Is plunge milling still viable?

Certain high-feed face mills, such as Kennametal’s 7792, also can be used for plunge milling. Photo courtesy of Kennametal.

Is plunge milling a lost art or an underappreciated but still relevant application? Or a bit of both? The main difference between plunging and traditional milling is the change in cutting forces from radial to axial.

“Plunge milling takes advantage of the machine’s strongest axis, Z, just like a drill,” explained Brian MacNeil, milling products and application specialist for Sandvik Coromant Canada. “However, plunge milling is not very common anymore. Modern high-feed tools provide good axial tool pressure and allow for extreme feed rates.”

Plunge milling also is becoming less common because it is seen as difficult to program.

“It’s not learned [by students] in machinist school. Only a handful of customers are using it, mainly in mould and die,” said Thomas Hagan, milling product manager at Iscar Tools.

So is plunge milling too old-school? Not if you have the correct tool.

“It was a growing trend before high-feed mills came out, and then was kind of left in the dust. However, cutting tool technology for plunging operations has greatly improved too. This makes plunging still a very viable process,” said Cullen Morrison, business development manager, threading and milling, for Komet of America.

Tony McClain, regional product manager for indexable milling at Kennametal, also disagrees with the notion that plunge milling is an antiquated technique best consigned to history’s dustbin.

“In our portfolio, we have other cutters that will not only do high-feed face milling, but also Z-axis plunge milling, circular interpolation, and ramping. I would say, a couple times a month, someone is asking us about Z-axis plunge milling,” said McClain.

It all depends on the shop and the part. Z-axis plunge milling is likely to be found more often in job shops than in large manufacturing companies.

Plunge milling takes advantage of the machine’s strong Z axis, especially in smaller, less robust equipment. Image courtesy of Sandvik Coromant Canada.

“Here in Oklahoma in the oil patch, where a lot of job shops don’t have the money to buy a million-dollar machine, they’re going to be doing Z-axis plunge milling because that’s the way they’re going to get the tool performance and metal removal rates on smaller machines with less horsepower. I don’t think [plunge milling is] a lost art. I think it’s still prevalent,” said McClain.

So when should plunge milling be used rather than more mainstream milling applications?

“There are several different circumstances where I would recommend plunge milling,” said McClain. “One is if you had a very long overhang; in other words, when your gauge length is very long. If you have a long hangout on a tool, Z-axis plunge milling is beneficial because it pushes the forces back up in an axial direction towards the spindle, which helps reduce vibration and deflection off the tool.”

According to McClain, another typical situation for plunge milling is when you need to achieve high metal removal rates on a small machine that has low horsepower. Also, the part’s configuration often dictates how it’s going to be machined. Plunge milling is useful if a part has a deep cavity that creates tool clearance problems.

While high-feed milling has become the predominant method of rough milling, the cutter technologies may have surpassed the ability of most machine centres – especially older, less robust models -- and the abilities of many programmers.

Take the Plunge

“This leaves a big opening for plunge milling. Modern high-feed mills can possibly achieve very high metal removal rates, but only if the machine can feed fast enough and the program path is smooth and consistent enough to keep the cutter engaged at high velocity without adding vibration. Plunge milling doesn’t have any of those problems, so in some ways it is more forgiving and much easier to program,” said Morrison.

Plunge milling has a reputation, deserved or not, for relatively low rates of metal removal. It’s another point of debate.

“I would say that under favourable conditions, using traditional milling tools to plunge is less efficient. However, if you’re using dedicated plunging tools in favourable or unfavourable conditions, plunging metal removal rates will be higher,” said MacNeil.

Komet’s KUB Pentron® drill also can perform plunge milling operations. Photo courtesy of Komet of America.

It all goes back to the type of application you have and what the process is going to be.

“If you are reaching inside a cavity or around a boss of the part or something of that nature, and you’re trying to run around it in a profile manner like you would with a 90-degree milling cutter, for example, you’re going to get a lot of radial forces pushing off on the cutter because you’re hanging out so far,” explained McClain. “You’re going to get a lot of vibration so your cutting parameters have to be reduced to be able to make the tool run. So in this case, a Z-axis plunge mill would definitely provide a higher metal removal rate.”

Coolant Usage

Coolant should be used only in two instances when plunge milling:

1. When working with specific materials, such as heat-resistant superalloy (HRSA) or high-nickel stainless steel
2. For clearing chips where air is not an option

“If you are going to involve coolant, the best case is to use the most you have available. Thermal cracking occurs when the insert is allowed to get hot in the cutting zone and then rapidly cool when it comes out of the cut. If you can force as much coolant as possible with high pressure into the cutting zone, you minimize the possibility of thermal shock to the insert,” said MacNeil.

McClain largely agrees with this assessment.

“If you’re going to have coolant, have lots and lots of coolant. Maybe high-pressure coolant is the best. In most milling applications we always recommend running dry with an air blast, but in some cases you need coolant to blow chips out of the way. If you run dry, you may end up hanging (double cutting) chips, which is detrimental to the cutting edge of the insert,” said McClain.

There’s not exactly a huge number of new plunge milling products on the market.  As for the future, the pundits ponder.

“I would say that in the future, the plunge mill probably is going to be a more versatile tool, which can be used to plunge mill, face mill, ramp, and circular interpolate. When it comes to a Z-axis plunge mill, you don’t pigeonhole it just for Z-axis plunge milling. With the economic situation that we’ve been in the last year or so, shops want to buy tools they can do multiple things with. I think that’s where we’re headed; Z-axis plunge milling is not going away anytime soon,” said McClain.

“The future? I think that plunge milling is still a very secure and effective way to remove metal that has been forgotten about by a new generation of machinists. It certainly does not have the ‘wow factor’ of watching a high-feed tool run around a part at 300 IPM,” said MacNeil. “[Plunge milling] also used to be somewhat cumbersome to program, but now most CAD/CAM companies have plunge cycles that have the best-practice programming rules built in.”

Having said that, does he ever anticipate seeing a plunge milling revival?

“I don’t know if it would be a revival, but [plunge milling] is certainly worth talking about because it can be very helpful. It can be a great problem-solver for a lot of different industry segments,” said MacNeil.

Contributing writer Nate Hendley can be reached at nhendley@sympatico.ca.

Iscar Tools, 905-829-9000, www.iscar.ca

Kennametal, 724-539-5000, www.kennametal.com

Komet of America, 847-923-8400, www.komet.com

Sandvik Coromant Canada, 905-826-8900,