How to Cut CNC Machining Costs - A Design Engineer's Perspective

There are ways to mitigate costs though—certain factors in part design drive CNC cost more than others. As a design engineer, being aware of these factors early on in your development cycle can minimize their presence and begin to chip away at that top dollar price tag.

Reduce Your Material Cost

If you’re going to make any sort of part, you of course need to decide what it will be made out of. Many excellent materials can be machined, each with its own unique set of properties, applications, and costs. The price of different stock materials can vary widely but is usually based on general availability, as well as the difficulty to produce that material in a specific stock type (tube stock or especially thick sheets, for instance).

In general, you should simply choose the cheapest possible material that still fulfills your design requirements. Just doing functional prototyping? Try to stick to cheaper options, like ABS and aluminum. On the other hand, if you’re designing a downhole tool, you’ll probably be forced to use pricier materials.

Reduce Setup Time

Besides thinking about what your part will be made from, it is also important to think about how it will be made. Here’s where things start to get a little tricky, as there’s often more than one way to skin a cat (machine a part). We’ll start by looking at how design dictates the amount of setup time prior to machining.  

Setup time, which consists of CAM (computer-aided manufacturing) programming, as well as machine setup and part fixturing, is commonly known as non-recurring engineering (NRE) cost. It typically makes up a large portion of a machining bill at the prototyping stage and should be minimized as much as possible.

It’s wise to assume your parts will be made on a 2.5/3 axis machine, due to their common availability. With this machine type, aside from the part profile, features can only be machined on one side at a time.

Most parts can be machined in six setups or less, as they are made from a six-sided, prismatic piece of stock material, such as a sheet, bar, or strip, that’s held in place by a vice or vacuum table. You should try to reduce this number as much as possible—1 or 2 is ideal—as each setup requires its own CAM program and fixturing step.

A Word on Special Fixtures

Occasionally, your part may have features that require non-standard fixturing, another source of added cost. These types of fixtures are varied enough to warrant a separate discussion, but below are two common ones and the features to avoid if you aren’t willing to shell out the extra dough.

• Soft Jaws—parts with complex or organic geometry cannot always be held reliably in a standard vice, so a set of jaws with geometry mirroring the part surface are machined for a better grip.

• Sine Bar—parts with features not parallel or normal to stock surfaces, such as a hole at a 30 degree angle, will need to be set up using this device, which allows the work piece to be held at custom angles.

Increase Quantity to Reduce Unit Cost

The good news about NRE cost, as its name suggests, is that it is non-recurring. That means if you’re making multiples of the same part, the same CAM programming and fixturing can be used over and over, or multiple parts can be cut at once from the same stock piece.

It is recommend to make more than one of each part, so that your unit cost is lower, but not so many that you’re making unneeded parts.

Reduce Machining Time

The last aspect of cost savings to consider is the actual time it takes to machine your part. For the most part, this is dictated by the two things you’ve already taken into consideration: material and part design.

1. Impact of Material Type