Designs We Love: CNC Machining

Posted On February 11, 2022 By Dan Robinson

Relationships, whether personal or in business, are all about mutual respect. We see tens of thousands of CAD files for machined parts every year. We can tell instantly when someone has been thinking carefully about machining in their designs. When we see that process understanding, we know we can produce that part quickly and efficiently, which works out great for both us and our customer. At the same time, when we see things that are problematic, it’s our responsibility to communicate those issues to the customer. Here are some things we’re always looking for in designs.

Milling vs. Turning: The Birds and Bees of Machining

Fundamentally, there are two types of machining: milling (using drilling methods) and turning (using a lathe). Very cylindrical parts should be designed with the lathe turning process in mind, and very square/orthogonal parts should use milling. They are, in many ways, precise inverses of each other. While both are subtractive processes, with milling the cutting tool rotates against the (typically) stationary material to remove unwanted metal or plastic. Turning does just the opposite. The part spins around a defined axis and the tool scrapes off material as it rotates. Your part may need to go through both processes to complete, and in normal situations, we should be able to tell from your CAD file what you want and how to machine it.

Machining Corners: Rounder is Better

We see this a lot, but if you take a moment and visualize it you can see how a round endmill can never create a 90-degree angle in a corner where two walls meet. Adding fillets to round out your corners shows us that you have that knowledge. I mean, sure, you can have us use smaller and smaller endmills to approximate a sharp corner, but it just gets you a smaller fillet, and a bigger bill. As your partner, we can save you some money if you’re OK with broader, rounded corners. Besides, they look pretty good, too.

Flatten Your Horizon: Where Walls Meet Floors

This is kind of the opposite of the previous section. Here, you’re aiming for flatness—a nice 90-degree angle where walls meet floors. When a wall meets a floor at a sharp 90-degree angle, the machining process is much more efficient. We do this with square endmills, which are just like any other endmill, but they have a flat, rather than a pointy, lower cutting surface. They are specifically made to mill out material to create a flat surface. One thing to remember is that when a fillet is applied and you end up with a rounded transition between the floor and wall, substantially more machining time must be spent contouring the fillet. This can make machining times much longer, and more costly, than necessary.

The Ins and Outs of Holes and Pockets

When designing for manufacturing, it’s important to know that those cool holes and pockets work for you and, in the long-term, your customer. We love to see features that are sensitive to that. Attention to each features aspect ratio is critical. As a rule, deep holes and pockets should be large diameter, and longer standing posts or ribs should be thicker. Generally, the smaller the LxD ratio of a feature, the better. Very few features should exceed an 8:1 LxD ratio unless you have a significant reason for the design.

Assemblies: Breaking up Isn’t Always Hard to Do

Sometimes it’s easier to simplify a design by breaking it up into two or more components and using bolts, welds, or other methods to join them together later. It’s great to see designer/engineers pay attention to tradeoffs between minimizing the number of components and the need to use assembly. With some CAD models, it makes more sense to reduce machined part complexity by breaking more complicated parts down into sub-components. This could have benefits for very complex parts.  

So, those are some of the things we love to see in CAD models for CNC machining. Read about the design elements we love in our other service lines: 3D printing, injection molding, and sheet metal fabrication.