For quality control of CNC machined parts, most machine shops use a Coordinate Measuring Machine. A CMM checks that the geometry and dimensions of a finished part match the original design, ensuring that the customer does not receive defective parts. It identifies even the most marginal errors and allows the machinist to rework a faulty part.

For a machine shop owner, however, the CMM is far from a perfect solution. While this kind of inspection provides security for the customer, it does not guarantee a high level productivity for the machinist.

Why?

Because identifying errors after machining means you’re already too late to recover costs. When a part has to be reworked because the CMM machine returns a negative result, the value of that part diminishes by half. Twice the expected time is going into it, as well as twice the material.

An alternative solution is on-machine inspection with a machine tool probing system. Machine tool probing systems are measuring devices fitted to the CNC machine itself, used to set up parts and measure dimensions during and immediately after the machining process.

This article explains how we use on-machine inspection in our CNC shop, and why CNC machine shops should follow suit.

1. Ease CMM bottleneck

Restricting quality control to a CMM has significant drawbacks and problems can escalate beyond simply doubling the labor time and material costs. In a busy shop, any given CNC machine will likely be assigned a new setup as soon as one set of parts are removed from the worktable.

If the CMM rejects some parts later on, the machinist will have to wait for a machine to become available — or worse, interrupt another job — then set up the machine all over again. Rejected workpieces can slow down production, producing a domino effect that delays many jobs.

Also, dependence on a CMM may result in long queues, as every job in the shop requires inspection from the same machine or group of machines. 

Incorporating inspection into the CNC machines themselves can ease the CMM bottleneck, because the inspection burden can be spread across multiple systems or, in some cases, handled wholly by the CNC machine.

2. Improve efficiency

Using a CMM is usually necessary for quality control, especially during precision machining, because it checks that workpieces meet geometric and dimensional specifications. But on-machine inspection does not have to replace the CMM; instead, it can work alone or in tandem with the CMM to make inspection — and the entire product cycle — more efficient.

A machine tool probing system, mounted on the machine spindle or turret, provides numerous benefits to a shop. The system can be used to identify and set up workpieces — a process that can take 10 minutes manually, but only seconds with a probe — and can measure features during the machining cycle and immediately after, when workpieces are still on the worktable.

Set up the workpieces fast

A probing system can also carry out tasks like monitoring the surface condition of the workpiece and initiating automatic offset correction. Additionally, many probing routines do not disrupt the machining process, so inspection can be carried out without disruption.

Probes are useful because they catch significant errors early, which immediately reduces wastage and, if a CMM is being used afterwards, reduces the chances of the CMM rejecting a part. If a CMM is being used, probes also allow quality control staff to reduce their sampling rate.

For example, they can inspect one in 50 parts on the CMM instead of one in 20, knowing that most possible errors would have been caught and corrected by the on-machine probe.

Introducing on-machine inspection can therefore reduce total inspection time while easing the CMM bottleneck, allowing parts to be shipped faster and increasing shop productivity.

In-Process Inspection for Machined Parts

3. Increase accuracy for post-machining

On-machine inspection can be beneficial in virtually all CNC machining situations, dramatically reducing the chances of having to scrap failed parts.

However, probe systems are especially valuable when precision machining features on cast or forged parts.

Machining parts from a workpiece of extruded metal is one thing, but machining on forged or cast parts is quite another.

Those metalworking processes tend to produce minor variations between units, so it is crucial that the CNC machine can calibrate properly with each slightly different part. An on-machine probe system helps the machine do just that.

The on-machine inspection system ensures accuracy when machining as a secondary process, even when there are geometric discrepancies between parts. It is especially useful for parts made with pressure die casting and investment casting.

4. Increase accuracy for molds

On-machine inspection is a particularly valuable asset in the production of molds for injection molding and other molding processes. Probes increase accuracy when machining cores and cavities, leading to better moldings and a lowered reliance on post-processing of plastic parts, where imperfections caused by low-quality molds must be fixed.

Mold making can benefit from an on-machine probe in several ways — before, during and after the CNC machining of mold components. Before machining, a probe can locate the workpiece, in addition to determining part orientation and rotation.

During machining, it can automatically realign the cutting tool and update offsets in reaction to changes in temperature. And 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 increases the accuracy of mold production and consequently the quality of moldings.

Use of on-machine inspection can therefore improve the quality, repeatability and accuracy of machined molds, while helping manage and combat variations caused by temperature change that can lead to features outside the desired tolerance.

In-Process Inspection for Mold Cavity

Improved precision machining

To summarize, on-machine inspection systems for CNC machines can be highly valuable for machine shops, as they provide the following benefits:

• Capture errors early
• Immediate feedback
• Automatic offset correction
• Reduce inspection backlog
• Ease CMM bottleneck
• Reduce CMM sampling rate
• Reduce total inspection time
• Reduce scrap
• Increase productivity
• Increase machining accuracy on cast and forged parts
• Increase machining accuracy on mold cavities

3ERP, a specialist in rapid prototyping and low volume manufacturing services, uses Renishaw machine tool probes for precision machining. Since adopting this strategy, productivity has increased, and the company has been able to ship parts faster.

Although the CMM remains a critical element in 3ERP’s quality control procedures, on-machine probes have eased the burden on the CMM while virtually matching the standalone machine in terms of inspection capabilities. Crucially, the move has benefitted both machine shop and customer.

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