Cutting Speeds and Feeds

§1The three parameters

Speed, feed and depth of cut are the three levers of every cut. They sound interchangeable — all three remove more metal when raised — but each one trades against a different limit, so they are never set alike.

Cutting speed (m/min) is how fast the edge passes the work; it chiefly governs tool life, and steeply (§2). Feed (mm/rev, or per tooth) is how far the tool advances each revolution; it chiefly governs surface finish (§3) and the cutting force. Depth of cut (mm) is how deep the tool is set; it chiefly governs the force and power, and how many passes a job takes (§4). Their product is the metal removal rate. The skill of setting up a cut is to raise the three as far as productivity wants while respecting the different ceiling each runs into — tool life, finish, or machine power — which is why roughing and finishing choose them so differently (§5).

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§2Cutting speed

Cutting speed is quoted for the material and tool, and converted to a spindle speed through the diameter; it is the parameter tool life is most sensitive to.

As the cutting-tools page set out, the spindle speed follows from N = 1000 V/(π D), and Taylor’s law V Tⁿ = C makes tool life fall steeply as speed rises — a small increase in speed can halve how long the edge lasts. Speed is therefore chosen first and conservatively: the material and tool fix a recommended cutting speed (carbide runs several times faster than high-speed steel), and that speed sits well below the maximum the edge could briefly survive, because the economic cost of a short tool life outweighs the time saved. Feed and depth are then chosen around it. Speed buys productivity at the sharpest cost in tool life, so it is spent carefully.

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§3Feed and surface finish

Feed controls how rough the turned surface is, through a simple geometric relationship with the tool’s nose radius — and because the relationship is quadratic, feed is the strongest lever over finish.

Ra ≈ f²32 r — f feed per revolution, r tool nose radius

Example 1 — feed against finish

The feed marks leave a scalloped surface whose roughness grows with the square of the feed and falls with a larger nose radius. Turning with a 0.8 mm nose radius at 0.2 mm/rev gives Ra ≈ 0.2²/(32 × 0.8) = 1.56 µm; halve the feed to 0.1 mm/rev and, because Ra depends on f², the roughness quarters to 0.39 µm (the hero shows this for two nose radii). A bigger nose radius smooths further — 0.4 mm radius at that feed gives 0.78 µm — though too large a radius raises cutting force and chatter. The lesson: for a fine finish, reduce the feed and use a generous nose radius; the quadratic means small feed reductions pay large finish gains.

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§4Depth of cut

Depth of cut sets how much stock a pass removes and, with feed, how hard the machine is worked; it is limited by cutting force, power and rigidity rather than by finish.

A deep cut removes stock in fewer passes, so roughing takes the deepest cut the machine and setup can stand. What stops it is force and power: cutting force rises with both depth and feed, and the power the spindle must supply rises with the removal rate (its own page), so an over-deep cut stalls the machine, deflects the work or provokes chatter. Depth barely affects surface finish — that is feed’s domain — so the usual strategy is to take the depth heavy for removal and control finish through feed and speed. Where the total stock is large it is shared across several roughing passes at full depth, leaving a thin, light final pass for finishing.

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§5Roughing and finishing

The clearest way to see how the three parameters combine is the split between roughing and finishing — two opposite settings for two opposite goals.

How roughing and finishing set the three parameters
Parameter	Roughing	Finishing
Depth of cut	heavy — remove stock fast	light — small final skim
Feed	high — productivity	low — fine finish (Ra ∝ f²)
Speed	moderate — protect tool life	higher — good finish, light load
Goal	maximum metal removed	size and surface quality
Roughing throws depth and feed at the job to shift metal, accepting a rough surface, while finishing uses a light depth and low feed for size and finish and can afford a higher speed because the light load barely wears the tool. The same three numbers, set oppositely, serve the two ends of every job.

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§6Choosing the numbers

In practice the three are set in order, each from what limits it, and adjusted by what the cut tells you.

Begin with speed, read from the material and tool — the figure that protects tool life. Set the depth next, as heavy as rigidity and power allow for roughing, light for the finishing pass. Choose the feed last: high for roughing, and for finishing from the finish wanted through Ra ≈ f²/(32r). Then listen to the cut — chatter means less speed, less overhang or a smaller nose radius; a poor finish means less feed or more speed; a stalling machine or blue chips mean less depth or feed. The starting numbers come from the material and tool; the final numbers come from the cut itself. Speed for life, depth for removal, feed for finish is the order to hold in mind.

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§7Quick reference

The working core of the page on one card rack.

