Common CNC Machine Failures

What Is a CNC Machine?	What Are Some Things That Can Cause Issues in CNC Machines?
How Does CNC Machining Work?	CNC Troubleshooting Tips

Have your CNC machines been behaving oddly lately? Do you notice a strange tick in their output, or in the way the machines are acting?

If so, you’re in the right place. We’re going to talk about a few of the most common problems in CNC machines, and how to fix these problems.

Before we get started, however, let’s make sure everyone is on the same page and understands the basics of what a CNC machine is and what it’s for.

What Is a CNC Machine?

A CNC machine is an electro-mechanical device used to manipulate and control machine-shop tools and devices using computer programming. In other words, controllers program the CNC machine to tell the machine shop tools what to do. The name CNC stands for computer numerical control.

The earliest CNC machines — or perhaps the forerunners of the CNC machine — were developed and used in the 1940s and ‘50s. These machines used a data storage technique known as punched tape. Of course, this technique soon became obsolete, and the systems quickly turned to analog and then digital computer processing methods.

How Does CNC Machining Work?

The basic function of CNC machining is to take a blank piece of material, such as a block of plastic or a similar item, and transform it into a finished product. The CNC machine does this by telling the machine shop tool exactly how to move, and exactly what pieces of the material to cut away, to eventually arrive at the desired result.

It has many similarities with 3D printing, in that a computer supplies digital instructions to tools that then work to create a finished product. Because the entire creation process is distilled into a set of precisely coded directions, the process is much faster, more efficient and less error-prone than if it were being done by hand.

Several of the technologies commonly used to actually create the finished products include drills, lathes, milling machines and other newer technologies like laser cutting machines, plasma cutting machines, water-jet cutting material, electron beam machining tools and many more. Materials that are commonly crafted with CNC machining include aluminum, steel, copper, titanium, wood, fiberglass, foam and plastic.

What Are Some Things That Can Cause Issues in CNC Machines?

No matter how fast and efficient CNC machines is, they’re not infallible. They develop problems and need maintenance just like any other type of machine or tool. And like any other tool, there are certain things you can do that might inadvertently damage these machines. There are also definite steps you can take to keep them in good working order.

While there are certainly any number of problems that can lead to issues and errors within CNC machining, some problems are more commonplace than others. These problems are also ones that are easy to overlook and easy to misdiagnose.

Let’s look at a few of the most common problems in CNC machines.

Poor or Improper Maintenance

CNC machines need to be carefully cleaned and lubricated on a regular basis — otherwise, problems can begin to arise. A lack of cleaning could lead to a buildup of dirt and debris. This might seem to be purely a hygiene issue, but it can have real implications in the machine’s output.

For example, if you begin to notice the material sliding around during the cutting process, a buildup of dirt could be preventing the machine from working correctly and providing the most accurate instructions. If this issue continues to go unchecked, it will create challenges as the machine struggles to locate and hold materials still. This leads to greater issues with accuracy and precision, and ultimately leads to more errors.

Improper lubrication can lead to machine parts sticking or not moving as smoothly as they should. This can lead to errors, as well as overheating and air blockage. Both overheating and a lack of air can also cause problems like material movement during the process.

Improper Settings or Tools

Perhaps one of your tools is becoming blunt, a cutting coolant or lubricant is failing to work correctly or a tool is moving at the wrong speed. All these issues will cause similar sorts of problems. The most likely result of one or more of these issues is the appearance of small burn marks around the edges and corners of the material.

The reason for this is simple. If the tool is moving too slowly, the material will be under the cutting edge for longer than it should be. This results in the burning and scarring you’re seeing. Similarly, if the coolant is not working correctly, things could be getting too hot, and the edges of the material might be getting singed as a result.

If one of your tools itself is burned, you’ll need to adjust the settings as needed for that particular tool so it isn’t scarring the material. Or, if the tool is blunt and not cutting correctly anymore, it could just be time to replace it altogether.

Aside from burned edges, some other visual signs of these types of problems to keep an eye out for are rough edges, visible cutting marks and raised edges. All of these are indicators that something is not working exactly as it should be in your machine.

Poor or Improper Programming

Another issue that could lead to serious problems in your CNC machining is incorrect programming. This is a simple cause-and-effect problem, since the programming directly controls the creation of the product. Therefore, if the programming is incorrect, the product will have problems.

