In this post, we’re going to outline what runout is and what you can do to avoid it.
Runout is one of the most significant sources of milling problems in the dental lab. If you have ever had abnormal tool wear or strange breakages you can’t explain, there’s a good chance you’re experiencing the effects of runout.
What is runout?
Basically, runout is the tendency for a tool to wobble as it spins. Any time that the tool spins around an axis that is not its center, it’s running out. It might not seem like that bad of a thing, but the forces that are in play when a tool is cutting need to be in precise balance. If they’re not, the results are anything but ideal.
How does it affect milling?
Efficient milling processes are all about balance. When a tool is running out, its performance will degrade drastically. The main reason is that runout creates uneven cutting loads on the tool. As a tool runs out, the chip load (amount of material the tool cuts) on the tool is constantly changing. This causes uneven cutting stresses and wears the tool unevenly. Excessive runout will cause more tool wear, more chipping of margins, and more trouble overall.
What are the common causes of runout?
In an ideal world, the tool would be in perfect alignment with the spindle axis at all times. However, this rarely happens in real life. There are many variables involved in how precisely a tool spins. Here are a few things you can check to help counteract the effects of runout:
- Collet health: The collet is responsible for centering the tool to the spindle. If it is worn or dirty it will easily cause runout. This alone is probably the largest contributor to runout related issues in the dental lab. All too often, the collet is ignored in the lab. It’s important to know that the collet is a wear item and does need to be serviced and replaced periodically. To avoid collet issues, make sure to follow the guidelines that came with your machine concerning collet maintenance.
- Tool quality: There are some low-quality tools on the market that aren’t made to the exacting standards that higher-end tools are. This results in a tool that can run out on its own. The ability to hold tight machining tolerances is the most important part of creating a quality tool. Just be aware: cheaper tools might have runout built right in.
- Spindle health: Unfortunately, spindles wear out. The bearings will wear and the spindle itself will start to runout. If the spindle is running out, so is the tool. Anyone who has run a milling machine for a significant amount of time will tell you that they’ve had to replace a spindle. It’s a huge headache, but if you’ve got a worn one you have to replace it.
- Milling strategy: Believe it or not, the strategy can cause runout too. The milling strategy tells the machine how to run the tool. It programs spindle speed, feed rate, depth of cut and other aspects of the milling process. If the strategy is not designed optimally, it can cause the tool to deflect (runout), and otherwise perform badly. In order to avoid this one, make sure you always have the most up to date milling strategy from your supplier.
How do you know if runout is your problem?
If you’re having milling issues there’s always a long list of potential causes. The best advice here is to work closely with your machine supplier and have them help you make the diagnosis. If you do have a runout problem, it’s likely you’ll go through several troubleshooting steps to get to it. This is because when troubleshooting a machine, the low hanging fruit is picked first. They’ll usually go through your software, calibrations, etc before attempting to diagnose a runout issue. Be aware that replacing tools or updating software may mask a runout issue short term, only for the problem to come back in short order.
If a runout condition is bad enough, it will show up in the tool wear pattern. Look at the tool under the microscope. If it has more wear on one side than the other, it’s very likely that it has been running out.
The sure way to see if you have a problem with runout is to measure it. Some machines have indirect ways of measuring runout, but the most reliable way is to use a dial indicator and measure it right at the tool. Usually, this is something that a service tech will do on a field call. Every machine has a runout tolerance that is considered acceptable. If it’s outside the acceptable limits (ask your machine tech what these are) it will be time to replace the collet and/or the spindle on the machine in question.
Regardless of what causes it, runout will erode your production efficiency. It will either creep up on you as your equipment wears or rear its ugly head suddenly. It’s best to be vigilant. Hopefully, you have gained a bit of knowledge about the characteristics of runout and increased your ability to troubleshoot on your own.