By Jeff Day
Over the past 5 years, the number of waterjet sold worldwide, both abrasive and pure water, has increased by an average of 18%. That growth is projected to continue for the foreseeable future. As more waterjets are purchased, current and new waterjet owners must look for ways to increase profitability. This article will attempt to point out a few ways that waterjet cutting can be more profitable. This article focuses on abrasive waterjet cutting.
Following is the typical representation of the breakdown of costs per hour for an abrasive waterjet, with abrasive being by far the largest portion of the cost.
Figure 1
While this is certainly true when looking at variable costs, when you consider fixed costs and labor costs, the picture changes dramatically.
Figure 2
With this more complete picture of costs associated with waterjet cutting, abrasive is still the largest cost-driver of waterjet cutting. Certainly anything that can be done to reduce abrasive consumption will have a great effect on cost per part. However, as an end-user, you can also focus on the other big factors: labor and the fixed costs of depreciation, interest and overhead.
Using a smaller water nozzle/abrasive nozzle combination can reduce abrasive consumption dramatically. Generally people will cut with a 0.014” water nozzle and a 0.040” or 0.043” abrasive nozzle and use anywhere from 1.2 to 2 pounds of abrasive per minute. By using a 0.010”/0.030”, abrasive consumption can be reduced to about 0.75 – 0.9 lb/min.
Most users are loath to do this though because a smaller water nozzle/abrasive nozzle combination means fewer parts per hour. While this is true if you can only cut with one head, cutting with 2 heads and the smaller nozzle combination will produce more parts per day at a lower abrasive cost per part. Not all parts are suitable for 2-head cutting because of part size, but with a 5’ x 10’ size table or larger, most waterjet users will find that the 2nd head can be used a large majority of the time. As an example, if you cut with the 0.014”/0.040” combination with one head at 20 inches per minute and 2 lb/min of abrasive, with the 0.010”/0.030” set-up, you would cut at 12 ipm and 1 lb/min of abrasive for similar quality. The cut speed is reduced by 40% and the abrasive is reduced by 50%. When using 2 heads, effective cut speed is 24 ipm (2 x 12 ipm) and abrasive is 2 x 1 lb/min. At $0.25/lb of abrasive, the smaller nozzle combination gives a cost of $0.021/in ((1 lb/min / 12 in/min)=0.083 lb/in * 0.25 $/lb = 0.021 $/in). The larger combination would be $0.025/in ((2 lb/min/ 20 in/min)= 0.1 lb/in * 0.25 $/lb = 0.025 $/in). This is a 16% savings per inch of cutting. If you consider how many inches of cutting you will do in a year, these fractions of a penny will add up dramatically.
Most intensifier pumps on the market are either 50 HP with 1.0 gallons per minute (gpm) of water output or 100 HP with 2.0 gpm. The 50 HP pumps will allow you to run either one head with 0.014” water nozzle or 2 heads with 0.010” at 55,000 psi. The 100 HP pumps can run either 2 heads at 0.014” or up to 4 heads with 0.010” at 55,000 psi. Another alternative on the market offered by some manufacturers is a 60 HP pump that puts out 1.2 – 1.3 gpm. The advantage of the 60 HP pumps is that you are able to run two heads with 0.011” water nozzles to get slightly higher speeds than with a 0.010” while not having to increase abrasive consumption significantly, if at all, since you will still use a 0.030” abrasive nozzle. The other advantage of the 60 HP pump is that you use much less electricity than a 100 HP pump.
CNC-abrasive metering can greatly reduce abrasive consumption. It takes less abrasive to cut with a “quality” cut than a “production” cut. A “Quality” cut might require 0.8 lb/min of abrasive, while a production cut might require 1.0-1.2 lb/min. If you have a part that requires higher quality in a particular area, but the rest of the part can be cut with a higher speed and lower quality, you can save 0.2-0.4 lb/min of abrasive by using abrasive metering and cutting the higher quality areas with less abrasive. Systems without abrasive metering cannot change abrasive feed rate during a part and will cut with the same amount of abrasive regardless of cutting quality desired.
Since only the bottom surface of the water jet stream is doing the work during the piercing process (Figure 3), CNC-controlled abrasive metering also allows you to reduce the amount of abrasive that is used during piercing. The amount of abrasive used during piercing can be reduced by 50% or more without increasing piercing time. Since it can take several seconds to pierce through thick material, reducing the amount of abrasive used during piercing can save several pounds of abrasive per day.
Figure 3 – Red lines show surface of abrasive waterjet stream that is eroding material. During piercing only a relatively small surface area of the waterjet stream is doing any work.
Abrasive metering in conjunction with variable pressure can also be used for etching of part identification marks or layout lines for downstream processes to reduce costs in those processes.
From a programming point of view, if you do parts that have long, straight edges, having the capability to do common line cutting can greatly reduce part times and costs. A 24 inch long straight edge on 2 adjacent pieces of 1” thick steel that only needs to be cut once with a common line cut, can save 6 minutes of cutting and abrasive consumption. At 0.9 lb/min of abrasive, this would save 5.9 pounds of abrasive on one common line cut.
