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How much does waterjet cutting cost?
August 26, 2013
What makes up the cost of waterjet cutting?
The cost to operate a waterjet is a common question, as people struggle to understand the difference between running a typical plasma cutting machine and a waterjet cutter. There are different consumables, different power requirements, cost of water, and most importantly, the cost of abrasive. Garnet abrasive can account for as much as 75 percent of the total hourly operating cost of a waterjet cutting system, so understanding what drives its use and how much it costs will be very important to the customer considering a waterjet machine.
To dig into this question a little deeper, I looked at six different waterjet system configurations, including:
- 60,000 PSI / 50 HP pump
- 60,000 PSI / 100 HP pump
- 60,000 PSI / 100 HP pump running two cutting heads
- 90,000 PSI / 60 HP pump
- 90,000 PSI / 125 HP pump
- 90,000 PSI / 125 HP pump running two cutting heads.
For each of these, I figured the cost of power, wear parts, water, and abrasive, on a cost-per-hour basis, as shown in the chart below. Each of these costs represents the maximum water and abrasive flow rate for the largest orifice that can be driven by each pump at it’s maximum pressure.
If you take the same data and look at each number as a percentage of overall cost, it looks like this:
Cost of water
The smallest slice of the pie is the cost of water. I’m using a national average of $3.51 per 1000 gallons, and I’m including both maximum cutting water flow and cooling water for the hydraulics. That cost is going to total up between $1,200 and $2,500 per year. That does not take into account any additional cost for sewer, so depending on your locale, that cost could be higher.
Cost of power
The next chunk of cost is the power to run the intensifier pump. Assuming an average of 7.46 cents per KWh, this is going to come in between $3,700 and $9,400 per year. This assumes the pump is running 6 hours per day, and looking at pumps between 50HP and 125HP.
Cost of wear parts
The next biggest cost is wear parts. This includes replacing the orifice, focusing tubes, pump seals, and the cost of miscellaneous pump parts calculated out on a cost-per-hour basis. This is going to range between $5 and $22 per hour, so using the same assumptions, it will total up to anywhere from $7,700 to $32,000 per year.
Cost of abrasive
The biggest chunk of cost comes from the abrasive. Typical garnet abrasive will cost somewhere in the range of 22 to 30 cents per pound, depending on type of abrasive, purchasing volume, and negotiated discount.
There are two main types of garnet: hard rock and alluvial. Hard rock, or crushed rock is just that, crushed down to a specific mesh size from larger chunks of garnet rock. Hard rock garnet is sharper, and therefore typically cuts a little faster. Alluvial garnet is mined from beaches and river beds, already worn down to sand by natural forces. Therefore, its grains are rounded, and cut a little bit slower. Obviously, you will pay more for hard rock abrasive than you will for alluvial. For an average, lets consider 25 cents per pound.
Next, consider the abrasive flow rate. Most metal plate cutting is going to use somewhere between 1 to 2.3 pounds-per-minute. Therefore, on average, you are going to be spending between $18 to $36 per hour on abrasive. Assuming 6 hours of cutting time per day, that's $110 to $220 of abrasive per day, or $550 - $1,100 per week, $2,200 to $4,400 per month, $26,000 to $55,000 per year.
Now, to make these numbers useful, we need to take into account the differences in cutting speed, so we can calculate a cost-per-inch, or cost-per-part, which is much more useful in comparing one process to the next. For this calculation, I assumed a 4” x 4” part cut from 1” thick mild steel, for a total of 16” of cut length. The resulting cost-per-part is shown below.
The actual values will change depending on the material type and part size, but the relative cost will stay the same. This chart looks as if it is useful to illustrate the cost savings advantage of higher pressure pumps: even though the actual cost-per-hour is higher, the resulting cost-per-part is lower.
However, one important piece of the puzzle is still missing, that will make a big difference. The above chart does not include labor, overhead, or burden rates. Once an hourly rate is included for those factors, we can calculate the difference in ROI from one type of pump to the next, which is helpful in justifying the purchase of the higher priced pumps.
In the chart below I have added an hourly rate to represent labor & overhead, then spread that rate over the total number of parts that could be cut in a period of time. The resulting chart below shows how the increased speed of cut with the higher pressure pumps impacts the overall cost.
Of course, the calculations don’t have to stop here. The next thing to look at is the increased number of parts that can be cut in a day, and then the increased profit that could be made from those parts, and then we need to factor in the difference in equipment cost, all leading toward a final ROI calculation. But that will have to be the subject of a future post.