High Feed Milling Cutters: Why to Use Them and How to Choose Them
When machining components, particularly from raw stock, one of the first operations is usually some sort of rough milling operation. This could be to create qualified surfaces, mounting positions, or simply to start machining the stock into the general form of the finished part. For these applications, High Feed Milling cutters are very efficient tools due to the high advance per tooth feed rates that they can achieve. While they might not always be the best choice of cutter style, there is a good chance that they can reduce your overall cycle time. Let’s explore what makes this style of cutter work and what are the circumstances that would lead us to select one for our machining application.
How it Works
High feed mills take advantage of and exploit a concept known as axial chip thinning. This is a geometrical anomaly that occurs because of the milling cutter having a lead angle less than 90 deg. Axial chip thinning shows us that while the lead angle of the milling tool becomes lower, the actual chip thickness is less than the advance per tooth. And, the lower the lead angle, the lower the actual chip thickness, or the greater effect from axial chip thinning occurs. For example, if we feed a milling tool at .006” advance per tooth with a 90 deg shoulder mill, we would expect the chip thickness to be equal to .006”. However, if the milling tool has a lead angle, we would expect the chip to be less than the .006” by a factor equal to the sine of the lead angle.
Here is the calculation applied to some lead angles that we see in typical tools. High feed mills are typically going to have a lead angle below 20 deg.
You can now see how dramatic the axial chip thinning effect can be from using extreme lead angles to create high-feed milling tools. In the case of a 20 deg lead angle, we would need to feed the tool 3x faster compared to a standard shoulder mill to generate the desired chip thickness.
Of course, having such an extreme lead angle would greatly diminish the DOC that is possible. The cutting edge of the insert is closer to being parallel to the face of the part as opposed to being perpendicular to it. This limits the DOC due to the length of the insert cutting edge.
The other limiting factor when using high feed milling cutters is in the capability of the machine. It’s pretty obvious that machines cannot start and stop instantaneously, they have to accelerate and decelerate at some finite value. This means that we have to consider if the tool is actually running at the programmed speed most of the time. If the space we are machining is too small, such as in a small pocket or on a small surface, the machine may not actually be reaching the programmed feed rates. It would actually instead be in continuous acceleration or deceleration because of the constant need to change directions or possibly even reverse and travel the opposite direction (in this case, the feed rate of the tool would actually reach 0 in/min for an instant). Therefore, we must check that the actual desired feed rate is being reached.
High material removal rates, resulting in shorter cycle times, is the main advantage of utilizing high feed mills in the roughing application. But there is a second advantage that can mean the difference between success, and slogging through your parts at a snail’s pace. And that advantage is in how the high feed mill manages the direction of the cutting forces.
The cutting forces that are applied to the tool body react perpendicular to the cutting edge. For example, if we start with a 90 deg shoulder mill, the majority (almost all) of the cutting forces are applied orthogonally to the axis of the tool. I like to think about it in terms of trying to push the cutter body off its centerline. This can have a tendency to cause chatter, or even temporarily bend the cutter setup causing poor surface finish and poor tool life. Counter to that, if we have a high feed mill, which we know is going to use an extreme lead angle and position the insert more parallel to the face of the part, that will direct the majority of the cutting forces in line with the axis of the tool, tool holder, and spindle. The tool setup is very strong and rigid in this direction since we are pushing directly into the head of the machine, not across it. Apply this to a tool setup that has a long overhang, and the benefits become even greater. So it’s very easy to say that for deep pocketing applications, or any tool setup that requires a long overreach, the high feed mill design can dramatically help us reduce chatter, as well as allowing us to machine at higher feed rates due to the way it manages the cutting forces.
Directing these cutting forces to be in line with the axis of the spindle is generally considered to be desirable. It is normally considered to be more rigid and stable compared to shoulder milling. However, we have to remember that “for every action, there is an equal and opposite reaction”. This means while the machine can handle large forces directed through the spindle, there is an equal force pushing back down on the part. We need to be aware of this in the case of smaller parts, parts with thin walls, or poor fixturing. If the part cannot also handle the cutting forces being applied in this general direction, it could have an adverse effect on the operation.
