Making A Slotting Tool In Freeform Machining Process
Making A Slotting Tool In Freeform Machining Tools
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Making A Slotting Tool In Freeform Machining Tools

All the aspects of high speed machining can be a real challenge to wrap your head around. Filtering through what’s real and what’s just a buzzword can be a chore in and of itself.

So is peel milling a technique worth learning?

Machining a groove or slot, often called full slotting, involves three machined faces: Slots closed at both ends are pockets, requiring end mills that can work in the axial direction; Full slotting with an end mill is a demanding operation. The axial cutting depth should be generally reduced to. Hole-making is a class of machining operations that are specifically used to cut a hole into a workpiece.Machining, a material removal process, creates features on a part by cutting away the unwanted material and requires a machine, workpiece, fixture, and cutting tool.Hole-making can be performed on a variety of machines, including general machining equipment such as CNC milling.

The cutting tool used in milling operation is called milling cutter, which consists of multiple edges called teeth. The machine tool that performs the milling operations by producing required relative motion between workpiece and tool is called milling machine. It provides the required relative motion under very controlled conditions. We can see that once the cutter hits the corner, the tool engagement angle, which is the number of degrees of the cutter’s circumference engaged in cutting hits a peak of 126 degrees. For comparison, slotting is a 180 degree engagement angle and is about as bad as you can get (well, plunging is. CNC machining is a highly utilised subtractive manufacturing technology. Computer numerical control systems offer less need for manpower and higher levels of automation. One of these automated fabrication methods is CNC milling.It is a process where rotary cutters remove material, which makes it the opposite of CNC turning. The milling centres do not just perform the cutting automatically.

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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 milling is a particular type of motion – a circular high speed maneuver that is excellent for carving out deep slots and other narrow features. Incorrectly used, though, it can waste a lot of time.

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

The basic idea here is to use a small stepover, usually around 10% of the tool’s diameter, but max out the Z depth.

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″.

Now if we look at the area of cutting engagement, we can work out what kind of stock removal we can get.

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.

For the peel milling approach, we’re cutting an area that’s 0.050″ x 1.000″, or 0.050 square inches.

At this point, it’s not too impressive, is it? Peel milling doesn’t have nearly the same engagement as the traditional approach.

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.

According to those sketches, you can cut the material with a 70% higher feed rate and have the same chip thickness as the “traditional machining” model.

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

One of the real disadvantages of the traditional roughing approach is that wear is very concentrated to the bottom of the end mill.

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.

To put it simply, the actual working time of the cutter’s flutes is about 1/3 of the time for peel milling compared to traditional milling.

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.

Trochoidal Peel Milling Toolpath

In the previous examples, peel milling was used to clear out easily accessed material. But what if you need to cut a deep slot? Can you still use the principles of peel milling?

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To begin, let’s look at what a trochoid is.

The Trochoid

Here’s the basic concept of a trochoid:

Imagine swinging a weight over your head at the end of a rope while you walk forward. That basic motion of a point rotating around a center and moving forward is a trochoid.

Here’s an illustration of the motion:

What you essentially get is a carving motion. For toolpaths, the forward advancement is low, but the “spinning” motion is done at high speeds. This is what keeps the cutting pressure light.

Here’s an example of what one of these toolpaths looks like in a CAM system:

As you can see there, rapidly feeding the tool forward with sweeping motions can maintain a consistent radial engagement.

Chip Clearance

Trochoidal milling is a really effective approach for tight area

Making A Slotting Tool In Freeform Machining