Grooving the small stuff

The challenges of cutting small diameter parts

November 24, 2010
News Release
Metalworking

Iscar Tools’ new MinCut inserts for internal grooving, undercutting and threading.

How best to approach grooving a small part has a lot to do with the materials involved: repeat, high volume applications tend to be with softer metals, with specialty one-offs applying to higher value applications, often with tougher alloys.

“Exotic materials are not for production jobs—these are low volume applications,” says Tom Ficker, regional sales manager for Cogsdill Tool Products, Inc., with responsibility for Canada. “The problem with precision grooving in high nickel alloys is that you beat down on the grooving insert almost from the get-go.”

As a result, Cogsdill’s tools are better suited to high production lines, an area that has seen some big changes of late. Only a few years ago, small bores were typically 4 mm, but now leading vendors are delivering off-the-shelf bore grooves to 2 mm, with specialty applications going much smaller. This has resulted in new abilities—and challenges—when it comes to grooving very small parts, as well as increased demand.

“There are companies that require face grooves with major diameters as small as 0.2 mm,” says Duane Drape, national sales manager for Horn USA Inc., in Franklin, TN. “This is mostly aluminium, but I have had customer requirements to get that small with steel at 50 + Rockwell.”

Hydraulics, given their need to control pressure, create demand for inside diameter (ID) and outside diameter (OD) grooving seals and threads, as well as relief grooves. In the automotive sector, miniature grooving applications include fittings for air bags and fuel injection components.

“The challenge with grooving is that below 1.5 mm it is difficult to make a tool that is strong enough,” says John Stewart, VP of engineering at Bokum Tool in Madison Heights, MI. “The overall determining factor is: What does a groove look like in a hole that size? If it is proportionally very similar to a larger hole then it is theoretically possible to get very small, but it really comes down to the strength of the tool itself.”

ID grooving in particular can be an issue with small parts, says Steve Geisel, senior product manager for Iscar Tools Inc., Oakville, ON.

“As a tooling manufacturer, we are being pushed to produce tooling able to groove in smaller, and smaller diameters. Our PICCO line can produce grooves in diameters from as small as 2 mm (.08 in.) and we can go smaller by special request.”

Geisel concurs with John Stewart that the challenge for tooling suppliers is to make a strong, rigid cutting tool or insert.

“The cutting process produces pressure on the work piece and on the tool. Controlling the pressure during machining dictates how successful we will be [in producing an effective cutting tool]. We can control cutting pressures by controlling the width of the grooving insert; the wider the insert, the more pressure it will produce. We also grind positive rakes on the inserts to control the pressure and to act as a chip former to control the chip shape and size.”

Inserts or solid carbide

A frequent issue is whether or not to go with inserts or solid carbide tools. Inserts can be indexable or have only one edge for single use. Inserts tend to be less expensive, but usually can’t get into the smallest bores, and are not as strong as solid-carbide tools.

“The grades Iscar uses for our inserts depend on how fast the machine spindle can spin,” says Iscar Tool’s Geisel. “10,000 rpm sounds high but remember you are working with very small parts, around .100 in. in diameter. So that 10,000 rpm with a .100 in. diameter works out to be about 260 sfm…we need to design and produce the insert out of carbide grades with coatings that can run at such speeds.”

“When things get small you have a length and diameter ratio to contend with,” says Ficker from Cogsdill.

“It would be preferable to take an indexable insert into small grooves, because it would be more economical. But you often have to go with a one piece design, using a brace tool with a carbide or carbon steel cutter brazed to it.”

Solid carbide grooving tools are usually L-shaped. Part of the challenge is that even the non-cutting, or “leg” part has to fit into the bore diameter. Given that the bottom of the L, or the “foot” has to be even smaller, strength becomes an issue, as does the fact that groove depth is limited. As a result, with bores of less than 2 mm in diameter the best tools are now constrained to a groove depth of around 0.2 mm.

But replaceable inserts, single edge, can still get to some small diameters. More cost effective than a solid carbide tool, they can also deliver better axial depth. That said, solid carbide tools are usually more expensive, toolholders aren’t, and there is a risk the holder might break after a dozen or so uses.

Slower feed rates

Small grooving tools can require much lower feed rates due to breakage concerns.

“It is important to keep speeds as low as possible when you have small-sized parts,” says Igor Kaufmann, a member of Kennametal Inc.’s global team for turning tools. When grooving miniature parts, Kaufmann emphasizes that the tools still need to fit axially and apply radial force to the bore. As a result, it is hard to ensure that the tool is strong enough, which then leads not only to chatter issues but also challenges with breakage and chip evacuation.

“Slow feeds can be a good idea, depending on the material,” says Bokum Tool’s Stewart. “With smaller sizes a weaker tool neck will want to deflect, especially with carbide, and with increased distance to the groove, feed speeds need to be adjusted downward.”

The problem with carbide is that reducing deflection increases the risk a tool will break, so it’s important to preserve the strength of the neck of the tool. Tool monitoring is also recommended, because with small cutting edges it can be hard to see or hear when a tool breaks.

A chip off the old groove

When grooving small pieces, cutting pressure tends to be relatively high. Aside from slowing feed rates, it can also help to use high lubricity coolants that work well in tight spaces.

“If you are cutting to small major diameters on the face, where the grooves generally aren’t that deep, the chips are not an issue,” says Drape. “But in ID grooving applications where the groove is deep into a bore, then we need to get coolant in, and also need to break down the chip so it can evacuate.”

Chip size, adds Iscar Tool’s Geisel, can impact performance when working in small diameters.

“You need the chips to be able to freely get out of the bore and away from the cutting action so they do not interfere with the machining process.”

Managing chip generation in tight spaces has always been a challenge, because the smaller the hole the harder it is to get the chip out. One approach is to plum or port the tool to get coolant out. There are also advanced tools, like Kennametal’s AF-GUP insert geometry for grooving and turning, that are specially designed for these challenges.

“This is a double-sided insert that works well with slow cutting forces,” says Kaufmann. “The GUP insert geometry is advanced and versatile, with excellent metal removing rates.”

If the chip created by the grooving tooth is wider than the finished groove, then you can minimize the possibility that the chip will stay in the groove. But when it comes to harder metals the challenge isn’t chip control so much as creating the chip in the first place. “At 1.5 mm you need to have a tool that is strong enough to create the chip,” says Stewart. “But from there, with a strong alloy like Inconel, it is relatively easy to control the chip. In the case of aluminium and softer materials, where the chip is continuous, it creates greater headaches because the material balls up.”

The risk with these softer metals then becomes that the chips pack the hole and break the tool.

“The geometries are not standardized, but there are ways to create space at the head of the tool to promote chip flow,” says Stewart. “Sometimes the actual grooving tooth on the cutting tool will have a chip breaker on it—this will help the chip move out and get flushed with coolant.”

Another approach is to compress chip width, thus ensuring it doesn’t lodge in the groove and result in tool failure. Either way, when it comes to grooving small parts, the combination of slow feed rates, proper tool selection, and a strategy for chip evacuation is the best way to ensure success.

Tim Wilson is a freelance writer and regular contributor based in Peterborough, ON. CM