Choose the right design
Deciding if high feed milling is the right choice for the application does not end the decision process, it’s not the only decision to make. Choosing the right style of high feed mill can also affect the level of success. Fortunately, these decisions start to fall in line with the choices required for selecting a standard indexable milling tool. The advantages of a particular insert style need to be held up and compared to the requirements of the application, or the overall goals of the machinist. If you’re not familiar with this process, a great infographic can be found here.
Basic Insert Design
We see several different insert designs incorporated into high feed mill tool bodies. And, of course, they all have slight advantages and disadvantages.
A button insert is how the concept of high feed milling was introduced to machinists. Since the cutting edge is curved, the lead angle really depends on the DOC being applied. The lead angle, in a case like this, is taken as a line tangent to the cutting edge, in the middle of the DOC. For example, if the button insert is at maximum DOC, the lead angle would be calculated at ¼ the diameter of the insert, and the tangent at this point would be 45 deg. However, if the DOC is only 10-20% of the insert diameter, we get an extreme lead angle and the tool behaves as a high feed mill. Additionally, if you are only using a small percentage of the insert cutting edge, this will allow more cutting edges per insert. Possibly even up to 8 indexes per insert like this one.
The button insert design can be slightly limiting. A larger radius will allow us to make the effect of the lead angle even more extreme. But an extremely large button insert is not economical, nor practical. Tool designers started adding large radius for the insert cutting edge but only including a small arch of the radius that is required for machining. This allows for a large radius to be included on a small insert footprint, many times these inserts are triangular or trigon shaped. Further, these trigon-shaped inserts can be positive basic shape or negative basic shape. The tools will perform completely different based on this basic insert shape and so the choice needs to be made between a light cutting and versatile high feed mill or is a stronger insert with higher tool life more advantageous.
High feed mills with 4 edged square inserts are another common design. The square insert is simply spotted at the desired extreme lead angle. A couple of notable differences compared to the other designs being discussed are that this is the first design that uses a constant lead angle. The button insert and triangle-shaped inserts with large radius all depend on the depth of cut to define the average lead angle. But, one of the main advantages is that it incorporates a very simple and common insert design. This would typically translate to a lower cost of the insert due to either smaller size, or the lower cost to manufacture. It may also be possible that this common insert may be used in another cutter body design, somewhere else in the shop.
The latest improvement to high feed mills with square inserts is to modify the insert design in an effort to increase the performance. While this is getting away from the advantage of selecting this insert style, improving performance is always a good thing. The focus of the design change is to increase the thickness of the insert. Making the insert stronger and more stable by the inclusion of more carbide allows higher advance per tooth and increased material removal rates. The new design can actually include a large radius along the cutting edge mimicking a design concept from the other feed mill styles. This will ease the way the insert enters the material and have the effect of making a softer cut with the tool.
Another advantage of the square insert design is that the insert can typically be very small. The triangle inserts mentioned earlier and even the button inserts, can typically be somewhat large. This limits the number of inserts that can be included in the cutter body. The large inserts require large pockets and deep chip flutes consuming a large percentage of the tool diameter per pocket.
To combat this, tool designers are once again trying to improve the insert design. Using a narrow insert allows smaller pockets and shallower chip flutes which results in higher density cutters. Although, making the insert non-symmetrical can limit the number of cutting edges. By making the insert a negative basic shape, the number of cutting edges can be doubled allowing the insert to be indexed 4 times. The advantage now becomes using a high feed mill, capable of extreme advance per tooth, and incorporating a large number of teeth in the tool. The overall feed rate of these tools can be extremely high. And, it is recommended to check the machine’s capability before selecting this style of high feed mill.
As you can see, there is a lot to consider when opting to use high feed milling for the rough milling application. There can be a lot of advantages due to the extremely high material removal rate. However, it’s not necessarily a foregone conclusion to go in this direction. There is no doubt that the future of machines is moving towards light, fast, and agile machine movements. High feed mills are absolutely in line with this trend. There are, of course, a lot of machines out there with not a lot of speed capability, yet tons of torque available. These machines have a lower ability to make high feed milling the most efficient process.
The design of the tools/inserts also has to be considered. Larger inserts have more DOC capability, but they will typically be less dense in the number of teeth. Different insert designs can improve the density, they can be more or less expensive due to their complexity, and they have a different number of cutting edges per insert. All of these factors need to be held up to the requirement of the application to determine which high feed mill is right for the application.
Author: Luke Pollock, Product Manager at Walter USA, LLC.