Attention to detail in tooling a must for small part grooving
by Tim Wilson
Grooving applications for small parts is broad-based, with demand being seen in medical, aerospace, turbines, the electrical and hydraulic industry, job shops supporting the automotive sector, and even watches. Applications include face grooving and small internal and external grooves–O rings, seals, snap rings–that are often machined off of Swiss-style lathes. These machines can groove down to just a few microns, the equivalent of thousandths of an inch.
“There is a lot of small grooving going on out there,” says Todd White, sales director at Scientific Cutting Tools in Los Angeles, CA. “One tool I am looking at now goes into a 1/8th bore, but is ten thousandth of an inch wide and only five thousandth of an inch deep.”
This is a very fine groove made out of 303 grade stainless steel for a Swiss part. In such a situation, White will offer a tool quote that serves the customer’s specific requirements.
“In this example they’ve sent me a tool drawing with width and a description of their needs,” he says. “We see that quite often.”
With such detailed applications, it is crucial that the tool is ground to a fine surface finish; you don’t want chips sticking because of grind marks. As well, at such small widths a tool has to be made correctly so that there is clearance and the chips don’t pack into the bore.
“You can grind the shank to give it more strength, and instead of having a round shank you can have one that is more elliptical,” says White. “This way you can get into a small bore, but with more strength.”
To get the right tool for a small grooving application, it makes sense to shop around with vendors. Kennametal, for example, launched an expansion of its small reinforced toolholders last year. The toolholder works well in Swiss machines utilizing the company’s A4TM double sided grooving and cutoff inserts.
“We launched an A4TM club head toolholder specifically designed to fit into smaller type, Swiss machines,” says Scott Etling, recently Kennametal’s global manager for grooving and cutoff, and now the company’s manager, global product management, for indexable milling.
“The new toolholders offer shank sizes that fit the Swiss style machines’ tool blocks. The design offers extra material under the insert to maximize stiffness. This will directly affect the surface finish on the workpiece, and increase tool life.”
For operations that require small groove widths on small diameter workpieces, selecting the correct geometry is important to keep cutting forces at a minimum and to optimize chip control. As in many machining applications, the right combination means understanding the trade-offs.
“Depending on the applications— and this includes the workpiece material type, the type of machine, overhang of the toolholder, and size of work piece to name a few—it is very important to choose the best tool for the operation,” says Etling.
Kennametal’s A4 –GUP geometry is designed to lower cutting forces as well as offering excellent chip control. By minimizing the cutting forces, the tool pressure will not deflect the work pieces, and will keep the heat at the cutting edge to a minimum, directly improving tool life.
Etling also has some advice if the tool wears prematurely.
“If the insert failure mechanism is tool wear, apply a harder grade to reduce the time the insert wears. If the cutting edge is chipping, then apply a tougher grade,” he says. “Always remember to change one variable at a time to zero in on the best cutting tool combination for your particular application.”
When grooving small parts in Swiss-style machines, having the latest and greatest coating is also plus. From Kennametal’s perceptive, that would be the KCU25 PVD coating.
“The recently launched KCU25 Beyond grade is a tougher grade with an advanced PVD coating that maximizes performance in those harsh applications. In fact, this grade should always be our customer’s first choice when grooving and cutting off. A more wear resistant grade would be KCU10” says Etling.
The whole picture
As people look to reduce costs in small grooving applications, they have their sights set on new products with more wear resistant grades, longer tool life, and better overall performance.
“In our newer products there are extra fine, sub micron carbide grains that have extra sharp cutting edges,” says Alex Livingston, a product manager for Tungaloy America, Brantford, ON. “With this technology you reduce the risk of having cutting forces that move the part as you machine it.”
Because the grooves are so small —whether for inside, outside, or face grooving—having micrograin carbides has become the norm in order to provide thermal protection, and also to give a tough edge line to prevent chipping. All of this has to happen in a stable environment to ensure optimal speeds.
“Grooving small parts is generally about tool stability and selecting the right cutting speed,” says Dave Andrews, product and application specialist with Sandvik in Mississauga, ON.
Sandvik Coromant’s tools specific to grooving small parts, both internal and external include the CoroCut XS, which helps with external parting, grooving, turning, backturning, and threading. It has the CoroTurn XS for small internal applications.
“Grooving is a very demanding application,” says Andrews. “You have a lot of insert engaged in the cut. A three millimetre wide cut has a lot of engagement, and can present problems with tool pressure on the component.”
When grooving small parts, a tool has to be as rigid as possible for best performance. But stiffness and rigidity don’t go well together.
“The harder it is, the more fragile,” says Steve Geisel, senior product manager for Iscar Tools Inc., Oakville, ON. “We’re making cutting tools out of very tough pieces of carbide, which gives us strength but increases fragility. Some of the biggest challenges when talking about grooving parts this small is spindle speed, and also chip evacuation so that the tool doesn’t snap.”
Surface feet per minute (sfm) can be very low when grooving small parts because bore diameters are so small. As a result, an operator may find that he is running at rpms well in excess of what is required.
“People often overlook this,” says Geisel. “They know they should be running at 4,000 to 5,000 rpm, but if a bore diameter is at 100 thousandths of an inch, they could find themselves at 20,000 RPM, which is way too fast.”
How small is small?
When discussing grooving applications, industry players can have differing views as to what constitutes small part machining.
“I would say anything under an inch and a half in diameter,” says Scott Etling from Kennametal. “The segment is very broad; you can get down to ten thousandths to twenty thousandths of an inch
in diameter. This definition can change for many different manufactures that fall under the small part machining umbrella.”
Alex Livingston from Tungaloy puts the cut off a little lower.
“I would say a small groove has a width of under two millimetres (mm), or 80 thousandths of an inch,” he says.
In fact, Tungaloy has both three corner and two corner inserts that can go well below two millimetres —all the way to a .3 mm groove.
“These are usually steels and stainless steels,” says Livingston. “You also see titanium for medical components like dental implants.”
In fact, Tungaloy has just launched its Tiny-Turn product line for small components in boring, threading, and grooving.
“With Tiny-Turn, widths will focus on 2 mm down to .5 mm,” says Livingston. “A big percentage of this market will be in production environments.”
Dave Andrews of Sandvik and Steve Geisel at Iscar both say that small would be defined as under 1 mm, or about 40 thousandths of an inch.
“For us twenty thou is pretty standard, and we can go smaller upon request,” says Geisel.
For Horn USA, anything under 10 mm in diameter is a small part.
“When you get to under 2 mm you’re getting into micro machining,” says John Kollenbroich, a technical manager with Horn USA. “We can do that as well.”
Kollenbroich says when it comes to micro machining, accuracy is a must.
“The centre line plays a critical role with small parts,” says Kollenbroich. “Being off centre by even a thousandth can break a tool; there is much less room for clearance.”
To cut a .2 mm hole, for example, if you are off by even .05 mm, that is too much depth of cut; you’ll break the tool right away. As a result, knowledge, experience, and a high quality tool are required to ensure that it goes back into the pocket at the same place where it was first set up. SMT
Tim Wilson is a freelance writer based in Peterborough, ON.