Fillets, Chamfers, Edge Breaks Oh My

A question we often see is: “What features are just finishing, and what steps are required for functionality?” Chamfers, Fillets and Edge Breaks are usually the sources of these questions. We hope to help you understand their purpose and when to include them.

Without them, parts would be out of dimension during manufacturing, cracking would become more prevalent, and handling them would be a bloody mess. Conversely, with too many; parts won’t mate correctly, fastener heads would have diminished contact area and manufacturing costs would be through the roof. So, what are these features and why are they so important? Which one should you use in which scenario?

We’ll address each and answer these questions in the process:

Edge Breaks:

These are the simplest, and for manufacturing, the most imperative of the three. The edge break is exactly as it sounds, a cut on a corner of a part to remove (or break) sharp edges created during cutting operations. This feature is usually called out in a general note on a manufacturing drawing as “Break all sharp edges” and sometimes a tolerance is included like 0.005”/0.015”. This hints at the first key about the edge break: it is intended for every cut on the part, and it is very small.

The first functional reason for an edge break is to remove any extra material from a corner that was pushed over or otherwise inadequately removed. These burrs could cause errors during the next operations setup. In addition, the sharp corner can prevent the part from fitting in a tool properly. By removing a small amount of material from each corner, we eliminate these concerns.

Another reason to break edges is for handling. Machining or other manufacturing processes leave razor sharp edges. If the edge is not broken, operators and end users would likely cut themselves on the object. In some materials, stacking parts with unbroken edges can scratch other pieces in the lot, causing quality issues. Think razor sharp steel cube hitting an aluminum mating flange.

Luckily breaking an edge after a machining operation is quite easy. In metal, a quick pass with a file or deburring tool is often adequate. In wood or plastics, a hand knife is often employed. There are even automated vibratory or tumbling machines which can handle large batches of edge breaks all at once.

Chamfers:

One of my favorite YouTube content creators has a habit of saying “Chamfers separate us from animals.” In many ways that is true, however they serve a functional purpose as well. These features are often called out with a linear dimension and an angle. For instance, 0.050” X 45deg. The linear dimension tells us the “leg” of the triangle created and the angle tells us the angle of that triangle. 45 degrees is most common, meaning the legs are equal in length.

Typically, they are used for creating space where two parts would meet. Such as the lead in chamfer on a screw, it helps the female thread start on the male thread. The same is done for pins or bushings to facilitate their installation, providing a centering action during pressing.

This is also the case in scenarios where there may be a radius cut at the bottom of a blind bore. This could be the result of the drill bit used. Instead of squaring the bottom of the bore, a chamfer is cut into the pin or busing to be inserted. Squaring the bottom of a bore is a risky operation, especially if the bore is deep. Chamfering a pin is a much safer and more cost-effective approach.

Chamfering is accomplished largely the same way in machining and wood working. For lathe operations, chamfers are easily cut using a standard cutting tool and cutting at an angle. In milling or drilling work, special cutters are used. Some milling machines can angle the head for chamfer cutting using a standard end mill, however this is usually a suboptimal setup compared to using a chamfer cutter (5 axis CNC machines laugh at us all and can do whatever they please).

Fillets:

Fillets are different from the other two, as they mostly impact final part strength and appearance rather than manufacturing. The main impact from manufacturing that leads to fillets are cutter radii allowances. Cutting tools do not have perfect 90-degree angles. As a result, they cut with a tiny radius, leaving a fillet behind. The allowance for these are usually shown as 0.050” R. This is a typical radius for an end mill or carbide insert used in metal removal. The allowable radius from a drill bit will be much higher.

This is also a major consideration for sheet metal piece parts. Press tooling requires a radius at each bend. The radius changes with a variety of factors. Pressing too thick of a sheet, with too little of a radius will cause cracking. Hardness of material, thickness and radius all have to be selected in unison to prevent defects.

Aside from cutter corner radius allowances, fillets are usually added to designs to eliminate stress risers. In a simple analogy, stress flows through the part like a fluid. Anywhere where a sharp change in the flow path occurs, stresses peak. This is the concept of a stress riser, and sharp corners are the predominant form. Fillets eliminate these by providing a smooth flow in the part geometry. By eliminating the stress risers, strength and fatigue life can be improved dramatically. A functional fillet for part strength is likely to be quite large. 1.000” radii are not uncommon for such a feature.

They also look nice.

Conclusion:

When designing a part, these features are critical to consider early on. Picking the wrong sizes or types of edge treatments can lead to weak parts, fitment issues and huge manufacturing costs. By picking standard sizes for edge breaks, chamfers and fillets, it is possible to get better functionality without increased cost. Picking non-standard sizes, however, leads to custom tooling or advanced programming. I don’t need to tell you how much cost that can add to a part.

This is where MFG Concepts seeks to help our clients. We will optimize your part for highest functionality, with lowest manufacturing costs. We have seen how a simple change in corner radius can save a company thousands per production run. Our goal is to give the client a drawing and design that can truly succeed, with no modification! Contact us today to see how we can help.

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