Customers of forge shops are increasingly requiring their raw forgings to be rough machined before they are shipped. They may be squared in a milling process or rounded in a turning operation. To speed machining processes, new materials have been developed that cut faster, such as coated carbide. Recently, a phase-toughened ceramic material called XSYTIN™-1 has been introduced to improve cutting speeds even more.
It used to be quite easy. The customer ordered a forged component, which was manufactured to the provided specifications, and then off it went. Our competition was local or at most regional. Life was simple, but it wasn’t long before competition came from across the nation, then from across North America. Now, we are living in a global economy, and competitors come from anywhere and everywhere.
What else has changed? Increasingly, customers are requiring their forgings to be rough machined. They either need to be squared in a milling operation or turned for round forgings. This does not need to be viewed as a “necessary evil.” It can also be used as a competitive tool. By being proactive and developing a strong machining process before the customer asks, we can gain additional business while protecting the business we already enjoy.
You know everything about forging but are not up-to-date on the latest technology for maximum productivity machining. So, where do you start? First, look at the kinds of components and materials you are working with and determine the machine type, size and specifications you will need. For example, if all your components are larger, then a small lathe will not do you much good. Nor would it make sense to use a high-speed milling machine because more than likely you will need torque, not speed, to drive a larger milling cutter.
Next we get to the cutting tools. Looking at the historical development of cutting tools as it relates to typical forged components, everything has changed as new materials forced us to rethink the machining process.
Not following the common thinking has been the hallmark of innovation throughout history. Consider the famous quote by Albert Einstein: “We can’t solve problems with the same kind of thinking we used when we created them.” Knowledge is critical, but continuously using the same logic over and over means never progressing.
Introduction of Coated Carbide
Greenleaf Corporation introduced the first coated carbide to the U.S. market in 1971. This changed how machining could be done. It allowed for higher speeds, higher temperatures and enabled significantly longer tool life. The development of the coated carbide was in direct response to new, difficult-to-machine materials becoming increasingly common, such has high-alloy and stainless steels.
A revolution in the metal-cutting industry occurred in 1973 when Greenleaf introduced the first viable ceramic cutting tool, a hot-pressed ceramic that could retain its hardness at high temperatures. The hot-pressed ceramic increases the cutting speed significantly and requires lower chip load (depth of cut times feed rate).
In March 1985, WG-300® ceramic cutting tools were introduced. These were the first whisker-reinforced ceramic tools, allowing not only for higher speeds but also high strength. This resulted in a combination of cutting speeds up to 10 times that of conventional carbide with retained feed rate and depth of cut.
This cutting-tool development meant that forgings could be machined at very high cutting speeds, increasing productivity significantly. As a result, the need to receive the forgings in a timely manner became critical. Having too few meant the machines were standing still and incurring costs, and having too many meant insufficient space to store them or inefficient production flow.
Recently, we introduced another new cutting-tool material called XSYTIN™-1, which is a phase-toughened ceramic that offers a significant improvement in toughness behavior.
When it comes to forgings, especially larger ones, the available cutting speed may be a limiting factor. This has, at times, forced the use of carbide cutting tools where high cutting speeds are less of a factor. The only way to gain productivity was to increase the feed rate and increase the depth of cut by utilizing larger inserts. Until now, ceramics simply were not an option because the speeds needed for an efficient cutting process could not be reached.
XSYTIN-1 changes this by closing the gap between the typical high speeds of ceramics and the traditional slower speeds of carbide while retaining the feed rate and depth-of-cut capabilities of carbide. This versatility accommodates a wider range of applications, gaining productivity without the necessity of high speeds.
In a recent test at one of the largest forging companies in Pennsylvania, our sales and service engineer, Aaron McKeown, came across this exact challenge. The customer is using ceramics on the smaller forgings but has been unable to gain the productivity promise of ceramics on the largest of the forgings due to cutting-speed limitations. By utilizing the new cutting-tool material, McKeown recollects, “It was a forged part with heavy scale, interruption and limited speed capabilities. Testing with XSYTIN-1 showed a considerable productivity gain over carbide.”
