Theoretical Surface Roughness Ra Calculator for Threading

Optimize your machining processes with our online cutting parameters calculator. Select the operation type (turning, milling, drilling or threading) and instantly get the critical calculations for your shop: cutting speed (Vc), revolutions per minute (RPM), feed per revolution or per tooth, machining time and the theoretical roughness (Ra / Rz) based on the insert nose radius. To fine-tune those values live at the machine, use the interactive machining calculator.

Cutting calculator

Step 1

Which operation are you machining?

TurningRPM per part diameter and feed per revolution.MillingRPM per cutter diameter and feed per number of teeth.DrillingRPM per drill diameter and axial feed per turn.ThreadingRPM, synchronised feed, lead and time for tap, lathe threading or thread mill.

Paso 2

What do you want to calculate?

Cutting speed VcCalculate Vc from diameter and RPM.RPMCalculate revolutions from Vc and diameter.FeedWork out the programmable feed or its linear equivalent.Theoretical roughnessGuideEstimate Ra/Rz with feed and tip or edge radius.

Paso 3

Calidad superficial en roscado

In threading, roughness and thread quality cannot be predicted well from feed and radius alone. They depend on the tap/cutter/insert, coating, lubrication, material, chip evacuation and compensation. Check with a gauge, a profile or the drawing criterion.

Formula

Calidad = herramienta + material + lubricación + compensación

Result

Recomendación

Verificar con galga/perfil

Material, power and warnings

Pick material and tool: that gives you the Vc and feed ranges, and also whether your machine can take the cut.

ToolingCut typeCondition

Do you want to adjust these values live?

Here you get the recommendation (Vc, feed, roughness, threading). In the interactive machining calculator you can move Vc, diameter, RPM and feed with linked scales, locks and time/power estimation.

Open interactive calculator →

Surface roughness in threading: formula, example and table

Ra ≈ f² × 1000 / (32 × r)

Theoretical Ra in µm from the feed f (mm per revolution or per tooth) and the nose or corner radius r in mm. The real finish is worse with vibration, built-up edge or a worn insert; take the table as the best case in threading.

Worked example: f 0.15 mm and r 0.8 mm → Ra ≈ 0.15² × 1000 / (32 × 0.8) = 0.88 µm.

Ra 0.8 µm is a good turned finish; Ra 3.2 µm, a normal machined surface.
Feed f (mm)	Ra r 0.2 (µm)	Ra r 0.4 (µm)	Ra r 0.8 (µm)	Ra r 1.2 (µm)
0.05	0.39	0.2	0.1	0.07
0.1	1.56	0.78	0.39	0.26
0.15	3.52	1.76	0.88	0.59
0.2	6.25	3.13	1.56	1.04
0.25	9.77	4.88	2.44	1.63
0.3	14.06	7.03	3.52	2.34

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