“When the pressure is very high, for example in what we call the ultrahigh pressure coolant range [2,500 to 5,800 PSI], you need a third-party pumping system, machine tool, and tooling that can all handle the pressure,” said Geisel.

Point loads that are applied in the vicinity of tapered end cuts are limited to certain magnitudes. If those magnitudes are exceeded, then the cut will fail. Point loads that fall within the support length are not counted toward the limits. Point loads within the cut length or distance equal to product depth away from inside face of support, will adhere to the point load limit.

Carbide.  Carbide tooling can cut material with hardness up to 55 on the Rockwell hardness C scale (HRC), but cutting speed must be greatly reduced. Tool life also is very short. However, carbide inserts are the most inexpensive of the three to purchase.

CBN. When hardness exceeds 60 HRC, CBN inserts become the best choice because they can be run at the lathe’s highest spindle speed. Available with one cutting edge or as a multitip insert, it is also the most expensive option.

“If your insert wears, you are no longer cutting, you are work-hardening the material,” said Geisel. “You will no longer be at the hardness you think you are at.”

“Insert geometry is important,” said Andrews. “Most CBN inserts have a small, negative hone placed on the edge to provide strength for this operation. The angle and size of hone can be modified for different operations, like interrupted cutting.”

CBN inserts forhard turning

“This is why it is important to understand the function of the component before making the decision to grind or hard turn,” said Andrews.

“All parts must be held very tightly because of the cutting forces generated in hard part turning,” said Geisel. “As always, the part will dictate what tool should be used. Look at the material, hardness, size, and geometry part and work backwards from there.”

When hardness exceeds 60 HRC, CBN inserts become the best choice because they can be run at the lathe’s highest spindle speed.

When a hardened part is machined, the chip produced is different from a traditional soft turned chip. In this case, a very thin, ribbonlike chip is produced. If the insert does not have a chipbreaker, chip control can be difficult.

“We tell our customers to either use no coolant at all, or to use a lot of it. There is no in-between in hard part turning,” explained Iscar Tools Senior Product Manager Steve Geisel.

Hard turning insertfor steel

The chips created by this type of setup are similar to those created when cutting normal steel with the creation of the familiar “6” and “9” chips. These are handled easily by the machine and will not damage part, toolholder, or insert.

Proper chip creation is also a safety factor for the operator. When the system no longer produces long, stringy chips, commonly referred to as birdnests, the operator doesn’t need to reach into the machining envelope to remove them.

Turning insertfor hardened steel

While the surface finish (Ra factor) produced from hard turning can be similar to grinding, it is important to note that the finish itself differs. When a part is ground, the wheel produces a random pattern in the surface with no directional cutting marks. When hard part turning is used, a spiral feed pattern on the part’s surface is produced. This can create sealing problems in some parts.

The application of high-pressure coolant requires machine tools that can handle the volume and pressure of the coolant coming from the pump.

Turninghardened steel

Hard part machining, which can be done on nearly any lathe provided the machine is rigid enough for the forces created, is becoming more popular in Canadian machine shops because it can prove to be more economical and productive when compared to grinding. Also, the technology no longer limited to turning; grooving and boring also can be performed if cubic boron nitride (CBN)-tipped inserts are used.

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Point load limit is determined by combining all the point loads regardless of load type and ignoring negative loads, the summation of the loads cannot exceed 310 lbs where LDF is not applied. Applies to IBC and NBCC.

“Small-diameter grinding wheels for internal operations can be expensive and need to be dressed often, but a CBN insert in a boring bar could literally last for hundreds of components,” said Andrews.

Some tooling manufacturers not only recommend the use of coolant, but recommend its application at high volumes and high pressures.

According to Andrews, hard part turning does not necessarily require coolant, so that cost can be eliminated completely. Another saving arises when comparing the cost of inserts and grinding wheels.

However, added Geisel, there are benefits to be gained from using high-pressure coolant, especially at the cutting edge.

In hard turning most of the heat is removed in the chip itself, however, some residual heat will be trapped in the workpiece and tooling. CBN tooling is designed to run at very high temperatures, which is why it is a good choice for hard part machining.

“It’s all about chip control, tool life, and speed,” said Geisel. “If you apply coolant with a high-pressure coolant system, you can run two to three times faster, while at the same time getting longer tool life.”

One way to increase tool life in hard turning is by applying a coating to the insert. This allows higher spindle speeds and even a little extra feed rate to be used.

This “rehardened zone” is a burning of the outer surface of the workpiece. To avoid this, inserts should be changed early and often enough that excessive wear and rubbing do not occur.

“Predictability of tool life is extremely important in hard part turning,” said Geisel. “Since hard parts are usually at the end of the part-creation cycle, there is a high amount of value in the part already. You need to be able to know how often inserts need to be changed. You can’t run the risk of breaking an insert and damaging the part.”

“Poor chip control can lead to a poor surface finish due to chips rubbing on the finished part surface,” said Andrews. “Increased heat in the workpiece due to hot chips building up on the component and, in extreme cases, insert breakage if chips are trapped between the workpiece and the insert cutting edge [can all damage parts].”

As materials get harder, the choice of tool material becomes more important. The three main choices typically are carbide, ceramic, or CBN inserts.

Because inserts with positive geometries cannot be used, the negative land on the insert takes the brunt of the force. The harder the material, the higher these cutting forces are. These forces have to be taken into consideration during workpiece setup, when stability is critical to achieving the desired results.

In hard turning the tolerance and surface quality are very important, so tools need to be able to provide close tolerances while engaged for a long time in the cut. The life of CBN inserts typically is more predictable than that of ceramic inserts.

Whichever type of insert is being used, it’s very important to monitor the tool. If worn inserts stay in service beyond their usable life, it is possible that they will negatively affect the outer layer of material on the workpiece.

As materials get harder, the choice of tool material becomes more important. The three main choices typically are carbide, ceramic, or CBN inserts.

Ceramic. Ceramic inserts work well in the range between 50 and 55 HRC and at cutting data comparable to CBN.  Higher spindle speeds can be achieved with these inserts but at a higher price than carbide inserts.

One tip is to machine the part with the insert in an upside-down position so that gravity can help remove chips. While not necessary, coolant can aid in chip removal, and some machinists even use compressed air to vacate the chips from the workpiece area.

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“The cost of producing a part can be significantly less than when grinding,” explained Sandvik Coromant Canada Product and Application Specialist for Turning Products David Andrews.

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“Coolant use is not recommended for hard part turning as it offers no benefit to tool life,” said Andrews. “It can be used, however, to keep parts cool for accurate measuring and to clear away chips from the workpiece.”