08/05/2025
Here is an article I wrote for Practical Machinist 16 years ago when high feed cutters where just starting to gain popularity and there was a lot of mystique about them. It’s an easy read and explains how the depth of cut is extended out over a longer cutting edge.
started to hijack Slapstick's thread with this but I've had several inquiries from some of you and thought I would post this as a separate thread. Because it's technical nature.. I think it can stand on it's own.
I did a presentation on high feed milling for Lockheed last year and there is some mystique about these cutters so I'll throw out a condensed version for you guys. If you're using these tools already then you will be familiar with the theory and parameters of making them work successfully.
These are a further evolution of the strides in productivity pioneered in the mold and die industry. At some point they discovered "button cutters" could be run at much higher feed rates because of the huge radius on the inserts which gave them strength and the benefits of radial chip thinning. But round inserts have a tendency to retain heat and tools failed with little or no warning. The cutting forces change rapidly with increased depth of cut.
Pic 1 below illustrates the differences that depth of cut makes on a high feed milling insert and a round insert. Depth of cut is expressed as "Ad".
Notice how the load is spread out along the cutting edge of the feed mill insert? A ten degree inclination angle of the cutting edge produces a 6:1 ratio of chip load to chip thickness. What does this all mean in the real world?
It means that a .040 to .060 chip load per tooth produces a chip thickness of only .007 to .010. There's the high feed rate explained.
High feed cutters exploit radial chip thinning to the extreme. Remember how a 45 degree face mill spreads the depth of cut over a longer length of the cutting edge? In traditional milling methods the force is highest when the insert enters the cut, and tapers off from there. High feed cutters "sneak up" on the cut. They employ a soft entry, followed by an increase in the chip load, then the cutting forces taper off as the corner rad and the periphery (which has back draft) exits the workpiece. In carbon steels most, if not all, of the heat is carried away in the chip.
Typically the inserts have a large hone for strength and sit in the cutter with negative axial and radial rake. This does several things. The negative radial rake kicks the chips away from the cutter so that in deep cavities where chip evacuation is a problem the cutter won't re-cut chips. I have a video of a test we ran using a feed mill on an 8 inch extended arbor in a mold and you cannot see the tool at all because it was buried under 5 inches of chips.
The negative axial rake takes some of the twisting moment off of the tool by directing the forces up into the spindle. Helps stabilize the cutter.
Most of these cutters won't exceed .060-.080 depth of cut. They are made for high spindle speeds, light DOC's, and insane feed rates. I use the 30/30 rule for the first pass and adjust from there: .030 DOC, .030 feed per tooth. Your material group dictates SFM which will give you the RPM
Don't even think about running these tools in Ti or nickel bases alloys. The huge hone, lack of relief behind the cutting edge, and negative geometry of the insert will heat treat the surface of the part and you'll swear what's at the end of the spindle has transformed itself into an arc welder. Edit: This article was written in 2009. Newer “positive/negative” geometries have been introduced for high temp alloys
