CNC turning internal bore finishing achieving required roughness on deep holes

Content Menu

● Why Roughness Matters in Deep Bores

● Main Obstacles in Deep Hole Finishing

● Parameter Selection That Actually Works

● Tooling Solutions Proven in Production

● Practical Examples from the Shop Floor

● Measurement and Feedback Loops

● Conclusion: Putting It All Together

● Q&A

 

Internal bore finishing on CNC lathes stands out as one of the tougher jobs in precision machining. When the hole depth goes beyond five times the diameter, everything gets harder. Tool deflection grows, chips refuse to leave, coolant pressure drops at the bottom, and vibration shows up exactly where you least want it. The goal remains simple: hit the specified roughness, usually Ra 0.8 µm or better, sometimes down to Ra 0.2 µm for hydraulic seals or bearing seats, and do it repeatably on every part. Shops that figure this out keep the contract; those that don’t end up with scrap piles and late deliveries.

The problem is real. A 60 mm bore that runs 400 mm deep in 4340 steel can start at Ra 0.6 µm near the entrance and climb to Ra 2.5 µm halfway in if the setup is wrong. The same part in stainless 17-4PH behaves even worse because work hardening adds another layer of trouble. Over the years, trial-and-error has given way to systematic approaches backed by published work. Engineers now lean on parameter studies, damping solutions, and smarter tool designs instead of guessing spindle speeds.

Why Roughness Matters in Deep Bores

Surface finish inside a bore affects more than appearance. In hydraulic cylinders, a rough wall increases leakage past the piston seals. In fuel injection components, high spots break off and damage downstream valves. Fatigue cracks start faster when stress risers hide in deep valleys. Specifications therefore get tighter every year. Ra 1.6 µm was acceptable ten years ago; today many prints call for Ra 0.4 µm or less, especially on parts that see millions of cycles.

Measurement itself is part of the challenge. Pulling a stylus profilometer 500 mm into a 50 mm hole takes time and risks damaging the surface. Shops now use bore scopes with optical roughness sensors or air gauges tied to CNC probing cycles to catch problems before the part leaves the machine.

Main Obstacles in Deep Hole Finishing

Tool Deflection and Vibration

The boring bar acts like a long cantilever beam. Cutting forces at the tip create bending moments that increase with the cube of the length. A bar that is stiff enough at 3×D becomes springy at 8×D. The result is chatter marks that look like washboard patterns under magnification. Frequency analysis often reveals modes between 80 Hz and 300 Hz, depending on bar diameter and material.

One shop running 75 mm bores in landing gear forgings saw clear helical marks at 180 mm depth. Accelerometer data pointed to a 142 Hz resonance. Switching to a carbide bar with an internal tuned-mass damper knocked the amplitude down by 75 % and brought roughness from Ra 1.9 µm to Ra 0.38 µm without changing speeds or feeds.

Chip Flow and Coolant Delivery

Chips generated deep inside have to travel the full length of the bore to escape. Long stringy chips wrap around the tool and scratch the fresh surface on the way out. Coolant that works fine at 100 mm depth loses pressure and direction at 400 mm. Through-tool coolant at 70–100 bar helps, but nozzle angle and flow rate have to be right.

A valve manufacturer finishing 42 mm bores in ductile iron cut chip scratching entirely by adding a second coolant outlet angled backward on the boring bar. Roughness dropped from Ra 1.4 µm to Ra 0.55 µm, and tool life doubled because chips stopped welding to the insert.

Heat and Insert Wear

Temperature rises faster than coolant can remove heat in deep holes. After twenty parts, flank wear on a carbide insert can add 0.5 µm to the Ra value. Coated inserts delay the problem, but eventually crater wear dominates. Cermet and ceramic grades often hold edge sharpness longer in finish passes.

Parameter Selection That Actually Works

Cutting speed, feed rate, and depth of cut interact strongly in deep holes. General rules still apply, but the safe windows shrink.

Cutting Speed

Higher speeds reduce built-up edge and give cleaner shear, yet they also excite vibration. Most carbon and low-alloy steels finish well between 110 m/min and 160 m/min when the bar is properly supported. Stainless grades drop to 80–120 m/min. Titanium stays below 70 m/min unless you run ceramic or whisker-reinforced inserts.

