To produce parts with accuracy, precision and high-quality surface finish, it is important to minimize tool deflection. Tool deflection occurs when the force of cutting overcomes the stiffness of the tool, causing the tool to bend. The tool may not perceptibly bend during the operation, but the proof is in the final measurements. Parts made with a tool that is deflecting may be misshapen, out of tolerance, suffer from variability or exhibit a ‘chattered’ surface finish.
Tool deflection also accelerates tool wear and increases the likelihood of tool breakage. So, in addition to improving part repeatability and quality, preventing tool deflection saves money by reducing tool replacement frequency and machine downtime for tool changes.
What is tool deflection?
A cutting tool is held tightly in a chuck. All the parts of the chuck and the machine are stiff and tight; they prevent the tool from moving and limit its ability to bend inside the chuck. Similarly, the part being machined is ideally immovable and tightly clamped and supported on the stage.
Between these two extremes is the unsupported and cantilevered cutting part of the tool. During a cutting operation, the tool pushes against the material. But the material is immovable. Some of that force goes to cutting the material, but some of it pushes against the cantilevered cutting part. This pushing causes the tool to bend away from the centerline like the tip of a fishing pole bending under the weight of a fish on line.
This article details factors affecting tool deflection, but how much the tool bends can be summarized with two key factors:
- Unsupported Length – the amount of tool hanging out of the chuck.
- Tool Stiffness – this is also influenced by several factors including:
To minimize tool deflection, the amount of cantilevered length should be minimized. The amount of tool hanging out the chuck should be the least required to accomplish the cut. More passes and smaller cuts reduce tool deflection. These factors should be balanced with specifics of the part geometry and finish requirements.
In the category of tool stiffness, there are multiple influences including tool condition, tool material, and tool size and shape – number of flutes, core diameter and shank diameter. If the tool is worn and dull, it cuts less efficiently, and more force is required to achieve the same cut and finish as a sharp tool. So, more tool deflection occurs. High-quality tools made with stronger and stiffer materials result in less tool deflection. Finally, a larger diameter, more specifically a larger core diameter tool is stiffer. So, the largest tool allowable for the part geometry should be used.
It may not always be possible to use a larger diameter or shorter tool. Some operations may require a long reach or long flute tool. In these cases, tools should be matched to the operation. For a deep feature that requires minimal cutting surface, a long reach tool with a larger diameter and thus stiffer shank is more appropriate. An operation for a seamless slot wall, on the other hand, calls for a long flute tool.
In summary, optimizing tool selection and set-up geometry to minimize unsupported length and maximize tool stiffness results in less tool deflection for improved part quality and lower tooling cost.