Originally Published on "Make it from Metal"
Jonathan has been working in manufacturing and repair for the past 12 years. His specialty is machining. He manages a machine shop with multiaxis CNC machines for aerospace and medical prototyping and contract manufacturing.
So is peel milling a technique worth learning?
Peel milling is an approach that uses high feed rates, low radial depth of cut and high axial depth of cut. It relies heavily on the principle of chip thinning, using a tool path that maximizes tool wear along the entire flute length.
Trochoidal peel milling is a particular type of motion – a circular high speed maneuver that is excellent for carving out deep slots and other narrow features.
Ok, that was pretty packed with information. Let’s break that up and use a few diagrams to explain what’s going on, how to do it properly, and how to know when your application justifies it.
Peel Milling Principles
Of course, when using this method in the real world, you’re best off consulting the cutting tool manufacturer for recommended cutting parameters, but these numbers are usually pretty realistic.
For example, if you have a 0.500″ endmill, you’ll cut at a depth of 1.0″ but with a stepover of 0.050″. Compare that to a standard approach of cutting with a depth of 0.250″ and a stepover of 70%, or 0.350″.
For the standard milling approach, we’re cutting an area that’s 0.250″ x 0.350″, or an area of 0.0875 square inches.
But there’s a secret sauce.
Peel milling can take advantage of something called chip thinning.
If you take a look at the size of chip that you get with such a low radial engagement, you’ll realize that it’s actually super thin. What this means is that you can crank up the feed rate to get a normal chip thickness.
Now let’s convert those previous examples from 2D area to 3D volume.
To do this, we’ll need to add some material data to come up with realistic feed rates. Let’s say that we’re cutting 4140 HTSR. We’ll use a cutting speed of 400 SFM for standard machining. Let’s see what that stock removal looks like.
For conventional machining, the RPM works out to 3200 RPM. We’ll use a feed rate of 0.003″ per tooth, using a standard 4 flute endmill. This means that we’ll be feeding the cutter at 38.4 inches per minute.
Taking that same cut of 0.250 deep x 0.350 stepover, our 2D cut is 0.0875 square inches. To convert that to cubic inches per minute, we’ll multiply that by our feedrate.
0.1875 square inches x 38.4 inches per minute = 3.36 cubic inches per minute.
Now let’s compare that to peel milling. To maintain the same 0.003″ chip thickness at 0.050″ stepover, the feed can be increased to 0.0051″ per tooth. Another perk of peel milling is that the RPM can also be bumped up.
So let’s increase that RPM to 500 SFPM, using a 0.0051″ chip per tooth. This works out to 4000 RPM, and a feed rate of 81.6 IPM. As mentioned previously, the 0.050″ stepover and 1.000″ depth of cut works out to an area of 0.050 square inches.
0.050 square inches x 81.6 inches per minute = 4.08 cubic inches per minute.
That’s about 15% faster material removal than the traditional approach. But the advantages of peel milling don’t end there.
Maximizing Tool Wear
Using that same previous example, what would our endmill look like after an hour of work?
The bottom 0.250″ would be worn out, and the top 0.750″ would be totally fresh. Not really maximizing the use of the tool, is it? Now aside from the cost of the endmill itself, add the down time of the machine operator swapping the tool for a fresh endmill. Overall, it’s just not an efficient method.
Now compare that to the tool that’s used for peel milling. What would that tool look like after an hour?
Instead of all the wear focused on 25% of the tool, the wear would be evenly distributed across the entire flute length.
Also, because of the small radial engagement, the individual flutes are actually in the cut for less time. Let me illustrate.
Does this mean that your cutters last 3x as long? Not always, in my experience. But I do tend to improve tool life noticeably, especially in hard to machine materials like titanium, inconel and cobalt chrome.