Thomas Ligon Group G Power Consumed using a Manual Vertical Mill The vertical mill’s power consumption can be broken down into two states: idle time and milling time. It was found that material removal rates of less than 3 in3/s of aluminum consumes approximately the same amount of power as the spindle spinning freely. MRRs greater than 3 in3/s is not suggested and will consume significantly more power. The use of compressed air chip removal also does not require significantly more power. The power usage for these two states was found to be 240 Watts during idle and 1250 Watts during milling. Multiplying these values by the length of time which the machine was in each respective state will determine the total energy used by the mill. The vertical mill tested was a Bridgeport EZ-Track. Vertical milling’s energy consumption is primarily governed by the power drawn by the motor which spins the spindle. However, measurements were taken at multiple states to account for the entirety of the mill’s energy usage. These states included operating only interface screen computer, spinning the spindle while not under load, only auto-feed traversing the bench, and operating the mill under two loading conditions, both using auto-feed. The power used while also running the compressed air chip removal system was also tested. Energy usage can be easily calculated using the equation: E = T V I Where E is the energy, V is the measured RMS voltage, I is the RMS current drawn, and T is the length of time which expired while using the machine. Calculating the power using the RMS voltages and currents is not strictly correct because it does not account for the inductive loads in the system. This might be cause for some concern considering the primary load is a motor which almost only comprises of wire coils. However, for this analysis we will consider the RMS power to be sufficient. The results are presented in the table below: Test Conditions Voltage (V) Current (Amps) Power (Watts) Computer Screen Only 480 0.5 240 Spindle Spinning Freely 480 2.4 1150 Free Auto-Feed Traversing 480 0.65 310 Milling @ MRR = 3 in3/s 480 2.6 1250 Milling +Air @ MRR = 3 in3/s 480 2.62 1260 Milling @ MRR = 6 in3/s 480 3.2 1530 Both milling conditions included a feed of 0.02 in/rev, a 3/4” end mill, operated at 1600 rpm, and were cut out of an aluminum block. The only difference was the first cut depth was 1/8” and the second cut depth was 1/4”.The results provide some very interesting insight into what should be accounted for when using the vertical mill. First, traversing using auto-feed while not under load does not draw significantly more power than when the machine is idling with just the computer screen on. Second, if you are making cuts with only moderate material removal rates, the time the spindle is freely spinning can just be added to the cut time. This is important because a MRR of 6 in3/s of aluminum is extremely fast and most cutting operations will consist of MRRs less than 3 in3/s. It is also clear the use of the compressed air for chip removal is negligible. With these observations in mind, the power usage of the vertical mill can be approximated by accounted for the amount of time the machine was actively milling the part and subtracting it from the total time the user was on the machine to determine the idle time. The power usage during milling can be approximated as 1250 Watts, and the power usage when the machine is idle can be approximated as 240
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