How to Write a CNC Part Specification
A well-written specification sheet is the single most important document in the CNC quoting process. It eliminates ambiguity, reduces quote turnaround time, and prevents costly mistakes during manufacturing. Whether you are a procurement manager sourcing a new vendor or an engineer handing off a design, the spec sheet bridges the gap between what you need and what the shop produces.
Start with the Material
Material selection drives cost, lead time, and machinability. Always specify the exact alloy, not just the category. "Aluminum" is not a specification — "6061-T6 aluminum" is. If your application requires specific material properties like corrosion resistance, conductivity, or hardness, call those out. Include heat treatment requirements if the part needs to be hardened, annealed, or age-hardened after machining. Shops stock common alloys like 6061, 7075, 303 stainless, and 4140 steel. Exotic materials like titanium or Inconel will have longer lead times and higher costs.
Define Tolerances Clearly
Tolerances tell the shop how precisely each feature must be machined. Standard CNC tolerances of ±0.005″ cover most non-critical dimensions. Only call out tighter tolerances (±0.001″ or ±0.0005″) where functionally required — on bearing bores, mating surfaces, and seal grooves. Over-tolerancing the entire part is the most common and most expensive mistake buyers make. Use GD&T (Geometric Dimensioning and Tolerancing) callouts on your drawing for position, flatness, perpendicularity, and runout where they matter.
Specify Surface Finish and Coatings
Surface finish affects both function and appearance. A standard as-machined finish of 125 Ra works for most structural parts. Sealing surfaces and O-ring grooves typically need 32 Ra or better. Cosmetic parts may need bead blasting, anodizing, or powder coating. Always specify the finish requirement per surface if they differ — for example, "125 Ra general, 32 Ra on sealing faces." This lets the shop optimize their process rather than finishing the entire part to the tightest callout.
Include Thread and Feature Callouts
If your part has threaded holes, specify the thread size, class of fit, depth, and whether the hole is blind or through. For example: "4x 1/4-20 UNC 2B thru" or "2x M6x1.0 6H blind, 12mm min full thread depth." Call out countersinks, counterbores, and chamfers with their dimensions. If you need threaded inserts (Helicoils), note that on the spec sheet so the shop can drill and tap to the correct insert size rather than the final thread size.
Do Not Forget Inspection and Certifications
Your inspection requirements affect both price and lead time. Standard visual inspection is included with every order. Dimensional inspection reports, CMM reports, and First Article Inspection (FAI per AS9102) add cost but provide documented proof that parts meet spec. For aerospace and defense work, require AS9100 certification and ITAR compliance. For medical devices, require ISO 13485. Always request a Certificate of Conformance (COC) and material certifications (mill certs) for critical applications. These documents become part of your quality record and are essential for traceability.