Tunable Boring Bar Enables Fabrication of Long-Length, Thin-Walled Liners

Problem:
For more than 60 years, Los Alamos National Laboratory has served the safety and security of the United States by developing and applying the best science and technology possible. The people and the work environment at Los Alamos enable unparalleled science through leadership, innovation, best business practices and a focus on safe and secure operations in order to make the facility the premier national security science laboratory for the 21st century.

Los Alamos’ Material Science and Technology Division, Polymers and Coatings Group, needed to fabricate long-length, thin-walled aluminum liners used in a series of high-energy density experiments. Specifically, the liners are used in spallation experiments. Spallation is a process in which fragments of material are ejected from a body resulting from impact or stress. In the context of impact physics, it describes ejection or vaporization of material from a target surface during impact by a projectile. The liners act as the driver projectile converging energy onto a target.

In the experiment, electrical current is pulsed through the liner in a split second, where it implodes, moves towards a target at the center and then hits the target. The spallation is a tearing apart of the target material itself.

Frank Fierro, a journeyman machinist at the Laboratory, is the master of understatement when he says the liner had to be “as round and with as nice a finish as possible.”

In fact, the liner, which is cylinder-shaped and made of almost pure aluminum (99.99 percent), had to meet exacting tolerances. The combination of a thin wall at only 3 millimeters (~1/10 of an inch) and a length at 234 millimeters (over 9 inches) caused manufacturing issues with concentricity and surface finish. The entire liner had to measure up to tolerances on length, diameter and circularity to within 12 microinches overall.

The inner finish was even more critical. The use of conventional boring bars and even semi-high-tech bars and inserts would develop harmonics, inducing chatter at any depth in the bore. Chatter would be a particular problem at the far end of the liner that is unattached during the boring process, leading to a poor finish with a roughness average of up to .711 µm (28 microinches). The scientists had set the roughness average at a maximum of .203 µm (8 microinches) throughout the length of the inner side of the liner.
 
The nearly pure aluminum made the task more difficult. “That material is pretty soft and gummy,” Fierro says. “It’s movable, and with a little pressure you can move it. Especially with such thin walls and long length, you can deform it pretty easily.”

Not making the challenges any easier were the doubters within Fierro’s own building. Given the length and material, the scientists themselves wondered if a liner of such tolerances was possible for the experiment to be held at all. “There were people here with serious doubts that we could do that,” says Fierro.

Solution
Looking through a tool catalog from Kennametal, which Fierro says provides 99 percent of all his metal tooling, he began looking for a “long and beefy” boring bar to use on his relatively inexpensive HAAS CNC TL-1 Lathe. “As I was looking I saw a tunable boring bar that claimed to eliminate chatter, vibration and harmonics. I called Jai Prasad (Metalworking Applications Engineer) at Kennametal and he confirmed that was what I needed (along with a polycrystalline diamond cutting insert).

“Jai actually came out and we did a little test with the boring bar on the machine, and it worked out great. His knowledge and help make a big difference in the shop.”

Fierro first created a liner longer than 10 inches as a test piece to prove the tunable boring bar would do the job. Satisfied that he had the right tools, he got to work on the real pieces needed for the experiments.

Fierro began machining a rough liner with a flange on one end, as opposed to using clamps. Clamps would have caused distortion. Instead, the flange was bolted directly to the fixture plate on the lathe in order to eliminate external stresses on the liner. First, he rough machined the internal diameter to a near-net dimension, using moderately deep cuts to save time. Fierro then placed an end cap that was lightly tightened onto the liner to significantly reduce vibration while machining the liner’s external diameter.

The finishing work to come was the critical and final part in the process prior to cutting off the flange.

“Going back to the inside of the liner, I made the finishing cuts. This is where the tunable boring bar made all the difference in the world,” Fierro says. “I needed finish consistency throughout the length of the part.

