Posted July 14, 2017
5 Ways to Design for Cost with Milling
Milling is a manufacturing process used across a variety of industries to create parts of all shapes, sizes and materials. Due to its huge variety of tooling, machine capabilities and decades of technological advances, milling is a process that every designer should understand how to utilize. However, costs can add up quickly because of material selection, part geometry, programming time, and many other factors. To date, milling is our most popular process here at Plethora, so we’d like to pass on some tactical advice about how to lower the cost of your parts. We understand that many of the below considerations are unavoidable based on design constraints you may have, but they’re worth noting for situations in which the design is flexible.
Plethora offers a diverse set of material options for milling. One of the key questions you’ll have to answer while designing a part is which material to choose. There are many situations that will require a certain material, in which case your decision is easy. For example, 316 Stainless Steel is a fantastic material choice in environments where corrosion is a concern.
In a situation where the material to use is not obvious, such as when you need a lightweight enclosure for a PCB assembly, there are a few options. This is the first situation where material cost should enter your brain - there’s no need to use pricey Stainless Steel where Aluminum will do, and there’s no need to use a plastic like Delrin when ABS will do. Here’s a quick pricing breakdown of the materials Plethora offers, based on prices from McMaster-Carr:
Another consideration to keep in mind is your material’s machinability, since harder-to-machine materials will be more expensive. Metals and plastics have different scales for machinability, but in both of these, the materials with the higher machinability ratings are easier to machine.
TOOL ACCESSIBILITY & OPERATIONS CONSIDERATIONS
As I wrote about in a previous post, it’s important to consider how your part’s features are going to be milled. The location, size and profile of a pocket, for example, can add tool changes, cycle time, and even additional operations to the milling of your part. Feature complexity and tool changes require more time preparing for machining in CAM as well. All these make your part take longer to make, and time in the machine costs money.
This part will require multiple milling operations to complete: one to mill the pocket in the box, and another to access the hole in the side wall, as shown.
For example, a particularly narrow pocket may require a smaller tool than is being used elsewhere, therefore necessitating a tool change, which takes time. Once the new, smaller, tool is being used, it cannot remove material as quickly either. A feature placed on the side of a part may require an additional milling operation, which in turn requires an operator to interact with the milling machine and spend time generating additional tool paths. Minimizing the number of operations your part requires will help minimize cost.
Although including a feature that necessitates an additional operation can be very costly, designing in a complex part profile may also add additional cost in the way of fixturing. Consider a simple block, and how easily it can be held in the milling machine with a simple closing of the standard vise. Now consider a part that’s very long and thin - although it can still be held in the vise, additional fixturing will likely have to be added to support the part on one end or both, to mitigate vibration.
In this fixturing example, the thin walls of the part, combined with the huge gripping force of a machine vise, may cause the part to deform as it is milled.
As with all of these, sometimes it’s unavoidable, but if the part can be designed in such a way that will make it easy to fixture in the machine, this will save you money.
Sure, we all want our parts to look nice and feel comfortable in the hand, but adding certain aesthetic features may quickly jack up the price of your part. Fillets, for example, are notorious for being difficult to machine, yet are often added to edges of a part in CAD, without hesitation. Consider the challenges with this: a new, specialty tool may need to be used to cut the rounded edge, and tool changes add cycle time. Another option is to interpolate the rounded edges and mill them with a series of simple traverses at various depths, but this adds both CAM and machining time.
An example of a specialty tool that may be used to mill rounded edges on a part.
Features like this, despite adding a nice appearance and feel, often add cycle time, a tool change, and extra headaches for the machinist, so they’re best to avoid if possible. They’re certainly more understandable in production situations, where the purchase of a specialty tool to add a rounded edge to a high quantity of parts makes more sense.
Plethora provides standard tolerances of ±0.005, but if you’re looking for something tighter than that, you can send in your CAD files for a manual quote. In this situation, it’s certainly possible to rack up dollars quickly in tolerancing cost, and care should be taken in correctly tolerancing your features. A feature toleranced at ±0.001” will cost about twice as much as a feature with our standard ±0.005” tolerance.
For example, if your application requires the precise location and assembly of multiple components, consider building precision into a single assembly fixture rather than requiring tight tolerances on every milled component. If tolerances are needed at all, though, attempt to keep them as loose as possible, to reduce cost.
In short, the main costs associated with milling are the upfront material costs and any extra time required by the machinist. Sometimes the material cost is unavoidable, so if there’s a single overarching tip we can provide to help keep cost down in milling, it is this: consider the time required to achieve what you’re asking. Time is money!