of per part,
v
TOT total volume of all test pieces.Appendices IV and V represent the results of energy calculation in CNC machining and LAM respectively.
6.5.3 Mass loss calculations CNC machining
The material that were lost during the CNC machining were the stainless steel material and cutting fluid used for cooling and lubrication. The volume of removed material (chip) from the stock material were determined as equation 9 shows.
(9)
where Vremoved volume of removed material, Vinpu
t
volume of starting material, Vpart volume of final part.The mass loss due to raw material, mr, was estimated using amount of removed volume of material per part (Vremoval, [cm3]) multiplied by the density of material (ƍ, [kg/cm3]). Volume of starting bar were estimated using solid works. The equation 10 was used to determine amount of removed mass. Appendix IV represents results of removed mass and volume of material.
. ƍ
(10)Where mr removed mass,
Vremoved volume of removed material, Vinput volume of starting material, Vpart volume of final part,
ƍ316L density of stainless steel 316L.
6.5.4 Specific energy consumption
In order to determine the SEC for producing each of the test pieces, it was necessary to determine the amount of mass and volume deposited. The deposited mass were determined using SolidWorks software, LAM system and measuring scale. The SEC for producing the sample pieces were estimated using equation 11.
(11)
where SEC specific energy consumption,
ELAM energy used to deposited or remove amount of material,
Md deposited mass.
Appendix IV shows the amount of removed mass and volume in CNC machining and the amount of deposited mass and volume in LAM parts are shown in appendix V.
The SEC in CNC machining was not included in this studies however its inclusion into further studies may offer a more appreciative energy consumption comparison.