Checkpoints & Defects for Inspections of Mechanical Parts

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, endurance
• Safety & regulatory

What about precision mechanical parts, be they in metal, plastic, resin, or another polymers?

When it comes to classifying defects for mechanical parts, we generally use the same 4 categories but of course with a few adjustments.

1. Appearance

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 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 specification (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 sample products approved 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 (by example the manufacturer has to show a test report corresponding to the batch of material that was used for the order being inspected) – it 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

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 for 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 caught between:
  • 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 – Integrity of some parts must be checked without destruction (e.g. a die cast fan blade used for air movement in traffic tunnels)

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. It is logically organized. It is at the basis of the “magic triangle” of specifications, sample reviews, and inspections.