What is the true cost of gate removal and cleaning? Eagle Alloy recently challenged the traditional notions of evaluating cleaning costs to determine the best way to remove gating systems in its steel casting plant.
Eagle Alloy is part of the Eagle Group of companies, which includes Eagle Alloy, Eagle Precision Cast Parts (an investment casting facility primarily pouring steel) and Eagle Aluminum Cast Products (a permanent mold and green sand aluminum plant). With experiments centered at the steel shell molding plant, the ultimate question was: what gating removal and cleaning method yields the best routing value (with value being defined as quality divided by price)?
While the facility could not determine a standard cost per square inch of gating material removed because of the different methodologies used and operator variability, it was able to rank each of its cutoff methods by economic effectiveness.
CUTOFF OPTIONS CONSIDERED
Eagle Alloy first considered the oxy-fuel torch cutoff method in its in-house experiment. Initially, the facility used this method with acetylene and oxygen for cutoff. It then began using MAPP gas, lowering costs considerably. About 20 years ago, a gas compressor was made available that would increase the pressure of the natural gas traditionally held in a plant from 3 psi to 12 psi so it could be used with a torch system. Eagle Alloy negotiated with the gas company and was able to install a separate natural gas line from the meter to the cutoff area at 12 psi, enabling torching with natural gas and again lowering costs.
Eagle Alloy traditionally uses the chop saw gate cutoff method, when the gate configuration allows it. The company uses a 20- or 22-in. abrasive cutoff saw and has experimented with many minerals, as well as a hot press vs. cold press wheel. Fig. 1 shows a coolant system that was added recently for cutting crack-sensitive materials. The system has dramatically reduced cracking.
[FIGURE 1 OMITTED]
Friction Band Saw
Eagle Alloy used a friction band saw for many years, and the company's investment casting facility, which produces primarily stainless steel castings, still uses it today. The saw cuts stainless more easily than carbon steel, but with the gate sizes used at Eagle Alloy, the group deemed it unsafe and eliminated the operation.
Auto Band Saw
Eagle Alloy has experimented with a traditional band saw on several carbon steel parts. The saw allows an operator to perform different tasks while it is in operation. Also, the consumable costs associated with the saw are significantly less than other traditional methods. The downside is the product must be normalized before cutting in order to optimize the use of the consumables.
The shell molding facility also has experimented with a feeder band saw, generally used to cut tubing in fabrication shops. In this application, the part's gating is cut off from one side, and if the cut is parallel, the saw automatically moves to the next cut on the other side of the gate. This method is particularly effective for higher volume work.
CLEAN UP OPTIONS CONSIDERED
The Abrasive Wheel
For the size of castings produced at Eagle Alloy, the abrasive grinding wheel is the traditional cleaning implement. The facility uses a 30-in. snag grinding wheel with a bar grind. With this tool, operators can use their body to create more pressure against the wheel (Fig. 2). Eagle Alloy also has used a swing frame grinder, as well as cup grinders using the abrasive wheel methodologies.
[FIGURE 2 OMITTED]
Depending on the size of the rod, as well as operator sensitivity, Eagle Alloy cleaning room operators also use arc welding to smooth gate areas. The method consumes more energy but can be equivalent to a snag grinder in some cases and maintains a high level of quality.
Eagle Alloy also has had success with belt grinding gate contacts. The facility currently uses a belt grinder that is safe and requires an acceptable amount of pressure (the grinder puts 400 psi against the belt). However, the method is compatible with only certain gate configurations. In Fig. 3, a three-on fixture that is compatible with the belt grinding method is shown.
[FIGURE 3 OMITTED]
TWO WHEEL TESTS
In order to determine how much it costs per part (and square inch) to remove gates and clean contact areas, Eagle Alloy first used a cold wheel test. The wheels at the time of the experiment cost $19.87 (note: prices in this article represent an adjusted value and should he used for comparison purposes only) and offered 235 sq. in. of usable wheel, making them $0.08 per usable square inch.
The facility cut 1,604 sq. in. of the first part in the experiment (part A) with 1,125 sq. in. of usable wheel: (1,125 * $0.08)/1,604 = $0.06/sq. in. = $0.24 per part (4 sq. in.)
