Overcoming five common challenges when turning

While it’s nearly impossible to quantify the most common challenges or most important considerations when turning, experts agree that the five aspects of turning discussed below should be considered.

It’s important to note that each factor has the potential to impact the others. So, instead of looking at each issue individually, it’s recommended machinists take a holistic best-practices approach to turning.

Application of coolant directly to the cutting edge is important for heat management, chip evacuation and surface finish. Image courtesy Seco Tools.

1. Heat Management

“You absolutely have to figure out a way to disperse heat,” said Scott Leitch, sales and marketing manager at Exsys Tool Inc., San Antonio, Fla. “If the toolholder doesn’t have coolant either running through it or running on top of the assembly while it’s in operation, heat will build up and can have negative effects on tool life, surface finish and part quality.” Leitch added that sealed toolholders (for the application of through-tool coolant) help the coolant disperse heat more efficiently.

Kurt Ludeking, product manager at Walter USA LLC, Waukesha, Wis., noted most machines are supplied with coolant ports and, in theory, operators just need to direct the coolant to the cutting edge for proper heat
removal.

“That works … sometimes,” he said. “It depends on a lot of things, and the coolant flow can get misaligned during routine maintenance, or even knocked out of the way by chips flying around. If you don’t have the coolant directed properly, it won’t be effective and you’ll wind up getting a lot of crater wear and flank wear.”

High-pressure through-coolant is typically regarded as the best solution, but it’s not always an option, such as when clearances don’t allow for a bulky high-pressure apparatus, or when the part is too thin or delicate to withstand the pressure.

Absent high-pressure coolant, using toolholders that direct the coolant through the toolholder and straight to the cutting edge helps, Ludeking said. “Even if it’s relatively low pressure, around 150 psi or so, which is pretty typical for most modern machines, you get a better cooling effect, because it’s always directed right at the cutting edge.”

However, while heat management is certainly important, not everyone agrees it’s a major concern.

Don Graham, turning product manager at Seco Tools LLC, Troy, Mich., said the real issue is tool life, and, while heat management certainly plays a role, its impact has been drastically diminished by modern tool geometries and
coatings.

“Tool life relates to heat management, but with modern tools from any good cutting tool company, heat shouldn’t be a problem,” he said. “I fear that people who worry about heat management are probably just not using the right tools at the right speeds and feeds.”

2. Setup Selection

Rather than focusing on heat management, Graham recommends operators optimize tool and toolholder selection—although he admits that can be easier said than done.

“I don’t think any cutting tool company, including Seco, has an easy way to select the ‘best’ cutting tool for a job,” he laughed. “When it comes to getting the tool that’s appropriate for the design of the part, there are around eight to 10 common insert shapes, each of which can be held in a number of different orientations. So, first of all, you need to determine the right insert shape, then figure out how to orient it for the job, then figure out what the proper feed, speed and grade would be for the material you’re working with. It’s all application-specific, but if you get it right, you should have very few issues to address.”

Hurco’s TMi series CNC lathes are for universal turning applications and offer an uncomplicated transition from manual numerically controlled turning, according to the company. Image courtesy Hurco North America.

Milton Ramirez, product technical specialist for CNC lathes at Haas Automation Inc., Oxnard, Calif., pointed out the importance of workholding and noted, especially when turning, a steady rest may be a necessary setup element.

“Workholding is always critical,” he said. “Whenever possible, use collets because they offer more gripping surface and, therefore, more rigidity. The next-best option is to machine jaws to the material or part size so the gripping diameters match the part and jaws.”

He also recommends a tailstock for support, if the part allows it, and noted the best type of support for long and difficult turning is a follow rest. However, this type of rest is only available on twin-turret machines, wherein the part is machined by one turret and supported by the other.

For lathes with only one turret, he explained, the most common workholders are steady rests, which have a body that does not move. “Only the grippers are programmable for gripping or releasing the part.”

Once the tool and the part are held in place, said Michael Cope, product technical specialist at Indianapolis-based machine tool builder Hurco North America, the next essential step is choosing the proper insert geometry.

“Insert selection can sometimes go beyond the typical decision between corner radius and the grade of a particular insert,” he noted. “For more-complex operations than simple facing and shoulder turning, picking the right insert geometry is a key component. Increasing the lead angle of the cutter can also help. Higher lead angles and positive-raked inserts produce a better surface finish than tools with a shallower cutting angle.”

3. Surface Finish

In any machining operation, part quality is of paramount importance, and surface finish is a major component. Besides higher lead angles and positive-raked inserts, how can machinists impart the best surface finish from their turning operation? Use wiper inserts, according to Walter’s Ludeking. They are designed to leave the surface with half the roughness of a standard turning insert run at the same feed rate.

Precision coolant toolholders provide coolant as close and horizontal as possible to the effective working area under the chip, benefitting chip control and tool life and increasing speed. Image courtesy Walter USA.