The Complete Guide to RFQ for Custom CNC Parts: What to Include and Avoid
Introduction: Why Your RFQ Matters
Every precision CNC machined part begins not with a cutting tool, but with a document — the Request for Quotation (RFQ). For procurement managers, engineers, and product developers sourcing custom CNC parts, the RFQ is the single most important communication tool in the entire manufacturing process. A well-prepared RFQ can reduce quote turnaround time by 40–60%, lower machining costs by 20–40%, and eliminate weeks of back-and-forth clarification emails.
Yet many buyers submit incomplete or ambiguous RFQs, forcing suppliers to make assumptions, add risk buffers, and deliver pricing that reflects uncertainty rather than actual manufacturing cost. Studies estimate that shops add 15–25% to quotes when key specifications are missing. This guide covers everything you need to include in your CNC machining RFQ — and the costly mistakes to avoid.
What Is an RFQ for Custom CNC Parts?
A Request for Quotation (RFQ) is a formal document that communicates your manufacturing requirements to a CNC machining supplier. Unlike a simple price inquiry, a complete RFQ functions as a technical communication package that enables the shop to accurately estimate machining time, plan tooling, evaluate manufacturability, and assess quality requirements.
An RFQ is not a generic procurement form — it is a technical document that tells the shop exactly what to program, fixture, machine, inspect, and finish. Missing any of these elements forces assumptions, and assumptions always cost you money.
The core components of any CNC machining RFQ include: a 3D CAD model (STEP format preferred), a 2D engineering drawing with GD&T per ASME Y14.5-2018, complete material specifications, tolerance requirements, surface finish specifications, quantity tiers, inspection expectations, and delivery logistics. When all these elements are present, shops can quote accurately on the first round.
Key Benefits of a Complete RFQ Package
Faster Quotes
Complete RFQs can be quoted within 24–48 hours, compared to 3–5 days for incomplete packages that require clarification.
Lower Costs
Complete specifications eliminate risk buffers, reducing quoted prices by 15–25% compared to vague requests.
Apples-to-Apples Comparison
Consistent RFQ packages allow you to compare quotes across suppliers on a like-for-like basis.
Fewer Errors
Clear specifications reduce the risk of manufacturing errors, rework, and costly production delays.
1. 3D CAD Model — The Universal Starting Point
Every CNC machining RFQ must include a 3D CAD model. The universal standard accepted by virtually all CNC shops is the STEP format (.stp or .step). STEP files open reliably across every major CAM system and avoid version-compatibility issues that arise with native CAD formats. If you are using SolidWorks, Inventor, or Creo, export a STEP file alongside your native format.
The 3D model is used by the shop's engineering team to analyze geometry, estimate stock size, identify machining features, plan setups, and generate toolpaths for CAM programming. Without a clean solid model, accurate quoting is impossible.
Pro Tip: Before sending your file, confirm it represents the latest approved revision. Include the part number and revision in the file name so the quotation, drawing, and production order all reference the same design.
2. 2D Engineering Drawing — Defining Manufacturing Intent
A 3D model shows geometry, but only a 2D drawing communicates manufacturing intent. The 2D drawing is the contractual document that governs production — if the 3D model and drawing conflict, the drawing wins. Every complete RFQ must include a fully dimensioned PDF drawing with GD&T per ASME Y14.5-2018.
Your 2D drawing must include:
- Title block: Part number, revision, material, surface finish, drawing scale, and standards used for tolerancing
- Critical dimensions with tolerances: Clearly marked on bearing bores, locating holes, threaded holes, mating faces, sealing surfaces, and press-fit areas
- Datum references: Identified for GD&T callouts
- Thread specifications: Size, pitch, class, and depth for all threaded features
- Edge break and deburr requirements: Specify "break all edges 0.1–0.3 mm" rather than "no burrs allowed"
- Surface finish notes: Ra values for critical surfaces and coating specifications
3. Material Specification — Never Leave It to Interpretation
Material choice directly affects machining cost, strength, weight, surface finish, corrosion resistance, and lead time. Writing only "aluminum" or "stainless steel" forces the shop to guess — and they will quote based on the lowest-cost option. Always specify the exact grade, temper, and governing specification.
| Material | Correct Specification | Typical Applications |
|---|---|---|
| Aluminum | 6061-T6 per AMS-QQ-A-225/8 | General industrial, automotive, aerospace |
| Stainless Steel | 304 per ASTM A276 | Food processing, medical, architectural |
| Stainless Steel | 316L per ASTM A240 | Marine, chemical, pharmaceutical |
| Titanium | Ti-6Al-4V per AMS 4928 | Aerospace, medical implants, racing |
| Steel | 4140 per ASTM A29 | Gears, shafts, structural components |
| Brass | C36000 per ASTM B16 | Fittings, valves, electrical components |
If you are unsure which material is best, explain the application and operating environment in your RFQ. A good supplier will recommend the optimal material based on the part's functional requirements.
4. Surface Finish and Post-Processing Requirements
Surface finish is one of the most misunderstood aspects of a CNC machining RFQ. Unless you specify the finish requirement, the supplier will assume an as-machined finish with standard roughness (typically Ra 3.2 µm / 125 µin). If your application requires finer finishes, coatings, or secondary processes, state them explicitly in the RFQ.
