Simply put: chamfers and fillets make solid models look good. Not only that, but they are quick and easy features to add to 3D CAD models. This is why many engineers frequently use chamfers and fillets to elevate their designs — but at what cost?
While chamfers and fillets are aesthetically pleasing and simple to design, including these extra features can significantly increase machining time and production costs for your part. Design for machining requires some knowledge of how the part will be manufactured and the skills to adjust your design to keep production costs low.
With that in mind, here are some questions you should ask yourself before adding a chamfer or fillet to your design:
· What is the function of the chamfer or fillet?
· Is this chamfer or fillet necessary for the part to function?
· Is it cost-effective to add this feature for the quantity I’m producing?
· How will the tolerance be checked?
To help you better understand why these design and cost considerations are important, let’s walk through some examples.
First, let’s compare how fillets and chamfers are made in the CNC machining process. Chamfers are typically made with a tool that’s ground to the correct angle, such as a spot drill, countersink, or chamfer tool. A spot drill or countersink tool is used to make interior chamfers, while a chamfer tool is used to make exterior ones.
Fillets are typically created with an end mill that machines the part profile. If the fillet is along a part edge, it will likely be made with a corner rounding end mill. However, setting them up can be quite time-consuming. The cutter must blend the radius into the part’s top and side faces so that there is no step remaining.
To accomplish this, fillets along part edges (like the red fillet in Figure 1) are made with a ball-nose end mill. This cutter is programmed to closely follow the part edge and make very small step-overs. With this method, you can make high-quality fillets.
But this introduces added costs. Adding more chamfers and fillets and requiring higher levels of precision and quality significantly increases your machining time. And longer machining time means higher production costs. Before adding another chamfer or a fillet to your design, consider if it’s necessary for the part’s function and worth the additional cost.
Another factor that increases costs is the production volume. If only a few parts are being made, then the cost per part will already be high on top of the added fillet and chamfer costs. But if you are planning a larger production run and the costs per part will be low, then adding a chamfer or fillet could be cost-effective. Plus, with a large production run, you might be using a casting or forging process. In this case, the fillet would not only be cost-effective but also necessary.
If you’ve evaluated those factors and decided to add a chamfer or fillet, the next consideration is how to machine the part while keeping costs low. In Figure 2 below, the blue hole on the top of this part is a 5/8 in. -11 tapped hole with a 0.025-inch chamfer. This chamfer is important because it enables the fastener to start more easily into the hole.
When designing a chamfer like this one, keep your tolerance in mind. The tighter the tolerance, the more costly the part. In the example shown in Figure 2, the chamfer diameter is called out to three decimal places. This means that the diameter of the chamfer can only deviate 0.005 inches above or below the nominal size. A tighter tolerance means that the chamfer will need to be checked more frequently. It’s also more difficult to check: with a three-decimal-place accuracy, a chamfer gauge would technically need to be used. More checking and more difficulty? Once again, that means a more costly part.
If you were to change the chamfer to two decimal places for accuracy, you would double the allowable deviation to 0.01 inches above or below the nominal size. This would allow you to check less frequently and use a vernier caliper. If this kind of adjustment makes sense for your part, it can help you save on production costs.
You can also consider removing chamfers from the hole callout to make things simpler and most likely less expensive. In Figure 3, the function of the chamfers in the purple holes is to make it easier to assemble the pin into each hole. If the exact size isn’t critical, you could leave the chamfer feature off the solid model and add a note to the drawing stating to “remove all sharp edges.” Alternatively, it could be called out with a minimum and maximum feature size and generous tolerance as shown in Figure 3.
In some cases, you may even want to swap a fillet for a chamfer. Chamfers around part edges, such as the green chamfer in Figure 3, are usually more affordable to manufacture than fillets. A fillet radius tool requires more setup time than a chamfer tool because the toolmaker has to make sure it blends with the surfaces joined by a fillet. Also, chamfers require less machining time than fillets because fillets need multiple passes with a ball-nose end mill. These time-savings add up as cost-savings!
If a fillet is necessary and you can’t opt for a chamfer, try to make it a standard size if you are able to use a radius milling cutter. Standard radius cutters start at 1/16 inches and increase by 1/32 inches. Machining a fillet with this tool is always quicker than using a ball-nose end mill.
Understanding how your design decisions affect machining time and costs takes practice and experience, but with plenty of design for machining tutorials available online, you can get a head start on improving your skills and design intent.
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