In order to ensure that a product works well straight out of the box and performs reliably over its lifetime, it's important to consider a "break-in" or "burn-in" process for certain critical components. The difference between these two processes is subtle but important to understand when evaluating how and why to implement them. Both can play a critical role in maximizing lifetime and minimizing failures, ensuring a positive experience for end users.
When analyzing the likelihood of a component or system failure over time, the probability can be considered in three parts:
1. Manufacturing defects often manifest as failures very early on in the lifecycle of a component.
2. Wear- or age-related failures are more likely as a component reaches the end of its lifecycle.
3. Random failures may occur throughout.
The sum of these probabilities represents the overall likelihood of failure and is typically highest at the very beginning and end of a product’s lifecycle, while being relatively low in the middle. This characteristic is commonly called the bathtub curve.
The goal of a burn-in process is to eliminate early failures and get to the lower part of the bathtub curve by forcing the early failures to happen before a product is put in service, or even before a component is used in the production of a product. This can be as simple as operating a component for a few hours or days and then verifying that it still functions. The idea is that the component will warm up to (or beyond) operating temperature one or more times but won't be physically altered unless it fails. To accelerate the process, and increase the probability of exposing defects that could cause early failures, the component can be operated outside of its expected operating environment, for example at an elevated temperature or voltage.
While burn-in usually refers to a process applied to electrical and semiconductor components, break-in is more often associated with mechanical assemblies that have moving parts. Here the goal is usually to get surfaces that roll or slide on each other to work well together. In contrast to burn-in, after break-in, parts will be irreversibly changed from their brand-new state. This is usually achieved by running the assembly under a reduced load for a proscribed amount of time before subjecting it to normal service.
Burn-In and Break-In on the Othermill Pro
We perform both burn-in and break-in processes on various parts of the Othermill Pro. One of the most critical steps is the break-in procedure for the spindle (pictured above). The ball bearings that support the spindle, and the parts they mate with, all require micron-level tolerances to function properly. When they're assembled, the bearing races conform to the parts they're assembled with. As they're run for the first time, a small amount of wear occurs as they settle in, and the lubrication begins to be distributed inside the assembly. These processes generate heat, which could be enough to damage the assembly if we didn't start with a break-in period.
We designed a break-in program to start the spindles at a low speed and with no load, cycling them on and off at prescribed intervals. Heat builds up as the parts wear together, then the assembly is allowed to rest so the heat can soak through evenly. This cycle repeats, gradually increasing in speed and duration, over the course of a couple hours until the spindles are ready to turn at full speed indefinitely. After the break-in period, we take the opportunity to test the functionality of the spindle and verify that it meets our standards for runout and smoothness of operation.
To run the break-in program in production, we programmed an Arduino Nano (in the foreground of the image above) to send the necessary signals to a batch of eight spindle assemblies at once. And to control the spindles during break-in, we use the same electronic speed controller that we use in the Othermill Pro. Instead of keeping a set of eight controllers as part of the break-in station, we take the opportunity to use new production controllers that will end up in Othermill Pros. This gives us a thorough burn-in test for those components as well, with little additional setup time.Running burn-in and break-in on certain components can be among the most critical steps of an assembly. Burn-in runs a part through its paces, making sure it’s functioning properly, while break-in goes one step further to physically alter the parts, preparing them for normal operation. In assembling the Othermill Pro, these processes allow us to fix problems early on in the assembly process before the machine is built — and long before it's in the hands of a customer, when failures are much more disruptive and costly to fix.