End milling: An art or a science?

As I visit with customers and speak to operators, a common question I get is, “How do I choose the proper number of flutes for my end mill?” While this may seem like a trick question, it isn’t. In the old days it was rather simple, a 2-flute end mill was used for non-ferrous materials and a 4-flute end mill was used for ferrous materials.

This is no longer the case. We now have several choices in cutting tool material, helix angles, number of flutes, etc. In the past, and still today in some instances, end mills were made of high-speed steel. This gave way to cobalt and is now giving way to solid carbide cutting tool materials as well as coated materials. You can buy all the materials with coatings that allow the base cutting tool material to run at higher speeds. As we move toward higher speeds, tool balance and tool holding become more critical.

The key to ensuring end mills work properly is to guarantee you can evacuate the chip from the cut so that you don’t re-cut the chips. In processing the cut, don’t exceed the depth of the flute to facilitate chip evacuation.

Consider a milling cutter running on a high-temperature alloy. The diameter of the cutter is 0.75", the depth of the stepover is 0.060", the speed is 150sfm, and we’re using a 4-flute end mill. We’re running 764rpm at 0.005 chip load per tooth (cpt) and have 4 flutes. We are feeding our cutter at 15.3 inches per minute (ipm). Providing we can maintain the same stepover and we have sufficient horsepower to make the cut, we could move to a 6-flute cutter and feed at 22.9ipm. This would result in 33% reduced cut time.

So, is this science? Yes. You must make sure you have the correct horsepower available, the correct fixturing to hold the part securely, and the correct tooling to minimize runout and maintain chip thickness.

End milling is becoming more sophisticated as new tooling and coatings become available. Adding more flutes isn’t always the answer, so make sure you’ve done your homework and answered the questions asked above. However, once you’ve done this you can significantly reduce cycle times by applying the correct number of flutes to your end milling operations.

Whenever you’re looking to optimize shop operations, look at your end milling operations. There’s time and money spent here, and a simple change in the number of flutes used to machine parts may help you reduce your cost and be more competitive in quoting jobs to customers. This is a simple way to reduce cost and improve productivity. As always, work with your local tooling supplier and I’m sure they’ll help you choose the correct tooling for your job.