Rising to Zero
At Christopher Tool, "zero defects" is more than good intentions
Since machining its first aerospace part in the mid-1960s, Christopher Tool & Manufacturing Co. (Solon, OH) has always met the continuously tightening part-tolerance requirements of its customers. This success includes overcoming one particular challenge that began in 2007, when the initiative of “zero defects” was introduced into the aerospace industry. While some manufacturers supplying the industry took the concept with a grain of salt, Christopher Tool has taken substantial steps to attain that level of part quality.
The concept of zero defects is very literal, and the mandate is well enforced. So, aerospace shops continuing to be successful and competitive, such as Christopher Tool, are those now producing perfect parts that are always within specified tolerances.
“These days, aerospace OEMs will refuse any parts that deviate from tolerance,” said Patrick Christopher, president of Christopher Tool & Manufacturing Co. “All parts must be to print and documented with the proper paperwork, and shops must take initiatives and have corrective measures and benchmarking in place to ensure any past mistakes will not be repeated.”
All of this “mandatory continuous improvement,” he said, has significantly changed the aerospace industry landscape. Quality is paramount and must be maintained for parts that grow increasingly more complex. To fulfill these requirements, the shop constantly invests in expanding its technological capabilities—two recent examples being full simultaneous five-axis high-speed machining and rotary axis wire electrical discharge machining (EDM).
Because of these two machines, the shop can now process previously farmed-out parts in-house as well as shorten cycle times for many of its existing components. But most importantly, the machines make it easier to achieve better surface finishes and much tighter tolerances.
Propelled by its heavy involvement with the Joint Strike Fighter program, Christopher Tool added a GF AgieCharmilles HSM (high-speed machining) Mikron 600U LP full five-axis milling machine and Cut 200 wire EDM to its existing 35 or so pieces of CNC equipment. With a tilt/rotary table design, the milling machine delivers +30/-110° B-axis travel and 360° in C. The shop’s Cut 200 wire EDM features a rotary fourth (B) axis specifically for complex part geometries.
The HSM 600U LP’s high-speed comes from a 36,000-rpm 32-hp (24-kW) spindle and rapid traverse speed of 196 fpm (60 m/min). And for continuous production, it is equipped with an integrated pallet-changing system.
The large-size milling machine features linear motors on its main axes and optimized direct-drive motors for its B and C axes. Both the GF AgieCharmilles milling machine and wire EDM incorporate glass-scale positioning that provides Christopher Tool with extremely high precision and superior part surface finishing capabilities.
In addition to its involvement with the Joint Strike Fighter program, the shop has a worldwide customer base and supplies components to all the major aircraft jet engine builders. And while over half of the shop’s work is in aerospace/defense, it also manufactures components for the oil and gas industry, does prototyping for diesel engines and provides parts for robotics.
Production levels are low-to-medium volume, job lot sizes average between 30 and 50 pieces, and between 400 and 500 jobs are shipped per month—with 60% of those being recurring work, the rest new. In aircraft and aerospace work, the shop typically produces the prototype components and then gets the job to manufacture at production volumes.
Aerospace jobs can entail parts for pumps, actuation systems, fuel control systems and auxiliary power units for both defense and commercial aircraft. Most part sizes fall within a 10" (254-mm) cube, and they are machined from both raw stock material and castings.
Christopher Tool defines part complexity based on its geometric intricacy and tolerance requirements. The shop routinely works in true position tolerances between 0.0002 to 0.0005" (0.005–0.0127 mm), flatness/squareness tolerances to within 0.00005" (0.00127 mm)—some parts even 0.00002" (0.0005 mm)—and to Ra 2 surface finishes. Adding to the challenge, all parts are made from exotic and tough-to-machine Nickel alloys and hard to machine steels with low coefficient of expansion. Such materials include Monel, Hastalloy, titanium, carbide and powdered metal (CPM 10V) as well as precipitation hardened materials (17-4 and 15-5) and some high-speed steels such as T1, T15 and M2. And with all of its experience, the shop definitely categorizes itself as an expert in machining these materials and in constantly holding extremely tight tolerances in every part it produces.