Three levers

speed · feed · depth

Speed

→ tool life (Taylor, steep)

set first, conservatively

Feed

→ finish · Ra ≈ f²/(32r)

halve feed → quarter Ra

Depth

→ force & power

heavy rough, light finish

Order

speed → depth → feed

then read the cut

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Handbook application: from concept to controlled practice

Purpose. This expanded section turns the original page into a practical handbook. It preserves the supplied material and adds a repeatable way to apply, check and review Cutting Speeds and Feeds. It does not replace a contract, legislation, a controlled standard, competent engineering judgement or specialist advice.

The operating aim is to carry the subject from function and assumptions through design evidence, verification and controlled release. Read the original explanation first, then use the workflow and checks below to convert knowledge into evidence.

Apply Cutting Speeds and Feeds by beginning with the duty, not the component or software command. Convert the key ideas—cutting, three, speed, feed, finish—into measurable requirements and interfaces. Record operating and non-operating environments, duty cycle, expected life, loads, energy sources, human interaction and reasonably foreseeable abnormal conditions. When a value is not a project requirement or verified supplier datum, identify it as an assumption or illustrative value.

Create a calculation and evidence trail that another competent person can audit. Every input should carry a source, unit, revision and uncertainty or tolerance where relevant. Every model should state its boundary conditions and limitations. Keep nominal capacity separate from design capacity, and keep verification margin separate from an arbitrary safety factor. If a code or standard governs the work, confirm the applicable edition and contractual status rather than copying a number from a secondary summary.

Design for manufacture, assembly, inspection, operation and maintenance at the same time. A technically valid geometry can still fail because it cannot be fixtured, measured, cleaned, guarded, reached or replaced. Review process capability, datum or reference strategy, tolerance accumulation, access, error-proofing and changeover. Where people interact with plant, apply the hierarchy of controls and consult those who will operate, clean, maintain and recover the equipment.

Plan verification before release. Define the characteristic, method, equipment, sample or test condition, acceptance criterion, record and responsible person. Validation then asks a different question: whether the resulting system is effective and suitable in the intended use context. A passed drawing check or analysis does not by itself validate usability, maintainability or production performance.

Step-by-step operating method

Define the duty. Capture the required function, interfaces, operating environment, life, loads and unacceptable outcomes.
Establish the model. Identify governing principles, units, material or process data, assumptions and uncertainty.
Develop alternatives. Compare feasible concepts against performance, manufacturability, safety, maintainability and cost.
Verify the design. Use analysis, test, inspection or demonstration with acceptance criteria defined before execution.
Release and learn. Baseline the design, control changes, retain evidence and feed operating results into the next revision.

Illustrative design review record

Illustrative values only. Build a one-page record with the required function, input sources, assumptions, governing load or process condition, failure consequences, selected concept, verification method and acceptance criterion. Mark every numerical input as project requirement, verified supplier data, measured value, calculation output or assumption. Review the weakest evidence first. If an assumption can change safety, compliance, interchangeability or capacity, it must be resolved before release rather than buried in a calculation note.

Evidence class	Question	Release expectation
Requirement	What must the design do and under which conditions?	Approved and traceable
Input	Where did the load, property, tolerance or process limit come from?	Source, unit and revision recorded
Analysis	Which model and assumptions connect input to result?	Checkable calculation or simulation
Verification	How will conformity be demonstrated?	Method and acceptance criterion agreed
Validation	Will the solution work for intended users and conditions?	Representative use evidence

Common failure modes and recovery actions

1. Watch for

Starting detailed design before interfaces and operating limits are agreed.

Recovery: Return to the governing definition or requirement and restate the decision in one sentence.

2. Watch for

Using catalogue or typical values as though they were certified project inputs.

Recovery: Separate evidence from assumption, assign an owner and set a date for validation.

3. Watch for

Checking nominal performance while ignoring tolerances, degradation and foreseeable misuse.

Recovery: Run a small counterexample, boundary test, pilot or independent check before proceeding.

4. Watch for

Confusing verification of requirements with validation of user need.

Recovery: Record the consequence, decision and rationale, then update the controlled baseline.

5. Watch for

Releasing drawings or procedures without configuration, inspection and change controls.

Recovery: Escalate when the issue affects safety, compliance, acceptance, material value or an agreed tolerance.

Review checklist

What function and failure consequence govern this decision?
Which inputs are measured, specified, assumed or illustrative?
How will conformity be demonstrated and recorded?
What change would invalidate the current evidence?
Are mandatory requirements distinguished from recommendations and illustrative values?
Are sources, assumptions, units, dates and versions recorded closely enough to reproduce the decision?
Have safety, legal, ethical, stakeholder and operational consequences been considered at the appropriate level?
Is there a named owner and a trigger for review, escalation, change or retirement?

Questions for deeper application

What is the most important distinction a practitioner must preserve when applying Cutting Speeds and Feeds?