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Turning is where the finish model stands on real geometry
Every field on the turning tab, with its help text printed rather than hidden behind a hover. This is the operation where the surface finish calculation rests on a nose radius you actually have, rather than an assumption.
Turning — cut parameters
Worked example valuesISO 513 class, not a specific alloy. The letter is already on your insert packaging. It sets the base cutting speed range and the Kienzle constants used for force and power. Plastics sit outside ISO 513 entirely and are labelled that way rather than pushed into the nearest letter.
The driver variable. Ra is specified on the part drawing before any process exists, so it is chosen here and feed is back-solved from it. The slider snaps to the ISO 1302 preferred grade steps, because those are what a drawing actually carries — not arbitrary decimals. A grade is proposed automatically from your operation conditions; dragging the slider picks the nearest step and switches Set Manually on, so an overridden value is always visibly an override.
Picked from the ISO 1832 standard steps, addressed by the two-digit code stamped on the insert. Note that the radius is only one part of the edge: the same designation also carries a chipbreaker code, and that geometry is what governs chip formation, evacuation, force direction and vibration. This calculation covers the radius and says nothing about the chipbreaker. This is the cheapest lever on the whole tab: a larger nose radius raises the feed you can run at the same Ra. Rule of thumb — use the largest radius the setup tolerates; the risk you are trading into is chatter and vibration, not finish. Radii where the required feed would exceed the radius itself are flagged, because the chip would then be too thick for that corner. Set the lead angle to Round and this field becomes Insert Diameter: enter the diameter printed on the insert and the radius is handled internally.
Back-solved, not entered. Feed follows from Ra and nose radius. A fine finish with a small nose radius produces a very small feed, which is the usual cause of chip thickness landing below the rubbing floor — the edge deforms material under itself instead of shearing it.
Sets how feed becomes chip thickness: h = f × sin(lead angle). At 90° the chip is as thick as the feed. A shallow angle of 45° or below thins it further, which helps entry and exit shock but pushes you toward the rubbing floor. Round inserts are the worst case — the entering angle varies from zero at the tip, so the average chip is roughly half the maximum, and engaging deeper relative to the insert radius thickens it.
Scales removal rate and power almost directly. It does not change chip thickness, which makes it the safer way to add productivity when edge life is already marginal. Where the machine cannot deliver the power, the calculation inverts and solves depth back from what is available rather than leaving you to guess a reduction.
The full cost of one usable edge: the insert, plus its share of the toolholder, maintenance and spare parts. Feeds the economic speed calculation. Leave it blank and you still get speeds and feeds; fill it in and you get the speed that minimises cost per part rather than the one that finishes soonest.
How long it actually takes to index or replace the edge, door to door. Together with edge cost and the machine rate it separates the minimum-cost tool life from the maximum-production tool life. These two answers diverge sharply when the machine is the bottleneck.
Theoretical finish from nose radius and feed; real surfaces are also affected by built-up edge, vibration and insert condition. Verify against your insert manufacturer's data before cutting.
The relationships behind the fields
Knowing these tells you when a result is wrong.
Theoretical surface finish
Ra = f² / (32 × rε)
Feed appears squared, so halving the feed quarters the roughness — and the reverse is why a small increase in feed can put a part out of specification.
Inverted, it gives f = √(32 × rε × Ra), which is how the tool works: you specify the grade, the feed comes out. Because nose radius is in there linearly, moving from a 0.4 mm to a 0.8 mm insert buys about 40% more feed at the same finish.
Chip thickness
h = f × sin(κr)
Unlike milling, the chip thickness in turning is constant for the whole cut, so there are only two levers: feed and lead angle. That makes the rubbing floor easier to reason about — if you are under it, one of those two has to move.
Below roughly the edge hone radius the edge stops cutting and starts rubbing. Tool life then gets worse as the chip gets thinner, which inverts the usual intuition, and the force model is being extrapolated outside the range it was fitted over.
Taylor tool life
V × Tn = C
Rather than assume an exponent, the tool back-solves it from two of your own cuts: n = ln(V1/V2) / ln(T2/T1), then C = V1 × T1ⁿ. The two speeds need at least 20% separation, because two points close together will not fit a stable slope.
From n and C come the minimum-cost tool life, the maximum-production tool life, and the speeds matching each. Both runs must end at the same wear criterion or the exponent means nothing — which is where ISO 3685 belongs.
Run these numbersThe turning calculator back-solves feed from the finish grade and finds your economic and maximum-production speeds from a two-speed test.
Open the turning calculator