Power Cost for CNC Machining

      Juice to run the machinery is a small percentage of CNC operating cost, but it's not zero. October 14, 2010

Question
Does anyone have the formula in which I can compute the amount of electricity (watts) my machine is using in one hour so that I may compute my operating costs, per hour?

Forum Responses
(CNC Forum)
From contributor H:
What is the machine and how often do you use it?



From contributor M:
A watt is a unit of power, it is measured in energy (joules) per unit time (seconds). It can also be expressed as volts (which is potential) times amps (which is current).

Are you trying to figure this for a computer or a router? I assure you the computer uses a negligible amount of power, the router does not. For the router it is also complicated by the fact that load is not constant. The difference between idling with the vacuum off is very significant when compared to hogging a large chunk of material with the vacuum on. It may be easier to meter the situation than figure it from input information you are not too sure of.



I take an average power bill within the high and low of the year, divide it by operating days within a month, the divide the day rate by opening hours. This is what I figure I actually pay.


From contributor B:
It's pretty easy to figure out the most it could be by looking at the HP of your motors. Most of the power is going to come from the vacuums, the spindle and your air compressor. So, for example, if you've got a 10hp spindle, two 20 hp vacuum pumps and you're using 10cfm air (from your 15hp compressor which puts out 50cfm) your max hp draw is 53. 1hp is about 750 watts - your wattage is about 40,000 watts or 40KW. If your power cost is 10cents/KWh it then costs you at most $4/hr for power. Realistically, you're not operating the spindle at max power ever and the pumps probably aren't maxing out either - so your actual cost is probably less than half - or around $2/hr. You could fine tune this a lot more but I don't see the point. I think this simple analysis will point out your power cost is not a real material percent of your CNC operating cost.


From the original questioner:
You are right to assume that the cost per hour is somewhat negligible as long as I am running flat 2-D programming. However, when you switch to 3-D processing, machine times can go well over five hours for relatively small parts. So even at two bucks an hour at five hours I need to make sure that that $10 is factored into the cost of the product.


From contributor T:
In my opinion, it would be easier just to take a historical average of your utilities to figure into your total burdened rate. I encounter this often when someone is buying a large piece of machinery or equipment for the first time and they want to get a feel for their cost increases. The calculations provided do provide that. There are some hidden phantom loads and such also that are harder to computer. Upon follow up, more often than not, I am told they get very accurate results by monitoring their electrical bills, to the point of they can predict their bill based on machinery usage. Granted, this approach is not effective right out of the gate, nor if your margin is so tight that you canít buy groceries because of the difference!


From contributor H:
Regardless of whether you are doing 2D or 3D work, the rate is the same. As another poster wrote, "power cost is not a real material percent of your CNC operating cost." That's why I asked how often you operate your machine. My hunch is it's not enough to waste your time trying to determine your cost of electricity. Also, unless your electric bill is different than mine, a portion of the cost is a fixed price for simply having the electricity available.


From the original questioner:
There is a huge difference in the cost of the item being produced when going from 2D to 3D. I can process literally 100 pieces or more per hour in 2D cut-out part production. In that case the cost of electricity to produce one piece is negligible. However, start cutting a 3-D part, and it takes you five hours to process that one part, then the cost of electricity becomes a factor. As I stated above, if the cost of electricity is even $2 per hour and it takes five hours to produce that one part, then I have to factor in the $10 of electricity used to make that one part as a cost of that one part. That $10 electrical cost is just as pertinent as the cost of the material I am using. This could also be seen as a cost of labor. The machine is doing the work and I need to know how much that machine is costing me per hour to work, in order to compute selling price of the product being produced by the machine.


From contributor G:
If you are producing a part on the CNC that is taking five hours to make, it is still the $2/hr in electricity. If you are charging what you should for the CNC ($65-$200hr) this $2/hr is still in the neglect-able range.



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