BuTech Pressure Systems (Erie, Pennsylvania) often experienced pileups of unfinished valves and valve parts on the shop floor because of their square-insert face mill's lack of efficiency cutting heat resistant alloys. The company traded off either throughput or surface quality and insert edge security. Insert wrecks were all too commonplace when they tried to speed operations up. After switching to a round-insert high-positive rake face mill, they increased throughput by 65 percent with no compromises in finish or edge security. Moreover, edge life rose 68 percent, cutting insert costs by one-third.
"Anyone who machines heat resistant materials knows how the heat buildup problem can slow an operation down," says Ron Renner, BuTech tool room manager. "Since we were falling behind, we had to find a way to address that bottleneck at the milling end once and for all."
So Mr. Renner consulted Leo Enzo, their Sandvik Coromant area technical rep, who had solved other problems at BuTech before. Mr. Enzo proposed an in-house test on a representative workpiece with a fairly new face mill, a round-insert CoroMill 200. He also recommended a 90 degree approach to the workpiece, rather than running parallel to it, the shop's normal practice. The test was successful on the first pass. At 140 percent and 15 percent higher feeds and speeds, respectively, the number of parts per edge rose to 42 from 25. "Despite our initial skepticism, the results spoke for themselves. It didn't take much figuring to realize we would recoup our conversion cost in the first batch," says a satisfied Mr. Renner.
"Smaller machining centers are the trend today. The CoroMill face mill buys BuTech's low horsepower vertical mill the effect of a bigger machine," says Mr. Enzo. "The round `H' inserts reduce the power required to cut metal and the heat that is generated, especially when working with heat resistant alloys. At higher speeds and feeds, they still consume ten percent less electricity than with conventional tools."
After using the new face mill for a little less than a year, BuTech has standardized on the CoroMill system, which addresses all their current milling needs. They have added a CoroMill 245 to handle face milling of valve stems. During the first six months of operation, they have reduced their insert requirements for this operation by more than one-fourth.
A closer look at the gains of BuTech's simple switch to the new round-insert CoroMill 200 reveals just how they did this.
BuTech makes high-pressure valves and valve parts for sub-sea, pharmaceutical, air and space and petroleum applications. The sub-sea valves must be precision machined to perform defect-free for about 25 years. Non-corrosive materials used are most often 316 stainless steel and Inconel 625. The jobs run continuously 20 hours a day on two Haas VF3 15-hp, single-spindle vertical milling centers.
A representative workpiece is an Inconel 625 tee for small underwater valves. The workpiece comes to the milling station as a rectangular block approximately 2.5 inches by 3 inches. The milling operation consists of rough milling the side and top surfaces to size and making the block square (2.12 inches ±0.010-inch).
The previous cutter was a 3-inch, 45-degree indexable carbide face mill with four teeth. The inserts were square, positive rake, with no chipbreakers. The cutter also had small wipers for improved finish. The part finish met spec, but at the cost of slow speeds and feeds.
Tests on the Inconel tee were run with a combined 3-inch CoroMill face mill with four round H1OA grade inserts. Cutting speed was raised to 140 sfm from 119 sfm and feeds to 6 ipm from 2.5 ipm.
Test data was as follows:
BuTech's "secret weapon" for de-bottlenecking the milling operation is a fourteeth CoroMill 200 with H1OA round inserts. Round inserts, by geometry, are inherently stronger than square inserts, because there are no corners to cause stress raisers. The new insert's faceted design provides quick, distinct indexing for up to eight edges, doubling the number of cutting edges from before. BuTech's problems with built-up edge have been eliminated, because the round insert's positive rake requires lower cutting forces. Consequently, it generates less heat, and introduces more of a cleaving action at the cutting point. In addition, the modern carbide grade and coatings on the new insert are more durable.
Operators run each edge for 15 minutes of cutting time between indexing. Indexing and insert replacement is hassle-free, since the inserts are screw mounted. The screwed-in insert also provides greater stability to the cutting area.
The economic aspect of the switch proved better than Mr. Renner anticipated. "The investment vs. the gains was negligible," Mr. Renner adds. "It didn't even play a role in our decision." In addition to the minimal initial expenditure, there were no extra costs for toolholding. The new face mill fits on the same holder.
Consumption was reduced by 3.3:1. BuTech now uses 40 inserts/batch versus137 inserts/batch earlier. They also get extended life on their cutter body. That's because the inserts are protected by thick carbide seat pocket shims. In the event of insert failure, only the insert shim needs to be replaced,—a minor expense—as opposed to replacing the whole cutter. BuTech also added life to the inserts, because the new cutter's spring loaded quick-locking cassettes provide precise radial setting. This radial accuracy makes it easy to generate optimum chip thicknesses consistently on all the cutting edges. Insert loading is uniform ensuring longer tool life and better surface finishes.
CoroMill 200 Comparable 3" Cutter
Cutting speed (sfm) 140 119
RPM 175 150
Feed (ipm) 6.0 2.5
Axial depth of cut (inch) 0.0250 0.0250
Radial depth of cut (inch) 1.560 1.560
Cutting time/component(min) 1.20 2.10
No. of insert edges 8 4
No. of components/edge 42 25
Total Time/component (min) 1.47 2.43