Similar to a general turning operation, parting-off involves rotating a workpiece and cutting it with a stationary tool.
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Multiple factors must be considered when parting including the machine tool&#;s power and movement; the material&#;s hardness and machinability; and the cutting conditions -- required DOC, feed, and, most important, speed rate.
A common problem in parting operations is the &#;umbrella effect,&#; a situation that occurs when cut chips prevent coolant from reaching the cutting edge. When coolant does not reach the cutting zone, temperatures in this area rise and wear occurs more quickly, reducing the insert&#;s life.
&#;When you apply coolant in a conventional manner, you will get coolant landing on top of the chip, and the chip deflects the coolant away from the cutting zone,&#; explained Steve Geisel, product specialist for Iscar Tools Canada. &#;Effectively what is happening is that the coolant is cooling the chip, but not the cooling zone, nor is it lubricating the cut.&#;
Cooling the chip is not necessarily a bad thing, however. If the chip is cooled rapidly, it assists in breaking the chip. But if coolant can get to the cutting zone, the temperature is lowered and the insert lasts longer.
When materials such as stainless steel and high-temperature alloys are machined, the temperature near the cutting edge becomes extremely high. In addition, this kind of material tends to adhere to the tool&#;s cutting edge, causing built-up edge (BUE). Flank wear and cratering are also common.
This type of wear can be reduced by cooling the cutting edge.
Geisel explained that a new technology from Iscar enables coolant to be passed through the insert.
&#;What Iscar has been able to do is to press the insert with the coolant channels already in place,&#; he said.
According to the company, this design helps cool the insert from the inside, while also delivering an efficient coolant jet close to the cutting edge.
&#;These tools are especially good for cutting stainless steel and other &#;gummy&#; materials because they ensure that coolant gets to the cutting zone,&#; said Geisel. &#;Blades with coolant-through also are suitable, but if you can use these coolant-through inserts in your application, you can increase spindle speed and reduce cycle time.&#;
Materials such as titanium, INCONEL® alloys, and austenitic stainless steel tend to strain-harden during the cutting process and form long, tangled chips. Getting coolant to the cutting zone improves chip formation and reduces flank and cratering rates.
These materials often are used in the creation of high-value parts for the aerospace and power generation industries. It is the nature of the materials, however, that decreases their machinability. Breaking a chip depends on the chip absorbing and being softened by the heat generated in cutting. But some materials like titanium alloys, for example, are poor conductors of heat. This means that the chips will be hard to break.
Coolant-through tools can deliver lubrication and temperature control even in deep grooves. In deep grooves (long cuts), where temperature is much more of a factor than in short cuts. In these short, shallow cuts, external coolant often can be enough.
In addition to providing improved surface finish, selecting the proper insert and employing coolant correctly reduce BUE, especially when machining stainless steel and high-temperature alloys.
Improving chip control leads to better quality, process security, and a lower overall cost per part.
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Chip control is made more difficult because of the restricted, narrow nature of the cutting zone. A concern in any operation, it becomes even more vital when chips are hard to break. The operator must pay attention almost constantly.
Another benefit of creating consistently short, narrow chips is that it enables unattended cutting.
In a survey of end users, tooling manufacturer Sandvik Coromant found four common needs in parting.
1. Improve security. While the parting operation typically is only a small part of total machining time, if breakage occurs, it will mean a complete stoppage of the process. Keeping the cutting zone cool helps reduce the wear that can lead to a catastrophic failure of the insert.
2. Control chips. Long, stringy chips can wrap around the component during machining, which may prevent clamping into a subspindle for a second operation. If this occurs, the machine needs to be stopped and opened so an operator can remove the wrapped chips. This not only takes time, but exposes the operator to risk when clearing away sharp chips. Chips that have wrapped around the component also can destroy the surface finish, which may lead to a scrapped part.
Long chips also can be a problem for some chip conveyors.
&#;Chip control is one of the most important aspects to consider [in a parting operation],&#; explained Kevin Burton, product specialist for Sandvik Coromant Canada. &#;Chip width is narrowed to allow evacuation and improve wall finish. This is achieved by a &#;dish&#; in the insert&#;s geometry, which shrinks the chip width to narrower than the width of the groove.&#;
The geometry of the cutting edge creates a narrow chip so it can be evacuated smoothly. Consequences of poor chip evacuation are chip obstruction, which leads to poor surface quality, and chip jamming, which can lead to a breakdown.
Workpiece material is becoming more and more expensive, and scrapping a part can have a large impact on the bottom line.
3. Easy handling. With the introduction of high-pressure tools, installation and handling can be more difficult. A parting blade is exposed and must be turned regularly and replaced when necessary. In addition, inserts must be securely clamped to ensure process security but, at the same time, be easy to index.
Keeping an eye on the blade and the insert is important.
&#;It&#;s extremely important to read the tool wear if you want to improve and optimize an operation,&#; said Burton.
Tool wear can indicate if a harder or softer grade is needed or if a different geometry (sharp versus strong) should be used. It also can show if a change in cutting data or toolpath is necessary, he added.
4. Lower cost per component. A goal in all machining operations is to reduce the cost of the final part. An operation that is secure, has good chip control, and high uptime will help reduce the final cost.
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