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General Turning Formulas and Definitions


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When machining in lathes, turning centers, or multi-task machines, calculating the correct values for different machining parameters like cutting speed and spindle speed is a crucial factor for good results. In this section, you will find the formulas and definitions needed for general turning.

 
Metric values Imperial values
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/definition 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.

 
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