Final finish of a diamond-turned surface is affected by a variety of factors. Surface finish is described as the presence of defects with length scales of below 0.8mm in the machined surface. Longer defects are termed as form errors. This article discusses each factor affecting surface finish.
Compatibility of the material for the diamond turning process is crucial to achieve clean cutting with reduced surface damage. Grain structure and the presence of impurities in the material affect surface finish. Hence, selecting an appropriate material is important to attain the desired surface finish.
Diamond turning of electroless nickel can produce surface finishes better than 0.6nm Ra (Figure 1). The depth of cut and the surface speed during cutting can also affect tool wear and surface finish in some cases. However, the impact of surface speed is minimal for most diamond-turnable materials.
Figure 1. Typical performance achieved with SPDT of electroless nickel plated mold pin.
Diamond tools used in the turning process need to be sharp without chips on the nanometer scale. It is necessary to optimize the tool rake angle and tool radius for a given workpiece or material for a better surface finish (Figure 2). Proper tool lubrication and provisions for chip removal also help achieving better finish.
To avoid vibration in the diamond tool and workpiece, the diamond needs to be rigidly attached to a stiff tool shank, which then needs to be rigidly fixed to a stiff tool holder. Adequate support needs to be provided for thin workpieces by rigidly attaching them to the diamond turning tool.
Figure 2. Cutting tools’ cutting edge geometry. Rake angle is a and clearance angle is ß.
Achievable finish in diamond turning is theoretically limited by cusps formed on the workpiece when a circular shaped tool is used. Lowering the feed rate is one option, but is a key parameter to determine the cost of the turning process.
Cutting Forces and Dynamic Forces
Static deflection is the result of the static component of the cutting forces and does not affect surface finish, but affects form error. However, a diamond turning machine is affected by a variety of dynamic forces that are much higher than the cutting forces. The ability to defy these dynamic forces determines the surface finish capability of a diamond turning machine and is defined as dynamic stiffness (Figure 3).
Measuring the dynamic stiffness of a diamond turning machine is a more challenging task than static stiffness, and is measured by the machine’s damping, inertias and natural frequencies.
Figure 3. Dovetail/Boxslide in relation to dynamic stiffness
The spindle rotation affects surface finish by exerting significantly high dynamic forces on a diamond turning machine. These forces are mostly synchronous with the spindle rotation. However, the spindle exerts forces having an asynchronous component because of air pressure pulsations or electrical noise in the motor amplifier.
However, these forces do not affect surface finish, as they do not repeat each rotation. These forces can be tackled using a dynamically stiff machine and spindle, but the best solution is using a superior quality amplifier with low noise cables.
Environment and Peripheral Devices
A diamond turning machine is affected by a variety of other dynamic forces caused by the machine environment, such as machine vibrations due to sound pressure and seismic forces. These environmental influences can be isolated by employing pneumatic vibration isolators and acoustic enclosures. However, these measures are required only for machines having a low dynamic stiffness.
Nevertheless, even after the isolation of these environmental influences, dynamic stiffness is required to make a diamond turning machine impervious to other forces aboard the machine, including hoses, transformers, motors, pumps, compressors, and fans. All of these forces can affect surface finish if the equipment is dynamically compliant.
A diamond turning machine‘s motion control system also affects surface finish. A high servo bandwidth is required to obtain high dynamic stiffness in the drive direction, which then relies on the resonant frequencies of the machine, the position sensor’s response time, and the servo update rate.
Surface finish is also affected by timing errors in the controller as well as by the electrical noise from the slide actuator cables and power amplifier. Another factor that has an effect is the amount of structure between the workpiece and the position sensor, and between the tool tip and the sensor (Figure 4).
Figure 4. Tool contact and edge geometry
A diamond turning machine’s slide is capable of only holding position as well as sensing it. Hence, the noise level or resolution of the position sensing system needs to be low. Achieving a very fine resolution is possible with high levels of interpolation between grating lines on a scale.
However, these interpolation techniques are erroneous, thus causing finish problems different from those caused by electrical noise. In the case of diamond turning, sensor resolution is smaller by several orders of magnitude than the accuracy of the sensors over their entire range, or of the rest of the machine. In diamond turning, the sensor noise/resolution has a small effect on surface finish, as it is small.
Although surface finish is affected by all of the aforementioned factors, it is not correct to sum up the effects of these factors owing to the fact that some of these factors will partly negate others. Using the root-sum-square of the standard deviation of each error source is a better approach to determine the RMS surface finish or Rq, which often exceeds the practical value because of the averaging effect of the diamond turning process on machine-induced finish errors.
When the feed rate is slow enough and vibration is present, the diamond tool does not cut every revolution. The tool will only cut at the low end of each vibration cycle. At the high end, the tool will cut air because it is still so close to the pass when the tool was low. The averaging effect increases with slower feed rates and higher frequencies of vibration. The theoretical tool finish is below 1nm PV in most cuts, and is referred as Rmax, where the averaging effect becomes significant.
The occurrence of averaging theoretically increases with the slowing of the feed-per-revolution. Practically, surface finish will be affected by other low frequency error sources, such as the leveling response or temperature cycling of pneumatic isolators. As a result, an optimum feed rate needs to be set to reduce machine-induced finish errors. The averaging effect on the surface finish is fundamentally limited by the material properties of the workpiece.
This article discussed the factors affecting surface finish in a diamond turning process. The best achievable surface finish is a measure of the machine system’s noise floor, which should be considered along with all of the other factors in the selection of an appropriate machine for a specific application.
The newest controller, lowest resolution, or the stiffest spindle cannot provide the ‘best’ machine. The ‘best’ machine is a seamless package consisting of properly and harmoniously working components, enabling rapid production of superior quality optical components.
This information has been sourced, reviewed and adapted from materials provided by Precitech.
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