Diamonds are a Machinist’s Best Friend
"Pressed carbon"—also known as diamond—ranks among the hardest materials and is therefore ideally suited for machining the toughest metals. Especially when metals require specific properties, diamond delivers outstanding results, for example as a diamond burnishing tool to achieve perfect surface finishes. KEMPF from Reichenbach-Fils offers a wide range of diamond burnishing tools specifically designed for this type of surface finishing, covering internal, external, and even face machining operations, even in tight spaces.
More Than Just a Cutting Material
When people talk about diamonds, most envision sparkling and exorbitantly expensive jewelry rather than recognizing that diamond is a true marvel of nature and, along with various carbon compounds, ranks among the hardest materials found on Earth. In fact, it’s far too valuable to be used solely for jewelry or phonograph needles.
So what could be more logical than using this material in the machining industry? You might think this is nothing new, as diamond has been used for decades in the form of PCD and CVD cutting materials, primarily for turning and milling applications.
However, to achieve certain surface qualities, additional grinding, polishing, or smoothing and densifying with roller burnishing tools is usually required. This process works very well when machining defined workpieces with consistent dimensions and high quantities. But if production needs to be more flexible or if workpieces with varying dimensions must be produced in individual quantities, then diamond burnishing tools are the better choice.
Harder than any hardened steel and flexible in machining
Not only is the diamond material used, which comes into contact with the workpiece and densifies the surface, harder on the Mohs hardness scale for minerals at hardness 10 than any hardened high-speed steel roll. The entire tool design also experiences significantly less wear due to the elimination of moving parts.
Another advantage of diamond burnishing tools is their application flexibility. Depending on the tool type, when manufacturing rotating components such as rods or shafts, both external and internal machining—such as on tubes or bores—can ideally be performed with the same tool. The diamond used only needs to apply pressure contact to the workpiece—the type of machining process generally does not matter.
Diamond burnishing tools for external machining can therefore, under certain conditions, also be used for machining flat surfaces, often eliminating the need for additional tooling solutions.
When the Surface Needs to Be “Special”
By applying pressure to the surface, the surface structure—commonly consisting of “peaks and valleys”—is reshaped, and the roughness peaks are pressed into the valleys. This results in a smooth and additionally densified surface with a defined surface structure.
Such a surface is essential whenever specific properties like wear resistance or a particular optical surface finish are required. These requirements are especially critical for moving components, such as spindles or shafts. Often, these running surfaces are sealed by sealing rings to other hydraulic components. In these cases, the condition of the running surfaces plays an exceptionally important role.
These tools are also widely used in aerospace for many hydraulic components, as burnishing always produces the effect of a densified surface, making the respective components more resistant to external influences.
In the close-up, the "forming" is clearly visible. The peaks or "ridges" have been pressed into the "valleys," resulting in a more homogeneous and smoother surface.
Complete Machining on the Machine
KEMPF from Reichenbach-Fils, renowned especially for its deburring tools and burr-free machining directly on the machine, also offers a broad portfolio of roller burnishing and diamond burnishing tools as part of its goal for complete machining on the machine.
This approach helps eliminate subsequent, time-consuming grinding or polishing operations. The objective is complete machining on the machine—meaning from milling or turning the component all the way to the proverbial “finish.”
The diamond burnishing tools are particularly well suited for this, as they are highly flexible in application and, depending on the type, compact in design. In tight spaces, for example, short tools are used, while adjustable diamond burnishing tools can be employed for universal applications.
ARCA Regler Optimizes Tapered Spindle Production Through Diamond Burnishing
The listed advantages have also benefited ARCA Regler GmbH. The specialist in control valves and valve actuators employs around 500 people worldwide. The custom-made control valves are used wherever defined mixing ratios and adjustable flow rates are required.
For the production of the tapered spindles, a specific area must have an absolutely smooth surface after turning. The later running surface is wrapped with a sealing ring, so any excessive surface roughness can lead to hydraulic fluid leakage.
The mounted taper spindle at ARCA-Regler. On the right, the surface machined with diamond; on the left, the unmachined surface for comparison.