Basic Elements of Metal Cutting
All of this brings us to the present. Metal cutting is actually quite simple, with four basic elements to understand: force, stress, temperature and material.
- Force refers not only to the forces the forgings are exposed to, but also where those forces should be distributed. With a lathe, the strength will always be axial, from chuck to tailstock. In a vertical milling operation, the strength will always be from the spindle to the table. Knowing this, we can use these forces to our advantage. For example, a larger-radius insert, or a round, will yield the most beneficial tool life but will also increase radial forces at a shallower depth of cut, causing vibration tendencies. By increasing the feed rate, the axial forces are also increased and counteract the radial force, which gains stability and reduces or eliminates vibration.
- Consider what stresses will be put on the cutting-tool material at a macro and micro level, as well as what stresses there are in the component itself.
- Early cutting tools did not respond well to high temperatures, but modern cutting tools are designed to handle high, but stable, temperatures. This means that we need to produce high, but not too high, heat. Metal cutting is as simple as creating the proper amount of heat to plasticize the material in the cutting zone, keeping in mind that temperatures that are too low can be just as detrimental to the cutting process as temperatures that are too high.
It is very important to understand the behavior of the material being machined as well as the behavior of the cutting-tool material being used.
When working with forgings, looking only at the material we are going to machine is simply not enough. We need to also understand the condition of the material at the very first cut. Machining the scale of a forged component is commonly regarded as the biggest challenge for a few different reasons.
First, impurities tend to be drawn to the surface, causing the material to behave differently for the first cut than it does when it is clean. Second, it is a challenge to stay in the cut because out-of-roundness or the uneven nature of a forged surface makes it difficult, especially with larger forgings. Third, the stresses in the surface condition of a forged component are different than in clean material. This puts stress on the cutting-tool material, demanding it to retain bulk strength, abrasion resistance and stability at high temperatures. Modern ceramic cutting tools can withstand the stresses encountered from the inability to remain engaged in the cut, material changes in the surface structure of the forgings and the higher temperatures that modern production processes demand.
Simply put, material selection in the cutting tools, the basic tool shape and features such as radius dictate how productive you can be. This is only the first step. We also need to look at tool paths. When entering the cut, the initial contact with the forged surface tends to produce notch wear, which is a wear pattern in which a v-shaped notch develops at the depth of the cut line (Figure 1).
To reduce notch wear, do not enter straight into the component. Instead, follow a path that rolls into the cut (Figure 2). This will cause an initial notch that is outside of the cutting zone, resulting in longer tool life and increased process stability.
Once into clean material, use a machining strategy that removes the maximum amount of material in the shortest period of time. Select a large, round insert that will allow for a higher depth of cut and is the strongest geometry available. Take the first pass at 25% of the inscribed circle (iC), or in the case of a round insert, the diameter of the insert. Reduce the second pass so that the notch and flank wear from the first pass are not in the cut. As shown in Figure 3, do the same for the third pass and all subsequent passes. Keep in mind that when using a round insert, decreasing the depth of cut will effectively decrease the lead angle, resulting in a thinner chip. You can compensate for that by increasing the feed rate, which will result in a thicker chip.
Once you have combined the most advantageous cutting-tool material with the correct tool paths for your operation, you will achieve significantly higher productivity. Changes in the demands from your customers can be a blessing in disguise. By providing additional services, such as rough machining, you can thrive in the global marketplace.
Co-author Jan Andersson is global manager, Tech Team and Marketing, Greenleaf Corporation. He can be reached at 814-763-2915 or at firstname.lastname@example.org. Co-author Jack Kohler is application engineer, Technical Services, Greenleaf Corporation. He can be reached at 814-763-2915 or at email@example.com.