“At the beginning of the cut, right when you start cutting at the edge, that’s the point where you normally have the most vibration and harmonics. At that point the bar is hanging out there about 9 or 10 inches and is farthest away from where the liner is being held. The tunable bar came tuned from the factory and was set at the 10 to 1 length; I didn’t have to adjust it at all. I had almost no vibration there. The liner was rotating at 1,000 RPM’s. The cutter was stable. This was all done through computer numerical control using CAM software.”

The rpm rate Fierro used was selected through trial and error. “The general machining thinking is that when you’re trying to do a final finish cut, you run at high RPM’s and a slow feed rate that gives you the best finishes. But on this part if you were to try to cut with conventional tooling – whether slow or high RPM’s – the chatter and vibration was still there. With the tunable bar I tried lower and higher speeds, and the middle rate of 1,000 RPM’s worked the best. When time is money, you run higher rates on machining jobs. But this is special material and tolerances matter.”

Fierro doesn’t typically deal with pieces produced in volume. His shop produces a lot of prototypes. “It’s not like we’re doing hundreds of parts and trying to shave seconds off every cut. Prototypes can be harder, and often you don’t know how to approach a particular problem other than to use your experience and knowledge.”

He says the work went relatively easy, but he also had to worry about evacuating the metal chips so they wouldn’t build up and cause scratches inside the liner. “Visually, you’ll see the scratches. What helped was that on my final cuts, I took very small cuts, no deeper than 10 microns.”

Results
The final finish on the inner part of the liner was consistently .017 µm (6 microinches) throughout the more than 234-mm length – better than the .034 µm (8 microinches) finish tolerance set by the scientists – and proving to the doubters that it could be done. “I proved them wrong,” Fierro says with a chuckle. “The finish was actually better than I needed.”

He says the Kennametal tunable boring bar made the experiments to follow possible.

“I wouldn’t have been able to achieve the tolerances without this boring bar. And without the finish there wouldn’t have been a reason to make the part. That was the deciding factor on this job. If I couldn’t create this part, the experiments wouldn’t have happened. Or they would have had to design it in a whole different way. They would have had to scale down the size.”

The length of the liner enabled the scientists to get multiple experiments on the inside of the liner, something than wasn’t previously possible. With more experiments the scientists were able to learn more, and save time and money.

“The experiment went well, so I’m definitely happy and the people here are very pleased.”

Fierro says it’s also remarkable that the equipment he used, the Haas lathe, is relatively inexpensive. “It’s not a top-end machine, but it did what I wanted it to do.”

As a matter of fact, when Fierro made a presentation at an Institute of Electrical and Electronics Engineers conference, the attendees were surprised that he was able to achieve such high tolerances, given the soft aluminum and without using much more expensive turning equipment and pure diamond inserts.

Fierro says that, indeed, at first he believed he would need a more expensive machine or holder for the part, but Jai Prasad convinced him he could do the machining with the existing lathe. “Jai has been a really big help in my shop. He’s really knowledgeable. He always steers me in the right direction. In fact, he’s one of the reasons I’ve been so happy with Kennametal. He’s helped me out, especially on this job. I might not have spent the money on that bar if it hadn’t been for Jai.

“Kennametal is important to my operation. Ninety-nine percent of my tooling comes from Kennametal including all of my cutting inserts for all of my machines. And the service has been great. Every time I call them for quotes or ask them to send me things, I’ve never had a problem. The support is there, the people are there, and I’m very happy with them.”


About Kennametal Tunable Boring Bars
Kennametal tunable boring bars are manufactured with an internal dampening package designed to eliminate chatter in deep, hole-boring applications. They are able to hold tighter tolerances, reduce scrap rates and deliver improved tool life. Boring bars are pre-tuned from the factory for a 10:1 length-to-diameter ratio, but are capable of application settings from 6:1 to 10:1. At the machine, boring bars may be adjusted by hand tuning or tuning device to achieve optimal performance and productivity of the machine and tooling.

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