By Eric Gardner, Medical Segment Specialist
We work with customers from a variety of industry segments, including aerospace, automotive, energy, oil and gas and medical, and assist them in determining the best possible solution for optimizing their unique applications.
One highly successful project that comes to mind was helping a company improve the milling of its orthopaedic knee replacement components made from cobalt chrome (Co-Cr). While this material has great biocompatibility, meaning it works well in the human body, it’s tough stuff that can be the bane of many machinists.
Before we got involved, the company was struggling with tool life, averaging one conventional solid carbide end mill for every six parts. Add in the time required to change tools, process disruption and the perishable tooling cost per part was approximately $5 per workpiece.
We suggested the company apply high-feed milling (HFM) techniques using a high-performance high-feed milling tool as opposed to a conventional end mill. In doing so, the company brought its tooling cost per part down to around $2, without impacting production time. And perhaps the greatest benefit was increased process stability and predictability, which led to less scrap and higher quality machined products.
While a high-performance high-feed milling tool may cost a lot more upfront than a standard cutter, you can reduce your tooling costs substantially over the long haul by making the switch. That’s because there’s more to high-feed machining than cranking up the feedrate, and high-quality tools play a big role in successful high-feed milling.
Commodity end mills can’t handle high-feed milling requirements, namely extreme chips loads and the associated heat, pressure and cutting forces generated when flying through a workpiece at feedrates up to 10 times faster than seen with conventional machining techniques.
The high-feed milling tools we recommended to the company were made from a tough grade of micro-grain carbide and coated with a heat and wear-resistant TiAln coating. The geometry of the cutters involved shallow flutes with a neutral rake and very slow helix. And unlike general-purpose end mills, which typically have a square profile and zero-radius edge, our high-feed milling end mills have rounded ends, with corner radii upwards of 25% of the tool diameter. All of these features make for a tough cutting tool, one of which can withstand far higher feedrates in Co-Cr than less robust solutions.
Tips for Putting High-Feed Milling to Work for You
|Seco High-Performance High-Feed Milling Solutions|
Compared to conventional machining, where a relatively large depth of cut and lighter feedrate is the norm, high-feed milling relies on heavy chip loads and shallow depths of cut. You might be skeptical, but high-feed milling works because most of the cutting forces generated are directed axially, perpendicular to the cut and up into the spindle, which creates greater tool stability and reduces vibration, both of which increase tool life. Also, the smaller chip tends to carry the heat away from the cut, further increasing tool life and, in many cases, allowing for higher metal removal rates.
In high-feed milling, because the cutting tool is cruising along at such a high rate of speed, you have to pay special attention to your tool path. Constant cutter engagement is critical, which means you have to remove metal in a predictable and consistent manner. You can’t let your tool stall in the corner or you’ll rub it to death, and if you come into a corner that has the same radius as your cutter, all of a sudden your cutter engagement goes through the roof and you’ll break the tool every time. And you need to know how much material is in front of you at all times – when you’re driving a tool this fast, any speed bumps or potholes in the road can mean a broken tool.
All of this might be difficult to achieve using “traditional” or outdated programming systems, but there are several good CAM programs out there with high-feed milling in mind. Look for the ability to ramp in and out of the workpiece, so as to avoid shock to the tool. Trochoidal and helical milling paths, corner rounding capability and smooth 3-D contouring – all are valuable assets in the high-feed milling toolkit, ones which reduce sharp angular changes in the tool path while still providing constant cutter loads, thus minimizing adverse cutting impacts and extending tool life.
However, the best tool path and the best cutter will fail without a good machine tool. Because high-feed milling moves the tool rapidly down the proper tool path – one that maintains constant cutter engagement and avoids shock to the cutter – you need several things to support this. One of the primary requirements for any CNC machine tool is rigidity, which means high-quality guideways, rigid construction, and proper installation and environment. And since so much of the cutting force during high-feed milling is directed upwards in the Z-axis, you need a spindle of sound design and construction that can absorb those forces, preferably one fitted for HSK or dual-contact toolholders.
Also necessary is a modern machine tool control, especially for machining complex 3-D shapes. 3-D contouring of any kind generates massive amounts of data, and only those CNC controllers with high-speed processors and extensive look-ahead capabilities can keep up with these demands. And to serve the commands of the CNC quickly and accurately, the servo systems must be robust as well, and be able to maintain smooth and continuous motion even when traveling several hundred inches per minute as is frequently seen in high-feed milling.
High-feed milling is a complex strategy and the above tips are just a starting point. Speeds, feeds and setup techniques are some other things to think about. Just know that if you have questions or need assistance with a high-feed milling strategy, please don’t hesitate to contact us. Our expertise runs deep so whether you are in the aerospace, automotive, energy, medical or oil and gas industry, we can help optimize your process.
About the Author
As medical segment specialist, Eric provides technical sales and marketing support to Seco’s medical manufacturing customers. In his spare time, he enjoys spending time with his daughter as well as shooting sporting clays. Contact Eric at email@example.com.