Automatic Visual Inspection in Precision Machining
With CNC precision machining the last part to come off a machine should be identical to the first part. Despite their impressive repeatability though, inspection is still necessary. It verifies that the first part conforms to the print, and it finds defects if anything should go wrong during the machining process. Here’s how we go about protecting customers from receiving defective parts.
Committed to Quality
Certification to ISO/IATF 16949 shows that Impro operates an effective Quality Assurance (QA) system. This means formal, documented procedures are followed to eliminate variability and inconsistency and prevent mistakes. It also means Quality Control (QC) checks are performed as needed.
QC refers to the physical actions taken to verify parts being machined meet the specification. Many different activities come under the heading of QC, but one of the biggest is visual inspection.
Dimensional vs. Visual Inspection
Visual inspection refers to looking at a part to determine whether it has been machined as required. It’s different to using a coordinate measuring machine (CMM) to verify part features are the right dimensions and in the correct relationship to one another.
CMM inspection is the “gold standard” of dimensional inspection. It’s typically used to confirm that a CNC part program is correct, but often takes too long for use in series production. As a result, if something goes wrong while machining a batch of parts – tool breakage is the highest risk – it won’t be detected.
This is addressed by visual inspection. Historically, this job has been performed by people, either the machinists themselves or dedicated inspectors. They look for missed features or operations, like holes that weren’t put in because a drill broke. They will also pick up imperfections on machined surfaces caused by a chipped cutting insert and damage resulting from incorrect workholding.
Some parts are extremely difficult to inspect by eye though. They may have large numbers of features, multiple surfaces, or just be extremely small, so there’s a risk of defective parts slipping through. To prevent this, progressive machine shops are turning to automated visual inspection.
Automated Visual Inspection Technologies
One of the earliest visual inspection tools was the optical comparator. This is a projector system that throws a magnified image of small part features onto a screen. The person doing the inspection would use a template to see how the feature conformed to what was required. Comparators are still used today, primarily for checking features amenable to being backlit like chamfers, radii and screw threads.
The comparator can be automated by adding a camera and image analysis software. This may be used to determine presence or absence of small features and if calibrated, for measuring them too.
Advances on the comparator use also camera technologies, combined with scanning and sophisticated lighting systems. These can gather data for both presence/absence and dimensional checks. Some are capable of resolving and measuring features too small for the human eye to see.
For precision machine shops, multi-sensor inspection systems are the future. These combine mechanisms for holding and moving complex parts with cameras, various lighting technologies and software that determine if the part under inspection is good or bad. They are fast and reliable. Some even have probing technologies comparable to those of CMMs.
When a precision machine shop takes quality seriously they operate an accredited QA system and have QC checks in place to verify parts meet print requirements. These checks comprise both dimensional and visual inspections. Visual inspections may still be performed manually, but forward-thinking shops are adopting automated visual inspection technologies, up to and including advanced multi-sensor systems.
If you’re interested in learning how Impro verifies conformance of machined metal parts to the drawing, we’d like to talk. Contact us to start that discussion.