Hard turning process can eliminate grinding, reduce setup time, improve throughput

Technology advances in both machine tools and cutting tools are enabling manufacturers to turn a prehardened part faster and more efficiently.

Hard part turning—turning a part that is harder than 45 Rockwell hardness C (HRC)—is gaining traction in many Canadian shops. It is a growing trend in the automotive industry to hard turn a part either to completion, or to hard turn during the roughing stage and then grind to the final surface finish (Ra factor) requirements. In the production of aerospace landing gears, which typically are 55 HRC, hard turning has been used for many years. A more recent trend, however, is the hard turning of bearing races, which have curved surfaces that often can be profiled faster and more economically on a turning center than on a grinder. Grinding is especially expensive if a custom wheel has to be made to handle a complex part’s geometry. Also, multiple setups often have to be done on the same grinding machine, all of which takes time. Hard part turning is done on one machine, typically in one setup, with standard inserts. Technology advances in both machine tools and cutting tools are enabling manufacturers to turn a prehardened part faster and more efficiently.

Turning centers are much easier to integrate with both simple (bar feeders) and complex (gantry robots) automation than grinders, and it is easier to find qualified people to operate turning centers. However, grinding is still the operation of choice for components requiring the tightest of tolerances (±0.0002 in.). The fine finish of a grinding operation at these Ra levels cannot be reproduced even by the best hard turning operation. A spiral feed line will always appear on the part with a single-point turning tool as opposed to a multipoint tool like a grinding wheel.

Selecting the Right Insert

Insert selection is important in every turning operation, but in hard turning it is one of the first factors that needs to be considered. Asking a few questions can help you streamline the selection process:

1. What is the material and what is its hardness?
2. What is the surface finish requirement?
3. What are the tolerances I am trying to achieve?
4. Is the cut interrupted or uninterrupted?

Carbide inserts, the standard choice for most machining operations, can be used at the lower range of hardness (45 to 50 HRC), but at much lower cutting speeds. As materials get harder, cubic boron nitride (CBN) is added to the equation. “If you are machining a hard part with carbide, you will be limited to about 60 SFM; however, when using CBN tooling, you can reach between 450 and 650 SFM,” explained Steve Geisel, product manager for Iscar Tools Canada. “It’s not even close at these high hardness levels.” Hardened ferrous metals, including 4340, 8620, M2, and T15; abrasive ferrous metals, such as pearlistic gray cast iron; heat-resistant alloys; and superalloys all are candidates for CBN. “CBN is the first choice above 55 HRC,” explained David Andrews, product and application specialist for Sandvik Coromant Canada. “If a shop is hard part turning in high volumes, we will always recommend that CBN be used.”

While mixed ceramic and cermet inserts have their place in the 45- to 55-HRC range in certain types of turning, the hardest materials require CBN. “One nice thing about the mixed ceramics, however, is that they can handle soft and hard materials, whereas CBN inserts don’t work particularly well in softer material,” said Andrews.

CBN Explained

CBN tools have four properties that enable them to cut hard materials at high metal removal rates: high strength, high hardness, good thermal conductivity, and abrasion resistance. This is what enables them to cut hardened materials faster and deeper than carbide inserts. The three general categories of CBN tools to choose from are:

1. CBN-tipped insert. These inserts have a pocket machined into them that holds the CBN tip. While these inserts have only one cutting edge, it can be reground when it dulls.
2. Full-faced insert. These indexable inserts have a layer of CBN bonded to a carbide substrate.
3. Brazed-shank tool. These tools have a pocket with a CBN blank brazed into it.

The best use of CBN cutting tools occurs in situations that cause carbide and ceramic tools to wear quickly or fail entirely. “CBN tools improve efficiency, reduce scrap rates, and increase part quality,” said Geisel. “Because of the hard microstructure of CBN, cutting edges last longer.” According to Geisel, the main benefits of using CBN tooling for hard part turning include:

• Longer tool life
• Higher material removal rates
• Reduced machine downtime
• Increased productivity
• Better surface finish

Insert Technology

New to CBN inserts is the inclusion of wiper technology to enable a higher feed rate. “Wipers create an extended contact area between the tool and the part. The distance between the peaks and valleys produced by the insert are wider, and therefore the surface quality is better,” said Andrews. Sandvik Coromant’s new product, Xcel is specifically designed for high-feed hard part turning.

