May 2008 Edition

FROM THE SHOP FLOOR

Finish Pass Anxiety

Dealing with a tricky alloy and complex cutting takes theories, planning, and educated guesses, then its up to physics

Frequent readers know that Ansco Machine is a job shop. We’re a particular job shop; new stuff comes through the door every day that makes demands on us. The most recent challenge was a piece for a turbocharger. The specs showed it had tight tolerances, multiple steps, and undercuts. A series of labyrinth grooves in it, akin to the weakness of the Death Star, awaited us. A deeper look at the print kicked the degree of difficulty up a few more notches. This part is made of Incoloy 901.

Incoloy 901 is a super-alloy used in high-temperature applications. It is very hard, with a Brinell Hardness Number in the 300 to 380 range, but cutting can be very gummy. This is a machining paradox. We’d need a sharp edge to peel a chip, but the material is so tough and abrasive, the edge quickly wears out. What I know about super alloys I learned during the last two weeks, while machining this job.

I contacted one of my tooling sources, Jay-Z, to help me tool up for the job. After examining the part, we prepared for two eventualities: we were going to try some reinforced ceramic inserts to do much of the roughing and finishing. But, if the material proved too rough for the ceramic, we had some grades of carbide on which to fall back.

I planned to machine the part in three operations. The raw material was a closed die forging, and there wasn’t much flat on the piece to chuck. My first operation was to create a solid place to chuck the piece in a lathe. This provided an introduction to the material without having to hold any tight sizes or shapes.

Will Campbell, a machinist here at Ansco, implemented my plan and programs.

After a brief fight with the ceramic, Will and I decided to revert to carbide inserts. We had to run slowly, in the 50 to 75 sfpm range, but it held up to the rough skin of the forging better, and we made progress.

Using carbide tools, we finished the preparatory operation, then the first side of the part. The last operation, however, was the most difficult. The critical sizes and tricky grooves were on that side. Our plan offered flexibility. We left stock on the face so we could cut the grooves, measure them, and, after we read the results, remove the extra stock and cut the grooves to finish.

This seemed to work until we began to cut the grooves. The grooves appeared to cut just fine. The shapes looked good, and we had good finishes. But, once the part hit the inspection department we found problems. Due to the small tools needed to cut this shape, and the large amount of tool pressure required by the super-alloy, there was material left on most of the groove surfaces.

Too much material is better than too little. But, as we zeroed in on cutting the grooves to size, it occurred to us that even if we got the grooves perfect, we still had to cut them again in the later finish operation.

This material is expensive. This was a first inspection of the first piece. It would need approval before the production run. Since we took the first pass on the first operation, we thought we might have to scrap the piece.

We’re at a critical crossroads. We have to do the finish dimensions and finish the grooves. Will it leave material on all surfaces that will require small finish passes to bring everything in to size? We had anxiety about making that last pass. Every machinist knows the hollow feeling of scrapping a part. When a machinist reaches that critical juncture in machining an expensive part, that feeling sits in the stomach with each move.

Tomorrow we finish the part. Our guesses, theories, and plans will either be proved or disproved when we hit the green button on the control and physics take over. I’ll let you know the result.

Dave is responsible for programming, tool selection, and fixture design for Ansco Machine’s 20 CNC machines in Peninsula, OH. He’s been working in a machine shop since age 12, starting by drilling holes on a turret lathe and making simple parts on a Bridgeport.

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