Tackling angled pockets

Circle Segment End Mills feature unique design form with large radii, allowing a much larger axial depth of cut during pre-finish and finishing. A mold core or cavity with steep planar walls adapt to a nice flowing circle segment toolpath. EMUGE-FRANKEN USA

Pocket milling can be a time-consuming and oftentimes unavoidable operation. Working with closed or angled pockets adds a layer of complexity. These types of pockets can be tricky because of the many factors that must be considered when approaching these operations.

Milling closed or angled pockets is common in the mold and die world, especially in deep injection molding cavities, which can be seen in almost every part. Electronic enclosures and chassis are another area where a lot of deep pockets are produced. And of course, structural aerospace components also will feature geometries that have large pockets with small corner radii that tend to be deeper than the average tool can reach.

Tool and die shops often deal with lots of tapers or drafted walls, which are features to help eject plastic parts out of a mold. Molds can also be designed with tapered pocket and tapered cores that are typically used as a locking mechanism to accurately align at the final closing of both halves of the mold during the injection of the material. Lifter pockets are also a common feature if there is a need for additional part details. For example, perhaps a snap clip feature that would be used in a final assembly of interlocking parts. Lifters are also used to assist the ejection of the part from the mold. The pockets need to meet very tight tolerances to prevent the material from seeping/flashing into open gaps.

All of these pocket features can be problematic and producing them effectively will depend on specifications. How deep are the pockets? What are the tolerances needed? What type of machine will be used and how much torque does it have? What cutting tools are needed and what cutting strategy works best? Does your CAM software support the different cutting strategies?

Not all milling machines are created equal. Some are designed as economical options for shops looking for something that will perform a specific operation exceedingly well while others are loaded with features and specifications that expand a shop's capabilities.

"There is this misconception that if you can mill angled pockets on one machine, that you should be able to do it on any other," said Jason Rudbal, applications manager, Megatel CNC Solutions, Mississauga, Ont. "But that's just not true. Not all machines are designed to perform these types of operations. And while many machines can, not all make sense. How was the machine built? What about the spindle? What are the torque settings?"

A machine must have certain specifications to produce angled pockets effectively, and depending on the machine's features, it may need to be adjusted and calibrated for this application. There is no one-size-fits-all machine.

"It's important to adjust your strategy based on the power of the spindle, [and the] rigidity and stiffness of the machine that you have," said John Donald, operations director, Megatel CNC Solutions. "Also, you can't use a 5-axis machine in the same manner you would a 3-axis machine. The more axes, the more potential for flaws. It's important to keep that in mind."

Rudbal added that when it comes to angled pockets, some pockets on a one-angled tip can be milled on a 3-axis machine. However, when you compound angles, you immediately move to a 5-axis machine.

"With 5-axis machine tools and the appropriate software for programming, you are able to tilt a head over to avoid catching the toolholder as you go deeper into the pocket," said Ernie Dickieson, sales engineer, OPEN MIND Technologies, Boston, Mass. "It also means you can keep it short in the toolholder as you go deeper in your pocket. These things are critical when it comes to machining geometries like this. Being able to tilt automatically or with some sort of control into these geometries is critical."

The automatic tilting function in the hyperMILL Tangent Plane machining cycle enables the conical barrel cutter to machine the planar walls, as well as the smaller corner fillet. OPEN MIND Technologies.

However, regardless of the machine type, you do have to consider issues with chip evacuation. The higher the torque and horsepower, the more chip removal you need. If you push too hard, it creates big chips that can't be cleared out of the pocket, which can be problematic. Running high-pressure coolant, especially when cutting closed pockets, makes it a lot easier to mill the pocket, especially on 5-axis machines.

"Not all machines have through-coolant capabilities and it can be tough to rough out pockets like you would see in a boring mill setting," said Rudbal. "But when getting into angled pockets via 5-axis, beyond chip evacuation, cutter selection is important."

It's all about working smarter, not harder. The experts agree that it's important to avoid overloading the machine. Running it at 60 to 70 per cent is a good benchmark.

"Some shops want to take large cutters and make big, slow cuts, but the new style of cutters means that you can take a smaller pick and run it faster," said Rudbal. "This means that the machine is running faster, more efficient. And when you do the math, you are removing more material by doing it this way over the other option."

Having the right tool for the right job always is the best strategy. Spend time making sure the tool is correct. Do some research, talk to cutting tool and machine tool manufacturers to glean insights to make the process as smooth as possible.

When milling angled pockets, there are two common operating methods. The more traditional option is a small layered shallow depth of cut with a bull nose/torus end mill followed with a ball end mill to finish the tapered walls.

"The tool you select should have the geometry that is going to allow it to helical ramp into the material," said Dan Doiron, milling product manager, EMUGE-FRANKEN USA, West Boylston, Mass. "If that's not possible, then drill a pilot hole into the tapered pocket as an entry point for the tool to be set inside and then the machining process can start from there."

The depth of the pocket will affect the type of tool needed. Using a long tool with a long flute is not always a good idea, especially in a 3-axis machine when it comes to tool life when milling deep pockets. Most tools with long flute cut lengths are designed specifically for more modern tool path strategies such as a trochoidal climb cut peeling method that uses the entire length of engagement with a five to 10 per cent radial step over for stability. The combination of a tool with a long flute length of cut creating equally long length chips in an enclosed deep pocket will ultimately be disastrous resulting in chip entanglement that will ultimately cause the tool's cutting edges to break, due to the inability to flush out the long chips.