“Xcel is based on an existing insert style, but it has a unique corner configuration. It has a very shallow entry angle, which thins out the chip, allowing higher feed rates than in normal hard part turning,” said Andrews. “Instead of 0.004 to 0.006 in. per revolution, these inserts can be run at 0.016 per revolution.” The shallow entry angle spreads the work over a larger area of the insert. This spreads the heat out over a larger area, and also spreads the wear out over a larger area. This even wear pattern lengthens tool life, but, because of the shallow entry angle, the part needs to have open features or an undercut to work with this insert. And, just like a high-feed milling operation, stability in the setup is important to get the best results. “In any hard part turning operation you are presenting a very negative cutting edge to the workpiece, which requires much better stability in all aspects than a typical turning operation,” said Andrews.

Where Is the Wear?

The most common wear mechanism in hard part turning is crater wear to the insert. When setting up a job for the first time, it is important to keep an eye on the insert for this type of wear. During this fine-tuning of the process, it is important to determine how the insert will wear and how long it should be used. “I always stress to operators to check the insert often by using a microscope or eye loupe to make sure the wear patterns are what they think they are before changing to a different tool,” said Andrews. If some type of magnification is not used, the insert will simply appear broken. It is the heat and pressure created during the turning of hard materials that make a crater behind the cutting edge of the insert. It often is overlooked because the crater usually wears to the point where the entire edge of the insert fails and drops away. This can make it appear as if the insert has chipped prematurely, when in fact crater wear has caused the failure. “When this happens, people often make the mistake of moving to a different grade, repeating the process, and causing the failure to occur even more quickly,” said Andrews. Crater wear can be dealt with by reducing cutting data, mainly cutting speed. As the speed is reduced, tool life typically increases. Another choice is to switch to a more wear-resistant grade.

Much like typical carbide inserts, there are different CBN grades available to choose from, depending on the application requirements. “The CBN has to be matched to the workpiece that you are machining,” said Geisel. “The specific grade will depend on whether the turning operation is constant or interrupted, and also on the material being machined. Hardened exotic materials, for example, require a different CBN grade than hardened carbon steels or hardened stainless steels.” According to Geisel, Iscar Tools recently expanded its line of CBN grades to allow manufacturers to more finely tune their selection to each individual job. “No two jobs are identical, and no two shops are identical,” said Geisel. “As the technology grows, more and more shops will be able to take advantage of the benefits that come with hard part turning.” Chip removal is one of the major problem areas associated with hard part turning. During the machining process, a long, thin, ribbon-like chip is produced. Some CBN inserts can also have a chip breaker, which can aid in chip formation and removal, eliminating the birdnesting that is common in hard part turning. “Inserts that have a chip breaker create very small chips similar to those you get if you are machining steel in its normal state,” said Geisel. Adding a chip breaker to the equation may improve process security, surface quality, and safety. If a chip breaker is not employed, high-pressure coolant can help break the chip into smaller, more manageable pieces. “We have seen that standard-pressure coolant use does not really give any benefits,” said Andrews. “High-pressure coolant, on the other hand, can create 20 to 25 percent extra tool life in good conditions.” Many manufacturers will use high-pressure coolant not necessarily to extend tool life, but to aid in chip removal and improve process security. Warm parts and cold parts obviously will be different sizes because of thermal expansion. The use of high-pressure coolant can create thermal stability, keeping the part at a constant temperature. “Parts such as landing gear can be on a machine for hours, and without coolant the part can expand to the point that the needed tolerances can no longer be met,” said Andrews. “At this point it’s more about component security than it is about extending tool life or chip removal.”

Business First

In addition to the machining process benefits, turning prehardened material also can save a shop time and money in other ways. Roughing a part, shipping it to a heat-treater, and waiting for it to return takes time and costs money. “Being successful in business is about more than simply machining a part for shops today,” said Geisel. “Every time unfinished parts are sent out for heat-treating, production of those particular parts stops, a purchase order needs to be generated, and you wait on shipping. Conversely, if you bring in stock that has already been hardened, you don’t have these production delays and extra office costs.”