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Hard Turning Speeds and Feeds: Choose a Conditioned Start, Then Prove It
For CBN hard turning, choose the starting row by whether the edge stays engaged, then keep hardness, actual work diameter, CBN grade and edge condition, rigidity, and finish target attached to that row. A continuous cut can use only a continuous source row. Any loss of contact and re-entry calls for the source’s interrupted cut row. A fully interrupted part needs an additional check against a comparable toolmaker recommendation or a study with a matching condition tuple.
This article uses three operational classes. They are not an industry standard and have no numerical severity threshold in the sources used here:
- Continuous: the cutting edge remains in contact with the workpiece.
- Lightly interrupted: the edge loses contact and re-enters at a localized feature, such as a gouge. This describes the decision problem; it is not a standardized class.
- Heavily interrupted: repeated disengagement around multiple features. The fully interrupted study used below is one specific example, not a general setting for this class.
Classify continuity before choosing a number
Interrupted cutting means that the edge engages and disengages, whereas continuous cutting keeps the edge in contact. Grooves, splines, and gears can create that difference. Halnn's interrupted-cut definition supports the contact distinction, but its commercial guidance is not a parameter standard.
The practical question is not whether a part looks mostly round. Ask whether the edge will re-enter the material, how often it will do so, and whether the holder and workholding can repeat that event without movement. A forum case involving a chrome-coated hydraulic rod with gouges shows why a localized re-entry cannot be treated as uninterrupted turning, especially when hardness is only estimated. It is a condition-specific problem, not evidence for a general parameter rule. This is supporting context only.
Decision rule: If the edge re-enters the workpiece, do not begin from a continuous-cut value simply because most of the circumference is continuous. Use the interrupted row from the same supplier table, then inspect the edge and surface before changing one variable.
Build the condition tuple before opening the chart
Write down the work material and measured hardness, actual cutting diameter, continuity class, CBN grade and edge condition, holder and workholding rigidity, coolant practice, and finish target. These are not optional footnotes. CBN grade, workpiece hardness, cutting speed, and feed can all affect cutting force and surface roughness in the cited AISI H13 study, which covered 45, 50, and 55 HRC rather than every hardened steel. AISI H13 CBN study.
Rigidity belongs in the tuple because an inconsistent finish or edge failure can come from holder, workholding, runout, or stickout rather than the nominal feed. The sources do not supply a rigidity threshold, so a weak setup is a reason to prove a conservative start, not to invent a reduced feed number.
Select a supplier-specific starting row
The table below reproduces broad hardened-steel CBN starting ranges published by OnmyToolings. It is supplier-specific data, not SCT data, an ISO/DIN/ANSI standard, or a recommendation for every CBN grade, hardness, holder, or machine. Vc is cutting speed, ap is depth of cut, and f is feed per revolution.
| Operation and engagement | Vc (m/min) | ap (mm) | f (mm/rev) | How to use the row |
|---|---|---|---|---|
| Roughing, continuous | 80-160 | 1.0-2.5 | 0.1-0.5 | Supplier’s broad hardened-steel CBN starting range for a continuous cut. |
| Roughing, interrupted | 80-125 | 1.0-2.5 | 0.1-0.5 | Supplier’s broad interrupted starting range; use when the edge re-enters. |
| Final turning, continuous | 125-210 | 0.1-0.5 | 0.05-0.2 | Supplier’s broad final-turning range when the edge stays engaged. |
| Final turning, interrupted | 100-150 | 0.1-0.5 | 0.05-0.2 | Supplier’s broad interrupted final-turning range; do not transfer it to a fully interrupted study condition without checking the application. |
OnmyToolings' source table explicitly says to treat the values as starting ranges, inspect the cutting edge and surface, and optimize one variable at a time. Its distinction between continuous and interrupted rows is useful for the first choice. It does not define light versus heavy interruption, verify a particular SCT insert, or settle the correct grade and edge preparation for the job.
For lightly interrupted work, use the supplier’s interrupted row as the first source path. Do not create a midpoint between continuous and interrupted rows. For heavily interrupted work, first determine whether the actual material, hardness, insert condition, dry or coolant practice, diameter, and interruption pattern resemble a tested or toolmaker-supported application. A repeated re-entry process can fail for reasons that a broad range does not expose.
