Shop Doc – Improving Surface Finish

Today’s Machining World Archives June 2011 Volume 07 Issue 05

Dear Shop Doc,

We are turning a part made from PEEK (polyetheretherketone plastic) and need an 8 Ra surface finish on the part. We have tried carbide and a PCD insert. We can achieve around a 10 Ra finish but that is about the best we can do. Since it is a medical implant we can’t use coolant or abrasives. What process will enable the required surface finish?

Too rough

Dear Too rough,

You are on the right track using a PCD (polycrystalline diamond) for machining PEEK. The high hardness, abrasion resistance, and heat tolerance of diamond makes it an ideal tool material for machining medical grade PEEK.

However, in order to achieve very low surface finishes in soft materials like PEEK, or even metals like aluminum, you need a tool with a nearly flawless edge. Polycrystalline literally means “many crystals.” A PCD insert has a tip composed of small diamond crystals held together with a metallic binder. The random orientation of the crystals along with the metallic binder (usually containing cobalt) helps give the very hard diamond some toughness to resist fracture.

If you were to look at a micrograph of the cutting edge, you would see the diamond crystals do not provide a continuous, smooth cutting edge. In turning, each little crystal in the matrix will leave its “mark” on the turned surface. The solution is to use a monocrystalline diamond tool, which is a single piece of diamond crystal with a lapped cutting edge.

In addition to the better tool, you will need to address as many of the other variables that affect surface roughness as possible. Ideally your lathe would have a dynamically balanced integral motor spindle with ultra high precision ceramic bearings. The closer the lathe you run it on is to the ideal, the better off you’ll be. Choose the proper feed rate for the nose radius (see sidebar). Keep tool and work overhang to a minimum. Make sure your finish pass depth of cut is at least 60 percent or more of the nose radius.

Formula for Estimating Surface Roughness:
Ra= f²1,000,000/(24 r)
Ra= Surface Roughness in micro-inches
f = Feed rate in inches per revolution
r = Tool nose radius

If you are turning from bar, consider running short lengths of material and be sure to use a spindle liner that closely matches the bar diameter in order to minimize bar whip. Installing a close fitting bushing into the back of the collet can also help damp bar vibration.

Cool the work with a cold gun (vortex tube). Make sure you prevent chips from wrapping around the work. You can rig up a Shop Vac or use a compressed air gun mounted below the cutting area to draw the stringy chips away from the work.

Dan Murphy
REM Sales LLC

Dan Murphy is a regional sales manager for REM Sales LLC., a U.S. Tsugami importer. He can be reached at dmurphy@remsales.com

About Dan Murphy

Dan Murphy is a regional sales manager for Rem Sales LLC., a U.S. Tsugami distributor. He can be reached at dmurphy@remsales.com.

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