Machining requirements
Plastic machining tolerances depend on the part and its function.
An achievable tolerance is specific to the material, feature, geometry and agreed inspection conditions. A material name or machine specification cannot establish it on its own. The useful starting point is the fit, alignment or other relationship the finished component must retain.
A correct diameter does not establish a correct fit.
In rotating parts, diameter, roundness, cylindricity and runout describe different characteristics. In a milled part, a hole's relationship to a mounting face may matter more than a nonfunctional outside edge.
Datums provide the references used to communicate those relationships. The surfaces that locate the part in the assembly help explain which references matter. A drawing should distinguish critical functional requirements from other dimensions; applying an unnecessarily tight requirement to every feature can obscure that purpose and affect the machining request.
A feature can have the right size and the wrong relationship
Individual feature size
A bore diameter describes the feature’s size. Its shape and the mating clearance can also matter to function.
Position and datum relationships
The bore’s location and orientation relate to the surfaces that establish the part’s position. In rotating work, form and runout can matter separately from diameter.
The material can move before and after machining.
A dimension describes the part in a particular state. Material behavior, exposure and time can change that state without changing the original drawing.
- Elastic response
- A flexible feature may deflect under support, clamping or measurement force and recover when the force changes. Its free condition and its restrained condition can produce different observations.
- Temperature
- Stock, machining, inspection, assembly and service may occur at different temperatures. Thermal movement matters to the finished relationships, especially across a long feature or between unlike mating materials.
- Moisture
- Moisture uptake can alter dimensions and properties, particularly in nylon. A dry inspection condition may not represent a part exposed to humidity, washdown or immersion.
- Stock stress and material removal
- Residual stress in stock can contribute to distortion as material is removed. The stock form, section and removal pattern influence the part being evaluated; a material family alone does not establish its condition.
- Load over time
- Creep can change a seat, retained fit or loaded span during service. Inspection soon after manufacture cannot by itself establish long-term dimensional retention under sustained loading.
- Grade, fillers and orientation
- Filled or modified grades can behave differently from standard grades, including directional effects. Exact grade and stock information are more useful than a generic resin name when evaluating a critical relationship.
Geometry affects how the part responds to machining.
Two parts in the same material can present different dimensional challenges. The amount and distribution of material around a feature influence its support and response as stock is removed.
- Thin or asymmetric walls
- Reduced stiffness and unequal sections can make a feature more sensitive to force, heat or distortion.
- Deep pockets and holes
- Depth, access and the remaining wall section influence how the feature can be machined and supported.
- Long spans
- A long rail, shaft or strip introduces support and movement questions that a compact part may not share.
- Abrupt section changes
- Thin-to-thick transitions and sharp internal details can concentrate stress and affect local behavior.
- One-sided removal
- Removing much more material from one side can change the balance of the remaining section and its distortion tendency.
- Closely spaced features
- Nearby holes, slots or interrupted surfaces can leave less supporting material between functional details.
Milled features
Pockets, walls, mounting faces and hole locations must retain the relationships required by the assembly. See CNC plastic milling services for the supported process and the geometry relevant to a milling request.
Turned features
Diameters, bores, lengths and rotating relationships may require separate consideration of size, form and runout. See CNC plastic turning services for turned and turned-and-milled geometry.
Annealing, conditioning or stress relief can itself change dimensions. If your specification requires a treatment, identify it in the machining request so its availability and the finished-part requirements can be confirmed with the quote.
Connect inspection conditions to the installed function.
The drawing should state the conditions that apply when accepting the individual part: temperature, moisture condition, support and the relevant dimensional references.
Meeting a part drawing does not establish how the completed assembly performs in service. Equipment-level requirements and validation belong to the customer's engineering team; CDS reviews the specified finished-part and inspection requirements for machining.
A worn sample can help explain mating geometry, but it cannot establish original datums, unworn dimensions or the intended tolerance. The drawing and available equipment records remain important.
Inspection and service may place the part in different states
Free or supported at inspection
The accepted dimension applies to an agreed support and material condition. Measurement force can matter for a flexible feature.
Installed or loaded in service
Fastening, mating parts, heat, moisture and sustained loading may change a functional relationship after inspection.
Provide the specified dimensions for a machining quote.
CDS reviews the customer drawing, material and inspection requirements to confirm machining feasibility. The customer establishes required tolerances and acceptance conditions; CDS does not determine the component design.
- Controlled drawing: revision, feature sizes, tolerances, datums, feature locations and surface requirements, including thin walls or deep features.
- Specified material: exact grade, required brand or stock form, known condition and any permitted alternatives.
- Inspection and order: required temperature, moisture and support conditions, specified measurement methods or records, quantity and needed date.