Landing Gear Ups and Downs: the Eccentric Arc Surface in a Deep Hole

• 1
  Key takeaways
• 2
  Why the landing gear eccentric arc surface sits inside a deep hole
• 3
  What hardness above 53 HRC does to the cut
• 4
  Chatter is the real limit, not the material
• 5
  How to hold the work and reference the eccentric axis
• 6
  Parameters that hold the landing gear bore in tolerance
• 7
  Which tool and setup for which bore condition

A landing gear leg is not a straight column with a round bore. The upper and lower ends of the frame are offset, and the pin that carries the wheel or the drag brace has to swing about an axis that is not the bore center. That offset is what makes the bore surface eccentric. The arc radius is larger than the bore radius on one side and smaller on the other, so the wall thickness changes as you rotate around the axis.

In service the landing gear takes two loads at once. The leg bends from the ground reaction, and the eccentric surface takes a rotating contact load from the pin. The arc, not the round bore, is the surface that sets contact stress and wear. If the arc is off by a few hundredths of a millimeter, the pin contact moves to one edge and the load path is no longer what the stress analysis assumed.

That is why the eccentric arc usually carries a tolerance near ±0.005 mm and a finish in the Ra 0.8–1.6 μm band, with critical bands down to Ra 0.2–0.8 μm. The bore depth often runs several times the arc diameter, so the cutting tool reaches far from the spindle nose. Deep-hole geometry and a hard material together are the two constraints that decide the whole process.

Many landing gear pins and sleeves run at 53 HRC or harder, and some tool steel grades sit higher than that after heat treatment. At this hardness the material no longer forms a clean shear plane ahead of the edge. The chip comes off in segments, cutting pressure spikes, and most of the heat goes into the tool tip instead of the chip.

Coated carbide still works below roughly 45 HRC. Above 50 HRC the edge breaks down fast, and the failure is not gradual. The flank wears, the cutting force climbs, and then the edge chips. In a deep hole you may not see that until the arc profile has already drifted out of tolerance.

The practical answer is polycrystalline cubic boron nitride (PCBN) or a ceramic grade, run dry or with minimal lubrication, at surface speeds far higher than carbide would tolerate. A 60–120 m/min band is a normal starting range for PCBN in hardened steel, with depth of cut kept light. The trade is clear: the tool costs more, but it holds size across a long boring pass.

A boring bar that reaches 4× its diameter or more is a spring. When the cutting force varies, the bar deflects, the depth of cut changes, and the force varies again. That loop is regenerative chatter, and it shows up as a wavy arc surface with a pitch equal to the chatter frequency. In a deep eccentric bore the bar is also unsupported on one side, because the arc is offset.

Three knobs control it. Shorten the overhang if the part can be turned around. Increase the bar diameter until it nearly touches the bore wall, which also raises the bar's static stiffness. Then tune the spindle speed: a stability lobe diagram for the specific bar and holder will show a sweet spot where the same depth of cut becomes stable.

If chatter persists, reduce the radial engagement and take more passes. On a 5-axis machine with a Ø400 mm rotary table, the part can be indexed so the eccentric arc is cut with the shortest possible tool. That single setup change often removes more vibration than any speed adjustment.

Everything depends on the datum. If the part is clamped on the outer diameter and the eccentric axis is assumed to be concentric, the arc will be right on the machine and wrong on the CMM. The correct reference is the eccentric axis itself, established from the pin bore or from a machined face that the design already ties to that axis.

For hard, thin-walled gear legs, clamping pressure is a real risk. A three-jaw chuck closed hard will ovalize the bore, and the part springs back after unclamping. Soft jaws bored to the actual part diameter, plus a light clamp force, keep roundness inside the tolerance band.

A mill-turn center or a 5-axis machine with a rotary table lets the bore and the opposite face be machined in one setup. One setup removes the stack-up error that comes from re-chucking, and it is usually worth more than a tighter machine spec. We run this kind of work on 16 simultaneous 5-axis centers and 16 mill-turn centers, with a maximum processing size of 4,000 mm.

A workable starting point for a hardened steel landing gear bore: PCBN or ceramic insert, surface speed 60–120 m/min, feed 0.05–0.15 mm/rev, depth of cut 0.10–0.30 mm per side. Keep depth of cut below the bar's stability limit rather than pushing feed. Feed changes the finish; depth of cut changes the vibration.

Cooling matters more than it looks. PCBN generally runs dry or with air blast, because thermal shock from flood coolant can crack the edge. Ceramic grades behave the same way. If the setup needs lubrication for chip evacuation, use a directed minimum-quantity stream and keep it steady.

Then verify. Rough the bore, leave 0.2–0.3 mm, stress-relieve if the drawing allows it, and finish in a separate pass with a freshly indexed edge. Measure the arc on a CMM or with an air gauge set to the eccentric axis. One pass of roughing without a finish allowance is the most common way a good setup still produces an out-of-tolerance arc.