Reducing abrasive consumption reduces direct part cost. It also reduces the amount of work required to empty the abrasive out of the machine’s tank and reduces disposal costs of the abrasive. Abrasive removal systems can reduce part cost eliminating the downtime that is required to remove abrasive from the machine’s catcher tank since the abrasive is being removed automatically during the cutting process. Anyone who has had to do the back-breaking work of shoveling out the sludge from a waterjet will appreciate an abrasive removal system
Closed-loop water recycling systems can save on water costs if the cost of water in your area is high. From an environmental impact, if you are concerned about putting your waste-water down the drain, the recycling system can reduce that concern. Also, since all of the abrasive does not settle out of the waste water before it goes down the drain, there have been situations where companies with abrasive waterjets have clogged up sewer lines downstream from there facility. They have then received significant repair bills from their municipality. Closed-loop water recycling systems can avoid this. Finally, the better the quality of the water that you put into your pump, the longer the life of all of your pump and cutting components.
Increasing the number of cutting heads on a waterjet has the most dramatic effect on increasing your number of parts per day. Automatic head spacing on 2-head systems is critical in order to get maximum material utilization out of nesting with 2 heads. Automatic head spacing should allow one head to automatically be turned off so that one head can be used on small strips of materials. Without this capability, large amounts of raw material can be wasted.
Besides cutting with multiple heads, material handling is another huge opportunity to reduce downtime on a waterjet. New users are soon struck by the large amount of time their operators spend unloading and loading the machine. Automatic shuttle tables offer an efficient means of optimizing the machine’s cutting time. They provide higher usage rates because the operator can safely load raw material onto the shuttle table and unload finished parts from the shuttle table while the machine is cutting with the cutting table. This virtually eliminates any machine idle time for material handling. Since the shuttle table is transported completely out of the cutting area a sheet loader can be added without interference with the cutting bridge.
Another way to optimize cutting time is to have a machine with a larger cutting area, so that you can cut in one area while you unload and load in another area (see Figure 4 Right). To safely use a machine in this way it must be equipped with appropriate guarding, software, and safety systems to ensure that the machine movements cannot injure anyone performing the loading or unloading while the cutting bridge is in operation. Safe operation generally requires a reasonable “dead zone” between the cutting area and area designated for loading and unloading. Large-table machines require a higher investment and more floor space, but buying one large-table machine can be less expensive than buying two separate machines and offers the added capability of processing very large individual parts.
Height sensing can reduce part cost. The closer that you can cut to the surface of the material, the faster you can cut since less of the jet’s energy is lost. Optimal for most nozzle set-ups is to cut 0.040” – 0.060” off of the surface. Cutting at this height will allow you to cut faster at any given part quality desired. Cutting closer to the surface also yields a better part quality because there will be less rounding of the top edge of the part. To do this in real life though requires the use of height sensing, since it would be unrealistic to expect an operator to manually achieve such results on a consistent basis. Also, trying to control height manually would mean that the operator would have to spend 100% of his time standing at the machine. This reduces the labor-savings potential of allowing the waterjet operator to perform other tasks (e.g. loading/unloading) while the machine is cutting. Collision detection/protection in conjunction with height sensing can be helpful to reduce the likelihood of breaking abrasive nozzles on tipped up parts, reducing consumable parts costs.
Programming:
Many companies are now beginning to consider the “soft costs” related to operation of machine tools. A good programming software system can save significantly by reducing engineering labor costs, optimizing material usage, reducing turnaround time from order to finished part, and some systems can even automatically interface to other operations like bending if required. Consider the cost savings if programming time can be reduced significantly, cycle time can be reduced, or material usage can be improved.
Common-line cutting is just one example; If you cut parts that have long, straight edges, common-line cutting can greatly reduce cycle time and material cost. A 24-in.-long straight edge on two adjacent pieces of 1-in.-thick steel that only needs to be cut once with a common-line cut can save six minutes of cutting and abrasive consumption. At 0.9 lb./min. of abrasive, this saves 5.9 lbs. of abrasive on only one common -line cut and also eliminates the material “web” that is required with two individual cuts.
Additional maintenance and training of personnel can do wonders to increase machine output. Once a year, having a technician from the manufacturer come in to spend a couple of days with your people can be very helpful in covering things that may have been forgotten over the course of time, going over advanced application questions that may have arisen, or correcting improper maintenance habits that may have snuck into your company’s practices.
Reducing the weight of your used abrasive by allowing it to dry can reduce your cost of disposal. If your sludge removal system drains most of the water while it is removing the sludge, this can reduce or eliminate this problem. What sort of container is used for disposal can also affect your disposal costs. It is best to talk with your waste hauler about costs and options for the disposal of your abrasive before you purchase your first water jet so you are not surprised by it the first time you need a pick up. Your waste hauler will perform tests on your waste normally once per year to check for hazardous waste levels. Be sure to provide them with a sample of unused abrasive that they can use as a control sample against your waste.
Conclusion
While abrasive is the largest variable cost of waterjet cutting, there are several things that waterjet owners can do to reduce this cost. Users should consider the economic gains of cutting with 2 heads with smaller nozzle combinations to produce parts more cost-effectively. Perform tests with different nozzle combinations to evaluate speed changes relative to abrasive consumption for systems under consideration. When looking at new equipment there are also many features and options that a user can invest in to reduce the fixed costs per part of waterjet cutting. Beyond number of cutting heads, material handling options can be a cost-effective option to reduced fixed costs per part. Features such as height sensing and collision protection can help to reduce the labor content of part costs by allowing the machine to run in a lightly-manned environment.
Jeff Day is product manager—waterjet, Bystronic Inc., 185 Commerce Drive, Hauppauge, NY 11788, 631-231-1212, , www.bystronic.com.
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