Previously, this area was removed from the machine after turning and manually machined separately. The results were sufficient, but the process was unproductive and very time-consuming. By integrating a diamond burnishing tool into the machining process, significantly better surface roughness values were achieved after the initial trials. Additionally, a complex operation was eliminated, and the entire machining process became more flexible.
An unfinished control valve during final assembly: The already installed tapered spindle serves as the lift drive. The part with the burnished surface is located inside (below) the entire sealing sleeve and ensures absolute external tightness during the lift movement. If the surface were too rough (uneven), hydraulic fluid would leak.
Replacement for Complex Grinding and Polishing
The high flexibility of these tools was also decisive for PMW Präzisionsmechanik GmbH. The company supplies customers from various industries, including automotive, aerospace, as well as machinery and gearbox manufacturing.
For a steel disc with a diameter of 320 mm and a weight of approximately 13 kg, a surface finish requirement of Rz 1 µm was specified for the cylindrical surface. In the production process, only an Rz value of 3.5 µm was initially achieved, making subsequent smoothing necessary. This was previously accomplished through grinding and lengthy polishing processes.
Clamping situation during machining of the steel disc: Due to the large diameter, there was limited space for the diamond burnishing tool. For this reason, the compact SMDBS3 tool with a total length of only 125 mm was used here.
Below 0.8 µm Rz on the First Try
Since repeated intermediate measurements were necessary and consistent results were not achieved, the company sought an alternative. Together with KEMPF, a compact diamond burnishing tool was selected and integrated into the machining process.
The result exceeded all expectations. Without lengthy trials, an Rz value below 0.8 µm was achieved immediately. The cost-effective tool replaced time- and cost-intensive rework, ensured consistent quality, and significantly increased process reliability.
The mirror-smooth cylindrical surface, barely wider than a 5-cent coin, was also machined on the angled surface using the diamond burnishing tool. Thanks to the small radius of the diamond insert, even such machining operations pose no problem.
Pinion Shaft: Surface Quality at a New Level
Typical application: A pinion shaft made of alloyed steel with a diameter of 10 mm was to be burnished over a length of 16 mm.
After the preliminary machining, only an Rz value of 6 μm was achieved, whereas a final Rz value of 4 μm was required, which could not be reliably attained through turning. After using the DB1 tool with hex shank and cutting parameters of Vc = 120 m/min, f = 0.1 mm/rev, an Rz value of 1.04 μm was finally achieved.
Diamond Burnishing Tools Ideal for Flat Surfaces
Flat diamond burnishing tools are especially well suited for machining larger flat surfaces, for example on machining centers. The tool types DBFM63, DBFM80, DBFM100, and DBFM125 feature multiple spring-mounted diamond inserts that are precisely matched to each other. This allows surfaces to be reliably smoothed and densified in a single operation immediately after milling. The tools consist of a tool body and replaceable diamond inserts. This makes them economical and suitable for use on almost all materials—from carbon steel and tool steel to cast alloys as well as ferrous and non-ferrous metals.
The diamonds enable high-quality surface finishes while simultaneously densifying the surface. For example, in gray cast iron, an initial Ra value of 2–3 µm can be reduced to a final value of 0.2–0.4 µm.
A brief overview of the various diamond burnishing tools – on the left, the flat diamond burnishing tool DBFM63 with a pitch circle diameter of 63 mm and an overall diameter of 90 mm for smaller flat surfaces. Next to it on the right are the DB3 type and the slightly smaller DB1, primarily designed for external machining of rotating workpieces. On the far right is the universal burnishing tool, designed so that the diamond head can be adjusted by 180 degrees. This feature is ideally suited for machining components in small series production where specialized tools are not used. This tooling solution offers maximum flexibility for burnishing large bore walls, outer diameters, tapers, and surfaces.
Brilliant Results with a System
Diamond is not only valuable as a gemstone but also ensures that workpieces shine like a diamond—often more cost-effectively than expected. KEMPF offers a broad portfolio of Cogsdill diamond burnishing tools and can provide the right tooling solution for nearly every application.