To control the quality of CNC machined parts, most machine shops use CMMs. The CMM checks that the geometry and dimensions of the finished part match the original design, ensuring that the customer does not receive a defective part. It can recognize even the tiniest errors and allow the machinist to rework the defective part.
However, CMMs are far from the perfect solution for machine shop owners. While this type of inspection provides safety and security for customers, it doesn't guarantee a high level of productivity for machinists.
Why?
Because identifying errors after machining means it's too late to recoup your costs. When a part must be reworked because the CMM returns negative results, the value of that part is cut in half. It takes twice as long and twice as much material as expected.
Another solution is to use a machine tool probing system for on-machine inspection. A machine tool probing system is a measuring device mounted on the CNC machine itself that is used to set up parts and measure dimensions during and immediately after machining.
This article explains how we use on-machine inspection in CNC shops and why CNC machine shops should follow suit.
1. Alleviating the CMM bottleneck
Limiting quality control to the CMM has significant drawbacks, and problems can escalate beyond just doubling labor time and material costs. On a busy shop floor, any given CNC machine may be assigned a new setup once a set of parts is removed from the table.
If the CMM rejects some parts later, the machinist will have to wait for the machine to become available - or worse, interrupt another job - and then reset the machine. Rejected parts can slow down production, creating a domino effect that delays many jobs.
In addition, reliance on CMMs can lead to long lines, as every job on the shop floor needs to be inspected from the same machine or group of machines.
Incorporating inspection into the CNC machine itself can alleviate CMM bottlenecks, as the inspection burden can be spread across multiple systems, or in some cases handled entirely by the CNC machine.
2. Increased Efficiency
The use of CMMs is often necessary for quality control, especially in precision machining processes, because it allows checking that the part meets geometric and dimensional specifications. However, on-machine inspection does not necessarily require the replacement of the CMM; rather, it can work alone or in conjunction with the CMM, making the inspection, and the entire product cycle, more efficient.
A machine tool probing system mounted on a machine tool spindle or turret can bring many benefits to the shop floor. The system can be used to identify and set up workpieces (a manual process can take up to 10 minutes, but only a few seconds with a probe), and features can be measured immediately during the machining cycle, as well as while the workpiece is still on the table.
Probing systems can also perform tasks such as monitoring the condition of the workpiece surface and initiating automatic offset corrections. In addition, many probing programs do not interrupt the machining process, so inspections can be performed without interruption.
Probes are useful because they can detect major errors early, which immediately reduces waste and, if a CMM is used afterward, reduces the chance that the CMM will reject the part. Probes also allow quality control personnel to reduce sampling rates if a CMM is used.
For example, they can inspect one-fiftieth of the parts on the CMM instead of one-twentieth, knowing that most possible errors will be caught and corrected by the on-machine probes.
As a result, the introduction of on-machine inspection reduces total inspection time while alleviating CMM bottlenecks, resulting in faster part shipments and increased shop floor productivity.
3. Improve post-machining accuracy
On-machine inspection is suitable for virtually all CNC machining situations and can greatly reduce the chance of scrapping failed parts.
However, probe systems are particularly valuable when performing precision machining on cast or forged parts.
Machining a part from an extruded metal workpiece is one thing, but machining a forged or cast part is quite another.
These metal machining processes often produce small differences between cells, so it's vital that CNC machines are able to calibrate correctly for each slightly different part. A probe system on the machine helps the machine do this.
Even if there are geometric differences between parts, the on-machine probing system ensures accuracy during secondary machining. It is especially useful for parts made with pressure casting and investment casting.
4. Improve mold accuracy
On-machine inspection is a particularly valuable asset in the production of molds for injection molding and other molding processes. Probes can improve accuracy when processing cores and cavities, resulting in better molding and reduced reliance on post-processing of plastic parts, while defects caused by low-quality molds must be repaired.
Moldmaking can benefit from on-machine probes in a number of ways - before, during and after CNC machining of mold components. Prior to machining, the probe can position the part in addition to determining part orientation and rotation.
During machining, it automatically realigns the cutting tool and updates the offset based on temperature changes. Before the part leaves the CNC machine, the probe can perform mold-specific tasks, such as checking and verifying electrode condition. Each of these functions improves the accuracy of mold production and therefore the quality of the molded part.
As a result, the use of on-machine inspection can improve the quality, repeatability and accuracy of machined molds, while helping to manage and respond to variations caused by temperature changes that can cause features to fall outside of required tolerances.
Improved machining accuracy
In summary, on-machine inspection systems for CNC machines are valuable for machine shops because they offer the following benefits:
Early capture of errors
Instant feedback
Automatic offset correction
Reduce inspection backlogs
Relieve CMM bottlenecks
Reduce CMM Sample Rate
Reduce total inspection time
Reduce scrap
Increase productivity
Improve machining accuracy of cast and forged parts
Improved machining accuracy of mold cavities
