How to Polish Grooves in CNC Machining

• 1
  Key takeaways
• 2
  Why grooves resist polishing
• 3
  Pick the finish target before you touch the part
• 4
  Tools and consumables that reach inside a groove
• 5
  When to polish on the machine and when to do it by hand
• 6
  How to polish grooves in CNC machining: the sequence
• 7
  Groove finish targets by function and material

A groove is a slot with two walls and a floor. Access is limited, tool pressure is uneven, and the abrasive cannot follow a straight line the way it does on a flat face. On a 2 mm wide O-ring groove, a 6 mm polishing bob simply does not reach the corner. That is why grooves often come off the machine with visible tool marks while the rest of the part looks fine.

The geometry also decides the method. An open channel 8 mm wide and 3 mm deep can be finished with a mounted point on a straight grinder. A 1.5 mm wide thread relief groove needs a dental-style rotary tool or abrasive cord. If you try to force a large tool into a small groove, the edge rounds over and the groove width grows past tolerance.

Material behavior adds another variable. Aluminum 6061 loads the abrasive and smears, so you need a light oil or kerosene to keep the paper cutting. Stainless 316 work-hardens if you dwell too long in one spot. Titanium TC4 (Ti-6Al-4V) needs slow speed and fresh abrasive, or the surface galls.

The payoff is real. A polished groove removes the machining marks that act as stress risers, gives a seal a smooth path to slide against, and cuts the amount of secondary hand work on the assembly floor. This page covers how to polish grooves in CNC machining step by step, with the grit ranges and checks we use in our own shop.

Write down the function of the groove, then match it to a surface finish. A static O-ring seat in an aluminum housing usually seals at Ra 0.8–1.6 μm. A dynamic hydraulic rod groove running at pressure needs Ra 0.4–0.8 μm to keep the seal lip from wearing. A fluid channel where only flow matters can stay at Ra 1.6–3.2 μm as machined.

The target also sets the grit you stop at. Ra 1.6 μm is reachable with 600–800 grit. Ra 0.8 μm needs 1000–1200 grit. Ra 0.4 μm and below calls for 2000 grit plus a fine compound on a soft bob. Going finer than the function needs adds cost and risks rounding the groove edge.

Check the drawing for a corner radius or edge break callout. Many groove specs show a 0.2–0.4 mm radius on both lips. Polishing with a hard tool on a sharp corner will wear it into a larger radius, which changes the seal squeeze. If the corner is critical, mask it or use a soft backing that follows the form.

Decide who inspects and with what. A visual check under 10× magnification finds scratches but not waviness. A profilometer trace across the groove gives Ra, Rz, and the profile shape. Agree on the instrument and the cutoff length before the first part is polished, not after.

For grooves wider than 6 mm, a straight die grinder with a 3 mm shank mounted point works well. Use a rubber-bonded abrasive point for the first passes and a felt bob with compound for the final polish. Keep spindle speed in the 8,000–15,000 rpm range for aluminum and drop to 4,000–8,000 rpm for stainless and titanium.

For narrow grooves, abrasive cord or tape is the practical choice. Cord comes in diameters from 0.5 mm upward and follows the groove floor without rounding the walls. Wrap it around a wooden stick, pull it back and forth 20–30 strokes per grit, and rotate the part 90° between grits to cross the scratch pattern.

For blind grooves and cross-holes, use a rotary tool with a 1 mm to 2 mm abrasive ball. Dental labs use the same setup. Keep the speed low and let the abrasive cut. Pushing hard heats the tip and burns the binder, which leaves black marks on stainless and titanium.

Consumables to keep on the bench: 400, 600, 800, 1200, and 2000 grit in both paper and cord, a small bottle of light oil or kerosene, felt bobs in 3 mm and 6 mm, and a diamond compound in 3 μm and 1 μm. Label the grits. Mixing a used 400 grit bob with 1200 grit compound is the fastest way to put scratches back into a finished groove.

If the groove is on a 5-axis part and the surface is already close, a light in-machine finish pass can save a hand operation. A 6 mm ball nose at 0.05 mm stepover and 0.1 mm depth of cut leaves a surface near Ra 1.6 μm in aluminum. Add a high-speed finishing path and the tool marks become uniform, which makes the hand polish faster and more predictable.

Hand polishing still wins for narrow, deep, or interrupted grooves. A 5-axis tool cannot reach a 1.5 mm wide groove under a shoulder. For these features, plan the hand operation into the route sheet so it is not discovered at final inspection.

Control the environment. Polish stainless and titanium wet with a light oil to stop work-hardening and galling. Polish aluminum dry or with kerosene, and wipe the part between grits. A clean part shows the real scratch pattern; a dirty one hides it.

Track the pass count. Ten strokes at 400 grit, ten at 600, ten at 800, and so on is a starting point. Write the counts on the route sheet and adjust after the first part is measured. Without a count, two operators will produce two different finishes on the same job.