Adjust the Internal Corner Radii of Fillets for Faster Machining and Lower Costs

Can the fillets in your prototype model be achieved with standard tooling?

If not, you have a couple solutions: lengthen the tooling to achieve the desired angle or adjust the angle in the design for standard tooling. The simple choice is just to change the tooling, right?

Not necessarily. Going from standard to extended tooling can impact cost, machine time, and part quality. It might not be a risk you’re willing to take. In fact, in many cases, adjusting the design will actually give your project the best possible outcome.

If you’re not sure which solution works best for your prototype model, KAD can help. We have extensive experience using custom tooling and making design changes for more optimal machining. 

The Problem with Custom Tooling

Standard tooling uses the ratio of tool length at approximately 3x diameter for your inside radii. But if a 3x ratio can’t achieve the fillets you designed, one option is to lengthen the tool used for CNC machining. But if you need a custom, extended tool at 10x to 30x diameter, you can expect to face a few challenges:

• Higher tooling costs. The longer the tool is, the more material you’ll need to build it—and that’s an added cost in and of itself. Extended tools are also more difficult to make. Carbide, a common material used to make tools, is brittle. Longer tools are more likely to have a higher scrap rate for the tool manufacturer. As the customer, you end up eating these added costs. 

• Longer machining and finishing time. CNC machining with extended tools changes harmonics, increasing effects like chatter and taper that can compromise the integrity of the part and create an inconsistent finish. Machinists must go very slowly to minimize these quality concerns. If chatter does occur, additional operations may be required, which will add even more to your cost and lead time.

If custom tooling is your preferred solution, we suggest staying under 10x diameter. We can make exceptions and go higher, but doing so will increase cycle time and therefore total cost.

The Solution: Maintain an Appropriate Length to Diameter Ratio

If a small interior corner radius isn’t critical for the function of your part, we recommend increasing the diameter of the radius so that standard tooling can be used. Keeping the length to diameter ratio within 5x allows for the quickest machining, highest quality finishes, and most affordable tooling costs. As an added bonus, increasing the interior corner radius also makes parts stronger. 

If you’re flexible about the diameters of your internal radii, simply specify “internal fillets can be adjusted to best machining practices” when you request a quote. That note indicates to us that when our software flags issues with internal corner radii, we can adjust fillets accordingly and use standard tooling.

When you don’t have the flexibility to increase the diameter, there are alternative solutions that can still help you avoid custom tooling:

If you need very tight fillets, introduce a tapered wall. The tool won’t come into contact with the vertical wall for the entire machine time, reducing unwanted taper and chatter.

If you need a square corner, we may be able to use our in-house 5-axis CNC machining capabilities to machine it. Otherwise, we’ll send the part to one of our trusted partners for wire EDM.

The best way to avoid design adjustments or changes in tooling is to consider the function of your part from the beginning and stick with a standard length to diameter ratio of under 5x.
The team at KAD is happy to offer recommendations before you even request a quote to make sure your design is optimized for machining. Get in touch with us to discuss your part or go ahead and let us quote your next part!