These problems can be difficult to detect, especially when there are new or inexperienced employees involved. These employees may not have a completely accurate understanding of the machine’s process and may enter the coding incorrectly. They may not realize their error, although they recognize something is wrong.

To fix the problem, inexperienced employees may try to power the machine off, then on again. This can jump-start the machine and cause it to correct the problem. Meanwhile, the original problem — the incorrect coding — remains undetected.

To avoid this scenario, make sure every employee receives full and proper training on how to correctly code a CNC machine. This way, these kinds of easy-to-avoid mistakes will be less likely to happen, and you’ll be more likely to have a smooth and issue-free CNC machining process.

CNC Troubleshooting Tips: The Most Common Problems and How to Fix Them

No matter how well you maintain your machines, how well you train your controllers or how carefully you care for your tools, problems will still pop up. Some will be easy to solve, and some will be confusing, leaving you wondering what could possibly be wrong.

Fortunately, we’ve put together a list of the most common CNC machine failures, as well as their probable causes and the solutions to fix them.

What Do I Do When Something Goes Wrong During the Automatic Tool Change Process?

Solution: If you’re sure the problem is occurring at some point during the automatic tool changing process, the best way to troubleshoot the problem is to learn every step of the tool changing sequence. Once you are intimately familiar with the process, you’ll be better able to spot where the problem is happening and diagnose what is going on.

For your reference, the sequence should go as follows:

1. Axes move into position
2. Spindle aligns for the tool change
3. Doors open
4. Magazine pot gets into position
5. ATC arm moves to engage the tool on the spindle
6. Spindle releases tool
7. Arm removes tool from spindle
8. Arm puts tool into spindle
9. Spindle grasps tool
10. Arm returns to original position
11. Door closes

 How Do I Fix a Problem with the DC Motor?

Solution: If you’re experiencing problems with the DC motor, here are a few things to check and troubleshoot. Try these, and see if you don’t uncover the problem along the way.

• Remove the brushes and springs after removing the cover using a slotted screwdriver.
• Make sure the brushes are moving freely as you remove them.
• Make sure all the brushes have equal spring tension.
• Look carefully at the faces of the brushes — are they clean and shiny? If not, replace them.
• Look at how long the brushes are. When these brushes are new, they should be about 7.5 inches long. If they’re much shorter than this, there will be less spring tension.
• Shine a flashlight inside once the brushes are out of the way to see if there are any buildups. Clean the area out with an air cleaner and make sure to clean between all the grooves as well.
• Replace any grooves that are exceptionally worn.

What Should I Do If I Can’t Get the Emergency Stop to Clear?

Solution: Make your way through this checklist to see if any of these items might be the problem.

• Make sure the conveyor is plugged into the power source. Check the power cord for any signs of damage.
• Pull out or push in any emergency stops. Check for any problems with the conveyor, high-pressure unit, gantry and barfeeder. Make sure there are no blown fuses in the electrical cabinet.
• Check to see if there are any axes near the overtravels.
• Inspect the door interlock mechanism or any other interlocking modules inside the electrical cabinet. Check that the LED light is on and says POWER or something similar.
• Make sure the power supplies are active. Check to see that there isn’t an electrical shortage. Do this by looking at the LED lights and seeing if any are dimmed or burned out. If so, try removing the wires from it. If it changes after you do this, you have a short that will need to be fixed.
• Locate the E-string and follow it. Check to see if there is a problem with the voltage.

How Do I Fix It When the Machine Won’t Power Up, or Acts Strange as It Powers Up?

Solution: If no buttons are lighting up on the control panel, look behind the screen or panel for the power supply. Check to make sure the power supply is coming and going correctly. It’s possible a fuse back there is blown.

If the CRT doesn’t seem to be coming up, try moving the axes as you would if you were following the on-screen prompts. If they still move perfectly, the problem is with the screen display, not the machine. Check that the power is still being supplied to the screen.

How Do I Fix a Problem with the Power Supply?

Solution: In most cases, machines have multiple power sources. If you aren’t sure which one is causing the problem, try checking these locations:

• Behind the CRT screen
• Door interlock circuit
• I/O card
• Individual power supplies for the drives and spindle
• The external power supply

If you think there’s a problem with the DC power supplies, check for the power input and be sure the voltage is correct. Check the power output as well. If there seems to be no power, or if the voltage is too low, turn the power off and disconnect the output wires. Turn the power back on and check the output side. If there’s power and it’s the correct voltage, you were experiencing a short to the ground.

