February 2010 / Volume 62 / Issue 2|
Selecting coolant scientifically
By Mark W. Scherer, Master Chemical Corp.
| Using an independent laboratory to test different coolants can provide the impartial data needed to make the right choice for machining titanium.
Extending cutting tool life provides an important competitive edge in a difficult economy. Using the proper coolant for a process can increase tool life 15 to 30 percent. But how does an end user know which coolant is the right one?
Using the right coolant is especially important in the aerospace parts industry, which is intensely competitive. An uncertain economic climate only increases pressure on part manufacturers to keep costs under control. Programs are optimized, new tool technologies are tested and machines are upgraded.
Many aircraft parts are made from titanium alloys due to their strength, light weight and corrosion resistance. However, titanium is a poor thermal conductor, and heat generated by the cutting action can reach 2,000° F, quickly dulling cutting tools. Applying dull cutting edges can generate even more heat, further shortening tool life. The issue of adequate lubricity is central to effective titanium machining because poor lubri-city is a common reason cutting tools fail.
Courtesy of All images: Master Chemical Evaluating Effectiveness
How can you evaluate coolant to measure its effectiveness with respect to tool life and cycle optimization? According to Milton Hoff, vice president of strategic technology development for Master Chemical Corp., Perrysburg, Ohio, one way is to measure spindle motor load. “Spindle motor load can be monitored most effectively through measurement of the current change during a cut,” he said. “Most machine tool controllers can monitor the maximum amp load during a standardized pass or cut. If a machine doesn’t, you can set up a stand-alone ammeter to measure the change in current draw during a machining step.”
With either method, the starting current and maximum current need to be recorded.
Determining which metalworking fluids to use when machining titanium requires careful consideration. Synthetic fluids are low-foaming, have good filterability and cleanliness and dissipate heat, while soluble oils offer enhanced lubricity. Semisynthetics can offer the best of both worlds by providing effective lubricity and cleanliness while helping to extend sump life. In the end, multiple factors must be reviewed, including machine and operator requirements, water quality, fluid management, and fluid delivery and pressure.
The most effective analysis requires test cutting the same workpiece materials with the tools used in the process being evaluated. “You can machine a piece with everything else being constant except the coolant, and you can measure how much load is on the machine spindle by the current draw and determine which has more lubricity,” Hoff said.
Conducting these tests in-house is problematic because machine availability, different operators and inconsistent coolant maintenance create variables, which can distort results. Use of an outside laboratory for fluid evaluation provides the impartial scientific data needed for effective decision making. Master Chemical, for example, has partnered with the machining laboratory at Owens Community College, also in Perrysburg, to test machining fluids and processes.Problem Solving
A Tier 1 aerospace parts manufacturer was machining large quantities of Ti-6Al-4V at its various plants and observing rapid growth in its use of that titanium alloy. Each of its plants was applying a different metalworking fluid when machining titanium, and the manufacturer was interested in selecting one fluid to extend tool life and thereby reduce tool costs. To further reduce machining costs, the manufacturer wanted to select the most efficient titanium machining process.
When machining titanium, tool costs can be extreme, so it was vital to be able to maximize tool life without sacrificing part quality. The manufacturer did not possess data that showed which of the cutting fluids it applied was the best choice for standardization.
Working with Master Chemical, the aerospace manufacturer decided to evaluate the fluids at its plants using the Owens’ testing facility because conducting on-site tests was not an option. Using the college’s machining laboratory offered the manufacturer the ability to run impartial tests without tying up its own resources. The lab would run tests to determine which fluids provided sufficient lubricity by measuring the cutting parameters.
Machine tools for cutting titanium require fluids that minimize mist, reduce the chance for workers to slip on large machine tables when walking on them to sweep chips away and eliminate sticky residue that causes titanium chips to adhere to the machine tool. After the machining tests were completed, laboratory tests would be run to compare the fluids for critical noncutting functions, including biostability, sump life and health and safety concerns.
The scope of the test focused on applying a Weldon 4-flute, 0.750 "-dia., uncoated, M42 cobalt endmill with a 0.125 " corner radius. The tool was Crest Cut roughing style. The cutting parameters at full diameter were 0.350 " deep axially, a 60-sfm cutting speed, a chip load per tooth of 0.0045 " and a feed rate of 4.0 ipm. Climb cutting was employed.
