Volume Based Feed Rate Optimization: Can We Rely On It?

Feed rate optimization of NC programs has started to become a hot topic for the past 5 years. Most CAM companies have developed some type of an optimization tool to adjust feed rate along the tool path. The idea behind feed rate manipulation is to dynamically change feed rate along tool paths where geometry varies. This way, a long G01 command with one feed value can be split into a number of shorter G01 commands with different feed rate. The fundamental assumption that such systems are based on, called Volume Based Optimization, is that 

Process outputs such as spindle power/torque, cutting forces, tool deflection, etc... are directly proportional to the material removal rate (or the volume of material removed)... 

Since CAM based systems only have the geometric information obtained from tool path simulation and workpiece geometry update, this assumption seemed valid at the time. However, researchers specialized in metal cutting prove this assumption wrong! It is very likely that two different cutting configurations with identical material removal rate may result 50%-100% difference in peak cutting forces, spindle power/torque, etc...

In order to demonstrate this, we took two different cutting conditions for a tool with 20 mm diameter and 4 flutes:

Case1. Q = 1.2 lt/min; fz = 0.2 mm/tooth; n = 15,000 rpm; ap = 10 mm, ae = 10 mm

Case 2. Q = 1.2 lt/min; fz = 0.4 mm/tooth; n = 15,000 rpm; ap = 11 mm, ae = 4 mm

Note that both Cases have identical material removal rate of 1.2 lt/min. Both processes were simulated using an advanced process analysis tool, CUTPRO, and spindle power was calculated for these high speed machining cases. As expected, results obtained from the analysis were contradicting with Volume Based Optimization assumption. Despite identical volume of material removed, maximum spindle power varied more than 30% for these two cases, i.e., Volume Based Optimization had already introduced 30% error in calculating optimized feed rates. Such high percentage of error in feed rate can cause catastrophic failure during high speed machining.

Simulated Spindle Power with conditions given for Case 1.

Simulated Spindle Power with conditions given for Case 2.

This blog shows importance of Physics Based Optimization when feed rate optimization of an NC program is requested. MACHpro Virtual Machining; therefore, is an optimization tool of choice when it comes to optimizing feed rates based on in-depth analysis of cutting mechanics, which uses tool and cutting edge geometries, workpiece material properties, machine tool kinematics and dynamics to calculate the most reliable, and accurate process outputs and hence feed rates.