Home / Metal cutting / Turning
Turning, where the finish model rests on real geometry
The nose radius is stamped on your insert and the finish grade is on the drawing. Between them, feed is determined — not chosen. Everything else follows from there, including the speed that actually costs you least.
Results
Theoretical finish from nose radius and feed: pure geometry, and a floor rather than a prediction. Real surfaces come out worse. Geometry inputs are the lead angle, the nose radius and a rake class — rake corrects force and power only, never speed or feed. Chipbreaker and edge preparation are yours to choose and are not modelled. Verify against your insert manufacturer's data before cutting, and if you measure finish, see below.
Now find the speed that actually costs least
Cutting speed above is whatever you entered. To know whether it is the right one you need the Taylor exponent for your machine, material and grade — and that cannot be looked up. It has to be measured, which takes two runs and some counting.
RUN 1 — HIGHER SPEED
RUN 2 — LOWER SPEED
COSTS
Optimization
Enter two runs at clearly different speeds and press calculate. The speeds need at least 20% separation — two points close together will not fit a stable slope.
What the desktop application adds
The browser tool asks you for three cost figures. The application works one of them out and remembers the rest.
| Tool edge cost | Built rather than guessed — insert price over usable edges per insert, holder cost over its life in edges, off-machine labour for presetting and gauging, and spares per change. It also keeps the line between what belongs in Ct and what belongs in tc, which is where the common error sits: off-machine work costs money but not spindle time. |
|---|---|
| Machine cost | Built the same way — purchase price over its write-off period, floor space, maintenance, power and consumables, and the operator rate with burden, over the hours you actually sell. The last of those is the assumption that moves the answer, so the dialog shows what happens when it is wrong. |
| Machines | Saved profiles carrying rated power, maximum RPM, rigidity and cost per minute, so a job is checked against a real machine instead of a number typed in each time. |
| Units | Metric and Imperial throughout — m/min or SFM, mm/rev or IPR, mm or inch. Switching converts every entered value and every result rather than relabelling them, so a job set up in one system can be read in the other without retyping anything. |
| Operations | Turning, milling and drilling in one place, sharing the material, tool material, mode and rake settings so they cannot drift apart between tabs. |
| Reports | Save an operation report against a process ID, reload it later, and export the parameters and the reasoning behind them. |
Neither figure has to be precise to be useful. Machine rate reaches the model only as the ratio of edge cost to machine rate, and the economic speed compresses that further through an exponent near a quarter — being twice wrong on the machine rate typically shifts the answer by single digits. Edge cost is the one worth getting right, because the common mistake there is not an estimate but a category error: entering the price of an insert rather than the price of one edge, which overstates tooling by two to eight times and drags the economic speed well below where it belongs.
Theoretical finish is a floor, not a prediction
The Ra this page reports is pure geometry: the cusp the corner leaves at a given feed. Nothing else is in it. Every real surface comes out worse, and the size of that gap is exactly what nobody publishes.
What the geometry misses
Built-up edge, runout, deflection under load, chatter, and the edge steadily wearing through the cut. All of them roughen the surface, none of them appear in a cusp calculation, and their combined size depends on the material and the setup.
Why the gap is guessed at
Shops that own a surface tester measure finished parts and keep the reading. It stays in that shop, attached to that job. Nobody is collecting the parameters alongside the measurement, so the correction between theory and reality has never been characterised.
What it would take
Not much. If you already measure, record what the calculator predicted, what the instrument read, and the conditions. One line per job. The ratio between the two numbers is the whole payload.
Send a measured Ra
Material and grade, nose radius or cutter, feed, speed and depth, the finish the tool predicted, and what your instrument actually read. Enough of those and the theoretical figure can carry a correction that reflects real machines instead of pure geometry — and the milling side can finally replace its assumed corner radius with something measured.
Contributors get the correction factors back as they emerge. Nothing is published with a shop name attached.
The same part, two setups, two answers
Catalog values assume a rigid machine, solid workholding and a clean continuous cut. Almost nothing on a real shop floor is all three at once, and the gap between the catalog and your setup is where inserts get destroyed.
Rigidity, in four places
The workpiece itself, how it is held, the tool and its overhang, and the machine. A thin-wall part in soft jaws on a worn machine with a long tool is four weak links, not one, and they compound rather than average out.
Interrupted cutting
An intermittent cut hammers the edge on every entry. Keyways, cross holes, flats, welded seams, anything out of round. It is a toughness problem, not a wear problem, and the answer is to back off speed rather than feed.
Skin and material condition
Cast skin, forge scale and flame-cut edges are abrasive and often carry embedded sand or oxide. The first pass through them is a different cut from every pass after it. Heat treatment and prior work hardening shift the picture again.
These are scored, not just noted. The rigidity factors combine into a figure that drives which surface finish grade is realistic to propose in the first place — a setup that cannot hold a fine finish should not be asked to. Interrupted cutting, skin and material condition then combine with it into a second figure that scales the cutting speed up or down from the catalog value. A favourable setup earns speed above the book figure; an unfavourable one has it taken away before you ever see a number.
What the two speeds mean
They are rarely the same number, and the gap between them is the decision.
Economic speed
Minimises cost per part. Slower, longer edge life, fewer tool changes. This is the right answer when the machine is not the constraint — when there is idle capacity, running faster only spends money on inserts to finish work that then waits.
Maximum production speed
Minimises time per part, ignoring what tooling costs. Faster, shorter edge life, more changes. Correct when the machine is the bottleneck and every minute of spindle time has work queued behind it.
The band between them
Anything between the two is defensible; anything outside is not. Running above the production speed costs more and takes longer. Running below the economic speed also costs more and takes longer. That band is the useful output.