The tests provided two runs per fluid on 7 "×8 "×60 " Ti-6Al-4V billet plus one run per fluid on Ti-6Al-4V plates supplied by the aerospace manufacturer for a total of three runs per fluid. Machining was performed at the Owens machining laboratory on a Haas VF4 vertical machining center with a CAT 40 taper and computer-directed coolant flow.
Five machining fluids were evaluated, including Master Chemical’s TRIM MicroSol 585 and four competitive cutting fluids. Each fluid was run at 6 percent concentration, diluted in deionized water. Tool life was the determining factor. All chips and material were returned to the manufacturer upon completion of the tests.Measurements and Results
Running the parameters previously described, the expected tool life was 3 hours. The tools were examined microscopically with an OGP optical comparator, built-up edge was measured and photographs were taken. This happened after about every 30 minutes of cutting time, once a tool completed a pass through the material block to avoid workhardening. Readings of the machine spindle amp-load meter were also documented.
The original plan called for measuring flank wear. However, due to the configuration of the cutter, which was helical and scalloped and did not have a straight edge to enable the lab to use as a reference point for measuring actual flank wear, BUE was chosen instead. The manufacturer’s materials and process engineer observed the initial runs at the machining laboratory and concluded the test protocol was satisfactory.
The fluids were compared using maximum amp load as recorded by the machine tool controller. The motor amps increased during the course of the machining process. The average maximum motor amps were recorded during all of the passes, with the upper and lower natural process limits documented. Lower average and tighter natural process limits the implied, or predicted, longer tool life (Figures 1 and 2).
Tool life was also predicted by measuring the largest amount of BUE found on any of the four cutting edges after completion of a set number of passes. That’s because a tool with more BUE will be duller and therefore have a shorter life. Fluids A and B recorded the lowest overall average BUE and the tightest natural process limits, indicating the longest tool life (Figure 3).
All five fluids were tested for critical noncutting functions, including residue, bioresistance and corrosion protection. The best performer in each category was given a score of five; the next best a four and so on. In the residue test, Fluids A and E scored the best, while fluids B and D left significant amounts of residue on the machine, gaging equipment and surrounding areas. These fluids were determined to have a higher probability of leaving slippery residue on the machine table as well. Residue also results in higher coolant usage rates.
In the foam downtime test, fluids B and C showed excellent foam characteristics. In the test for ferrous corrosion in tap water, fluids A, D and F received high marks while fluid C produced significant corrosion on ferrous metals. In the biostability test, fluids C and D scored high marks while fluid B didn’t, meaning fluid B would likely have rancidity problems. In the fungal stability test, fluids C and E rated best while fluid B showed poor fungal stability. Fluids A and C scored highest in the overall score ranking for all noncutting and cutting functions (Figure 4).
The data was then provided to the aerospace manufacturer for review. “Evaluating the data from this test can help select the coolant that can maximize tool life for your machining processes,” Hoff said. “Of course, this is a controlled test, which doesn’t take into consideration some of the real-life variables that can and will occur. But it can point you in the right direction to select a fluid that has acceptable lubricity for your processes.”
Chuck Gee, Master Chemical’s aerospace segment manager, used the case study to illustrate the benefits of outside testing. “I was showing the case study to a manufacturing engineer who was evaluating our products for his firm,” he said. “The idea of making a decision based solely on scientific data had tremendous appeal, and he is scheduling an evaluation for his firm.”
While outside testing of metalworking fluids is still not common within the industry, demand is increasing. Master Chemical provides the service about once per month. “You can base decisions on data, put numbers, graphs and charts in front of an engineer, and when he knows it is an independent study, run in a controlled setting, he trusts the results,” Gee said.
“The benefit to customers is that they are making the decision based on science. Knowing which coolants will allow them to run a tool 20 percent longer represents a big savings of time and money,” Hoff said. “Understanding which coolants are most effective in preventing problems with foam, extending sump life and providing biostability improves the overall quality of the workplace.” CTEAbout the Author: Mark W. Scherer is manager-communications for Master Chemical Corp., Perrysburg, Ohio. For more information about the company’s cutting and grinding fluids, cutting oils, concentrated washing and cleaning compounds and rust preventives, call (419) 874-7902, visit www.masterchemical.com.
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