Precision boring of complex aerospace parts requires good tools, machines, and coolants, and a defined, repeatable process. With very demanding requirements, the difference between a good aerospace part and a scrapped one can be just a few tenths of an inch on a bore. The tolerances are not much different than in other industries, but the penalty for a scrap part due to an oversized bore is completely opposite. Aerospace workpieces are usually made from hard metals such as stainless steel, Inconel, or titanium – all very tough materials to precision bore – with some forgings costing more than $30,000 each before the first cutting tool starts to work. To make the situation even more difficult, the precision boring operations are usually the last to be done, so any scrap due to oversize holes means hours of previous milling and threading are lost.
LORD Corp., a Cary, North Carolina, manufacturer of expensive and complex aerospace parts was working on a prototype design to produce an engine mount at its Dayton, Ohio, facility. The company’s engineers had a difficult time producing precision bores on these parts in tolerance, and the cost for producing a single scrapped part was equal to the company’s entire monthly budget. The need to develop a process improvement became a top priority.
The engine’s forging – made from Inconel 718 – was heat treated to Rc40. The mechanical properties for this material classified it as a nickel-based superalloy, developed for high-strength, high temperature tolerance, and corrosion resistance. These characteristics make for very low machinability resulting in high cutting forces, generating more heat at the tool tip, leading to stringy and difficult-to-break chips. The material’s high nickel content also made it very abrasive, increasing tool wear. In addition, final machining and boring after heat treat intensifies the all-around difficulty.
LORD approached BIG Kaiser U.S. and Blaser Swisslube to find out how to improve boring process reliability because scrap due to an oversizehole was not tolerated under any circumstance.
“We need to instill the confidence in our engineers and operators that the parts can be bored,” says Greg Bernett, process improvement engineer for LORD.
BIG Kaiser engineers accepted the challenge and asked long-term partner Blaser Swisslube to participate and make thorough practical tests at its facility in Switzerland. Equipped with modern CNC machine tools and measuring equipment, Blaser Swisslube offered an ideal environment for the joint project. A sample workpiece of the same material and heat treat was procured, the test tools prepared, and travel arrangements by LORD and BIG Kaiser were made to Switzerland to participate in the tests.
The test procedure was based on real world requirements from LORD: Only after a hole was successfully bored in tolerance, could engineers move on to the next one. Each hole was considered a nearly completed workpiece, worth thousands of dollars, and could not be left undersized, have excessive taper, or have bad surface finish. Most importantly, it could not be bored oversize.
“Once the hole was in tolerance, we could move to the next hole and so on, establishing not only a repeatable procedure, but also expected tool life,” says Jack Burley, vice president of sales and engineering for BIG Kaiser U.S.
The key was finding out how bore results varied based on tools with different lengths and inserts with different geometries and sharpness.
Many parameters influenced the finishing process with the operation’s tight-tolerance requirements and low-machinability material. The solution was to manufacture every hole with two passes – with almost no adjustment in the diameter of the cutting edge in between. Due to radial forces, the first pass didn’t finish the hole, so a second spring pass reached the tolerance. The spring pass removed 0.0015" more diameter material than the first pass.
As inserts wear, bores get smaller with each successive cut, so when post-spring-pass bore is in the lowest quadrant of the specifications, the head adjusts to compensate. BIG Kaiser’s EWD digital head line enables these frequent adjustments.
“The heads give a zero setting that can always be a new starting point for a new insert, and each adjustment is stored in the head to give a total life for each insert, so every operator knows where the tool is,” Burley says. “Another difficulty was to reasonably predict tool deflection, something that is easier on softer metals. The longer the tool became, the more difficult it was. When tool wear is compounded with harmonics and sharper inserts, this material proves even more difficult to precision bore. We could do it, but the process was certainly much harder than with shorter tools.”
Stefan Appenzeller, head of product management at BIG Kaiser Switzerland, says the biggest challenge was to work out a rule for how and when to adjust the boring head. “Insert wear is about 0.0004" per bore, which helped us to predict when a tool needed to be adjusted.”
Bernett was happy that the tests were made as close to reality as possible. “Big Kaiser and Blaser Swisslube were asking for my feedback to see if each solution they proposed would be possible to re-create on the floor at LORD,” Bernett says, noting that it was interesting to see how the engineers could systematically and methodically approach the boring problems with solutions. “This was a true learning experience for me. Blaser Swisslube as our coolant partner brings in a huge know-how in regards to the whole boring process.”
“LORD is working with a coolant pressure of 80 bar. In the U.S. we have soft water and the coolant solution has to cope with the given circumstances,” says Christoph Wuethrich, head of manufacturing technology at Blaser Swisslube.
During the testing, Blaser was able to prove the current coolant is the optimal choice for the situation at LORD. Wuethrich says it is crucial to focus on the interaction of material, operation, coolant, and tool so the best results can be achieved.
Burley agrees. “I thought I had solved just about the all the toughest problems, but this material and the costs associated due to scrap were the most challenging tests I have ever made.”
BIG Kaiser U.S.
Blaser Swisslube AG