Industry standards indicate hot wheels are more durable than cold wheels, so the facility next performed the test using a hot wheel at $36.25 for 235 sq. in. of usable wheel, or $0.15 per usable square inch. Eagle Alloy cut 2,055 sq. in. of part A, with 1,202 sq. in. used: (1,202 * $0.15)/2,055 = $0.9/sq. in. = $0.36/part (4 sq. in.)
Through this calculation, Eagle Alloy determined hot wheels at the given price were 33% more expensive than cold wheels.
OXY-FUEL TORCH TEST
Eagle Alloy compared the cost of consumables with the oxy-fuel torch using a #6 torch tip. Many variables come into play when using this method. In addition to cut costs, at-rest costs are introduced, as the gas remains constant in both states. The oxygen, however, changes. At rest, oxygen flows at 55 cu. ft. per hour. While cutting, it flows at 350 cu. ft. per hour.
At the time of the experiment, the price of oxygen was $0.004 cu. ft., and gas was $0.00347 cu. ft. Following is a calculation of the cut cost for 100 pieces per hour with 20% cut time and 80% rest time:
Cut gas: [O.sup.2] = 350 CFH at $0.004/cu. ft.
CH4 = 20 CFH at $0.00347/cu. ft.
At rest gas: [O.sup.2] = 55 CFH
CH4 = 20 CFH
Cut: [O.sup.2] = 0.2 hour * 350 CFH * $0.004 = $0.28/100
CH4 = 0.2 hour * 20 CFH * $0.00347 = $0.014/100
Rest: [O.sup.2] = 0.8 hour * 55 CFH * $.004 = $0.176/100
CH4 = 0.8 hour * 20 CFH * $0.00347 = $0.056/100 $0.526/100 or $0.0053 per part
The cost per part at the time of the experiment was $0.0053, which is significantly less than the other methodologies. If the calculation were altered to assume 100% cut time, it would yield:
[O.sup.2] = 350 * $0.004 = $1.40/100
CH4 = 20 * $0.00347 = $0.07/100
$1.47/100 or $0.015/part
At 100% cut time, this method was still significantly less expensive than using cutoff wheels. With a 4 sq. in. part, the cold wheel cost $0.24/part, the hot wheel was $0.36/part and the oxy-fuel torch was $0.02/part.
AUTO-BAND SAW TEST
The auto-band saw cutoff method has shown an ability to save Eagle Alloy money primarily because the operator can leave the operation and perform other duties. Although the facility has to normalize the parts prior to cutting and the contacts have to be configured, it attempted a case study to determine whether it was less expensive than the typical torch/snag operation. Rigid fixtures were required to eliminate cleanup, and that allowed the experimenters to skip the abrasive wheel operation.
Labor costs for the torch method and snag grinder cleaning totaled $1.97 per hour, whereas the auto-saw cell at Eagle Alloy cost $1.36 per hour with two men working. The auto-saw method incurred an additional $0.32 for heat treating, whereas the torch method required none. The consumables costs were $0.35 per part for the torch method and $0.15 for the auto-saw. Taken all together, the total costs were $2.32 per part for the torch method and $1.83 per part for the auto-saw, a difference of $0.49 per part. The savings for the auto-band saw with consumables and normalization of the part before cutoff was $70,560 per year, given an annual volume of 144,000 parts.
SNAG VS. ARC CLEAN UP
When assessing the costs of cleaning gate areas after cutoff, the snag grinding operation was performed using a 30 in. wheel with different minerals. Employee variability occurs primarily due to wheel dressers, as Eagle Alloy found some employees preferred using wheel dressers, while others did not.
Two methods of measurements were used: the cost per square inch of the wheel and the cost per pound. Cost per pound was the preferred methodology, as it was easier to weigh the wheel before and after grinding the parts. The cost of the wheel at the time of the experiment was $3.19 per pound of usable wheel. The experimenters determined that grinding 118 pieces of part A used 31 lbs. of wheel, or 0.2627 pounds/part, a consumables cost of $0.84/part.
When arcing the same part smooth, the experimenters were able to complete 105 pieces using 21 arc rods. At $0.70/rod, this yielded a consumables cost of $0.14/part. Including labor, arcing was therefore $0.70, or 83%, less expensive than snag grinding.
Visit www.moderncasting.com for an article on cleaning room automation.
The auto saw offered savings over the torch for some parts.
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|Date:||Mar 1, 2011|
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