February 13, 2007
Thinking about purchasing a jet cutting system? Many factors should be considered before deciding which system to buy. This article discusses fixed and operating costs and productivity and provides the framework for making an intelligent buying decision.
The purpose in buying machinery is to make money with it. Buying nothing costs nothing, but makes no money. A premium jet cutting machine costs more than a standard machine, but the better machine usually is designed to enhance productivity. The equipment must match the work it is intended to do. When deciding which machine to purchase, a buyer must strike the balance between price, productivity, and machine capability.
A job shop and a manufacturer have slightly different considerations when choosing a jet cutting machine. The manufacturer knows exactly which parts he will be making and the production volumes. Some factors for the manufacturer to consider are:
A job shop owner is concerned with these factors as well, but he also may worry that his machine will be too small for the work his customers might request. He tends to buy a machine for the largest imaginable part. Machine accuracy sometimes is overlooked, but customers prefer precise parts that require little or no secondary processing. The shop's revenue depends on the ability to produce the parts the customer wants.
Costs include capital costs that are fixed, no matter how much the machine is operated, and those determined by the machine's operating time. Let's consider two shops to see the effects of these two costs. The first shop operates its machine part-time, about 24 hours each week. The second shop operates two shifts and runs the machine 70 hours weekly. We will use these operation times to calculate the hourly capital cost and the hourly operating costs.
Capital Costs—Say a machine costs $100,000 and is financed with a five-year loan at 8.5 percent. The monthly payment is $2,052. Operating at 24 and 70 hours per week, the hourly capital costs are $19.73 and $6.76, respectively. (To calculate capital costs for more or less expensive machines, ratio the price with the cost.)
Labor Costs—An operator might be paid as much as $20 per hour with a fringe rate for vacation, health insurance, and other benefits of 40 percent. This results in an hourly cost of $28. But, the jet cutting machine runs more or less by itself after the initial setup is accomplished, and one operator can run multiple machines or perform other useful tasks while the machine is completing its work. For this example, we allocate half of these costs ($14 per hour) to the part-making costs.
Abrasives—Abrasives represent the largest consumable cost in abrasive jet cutting. Garnet abrasive costs are highly transportation-dependent and range from a low of about 10 cents per pound to as high as 72 cents per pound in Hawaii. Costs also somewhat depend on the garnet grade and can vary by a factor of 3 based on quality. It generally is not a good idea to run a jet cutting machine with low-grade garnet full of dust and dirt particles, because these impurities cause frequent nozzle plugging and lost production. A moderate price for garnet is about 30 cents per pound.
Garnet flow rates are from about 0.5 pound per minute to about 1.5 lbs. per minute in the larger nozzles in use today. At an average flow of 1 lb. per minute with 30 cents per lb. of garnet, the hourly cost is $18 per hour.
Maintenance—Abrasive jet machinery has a higher maintenance cost than most other machine tools. Flowing abrasive wears out all parts that it touches, and in particular, the mixing tube in which the abrasive is accelerated to high speed by the water wears out in about 100 hours. Between $1 and $2 per hour should be allocated for replacing parts worn by the abrasive.
The high-pressure pumps and plumbing systems that provide water to the nozzle also require maintenance and incur costs. The higher the pressure, the higher the maintenance costs. Normal maintenance items in these components are seals and check valve seats. Occasionally other high-pressure parts fail from metal fatigue. A good budget for pump maintenance is about $5 per hour, giving an overall maintenance cost of up to $7 per hour.
Electricity—Many shops consider electric power as an overhead cost, like building rent, but it is a real cost of operating an abrasive jet cutting system. The cost depends on the pump type being used. At 10 cents per kilowatt hour, a 0.014-in. orifice operating at 55,000 PSI with a crank-drive pump costs about $2.48 per hour, whereas the less efficient intensifier pump costs about $3.26 per hour in electricity.
Many utilities charge a fixed rate for water usage that also includes the sewer charge. A price of about 1 cent per gallon is typical. The 0.014-in. orifice operating at 55.000 PSI draws just under 1 GPM for a cost of 60 cents per hour. An intensifier pump often is cooled with an additional 2 GPM of water, resulting in a total cost of $1.80 per hour.
Total costs for operating an abrasive jet 24 (Figure 1) and 70 (Figure 2) hours per week are:
|Costs||24 Hours/Week||70 Hours/Week|
Jet cutting costs are much greater than the costs for most other machining processes. So why is this process the fastest-growing segment of the machine tool industry? The answer is productivity. Job shops charge $100 per hour and up for nozzle time that costs no more than $65 per hour. Yet, the customer feels that the part price is low because the part time is low.
The major factor affecting part time is the software that drives the machine. The jet is not a rigid tool, and an optimum tool path is one that varies the speed according to the shape being cut. The jet exit point lags the entry point by an amount that depends on the cutting speed (Figure 3). A large lag is acceptable on straight lines, but it causes major accuracy errors on arcs and corners. Modern control software automatically adjusts for this effect and does a much better job than a human can (Figure 4). The part time difference between a control that runs at a constant compromise speed for corners and arcs and one that properly varies the speed everywhere along the path can be more than a factor of 2.
Software also controls the taper in the kerf to make a square edge. As the jet slows down, the kerf goes from widest at the top through parallel to widest at the bottom. There is a speed at which the taper is minimal. However, cutting the entire part at this speed takes a very long time. Through software, a two-dimensional flat part is input, but the machine automatically moves in five axes to compensate for and remove the edge taper. Again, the precise part is made much more quickly with the taper-removing software.
Why is part time so important? Consider a part that takes 10 minutes to make on a premium machine and only 30 percent longer, or 13 minutes, on a standard machine. Suppose that both machines operate at a $60 per hour or $1-per-minute cost. The part cost on the premium machine is $10, and it is $13 with the lesser machine. Now suppose that the machines use the entire 24 hours per week to make these parts or parts like them. The premium machine will make 7,488 parts in a year. The other machine will make only 5,760 in a year with the same operational costs. To make the same quantity as the premium machine, the standard machine would have to run an additional 374.4 hours (7.2 hours per week) at an additional cost of $22,464.
An even greater difference is apparent if the gross margin is considered. Suppose that the customer is willing to pay $20 for the part being made. The premium machine makes 7,488 of these parts at a margin of $10 each, for a total margin of $74,880. The standard machine makes 5,760 at a margin of $7 each, for a total of $40,320. The difference is an astonishing $34,560. The premium-machine owner could pay $30,000 more for his machine and get all his money back in less than one year. All these numbers are higher if the machine is used more than 24 hours per week.
Sadly, many buyers never take the time to do time studies on the machines they are considering. Some buyers have a part made on a premium machine and then buy a lower-priced machine, based on equipment cost alone, without ever having the test part made on the machine. These buyers learn what they have missed only when they are forced to compete with a premium machine. Buyers are urged to compare actual part times and make an economic analysis before purchasing any machine.