Boring Bar Speeds & Feeds Chart
Internal turning gets less rigid as overhang grows. Start with your normal turning data, then derate speed and depth of cut by L/D ratio before chatter forces the decision for you.
What This Page Covers Best
Use this chart when you already know the base turning parameters and need to scale them down for boring-bar overhang, bar material, and chatter risk.
Recommended Workflow
Start from a normal external-turning SFM/IPR, apply the L/D derating in the chart, then adjust RPM in small steps if chatter still appears.
Where Shops Go Wrong
Most boring issues come from copying external-turning parameters into long overhang bores without reducing speed, DOC, or nose radius.
Recommended Workflow
The Golden Rule: Always use the largest diameter bar that fits in the hole, with the shortest overhang possible. Rigidity increases with the 4th power of diameter. A 1.0" bar is 16x more rigid than a 0.5" bar.
Worked Example: 6×D Carbide Boring Bar
Suppose your normal external-turning start point in 4140 is 400 SFM with a moderate roughing DOC. A 6×D carbide boring bar should not start there.
The chart below puts a 6×D solid-carbide bar closer to 70-80% of base speed and 50-60% of base DOC. That means your first boring test should start nearer 280-320 SFM, then fine-tune from there.
If the bore chatters anyway, keep the feed stable first, shorten overhang if possible, reduce nose radius, and move RPM in small steps until the bar exits the resonant zone.
Parameter Derating by Overhang
| L/D Ratio | Bar Material | Speed (SFM) Factor | Depth of Cut (Ap) Factor | Typical Application |
|---|---|---|---|---|
| Up to 3xD | Steel | 100% (Normal) | 100% (Normal) | Standard Boring |
| 4xD | Steel | 80 - 90% | 70 - 80% | Limit of Steel Bars |
| 4xD - 5xD | Heavy Metal / Carbide | 90% | 80% | Deep Boring (Standard) |
| 6xD | Solid Carbide | 70 - 80% | 50 - 60% | Requires Carbide Shank |
| 8xD - 10xD | Carbide (Damped) | 50 - 60% | 20 - 30% | Silent Tools / Vibration Damped |
| 10xD + | Specialty Damped | 30 - 50% | 10 - 20% | Extreme Reach (Light finishing only) |
Need the Base Turning Start?
Start with our turning calculator, then come back here to derate speed and DOC for internal overhang instead of guessing from scratch.
Need Base Speeds?
Use the material SFM chart as your 100% starting point, then apply the boring-bar derating factors on this page.
Frequently Asked Questions
What SFM should I use for boring bars?
Start with the same material and insert grade data you would use for external turning, then derate it by L/D ratio and bar material. That is more reliable than memorizing one absolute boring SFM number. Example: if your normal carbide turning start in steel is 400 SFM, a 6×D carbide boring bar may need to start closer to 280-320 SFM.
What is the maximum overhang for a boring bar?
Steel shanks: 4×D maximum. Carbide shanks: 6–8×D. Anti-vibration (damped) bars: up to 10–14×D. Beyond these limits, chatter becomes uncontrollable regardless of parameters.
How do I prevent chatter when boring?
Use the largest possible bar diameter, minimize overhang, and keep the tool sharp with a smaller nose radius when reach is long. Start by shifting RPM in 10-15% steps and reducing DOC before making drastic feed cuts. Carbide or damped bars become much more valuable as you move beyond roughly 4×D.
What feed rate should I use for boring operations?
Use your normal turning IPR as the starting point, then trim it based on overhang, finish target, and chatter risk. Roughing may still live around 0.004-0.010" IPR, but once you get beyond 4×D it is common to reduce feed by roughly 10-40% and pair that with a bigger DOC reduction to keep the bar stable.
What is the difference between roughing and finishing boring?
Roughing boring removes bulk material with 0.020–0.100" depth of cut per side. Finishing boring uses 0.005–0.015" per side for size control and surface finish. Use a separate bar or insert for each operation.