Dovetail Milling Cutter - XD - single insert tools milling

“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].”

Easily access valuable industry resources now with full access to the digital edition of Canadian Fabricating & Welding.

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

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.

“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.”

“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.

“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.”

The Autofiltrex magnetic filter is the latest in market-leading magnetic technology, ensuring 24/7 "ultra-polished" machine tool fluids. With an integral waste reclaim and fluid recovery system, it operates with minimal user intervention. With a fine particle extraction performance, it provides huge cost savings in fluids and filter consumables.

Keep up to date with the latest news, events, and technology for all things metal from our pair of monthly magazines written specifically for Canadian manufacturers!

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

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.

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.

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.

“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.

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.

“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.

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.

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

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

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.

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

“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.”

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.

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.

“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.

“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.”

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.

“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.”

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

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.

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.

“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.”

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

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.

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.

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.

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.