If you’re looking to create a prototype or product in a cost effective and timely manner, one great option to consider is CNC machining. CNC machining allows you to scale the volumes of a product from one to one hundred thousand without investing in tooling. It’s economical, has a fast turnaround time, offers plenty of different materials you can work with, and is accurate enough for repeatable productions.
There are many ways to optimize and improve a design for better results at the lowest cost. Here are five things you should definitely consider for your next CNC machined project.
Only Specify Critical Tolerances as Necessary
Where possible, only specify critical tolerances where absolutely necessary to your design. For example, when specifying a radius on a 2D or 3D drawing, it’s only best to do so on features that perform a critical function for that part. By limiting the number of critical dimensions, it allows the CNC machinist more freedom in making your part and avoids costly set-ups and measurement for non-critical areas.
Don’t Go Too Small
On turned parts, try not to go smaller than a .4mm radii. Most CNC turning on a lathe is performed with turning inserts. They typically look like a triangle or diamond. Often, designers associate smaller with more precise, or somehow superior. In reality, smaller cutting radii make for a poorer surface finish. Larger radius cutters can be machined faster and will create a smoother finish where the high and lows are averaged out over a larger surface area.
Stick to the Standard on Inside Corners
Another good design tip to keep in mind when CNC milling is to go with a standard radius of 3 mm or larger for inside corners, as milling small internal radii is time consuming, expensive, and requires special set-ups. A ball nose cutter is often used when milling 3D shapes and radii. As with turning inserts, these rounded profiles create corner radii in their own size and shape. It is impractical to make a smaller radius because milling is done at right angles to a vertical face. Thus it’s difficult to mill small radii on inside corners without using very small cutters. And if you do that, the CNC mill will run much more slowly.
Know Your Surface Area
It’s important to keep in mind that a small, precise cutter must be fed into the workpiece at a substantially slower speed to prevent destroying the piece. To machine a given surface area requires that the cutter make multiple passes, removing small amounts of material each time which can increase lead times.
Deburr Your Sharp Edges
When creating the design, a “sharp” indication on the drawing where two flat faces meet on an outside corner is not normally specified with any numerical quantity. This is just the angle left over after the adjoining faces have been milled. An edge like this will easily chip, so they’re usually deburred by filing or otherwise rounded off to make the part more durable and easier to hold. Unless you request otherwise, these radii won’t need to be measured. Simple deburring is the fastest and most economical way to treat “sharp” edges.
If you’ve decided to use CNC machining for making a plastic injection or pressure die casting tool, many of these same design rules can apply. It’s best to keep the corner radii large, not only for easier manufacturing of the tool but also to improve the quality of the resulting molded part. Likewise, keeping critical dimensions to a minimum will expedite the making of the tool and will be more forgiving of the expansion and contraction that takes place when these tools are heated and cooled during production.