June 2008 Edition

FROM THE SHOP FLOOR

Finish Pass Anxiety – Part 2

A “twist” of fate, and a tool, creates and solves a mystery and helps alleviate finish pass anxiety

In my last column [Finish Pass Anxiety, "From the Shop Floor, MAN, May, 2008, Vol. 42, No. 5, p. 14], I was in the testing phase of an Incoloy job. We – Will Campbell, a machinist here at Ansco, and myself – were making steady progress on the job. At first sight we appeared to be hitting our sizes correctly. But, our quality department rained on our parade. Our grooves looked lovely, but according to our CMM, the sizes were inconsistent from one side of the tool to the other.

Checking these sizes, in itself, was a challenge. The grooves were part of a labyrinth seal. They were face grooves that have 10° of taper on each wall, and large radii at the bottom of the grooves. This meant that we used small width groove tools to machine them, and checking these diameters would be a challenge. After discussing the project with our quality manager Wes, we determined a way to check our sizes.

We would measure a plane on the piece, and measure the angled walls as a three-dimensional conical feature. Then we would have the CMM calculate the circle at which this conical feature and our plane intersected, thus we would know the diameters.

We discovered some odd things after inspecting our piece. When using the "left" side of the tool, our diameter was correct, but when cutting the other wall of the groove, using the "right" side of the tool, our sizes were too small. We were not removing enough material.

Dazed and Confused

We expected some of this, but it got confusing and disconcerting. After moving our offsets, we got unexpectedly inconsistent sizing results. Our CMM report said we were 0.015" too small. We moved our offset up 0.01", to leave 0.005" for a final pass. Upon rechecking, the diameter moved just 0.005".

Our first thought was that we did not have proper tool clearance, and got tool contact in unexpected places. The groove tools we used were not designed to be face grooving tools. But, since there were no actual face grooving tools available, we had modified the tools. We checked for clearance problems with bluing. After many checks and a solid model verification, we found that we did not have clearance issues.

This meant that we were getting a lot of tool deflection. At first, we were befuddled as to why the tool would deflect and cut more on one side than the other. But, it was clear this is what was happening.

A New Twist

I went back to the tooling catalog and found the groove tools with the smallest amount of reach that would complete the entire groove. In addition to using stubbier and more rigid groove tools, I ordered a tool in each twist. This let us use the more rigid cutting direction to cut both the tops and the bottoms of the grooves.

We separated each finish grooving pass in our program into its own operation. This let us run any pass we wanted without having to re-cut any other surface. This saved time once wasted in passes that were cutting air.

When the new tools arrived, we made slight modifications to let them face groove. This was achieved with an air sander and my brother Jeff’s golden touch. He’s Ansco’s lathe foreman. We loaded the tools, and began to cut again.

This time the task went better. Our finishes remained consistent, and our offset moves and sizes made more sense. We were able to dial-in perfect sizes. Due to our separated program, we could individually size each groove to ensure quality throughout the piece.

But, as I wrote in my last column, we were still not to finish sizes. Although we were much more confident, our "Finish Pass Anxiety" remained.

As we moved in to our final overall lengths, we took pains to go through each operation, taking nothing for granted. Each size was checked and rechecked.

We cut all of our surfaces with stock to spare and made our precise finish passes without a problem.

After all of this we still had to hit one key size, a size that we had not yet test cut: a diameter sized at 1.2510"/1.2505". To come this far and miss a diameter, albeit a tight one, would be a shame. As with our previous operations, we found a step-by-step approach, using machining fundamentals was the best thing to overcome finish pass anxiety.

Dave is responsible for programming, tool selection, and fixture design for Ansco Machine’s 20 CNC machines in Peninsula, OH.

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