Milling techniques that make the cut
- March 18, 2012
by Tim Wilson
When it comes to milling, in order to get the most out of your application, it’s worth knowing some of the tricks of the trade. When face milling, for example, about 70 per cent of the cutter should be engaged. This is ideal for correct chip formation and doesn’t generate too much heat; however, there are circumstances where full engagement is required, and this is when a thicker CVD coating is needed to withstand the added heat generated.
One method to improve tool life is the roll-in approach, which reduces a thick chip on the exit of the cut. On entry into the component this lessens some of the vibration – and consequently, tensile stress – caused when entering a cut straight on.
“The roll-in technique is very advantageous,” say Kevin Burton, product manager for milling at Sandvik Cormorant in Mississauga, Ontario. “Especially in difficult materials, if you go straight into the component it is detrimental to tool life.”
The idea is to roll the cutter clockwise, but without lowering the cutting speed or feed rate. The result is not only less vibration and more heat displaced into the chips, but also a doubling or tripling of tool life.
“The golden rule is thick to thin chips,” says Burton. “If you go straight in to the component you’ll have a thick to thick chip on entry, but if we roll in then we can have a thick to thin chip on entry.”
People are changing their techniques, too, and tending to come in more from the side, with less ramping.
“Ramping into the part would be for a closed pocket,” says Tom Hagan, Iscar Tools Inc. milling product manager in Oakville, Ontario. “Coming in from the side would be used for an open pocket. It all depends on the shape of the part that will demand the proper application to use.”
Hagan notes that with a large company like Iscar, operators looking for the right angle have the support of an electronic catalogue that offers thousands of combinations.
“For example, if you need a ½ inch cutter, and you’re shouldering a full slot at specific depth of cut, the catalogue might recommend four tools,” says Hagan. “It will give you the feeds and speeds, and tell you how fast to go.”
To get it right requires knowledge of how best to work within a range of machining parameters. Depth of cut is one parameter that is seeing some changes. Operators are finding that a lower depth of cut, at faster speeds and with more passes, has real advantages.
“Faster speeds and a lighter depth of cut is easier on the machine,” says Hagan from Iscar. “There is less wear and tear, which makes sense for people who are getting into smaller machines like the CAT-40s.”
For pocketing, a spiral morph tool path can also help, as it can both boost tool life and reduce – or virtually eliminate – vibration in both large and small pockets.
“This method is basically like a kaleidoscope or a corkscrew,” says Burton. “The technique is good for difficult materials like stainless steel and titanum – we will typically start with a very small radial engagement.”
Spiral morph can be used instead of conventional pocket milling methods. Unlike in these other methods, a high arc of engagement doesn’t result in heat and deflection issues.
“The advantage is that with spiral morphing you tend to keep the same engagement throughout the pocket,” says Burton.
Helical ramping can also be used for hole making. The advantage over drilling is it requires less horse power and is more constant; it is a benefit in difficult materials and reduces the need for many boring tools. This method is supported by some software programs, while also benefiting from vendor recommendations. Assuming that the plunge angle and radius are defined, it is then possible to have the right cutter size for the hole diameter.
“Our catalogues have a chart that shows the range of hole sizes for a specific diameter,” says Burton. “If the cut is too small, you can end up with a small cylinder in the middle, which isn’t good. As well, if the cut is too big it can cause a lot of problems, notably the melting of materials onto the cutter body.”
End mills with a tapered ball nose can also support innovative techniques. With tight cutting diameter tolerances of +/- .005mm or .0002 inch, as well a superior carbide and coatings, these end mills provide exceptional utility in both three axis and five axis environments.
“Our taper end mills can be used in a three axis environment to generate wall geometry that match the angle of the tool, or in a five axis machine to create virtually any angle or surface feature,” says Stephen Jean, milling tools product manager at Emuge Corp. in West Boylston, MA.
“In addition, these taper tools can be used in place of small diameter ball nose end mills in applications in which just the radius of the tool is engaged in the cut, such as trace milling or in finishing operations. Due to the robust nature of the taper design, with flutes along the taper and a larger shank diameter, these tools offer significantly more stability than ball nose end mill with the same diameter.”
At Emuge, advances in end mill design and development have also resulted in improved cutting tools for roughing applications. For example, the company’s MULTI-Cut end mills feature off-set flutes, rather than the typical 90 degrees flute arrangement on a standard 4 flute end mill. Also, the helix angles vary from flute to adjacent flute, which helps eliminate harmonic conditions that cause vibrations that lead wear, and permit the flutes to engage in the cut with added stability.
“Also, the cutting edge is ‘blunted’ to provide a slightly negative cutting geometry which further helps control wear,” says Jean. “This is as opposed to a sharp edge which would tend to chip and fracture, contributing to premature wear.”
Another company with the technology to support advanced machining techniques is OSG Canada, which has the new VGx variable pitch solid carbide end mills for chatter-free milling in a variety of applications, including slotting and side milling of steels, stainless steels, exotic alloys, cast iron and hardened steels.
“OSG Canada has recently released OSG Phoenix Indexable Cutting Tools to the Canadian Market, and the results have been very positive,” says Craig Ramsey, product manager at OSG Canada.
OSG Phoenix covers rough milling to finishing applications, with free cutting insert geometries. According to OSG, this can deliver on optimal chip formation.
“This enables efficient performance in all milling applications including slotting, side milling, helical as well as 3D contour milling,” says Ramsay.
With the right tools and some good advice on technique – often offered by the vendors themselves – milling can extend beyond the limited scope of past years. The result? Jobs get done faster, and easier, while tools last longer.