"Look for something that offers a long length of reach, and short flute lengths, with a high-feed geometry on the bottom of the tool," said Doiron. For example, a Duplex tool is a good option supporting two key functions. The smaller length of flute depth-of-cut will create smaller length chips that will be more manageable for removal with coolant or air.

The deeper the pocket, the longer the tool needed. However, longer tools require slower, less aggressive cutting to limit deflection, and chip evacuation is even more crucial as the chips tend to fall back into the pocket.

Emuge JET-CUT Duplex is a dual functional tool. The cutting face geometry is a high-feed design, which can take shallow depth cuts to shape a core or cavity at a very high-feed rate. It also can function as a traditional end mill, performing slot and trochoidal milling. EMUGE-FRANKEN USA

A tool that supports high feed machining methods enables a shallow depth-of-cut that could be used for adding the drafted walls of the pocket.

"Something like a high-feed mill can perform well," said Doiron. "If it's a deep pocket, definitely look for something with through-coolant. If the tool doesn't have through-coolant, look for a toolholder with coolant-through the collet capabilities. The goal is to look for a tool reach length that is as close to the depth of the pocket. This will increase milling efficiency for the roughing and the semi- finishing and finishing. For more advanced finishing, and if a 5-axis machine is available, introducing circle segment tools could further decrease cycle time."

There will be a mix of vertical and horizonal surfaces with a traditional ball nose cutting approach, so blending between those two areas is critical.

"With the right CAM technology, the programming can automatically extend surfaces and do a smooth overlap between them," said Dickieson. "With this extension, there is a gradual transition in the toolpath between the two areas. That's very critical. When working in the corner, you have to go back and do a secondary operation to remove the rest material in the corner where the larger cutter won't fit. Having a multiaxis machine, not just simultaneous 5-axis, and also controls such as auto index, which automatically looks at the corner and indexes the tool in an automated way, makes all the difference. It picks the right angle where the tool can fit from the top of the corner down to the bottom, minimizes the actual simultaneous motion and basically keeps it to a fixed orientation."

Depending on the CAM software, toolpath techniques can make milling angled pockets much easier. For example, 5-axis rest machining automatically detects corners where a larger ball nose or ball mill doesn't fit and creates a toolpath in that specific area. Or a secondary routine, like 5-axis corner rest machining, can pick the corners and, in vertical surfaces or simple geometry, create toolpaths for basic fillets.

"Roughing it out would probably be more in a 3-axis situation," said Doiron. "You can tip your part accordingly if needed, but most of the roughing will probably be done in the 3-axis. And then when it comes to finishing the side walls, you could do one of two things. You could use an end mill or insertable high-feed mill and then begin to bring in those walls on the taper, or you can supplement it with a circle segment tool (conical barrel cutter) to do the side wall finishing."

The tool helical ramps into the material, as it goes down and reaches a certain depth, it spirals out to the shape that is indicated in the CAD model. The tool goes down in sections or tiers and makes small Z-depth cuts putting in the tapered shape, ad can be used at a high-feed rate to makes the final tapered shape and smooths out the wall.

Rather than working with a traditional end mill, some shops opt for a conical barrel cutter strategy, which has the ability to machine side walls and then automatically tilting the smaller tip of the conical barrel into the corner.

"For this cutting tool strategy, the larger barrel radius of the cutter is used to machine the planar surface, and then automatically tiling the smaller tip of the cutter into the corner helps achieve that smaller corner radius," said Dickieson. "Five-axis tangent plane machining is a cycle that cuts the walls and then automatically tilts the smaller tip of the conical barrel cutter into the corner. Or 5-axis prismatic fillet finishing now uses the conical barrel cutter to automatically machine the fillet out, either using the tip of the cutter and stroking the long way up and down the fillet, or using the barrel radius to do that."

The geometry of the pocket will dictate the strategy needed. Conical barrel cutters aren't always the right choice. Any situation where you can get a larger tool to machine the side walls, but the corner fillet is smaller than the tool, a conical barrel cutter can be the right choice.

"If you can fit a larger cutter to do the side walls, but that cutter radius also fits into the corner, then a conical barrel cutter is not needed," said Dickieson. "Using a traditional approach of swarf cutting the side walls and then having the tool form that fillet radius is probably the more efficient approach."

Choosing the right tool and strategy for the angled pocket can mean the difference between success and scrap. However, it also is important to ensure that all other factors, including toolholding, workholding, coolant, and speeds and feed, are dialed-in appropriately.

"With a 5-axis approach, use the loads on all axes, not just the spindle," said Rudbal. "You don't want to overload the machine and have the Z-axis out of calibration. Errors compound with 5-axis, so it's important to calibrate and maintain the machine tool and monitor the loads on the axes. When you mill a pocket, you want to be able to repeat that process with the same results day in and out with precision and accuracy."

Associate Editor Lindsay Luminoso can be reached at [email protected].

EMUGE-FRANKEN USA, www.emuge.com

Megatel CNC Solutions, www.megatelcnc.com

OPEN MIND Technologies, www.openmind-tech.com