Almost all power supplies will be equipped with an LED indicator light. But don’t always take this light at its word. Check the power levels for yourself with a meter to be sure. If there is indeed a short to the ground, a fuse might blow or it the short will pull the voltage levels down.

How Do I Repair the Relay Board, and How Can I Tell If It Needs Repairing?

Solution: Although relay boards come in a variety of sizes and designs, they’re a common component of CNC machines. They all function in almost exactly the same way, so they’re easy to work with once you understand how they work.

Each relay board takes an I/O signal of 24VDC or lower and converts it into a signal that’s usually either 24VDC or 110VAC. This is important to do so the machine can handle higher loads and amperages.

Almost all relay boards are equipped with LED lights that turn on when the machine needs an output. When you see this light blink on, it means the output has reached the board and must be applied to the relay on the board. If the circuit is not fused correctly, this is where some problems could occur.

If you notice one of the traces on a relay board has been burned up, there is an easy fix. In most cases, you’ll simply need to replace it. Unsolder the burned trace and solder a new fresh one in its place.

The Machine Chuck Seems to Be Stuck and Won’t Clamp or Unclamp. What Do I Do?

Solution: Run through this checklist and see if any of these things might be causing the problem.

• Check to see if the hydraulic pump is working. If it isn’t, try powering the machine off and then on again. With the power back on, test the pump and see if it works this time.

 

• Look at the adjustment valve and check the hydraulic power. If it is nonexistent or not high enough, try setting the power higher.

 

• Inspect the foot pedal to see if it’s working. Look for the input signal for the switch on the screen. If there seems to be a problem with the input on the diagnostic screen, look more closely at the cable connections and contacts of the foot pedal. Check to see if any wires are broken, or terminals are corroded. Make sure you can press the pedal all the way down, and it isn’t being blocked by a chip or any debris.

 

• If the input doesn’t seem to be the problem, look more closely at the PLC output. If the output is not being created after the foot pedal is pressed, try to determine what is needed to turn on the output coil by following the ladder.

 

• Can you get the chuck to operate by using MDI M-code commands, but not with the foot pedal? This means the problem is almost definitely with the foot switch.

• Does the solenoid output seem to be working correctly? If so, your problem could be at the connector to the solenoid, where a wire might be broken or the connector could be loose. Try switching the connector with the coil and see if the coil works correctly.

 

• Lastly, look at the draw tube between the chuck and the hydraulic cylinder. Is it working? Check the tube by unscrewing it from either the chuck or the cylinder and removing it. While you’re doing this, it’s also a good idea to examine the body of the chuck. The internal wedge, in particular, ought to be greased regularly. If it’s dirty or hasn’t been greased, the chuck won’t be able to operate correctly.

 

 What Things Should I Check for in the Safety Circuit?

 

Solution: Follow this simple procedure to ensure all the necessary parts receive the check they need.

 

1. Ensure all the keys to the door interlocks are in rest position.
2. Look in the control cabinet to check the interlock modules. The LED lights in here will help you locate any potential problems.
3. Make sure the interlock release keys are in the proper position, and make sure you can remove them if necessary, as sometimes the vibrations can cause the keys to turn unnecessarily.
4. Tighten any connections that need tightening on the switches and interlock modules.
5. Look for the biggest connections in the modules and check these. Because of their size, they tend to experience a lot of vibration and are prone to becoming loose.

 

Put This List to Use

Hopefully, by following these instructions, you’ll be able to better understand your CNC machines, which will help you become better equipped to clean, maintain and care for them. It will be easier for you to prevent problems from occurring, and you’ll know how to recognize a problem when it does occur.

Perhaps most importantly, however, we hope you’ll be able to diagnose the problems that will inevitably occur, and better able to troubleshoot a solution to these problems. Even if you encounter a problem that requires you to call on an outside resource for repairs, you’ll still be one step ahead. You’ll most likely already know what the problem is, where it’s occurring and have some idea of how it can be fixed.

As time-saving as it is to be able to diagnose the problems in your machine and fix them yourselves, there will always be situations where the problems go beyond your capabilities to fix. When that happens, Global Electronic Services is ready to step in and help get your machine back to work. For more information about us, subscribe to our blog and follow us on social media. And if you have any questions, please don’t hesitate to contact us or call us at 877-249-1701.