The shop also has turning machines, vertical and horizontal machining centers, grinders and a coordinate measuring machine (CMM). But with the five-axis machine and rotary-axis EDM, many jobs previously done on the shop’s other CNC machines have been moved to these two newer machines.
“In addition to the impellers now machined in-house, we are processing several other parts on the HSM 600U LP, and doing so in single setups, allowing us to access all necessary part surfaces to shorten cycle times and further improve precision,” explained Craig Peck, vice-president of operations at Christopher Tool & Manufacturing Co. “Many of these parts previously required several different machines with multiple setups and operations. And with the rotary fourth-axis on the CUT 200 wire EDM, we can quickly and accurately burn extremely complicated features on round parts to eliminate any secondary operations on a turning or milling machine. Plus, we are NADCAP certified in EDM.”
He continued saying that the shop has experienced significant production improvement with about 30 part numbers now done on the HSM 600U LP. One part, for instance, required about four operations on a horizontal machining center and five separate ones on a knee mill. It is now machined complete in only two operations on the HSM 600U LP.
For an aerospace titanium end cap part that transferred over to the HSM 600U LP, processing previously required three CNC operations and three manual drilling operations. The five-axis machine not only completes the part in just two operations, but also reduces cycle time down to 75 minutes from a previous two-hour run time.
According to Peck, much of the shop’s part-processing improvements can be attributed to the shear speed of the HSM 600U LP, in addition to its five-axis capability.
“The machine’s 36,000-rpm spindle allows us to machine much faster,” he said. “We now take very light depths of cut, but at extremely fast feed rates. For instance, the heaviest cut we take is 0.125" (3.175 mm). However, the machine is feeding at 200 ipm [5080 mm/min]. This capability has significantly reduced cycle times for all the parts now done on the machine.”
Christopher Tool typically uses small-diameter solid-carbide tools on the HSM 600U LP, which are mostly ballnose end mills held by Rego-Fix powRgrip mechanical toolholders. For these particular endmills, the machine’s simultaneous five-axis motion (tilt/rotation) keeps them in a constant optimum position during the cut and prevents any machining with the “dead points” at the top of their radii. For an aerospace turbine shaft component previously farmed out, this capability will allow the shop to machine the part’s intricate vanes with a 0.046" (1.168-mm) diameter end mill, and do so at drastically shorter cycle times than would be the case if the part was done on a four-axis milling machine.
The HSM 600U LP processes all Christopher Tool’s impeller parts for the Joint Strike Fighter. For the engine, the shop manufactures about 20 part numbers and two main assemblies each with about 25 part numbers of their own. Before the five-axis machine, the shop was producing other parts for the project, but not the impellers.
“As the project ramped up, we realized we could no longer rely on outside vendors for our five-axis machining,” said Peck. “And when we looked at the HSM 600U LP, we knew its fast, high-accuracy linear motor technology would give us a significant advantage in machining speed and precision, but service and application support is what really convinced us. We conducted some inquiries, and many shop people we know praised the support from GF AgieCharmilles. For us, the company was very instrumental in getting us up and running quickly. They walked us through our first impeller, and by the second one, we were doing them on our own.”
Additionally, the machine has pallet changing. And according to Peck, the shop often has three or four completely different jobs—both long and short running, as well as multiple parts on single pallets—set up on the machine at one time. It then basically works around the clock (23 hours per shift), five-and-a-half days per week and mostly untended.
“Such capability to do more with less through state-of-the-art technology and automation,” added Christopher, “will further increase our agility to quickly meet changing customer demands. The disciplines we’ve acquired from our experience in the aerospace industry are benefitting us as we expand into other industries. For these new customers, we can provide them the same care and attention we give our aerospace work and at a much more competitive price, thanks in part to manufacturing innovations such as those from GF AgieCharmilles. ✈
Feature edited by Yearbook Editor Michael Anderson from information provided by GF AgieCharmilles.
This article was first published in the 2013 edition of the Aerospace & Defense Yearbook.
Published Date : 11/21/2013