When inspecting general consumer goods (except food), most companies classify defects and checkpoints in these 4 categories:
- Aesthetics & smell
- Conformance to specs (including measurements)
- Function, usability, reliability
- Safety & regulatory
But how about precision mechanical parts inspections, made from metal, plastic, resin, or other polymers?
Classifying defects & checkpoints when inspecting mechanical parts
When it comes to classifying defects for mechanical parts inspections, we generally use the same 4 categories but, of course, with a few adjustments:
What are the most common appearance issues (counted as defects) to keep an eye out for?
- Surface defects – porosity, scratches, dents or holes, orange peel, paint runs, blisters, debris contamination
- Color inconsistency – color match between two parts, shades, spotting, uneven surface, surface finish thickness, color match against master sample or reference data (RAL or Pantone)
- Burns – black specs and marks on the surface of the part due to overheating during the production process
- Flow marks – wavy or streaked appearance on the product surface (molded and cast parts)
- Haze – cloudiness on clear transparent parts
- Sink marks – depression on the surface of the product (molded and cast parts)
- Knit or weld lines – this is a visible line created at the intersection of the melt fronts meeting and trying to merge together (molded and cast parts)
- Flash – excess material at the parting line of a molding or cast part
- Texture – variation across the same part and between different parts of the same batch
2. Conformance to Specifications
- Critical parameters – check all critical parameters comply with specifications (these can be dimensions or other specific attributes of the product, and are usually indicated clearly in drawings and other documents)
- Check defect rates recorded in production (if possible) – higher defect rates indicate that approved sample products may include defective parts
- Weight – for certain products (castings for example) the weight is an important and easy point to check
- Tensile strength (if needed) – to test the elongation and fracture point of the part (stretched by moving the grips apart at a constant rate while measuring the load)
- Impact strength (if needed) – it is measured by allowing a pendulum to strike a grooved machined test piece and measuring the energy absorbed in the break
- Material hardness (if needed) – the metal often needs to be heat-treated in order to increase the hardness, therefore testing should be carried out on finished parts
- Chemical composition (for example the manufacturer has to show a test report corresponding to the batch of material that was used for the order being inspected) – is usually one of the critical parameters
- Corrosion resistance (if needed) – environmental testing including salt spray testing will verify if the part is susceptible to corrosion or not
- Environmental tests (if needed) – including wet, dry, hot, cold, vibration, acceleration, IP rating, UV light, etc.
- Packaging & labelling – if the parts have to be shipped out, they should be packaged (i.e. protected) and labeled the right way
We created video playlists about plastic testing and steel testing on Sofeast’s YouTube channel that explain the tests mentioned above in more detail.
3. Form, Fit, & Function
- Form – Form is the physical characteristics of the product. It includes things like shape, weight, color, material, etc. For example, you might describe a screw that will be used in your product as ‘SCREW, PAN HEAD, M3 x 0.5, 2mm Long, 316 SS.’
- Fit – refers to the ability of the part to interconnect, mate with, join, or link to another part or an assembly. If a part requires “fit”, it usually refers to having tight tolerances in order to match up to other parts or assembly.
- Function – refers to the action or actions that a part is designed to perform. In our example, the screw is intended to hold other parts of the product together.
4. Safety & Regulatory
Different industries will require different safety features to be met and these will be part of the product specification. Some of these would include the general attributes listed below:
- Sharp edges – Ensure there are no sharp edges unless they are required
- Pinch point – A pinch point is a place where it’s possible for a body part to be caught between, such as:
- Moving machine parts
- Moving and stationary parts
- Moving parts and materials being processed
- Getting clothing caught or tangled in moving parts
- Weight – too heavy to lift safely by a single person
- X-Ray requirements – The integrity of some parts must be checked without destruction (e.g. a die-cast fan blade used for air movement in traffic tunnels)
As you can imagine, most of these defects could be particularly troublesome if found during mechanical parts inspections as they could lead to injury or product failure.
Process controls in order to avoid defective parts
A process that is out of control will count plenty of errors. Here are a few examples:
- A poorly set injection machine will produce plastic parts with ugly weld lines, burn marks, and/or other defects.
- In a CNC cutting machine, if the tool is worn out and not replaced, it may start to produce unacceptable parts at high speed. (More info on preventive maintenance here).
- In a drilling machine, a worker can place the part upside down if not careful. It happens 2% of the time without being noticed, and the hole is misplaced.
- In an assembly line, screws are used as fasteners, but the torque of the screwdriver is way too low. The screws may get loose very fast.
This way of categorizing requirements is not perfect. There is some overlap — for example, many Conformance attributes might be subjected to safety and regulatory requirements.
However, we have found this approach useful for writing a list of checkpoints and thinking of potential defects for mechanical parts inspections and, indeed, other products, too. It is logically organized. It is the basis of the “magic triangle” of specifications, sample reviews, and inspections.
Brad Pritts says
As always, very helpful perspective. I would like to add one additional, but very important aspect regarding the “safety and regulatory” section. This is true for the auto and electronics industries and I believe similar concepts are used in aerospace.
The areas identified in the article reflect “process safety” aspects – risks to downstream assembly people. But we also have PRODUCT related safety issues. For these, there may be no immediate way for an inspector to identify them; in other cases they may be obvious. For example, brakes are one of my specialties. In brakes any nonconformity that would prevent the part from correct function is a safety critical item. It’s a little trickier for general purpose parts. For example, if a bolt is used to attach the brake system to the chassis, that bolt may be a safety critical part for that application. But if the same bolt is used in a different situation it may have no safety impact.
The implication of product safety issues is that the consequence of a defect is a risk to human life. To be more direct, if we screw up we can kill people. We must therefore put high priority on those particular parts. Many times in the auto industry we’ll insist on special automated QA – either automated manufacturing processing to do the actual operations, or automated 100% inspection/test.
And, for those producers either in the US, or selling to US customers, there are legal requirements that demand that we keep specific records of these inspections and controls.
So, if you are responsible for QA in these situations, it’s important to have guidance from the end customer on the safety aspects of the product, and develop the inspection plan accordingly.
Renaud Anjoran says
Brad, this is quite interesting. Many thanks for adding this to the article.