Operators at Cambron Engineering knew they had a problem workpiece when outsourcing vendors they approached turned down the project. Inconel parts do that. As a result, Bay City, Michigan, shop’s already full backlog was overflowing, creating strong company interest in cycle-time reduction for a particularly vexing feature on the parts.
A chance encounter with Horn USA application engineer Brett Kischnick satisfied that interest with a new tooling approach that cut cycle time from 59 sec. to 10 sec. for the feature, taking 19 hours out of the part's 80 hours total machining time.
The two parts are Inconel 600 (nickel, chromium, iron) plate, about eight feet in diameter and 1.875-in. thick, to be used as tube sheets in a shell-and-tube heat exchanger, an ideal application for this strong, heat/corrosion resistant material. Each disc must be drilled through with 1,400 1.5-in.-diameter holes, each with two 0.126-in. (wide) x 0.015-in. (deep) grooves milled into the circumference of each bore.
It's a touchy and tough piece of work, with the material alone worth about $60,000, according to Cambron tool engineer Bryon Christilaw, but it's well within Cambron's capability. The shop is a go-to supplier for nine GM plants, offering a climate-controlled 33,000 sq. ft. workspace shop and 47-person staff capable of design and manufacture of gages, dies, tooling, fixtures, and special machinery, as well as CNC machining of large fabrications.
"These parts come through several times a year, and on one occasion with our backlog already high, we needed to outsource the work," said Christilaw. "The material and features proved a major problem, however, and no one wanted to touch it."
Cambron machines these parts on a 25-hp Kuraki boring mill, and the interpolated grooving cuts were particularly difficult. "We were using a 3-flute, 0.125-in.-wide groove milling insert that was 0.697-in. in diameter," Christilaw explained. Running at 750 rpm and 9 IPM feedrate with 0.004-in. per tooth engagement, each groove took about 30 seconds – consuming 19 hours of the part's 80-hour machining time. "This didn’t include the operator’s time for in-process measurements, cutter compensation and insert changes," he added.
A part of that size and configuration naturally attracts attention when fixtured up, so Horn USA application engineer Brett Kischnick – visiting the shop to handle questions about lathe grooving – asked the operator about it and learned about the tool life and cycle time issues. He later proposed a solution with a Horn 713 12-flute groove-milling insert, 0.854-in. in diameter and 0.118-in. wide. The small differences prove what a “game of inches” machining can be, because the new insert reduced cycle time for each groove from 30 seconds to five seconds.
Here's how: The larger tool diameter – just 0.157-in. larger – reduces the length per cut from 1.830-in. to 1.337-in., about 27%. Increasing the number of cutter teeth from 3 to 12 allows a 400% increase in feedrate from 9 IPM to 36 IPM, while maintaining the same 0.004-in. per tooth engagement.
In addition, the smaller width of the 12-tooth tool (0.118-in. vs. 0.125-in.) leaves 0.008-in. material for the second pass on each groove. This produces a larger chip to minimize heat buildup in the tool, which is amplified when trying to "rub off" 0.001-in. "It is important to have enough stock to produce a good chip to carry the heat away," explained Kischnick. "Otherwise, the tool will absorb more cutting heat. The cooler the tool, the longer it stays sharp." In fact, the 12-tooth tool lasts nearly three times as long as the 3-tooth tool: 45 minutes versus 18 minutes.
The 713 12-tooth milling cutters produce a very smooth cut. They are designed for grooves up to 0.185-in. deep and 0.039-in. to 0.118-in. wide in holes as small as 0.886-in. diameter. They are screwed to the front face of a standard carbide shank, and utilize straight or staggered cutting edges, depending on the width.
"These tiny grooves had always taken a big chunk of the total cycle time, but this new tool cuts that down to a proportion that's more in line with their size," Christilaw added. "Anytime you can cut 25% off the cycle time for a part by simply going to a different style tool, it's a real game changer."