While recent advancements in machining centers have allowed for increased capability around high-volume operations, there are several factors that still necessitate the need for grinding. Grinding has many benefits over conventional machining, including longer tool life, finer surface finishes, and the ability to more effectively remove difficult-to-machine materials (such as new ceramics composites and carbide-impregnated metal alloys). This article highlights the benefits new grinding technology provides over conventional machining, and provides insight into considerations for converting current machining applications to grinding.
Grinding vs. Machining
Traditionally, machining is a very efficient process capable of removing material quickly. Driven by (relatively) large cutting surfaces and large chip sizes, the amount of frictional interactions (rubbing and plowing) between the tool and the workpiece when compared to grinding is minimal; this reduces the need for complex coolant systems. However, the large chip size can have a detrimental impact on the final surface roughness of the part, often requiring a subsequent grinding process to generate fine finishes. Recent advancements in abrasive grain and bond chemistries, and machine and coolant nozzle technologies have allowed grinding applications to compete and even exceed machining material removal rates on new and difficult-to-machine alloys. This, coupled with the added benefit of being able to rough and finish grind on the same machine platform, has become a cost-effective means of manufacturing. (Figure 1)
The benefits of grinding stem from having hundreds of cutting points on a typical grinding wheel (vs. a few on a typical mill or lathe). The many cutting points will tend to create smaller chips, thereby improving the surface finish and imparting a compressive residual stress on the surface of the part. In addition, the large increase in number of cutting points allows for a more distributed, uniform wear along the grinding wheel, leading to longer wheel life and fewer tool changes. A typical grinding wheel uses hard ceramic or super-abrasive grains, which are significantly harder than many of the machining tools on the market (although ceramic tools come close).
Considerations for Converting a Machining Process to Grinding
When considering whether an existing machining or turning application can instead be performed by grinding, the first step in the process should be to evaluate the process limitations, pain points, and materials. Common issues that can lend themselves to a grinding solution include:
What materials am I trying to machine?
Difficult to machine materials are now being pushed to higher volumes, often driven by advantages in performance over conventional materials. These include ceramic composites, carbon fibers, heavily alloyed aerospace metals, and some powder metals. They are notoriously difficult to machine due to their high hardness and chipping potential. This is where grinding, where smaller chips are generated and the surfaces are left in a state of compressive residual stress, can be an economic solution.
Does the application require both high stock removal and a very precise form and finish?
Through changes in wheels and/or dressing, the characteristics of the grinding action can be modified to either target a high-stock removal or generate a fine finish. The advantage with grinding is this can often be done using the same grinding wheel, or at least the same machine system.
Do I currently have a manufacturing line where a grinding operation is preceded by a machining operation? Could I move the process to a single grinder to save floor space and tact time?
Saving floor space is critical in high-production manufacturing, and truncating a process to fewer machines can have a large impact on the efficiency of the process. This also reduces the need to move parts from one machine to another, which can drive down overall process time.
Is tool life or tool changeover time on my machining operation affecting production rates?
The need to stop an operation to replace and re-sharpen the cutting tools can lead to higher cost per part and frequent stoppages. Using a grinding wheel can help minimize the down time and re-sharpening costs of machining operations. This is particularly true for new, difficult-to-grind alloys.
Are burrs an issue?
Larger burrs come from larger chips. Using a grinding wheel can often help lower the tendency for large chips and therefore minimize the need for deburring processes.
Are residual stresses in the part surface affecting form or function or rework?
One significant benefit to grinding is the compressive residual stresses imparted into the surface of the part during a machining process. If residual stresses are causing parts to twist, warp, or bend after your machining process, grinding is an excellent way to remove material and relieve some of these stresses in the part.
For new processes, selecting a grinding platform has several advantages over conventional machining platforms, including higher stiffness, dressing capabilities, and precision coolant systems. However, if looking to retrofit an existing process that uses machining equipment, it can often be achieved with some coolant modifications and electroplated grinding wheels, which require no dressing in order to maintain their form.
The retrofitting required in these situations is primarily related to converting the coolant system to grinding oil, which is typically optimized for a variety of conditions such as higher flash points, low foaming, and environmental/cleanup factors. The biggest change in retrofitting an existing process is to ensure adequate velocity to match the wheel speed. Compared to purchasing new equipment, these are relatively cost-effective measures for converting existing machining processes to grinding.
The benefits to electroplated plated wheels in these applications include the ability to plate nearly any shape, including drills, milling cutters, wheels, quills, etc, and the ability to hold fine tolerances (down to 0.0004” in some cases) where critical features need to be generated.
There are many examples of how customers have adapted this machine-to-grind concept for overall process improvements and cost savings. In one example, an energy company was ceramic turning dual bearing races for a turbine. The challenge was to ensure the races were identical to each other after being hardened to a 65RHC. Due to the wear on the ceramic inserts during turning, there was a significant amount of part rework to get the races to match. The solution was to implement an electroplated grinding wheel that could grind both hardened races simultaneously. Because the grinding wheel used cBN abrasives and had very low and uniform wear, this eliminated the rework. As a result, this brought the overall cycle time per part from 30 minutes to 11 minutes, saving the company more than $80,000 annually.
Another example is grinding gears from solid. It is common practice to mill or hob gears in a softened state, then harden, and grind the final finish. Some customers have adopted the approach of hardening the gears first, then rough and finish grinding on a single grinder with high-performance grinding wheels such as Norton Xtrimium or Quantum. In one example, a customer was able to cut cycle time in half by applying this technique, thereby lowering its overall manufacturing costs.
Not all customers will be able to modify or purchase new equipment to test whether grinding is a good solution vs. machining. In one case, a customer was using a machining process on a simple 3-axis machining center. They were having challenges with material tearing when running small ball mills on tight tolerance features. The approach they took was to order electroplated quills of the same size and shape as the ball mills to use for the finishing operations. This eliminated the material tearing (due to the smaller chip size) and required no change in the existing equipment to implement. Although the grinding tools have a higher price than the ball mill tools, this was justified by the significant cost savings in rework time by not having the tear outs in the material.
Many high-performance industries including those in aerospace and automotive sectors have made tremendous investments in next generation grinding cells in order to capture the capabilities of grinding and machining for optimizing processes. For those customers with low-mix, high-volume production, using the latest in high-performance grinding machine platforms and wheel technology can show significant improvements relative to machining. But for many smaller companies, these cells might not cost justify, especially for high-mix, low-volume facilities.
In these cases, converting an existing machining process to grinding with electroplated tools can have profound benefits in terms of cycle time, lower overall cost per part, improved tool life, and consistency in part quality.