Swiss Army Knife: Turning Performance

Courtesy of D. Nelson

Scott Butts checks the setup of one of TMCO’s Citizen Swiss-style machines.

Swiss-style turning can be complicated, so it helps to have an operator’s “tool kit” with multiple solutions.

Swiss-style turning requires careful consideration of which tools to apply and toolpath strategies, and there is no one-size-fits-all solution.

For example, machinists must be aware that heavy DOCs are typically required for one-pass machining, and that the definition of “heavy” changes with each machine. Typically, machinists must use tooling with sharp edges, positive rake angles and lead angles must fall within a certain range. Also, Swiss-style turning can create chip management nightmares that require different strategies, such as taking multiple passes and using high-pressure cutting oil and Y-axis toolholders. Mastering these strategies—and knowing which one to use—can be challenging, but they can achieve success.

Why Swiss?

Swiss-style machines have an advantage over conventional lathes because they virtually eliminate workpiece deflection. For example, turning an 8 “-long, ¼ “-dia. workpiece—a 32:1 length-to-diameter ratio—would be a major challenge on a conventional lathe.

A Swiss-style machine’s guide bushing makes this possible. The workpiece is gripped in a collet in the main spindle on a sliding headstock that feeds the bar through the guide bushing collet and past the cutting tools. The mechanical advantage of applying cutting tools only a few millimeters from where the stock is supported allows Swiss-style machines to turn high length-to-diameter workpieces.

While the guide bushing provides a rigid setup, it requires different machining strategies and tooling than conventional turning operations. A turned diameter usually cannot be pulled back in the guide bushing collet. If this happens, the workpiece can fall out of the guide bushing.

Multiple roughing passes can be performed as long as the length of cut is not greater than the length of the guide bushing—typically ¾ “. Scott Butts, manager, Swiss department for TMCO Inc., Lincoln, Neb., runs many parts that permit multiple passes. When the length of cut increases past ¾ “, things get challenging because the “multiple pass” strategy will cause the workpiece to drop out of the guide bushing. “We cut a number of parts where we take radial depths of cut under 0.2 “,” Butts said. “These turns are possible if the lengths of turn are shorter than the guide bushing. If the lengths are longer, the turn must be done in one pass.”

The “one-tool/one-pass” strategy is common in Swiss turning operations. However, the pass may be quite heavy—as much as a 0.5 ” radial DOC. This kind of DOC would suggest a roughing pass. Because there are no clean up passes, surface finish requirements must be met in the first pass. “As the depth of cut increases, we find that specific tooling geometries are required,” Butts said. “Sharp edges and positive rakes are an absolute must with these turns.”

What’s Heavy?

What is a heavy DOC? An exact value is difficult to determine due to different workpiece materials and various machine tool power ratings. As a result, a heavy DOC may be as little as 0.1 ” or as much as 0.5 “. Whatever the DOC, selecting the proper cutting tool geometry is the most important success factor.

Using inserts with positive rake angles is one requirement, because such free-cutting tools reduce cutting force. To get an idea of how much the rake angle affects the tool pressure required to make a cut, think about using a pocketknife. If the edge of the blade is angled to make a cut into the wood, the blade will cut easily. If the blade is turned perpendicular to the surface to produce a scraping action, far more force must be applied to the knife to take the same DOC.

A tool with positive rake angles makes a more efficient cut because it requires less power and force to make the cut than a tool with a negative or neutral rake angle. In addition, chips are pulled rather than plowed away from the cut surfaces, helping impart the required surface finish.

Another important characteristic of a turning tool used for heavy DOCs is the edge preparation, or hone. A hone—typically not greater than a 0.001 ” radius—dulls a turning tool, which strengthens the edge and protects it from chipping and wear. However, applying a honed edge requires more horsepower because of the increased force applied to the tool.

Hones are commonly used on conventional lathes to rough carbon steels. All molded inserts have some form of edge preparation. Inserts used to make heavy cuts on Swiss machines, however, shouldn’t have hones in order to minimize forces on the tool and to maximize the DOC.

The nose radius also has an effect similar to a hone. As the radius increases, tip dullness increases, and so then do the forces needed to make the turn. Inserts used to make heavy cuts on Swiss machines should use nose radii smaller than 0.031 “.

Choosing Lead Angles

Inserts are only part of the geometry equation. The lead angle on the toolholder is the most important thing to consider when selecting a tool for heavy turning, according to Brian Such, customer support group manager for Marubeni Citizen-Cincom Inc., Elk Grove Village, Ill. “When you take a heavy depth of cut, say 0.3 ” to 0.5 “, you might see something that looks like chatter on the workpiece,” he said. “You may think the tool is broken.”

The lead angle commonly used in conventional turning is 3° to 5°. This allows a tool to both turn and face, but the lead also changes the forces the cut exerts on the cutting tool. Feed forces are redirected by the lead angle into radial forces, pulling the tool into the cut in a radial direction. When turning less than 0.1 ” radial DOC, these radial forces are minimal. But Swiss turning can produce extreme cutting conditions.

Such illustrated extreme turning operations with two applications, each with heavy radial depths of cut. “To explain the concept, we used a one tool/one pass strategy with a 0.5 ” radial depth of cut on 17-4 stainless, 1.25 “-dia. stock. We turned it down to 0.25 ” over an 8 ” length.”

Such went on to describe a rectangular workpiece, measuring 0.75 “×0.25 ” and placed 0.25 ” off center using a special guide bushing and collets. This cut was more extreme because it was highly interrupted. “These are pretty tough turns and the principles of using the correct cutting tool geometry all apply. If you were to use a toolholder with a 3° to 5° lead in these heavy cuts, you might see chatter on the workpiece.”

If the turning tool has anything but a 0° lead, the X-axis servo has to pull back to compensate—a back and forth oscillation caused by the radial force on the tool creates this finish problem. “Put in a neutral toolholder and the problem goes away,” Such said.

High-Pressure Cutting Oil

Making the heavy turn might just be the easy part. Because the tool has a positive rake and a sharp edge, an efficient cutting action is created, but this leads to long, stringy, unbroken chips. As a result, chip management is one of the biggest issues Swiss-style machine operators face.

The chip must curl to break, and making a chip curl is partly dependant on the feed rate. When making heavy turns on a Swiss-style machine, feed rates rarely exceed 0.001 ipr. Under these conditions, a chip might not ever break, so the goal is to control the chip. There are a few methods to accomplish this.

Courtesy of D. Nelson