Turning formulas and definitions
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When machining in lathes, turning centres or multi-task machines, calculating the correct values for different machining parameters like cutting speed and spindle speed is a crucial factor for good result. In this section you find the formulas and definitions needed for general turning.
Cutting speed vc (m/min)  | Cutting speed vc (ft/min)  | |
Spindle speed n (rpm)  | Spindle speed n (rpm)  | |
Metal removal rate Q (cm3/min)  | Metal removal rate Q (inch3/min)  | |
Net power Pc (kW)  | Net power Pc (HP)  | |
Machining time Tc (min)  | Machining time Tc (min) |
| Symbol | Designation/defination | Unit, metric (imperial) |
|---|---|---|
| Dm | Machined diameter mm (inch) | mm (inch) |
| fn | Feed per revolution | mm/r (inch/r) |
| ap | Cutting depth | mm (inch) |
| vc | Cutting speed | m/min (feet/min) |
| n | Spindle speed | rpm |
| Pc | Net power | kW (HP) |
| Q | Metal removal rate | cm3/min (inch3/min) |
| hm | Average chip thickness | mm (inch) |
| hex | Maximum chip thickness | mm (inch) |
| Tc | Period of engagement | min |
| lm | Machined length | mm (inch) |
| kc | Specific cutting force | N/mm2 (N/inch2) |
| KAPR | Entering angle | degree |
| PSIR | Lead angle | degree |
Tangential force, Ft
Kc 0,4: Specific cutting force at feed 0,4 mm/r
mc: Constant, depending on material. Use 0,29 as general value.
Tangential force, simplified formula
When the entering angle (KAPR) is 75 degrees or longer, sin KAPR is ~1. Then this simplified formula can be used.
Rule of thumb: Ft should not exceed 90% of maximum load stated for the bar used.