A study on IS 2062 steel plates used entropy-weighted VIKOR to rank parameter combinations. The winning set was 140 m/min speed, 0.05 mm/rev feed, and 0.15 mm depth of cut, delivering Ra 0.62 µm with low scatter.

Feed Rate

Finish feeds for Ra 0.4 µm usually fall between 0.02 mm/rev and 0.06 mm/rev. Lower feeds produce smoother surfaces but increase rubbing and heat. One hydraulic shop found 0.035 mm/rev gave the best compromise on 90 mm cylinder bores 600 mm long.

Depth of Cut and Spring Passes

Roughing takes heavy cuts, 1–2 mm on diameter. Semi-finish leaves 0.3–0.5 mm. Final finish pass removes 0.05–0.15 mm with zero wear offset, followed by a spring pass at the same depth to remove deflection marks. Many CNC programs use G71 with a Q value for the spring pass; others write a separate G70 cycle.

Tooling Solutions Proven in Production

Damped Boring Bars

Bars with internal tungsten particles or tuned viscoelastic dampers cut vibration amplitude dramatically. Tests on 10×D bars showed particle damping reduced roughness by 40 % compared to solid carbide bars at identical parameters.

Insert Geometry

Positive-rake wiper geometries with 0.8 mm flat on the nose work well for steels. For aluminum, polished PCD inserts with sharp 15° rake give mirror finishes down to Ra 0.15 µm. Cermet inserts outperform carbide on cast iron by keeping a keen edge longer.

Modified Bar Designs

Some manufacturers machine asymmetric pockets into the bar to shift natural frequencies away from spindle harmonics. Finite element models predict the improvement before the bar is made. One paper reported 30 % lower roughness on 15×D holes with this approach.

Practical Examples from the Shop Floor

Aerospace contractor, 68 mm bore, 520 mm deep, 15-5PH stainless, Ra 0.4 µm required. Setup: 12 mm carbide bar with tuned damper, 90 m/min, 0.04 mm/rev, 80 bar coolant. Result: Ra 0.36 µm average, no chatter marks, 38 parts per insert.

Oilfield pump liner, 110 mm bore, 900 mm deep, 4140 QT. Setup: Two-stage process, rough with indexable head, finish with single-point cermet, spring pass. Result: Ra 0.78 µm consistent end-to-end, passed ultrasonic inspection.

Medical hip stem, 14 mm bore, 110 mm deep, Ti-6Al-4V. Setup: PCD insert, MQL, 55 m/min, 0.025 mm/rev, three spring passes. Result: Ra 0.19 µm, met biocompatibility requirements first article.

Measurement and Feedback Loops

Bore gages check diameter, but roughness needs profilometers. Portable units with 300 mm extension rods work for most jobs. Some shops mount a Renishaw sprint probe with roughness software directly on the turret for in-machine checks. Data feeds back to a macro that adjusts feed rate by 5 % if Ra drifts high.

Conclusion: Putting It All Together

Deep bore finishing on CNC lathes no longer relies on luck. Control vibration with damped or tuned bars, deliver coolant where it matters, choose feeds that balance smoothness and productivity, and measure every critical part. The studies cited here show reductions of 30–50 % in roughness simply by applying proven methods. Shops that adopt these practices see reject rates drop from 8 % to under 1 % on tough bore jobs. The next time a print lands on your desk calling for Ra 0.4 µm on a 10×D hole, you will know exactly where to start: bar selection, parameter window, coolant pressure, and a spring pass. Get those right, and the surface finish takes care of itself.

Q&A

Q1: What spindle speed range works best for finishing 80 mm bores in 4340 steel at 8×D?
A: 110–140 m/min with damped bar usually keeps chatter away and gives Ra under 0.5 µm.

Q2: How much does a tuned-mass damper inside the bar actually help?
A: Real tests show 35–45 % lower roughness and double the usable overhang.

Q3: Is high-pressure coolant worth the cost for 300 mm deep bores?
A: Yes, 70 bar through-tool drops Ra by 20–30 % and prevents chip packing.

Q4: When should I use a spring pass instead of just a light finish cut?
A: Always on holes deeper than 6×D to remove deflection marks left by the finish pass.

Q5: Which insert material lasts longest on 17-4PH stainless deep bores?
A: Coated cermet or ceramic grades hold edge sharpness three times longer than carbide.