How to Optimize Tool Selection and Cutting Parameters to Address Tolerance Issues in Custom Metal Machining

The functionality and performance of custom machining parts depend on whether dimensions fall under the desired range or not, called tolerance. This can be ensured using the right tools and machining parameters.

Let’s discuss how you can optimize the selection process for better tolerances;

Relationship Between Tool Selection and Machining Tolerances

Achieving precise machining tolerances requires not only advanced equipment but also the right choice of cutting tools. In metal machining, tool selection directly impacts accuracy, surface finish, and overall efficiency. Factors such as tool material, geometry, and coating influence how well a tool can maintain tight tolerances while minimizing wear and deformation. Understanding this relationship is crucial for optimizing machining processes and ensuring high-quality parts.

Tool Material

The tool should be strong enough to shear the material from a workpiece. Carbide, High Speed Steel (HSS), and Coated tools are popular options. Consequently, which tool material to use also depends on the physical and mechanical properties of the work material. For instance, aluminum or other soft metals can be machined precisely with HSS, but not titanium alloys.

The table below shows how different tool materials impact tolerances in the machining of aluminum, steel, copper, titanium, and brass. Tolerances are divided into low(≥ ±0.05 ), medium (±0.01 to ±0.05 mm), high (≤ ±0.01) mm.

The capability of machining equipment itself also impacts tolerances like 3 & 5 -and 5-axis CNC mills are more precise than traditional machines. Meanwhile, you can optimize precision with the right tool material, whatever the machine’s limit.

Tool Geometry

Single and multiple cutting points are two primary classifications of tool geometry. Tools with single-cutting points minimize the deflection due to consistent cutting forces and allow for controlled material removal. Other geometrical parameters like rake angle, end relief angle, nose radius, and cutting edge angle impact tool engagement with work and chip formation.

Machining Parameters and Tolerances of Custom Parts

The cutting speed, feed rate, machining depth, and all other parameters are critical for tight tolerances. For custom machining parts, these values need to be decided by considering multiple factors, such as geometrical complexity, involved tools, work material type, etc.

Cutting speed characterizes the movement of the cutting tool with respect to the fixed element, expressed in meters or feet per minute. High speed means a large chunk of material is removed quickly, which limits precise tolerances. On the other hand, slower speeds make accurate finish passes and allow for tighter tolerances.

Consequently, feed rate refers to the material removed by cutting the tooth during a cycle, expressed in feed per tooth. High feed rates not only reduce the accuracy but also cause material clogging and the risk of tool breakage.

Furthermore, cutting depth is the thickness machined by the cutting tool in a single cycle. Machining with larger depth increases the risk of tool deflection, heat buildup, and chattering.

You can perform a test run with the best possible values of cutting speed, feed, and depth before production.

Heat Effects on Tolerance and Strategies for Machining Stability

In metal machining, the tool continuously engages with the workpiece, which causes friction and heat generation. Inside machine components can also generate and transmit heat to the machining area.

This heat expands the work material and work hardens the machining tool. This expansion returns to its original position after cooling, so the machining dimensions become slightly smaller based on the thermal expansion coefficient of the work material. On the other hand, work-hardening of the tool leads to dullness and surface wear, further affecting the tolerances.

You can follow the following strategies to avoid heat effects and maintain machining stability;

Use of Coolant Systems: 

Coolants (Water or Oil-based) transfer the heat to the surrounding environment from the machining area to maintain a determined temperature. In CNC processes, the coolant flow is controlled by G & M-codes, whether it is a flood system ( flows through a pipe), a through-spindle system ( flows through internal channels of the spindle to tool), or a Mist coolant System ( spraying coolant).

Tool Path Optimization: 

Longer tool paths with more machining passes in linear patterns suit tight tolerances. Additionally, the machining path for the curved or irregular contours requires arc fitting to identify and slow the process. CAM simulation tools help to predict the effect and optimize the path.

Process Sequencing: 

It means in which order you perform multiple operations like cutting, drilling, contouring, threading, and turning. Additionally, process sequencing also involves avoiding repetitive heavy cuts in the same workpiece section and primary roughing.

The Critical Role of In-Process Measurement Tools in Detecting Deviations

Custom machining parts are not only inspected after production. Instead, in-process inspection and measurement are also carried out to detect dimensional deviations. This way, the manufacturer can control the desired tolerances and reduce the number of defective items.

Tools like touch probes, laser scanners, and infrared thermometers are used to detect dimensional deviations in CNC machining processes.

Conclusion

You must consider the right tool material, coating, cutting speed, feed rate, and thermal effects to achieve tight machining tolerances. At the same time, all setup and parameters should be according to the work material type and complexity of the CAD model that needs to be machined.