Convert cutting speed to RPM without turning it into a recommendation
For metric turning, convert a selected cutting speed to spindle speed with:
Where:
Symbols: n = spindle speed in rpm; Vc = qualified cutting speed in m/min; D = actual work diameter in mm; and π is the circle constant.
This is a unit conversion for a selected, conditioned cutting speed. It does not choose Vc, feed, depth of cut, CBN grade, or edge preparation. Recalculate when the diameter changes.
Hard-Turning Cutting Speed to RPM Calculator
Enter a cutting speed already qualified for the actual CBN grade, work material, hardness, cut continuity and setup, then enter the current work diameter. This calculator converts units only; it does not select a hard-turning speed.
Conversion only. Keep CBN grade, hardness, cut continuity, edge preparation, coolant practice and rigidity attached to the result.
Worked calculation: a fully interrupted SKD11 study condition
An IJRES experiment on fully interrupted hard turning used dry machining of a 60 mm diameter, 100 mm long SKD11 workpiece at 60-62 HRC with six grooves. The insert was reported as high-CBN content (90%), TiN-coated, and the experimental objective was flank-wear evaluation. The study reported an optimized model at Vc = 120 m/min, f = 0.167 mm/rev, and depth of cut d = 0.139 mm for its study condition; validation measured flank wear of 137.1 micrometres. The cited study is the bounded evidence source.
Using only its reported cutting speed and diameter:
Round only to the machine control’s usable resolution. The 636.6 RPM result is a derived spindle command for that fully interrupted SKD11, 60-62 HRC, 60 mm, six-groove, dry, TiN-coated high-CBN study condition. It is not a starting recommendation for another hard steel, CBN grade, interruption pattern, diameter, finish target, or setup.
Use the proof cut to choose the adjustment direction
Inspect the cutting edge and surface after the first condition-preserving trial. Hold the source row and setup record still long enough to identify what changed. Then change one variable, not speed, feed, depth, grade, and setup at once.
| Observed result | First check | Bounded adjustment direction | Evidence limit |
|---|---|---|---|
| Flank wear rises quickly or heat marks appear | Confirm continuity class, actual speed, and whether the application resembles the fully interrupted SKD11 dry-study condition. | If the application is comparable, examine speed first: the IJRES study found cutting speed had the strongest influence on flank wear within its tested design. Change one variable and inspect again. | This does not prove that lowering speed fixes every wear mechanism or material. |
| Chipping at re-entry | Reclassify the cut as interrupted; check insert grade/edge condition, holder, workholding, and re-entry feature before changing feed. | Correct the interruption/setup mismatch first. Then use the supplier’s interrupted row rather than a continuous row and prove one controlled change. | The sources do not prescribe a feed or depth reduction for every chipping event. |
| Poor surface finish | Verify the finish target, feed unit, continuity, hardness, CBN grade, edge condition, runout, and rigidity. | Use the source’s final-turning row only when it matches the engagement class. Change one variable after the mechanical checks. | The H13 study shows these variables influence roughness in its tested range; it does not calculate finish for every application. |
| Inconsistent finish or edge life on a flexible setup | Check holder/workholding rigidity, stickout, and runout before attributing the problem to the chart. | Stabilize the setup or reduce the trial’s risk through a verified application process, then repeat a single-variable proof cut. | The sources provide neither a rigidity threshold nor a numeric compensation rule. |
The fully interrupted study also reported that mechanical wear occurred at lower speeds and diffusion wear at higher speeds in the literature it discussed. That is a mechanism clue for a comparable hard-turning application, not a substitute for identifying the actual wear pattern or CBN condition. The study’s own experimental window was limited to 120, 140, and 160 m/min; it should not be extended below or above that window as though it were a catalog curve.
Record the qualified setting, then choose a tool family separately
Save the source row, units, actual diameter, calculated RPM, material and hardness, CBN grade and edge condition, continuity class, coolant practice, rigidity notes, surface result, edge condition, and the one variable changed after the proof cut. This turns a chart lookup into a repeatable process record.
Once those application inputs are known, the CBN turning insert range can be used to navigate candidate product families. The page is not evidence that a particular SCT insert has the required grade, edge preparation, interrupted-cut capability, availability, or performance. Confirm those details separately.
Choose the engagement row first, calculate RPM from the actual diameter, inspect edge and finish, then change one variable. That sequence keeps a useful source range tied to the actual application.
