Optimizing Metal Removal Rate in Aluminum Using a High-Speed Spindle

The Metal Removal Rate or material removal rate, denotes the amount of a material removed from the workpiece at a time during a machining operation such as drilling, turning, or milling.

It is an expression of the machining speed and is directly related to productivity and efficiency.

MRR (Q) =depth of the cut * cut width * the feed rate (measured in cubic inches per minute)

From the formula, you can notice that the higher the depth cut, feed rate, and cut width, the higher the MRR.

MRR also depends majorly on the material being machined. In the case of aluminum, machine shop production can experience some challenges during machining using a high spindle speed. This article aims to discuss Metal Removal Rate and how a CNC machine shop can optimize the Metal Removal Rate for aluminum using a high-speed spindle.

Aluminum and High-Speed Spindle: The Effect of Chatter

Using a high-speed spindle to machine a metal depends on factors such as material hardness. For example, when working with steel, the hardness ensures that the high spindle speed can only incur a light depth of cut.

For aluminum, its softness means there is a larger depth of cut and an increase in chatter (a self-vibration that reduces productivity and quality) when machining at a high spindle speed. As a result, aside from the tooling and machine, a machine shop working with aluminum and using a high-speed spindle further becomes limited by the chatter.

The presence of chatter means that machining at a high speed doesn’t equal the best Metal Removal Rate. As a result, machine shops must find a sweet spot that can offer an optimum speed that can assure an MRR. This is achievable using basic vibration analysis.

Understanding Charter

Many machining enthusiasts will say that chatter is the product of the cutting tool’s edge, however, this definition portrays chatter as a forced vibration even though chatter itself is a self-excited vibration. As a result, the better definition is that chatter is a self-vibration that occurs due to the interaction of impact and an already machined surface (even if smooth).

Every machined surface has waviness and together with vibration, we have the chatter. When both waviness and vibration harmonize, we can have the sweet spot which can increase the MRR during the aluminum machining process. However, when both don’t harmonize (1800 out of phase), chatter is at its worst.

Where Does Chatter Not Matter

There are three conditions where you do not need to find a sweet spot to get the right MRR

-If the spindle rpm is very low. This is because, at the speed range early end, the chatter will smother due to a strong natural process damping

-A light depth of cut (0.050 inch and below)

-All components of the process are very stiff. That is, the work holding is rigid, the spindle is dynamically stiff with a secure hold on the tool holder, the cutting tool is short, and the tool holder is compact. In this condition, the best speed is the top speed.

Using Chatter to Improve Metal Removal Rate

Getting the sweet spot is done by getting the sound at which the vibration and the waviness overlap. This is achievable in two ways as described below:

• Using software tools and analyzers

This involves using a microphone to listen to the cutting process.          Afterward, the optimum spindle speed that will generate the best MRR is calculated.

• Hammer Tests

Unlike the use of software, the hammer test does not require cutting the workpiece. Instead, the operator understands vibration analysis which a CNC machine shop might not have.

• Cutting tests

The cutting test requires a little cutting, an understanding of the CNC machining process, and what chatter/sound is all about. It gestures to the fact that getting the sweet spot is only possible by considering every component including the spindle, tool, tool gage length, and toolholder. Changing any of the components can alter the sweet spot. 

Below is the procedure for getting the optimum speed using the cutting test:

-Mill the workpiece at the maximum spindle speed

-Use any reasonable value for the chip load as it does not influence harmonic effects.

-Choose an arbitrary but constant value for the radial depth of cut until you find the sweet spot

–For the axial depth of cut, start at a low value and increase until chatter occurs 

-Calculate the metal removal rate using the maximum depth recorded before chatter. Use the equation (MRR= feed rate × radial depth of cut × axial depth of cut) 

-Reduce the spindle speed and check if there is a reduction in MRR. 

On reducing the speed, the MRR can decrease or increase. You will know you have found the sweet spot if the spindle speed allows a much deeper cut without leading to chatter. When you get to that level, that is the best speed to machine aluminum.

-Vary the radial depth of the cut and test to get the axial depth of cut

-Use the maximum depth before chatter to calculate the Metal Removal Rate

-Use the data to specify how to combine the cutting tools, tool holder, and machine for the optimum cutting.

Conclusion

Material removal rate or Metal Removal Rate is a measure of the productivity of the machining process. It is crucial in the CNC machine industry, and this article discussed different ways to optimize the metal removal rate in aluminum using a high-speed spindle for maximum aluminum.

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