Common surface roughness (Ra) values:
- Ra 3.2 µm (125 µin): Standard as-machined finish — acceptable for most non-critical surfaces
- Ra 1.6 µm (63 µin): Fine machined finish — suitable for sliding surfaces and visible components
- Ra 0.8 µm (32 µin): Precision finish — required for sealing surfaces and bearing bores
- Ra 0.4 µm (16 µin): Mirror/ground finish — requires grinding or lapping, used for optical mounting surfaces
Coating specifications should follow industry standards:
- Anodizing: "Anodize Type II, Class 2 (dyed black) per MIL-A-8625" — not just "black anodize"
- Passivation: "Passivate per ASTM A967" for stainless steel
- Powder coating: Specify RAL or Pantone color number and required thickness
- Plating: "Electroless nickel per ASTM B733, 0.0005 in minimum thickness"
Also identify areas that must be masked — threaded holes, mating surfaces, grounding points, and bearing seats should remain uncoated.
5. Tolerances — Apply Tight Only Where Needed
Standard CNC machining tolerance is ±0.13 mm (±0.005 in) for machined features. Tighter tolerances require additional fixturing, slower feed rates, specialized tooling, and more extensive inspection — all of which increase cost. The key to an efficient RFQ is distinguishing between critical and non-critical tolerances.
Common mistake: Applying a tight general tolerance to the entire part instead of marking critical dimensions individually. This can double or triple machining cost without any functional benefit.
When to specify tight tolerances:
- Bearing bores: ±0.013 mm (±0.0005 in)
- Mating faces and locating holes: ±0.025 mm (±0.001 in)
- Sealing surfaces and press-fit areas: ±0.05 mm (±0.002 in)
- General non-critical dimensions: ±0.13 mm (±0.005 in) — standard tolerance
On your 2D drawing, mark critical dimensions individually with their specific tolerance. Provide a general tolerance note for all other dimensions (e.g., "Unless otherwise specified, all dimensions ±0.13 mm").
6. Quantity, Lead Time, and Inspection Requirements
Quantity: Always provide both the immediate order quantity and anticipated annual volume. Quantity directly affects pricing — setup, programming, and tooling costs are distributed across the total run. Request tiered pricing (e.g., 1 / 10 / 50 / 100 pieces) to understand the cost-volume relationship and find your optimal order quantity.
Lead time: Specify a clear deadline rather than "ASAP." Standard lead times for custom CNC parts are typically 10–15 business days for simple parts and 15–25 business days for complex multi-setup parts. Expedited services are usually available at a 30–50% premium.
Inspection: Tell the supplier upfront if you require inspection reports. Common documentation includes:
- Certificate of Conformance (CoC) — standard at no additional cost
- First Article Inspection (FAI) — typically $200–500 per part number, essential for first production runs
- CMM inspection report — $50–150 per part for critical feature verification
- Material Test Report (MTR) — $50–150 per batch, required for medical and aerospace applications
Common RFQ Mistakes That Increase Cost and Delay
| Mistake | Impact | Solution |
|---|---|---|
| Missing material specification | Shop assumes cheapest grade; part fails in service | Specify alloy + temper + governing spec |
| Only 3D model, no 2D drawing | No tolerance, finish, or thread definition | Always include both 3D STEP and 2D PDF |
| Undefined surface finish | As-machined at Ra 3.2 may not suit sealing surfaces | Specify Ra value and coating per industry standard |
| No tolerance definition | Shop uses default tolerance; parts may not fit | Mark critical dimensions individually |
| Unrealistic lead time | Added expedite fees or rejected quote | Provide realistic deadline or request rush pricing |
| Incomplete quantity information | No volume discount applied; suboptimal pricing | Provide quantity tiers and annual volume estimate |
How SOMI Custom Parts Can Help
At SOMI Custom Parts, we understand that the quality of your RFQ directly determines the quality of your manufacturing outcome. Our experienced engineering team reviews every incoming RFQ against our ISO 9001:2015 quality management system, providing free Design for Manufacturability (DFM) feedback on every submission.
When you send your RFQ to SOMI, you receive:
- Engineering-led review: Our team of experienced CNC machinists and engineers reviews your drawings and provides actionable DFM suggestions to reduce costs without compromising function
- Transparent quoting: Detailed cost breakdown including material costs, machining and setup, inspection and quality control, surface finishing, and logistics
- Fast response: Complete RFQs receive a detailed quote within 24–48 hours
- End-to-end capability: From prototype to production, with 3-, 4-, and 5-axis CNC machining centers, CNC turning, grinding, and comprehensive surface finishing services
- Quality assurance: CMM inspection, FAI reports, and full material traceability for every order
Ready to get accurate pricing on your next CNC project? Submit your RFQ today and let our team put their expertise to work for you. Browse our CNC machining parts and product catalog to see the quality and precision we deliver.
Frequently Asked Questions
What file format should I send for CNC machining quotes?
STEP (.stp/.step) is the universal standard accepted by all CNC shops. Send both the 3D model and a 2D drawing with GD&T. Native CAD formats (SolidWorks .sldprt, Inventor .ipt) are also typically accepted, but always include a STEP export to avoid version-compatibility issues.
Should I specify the CNC process in my RFQ?
Generally no. Specify what you need (tolerances, material, finish, features), not how to make it. The shop will determine the most efficient process — 3-axis vs. 5-axis, milling vs. turning. If you have a specific requirement (e.g., "no EDM on this surface"), state it explicitly.
How do I compare CNC machining quotes from different suppliers?
Ensure every supplier receives the same complete RFQ package. Compare unit price, setup fees, material costs, included inspection reports, and included surface finishing. The lowest per-piece price may not represent the lowest total cost if inspection, finishing, or logistics are quoted separately.
What is a typical lead time for custom CNC machined parts?
Simple prototype parts: 5–7 business days. Complex multi-setup parts: 10–15 business days. Production batches (50+ units): 15–25 business days. Expedite services are typically available at a 30–50% premium for 2–3 day turnaround on simple parts.
Can I send an RFQ before my design is fully finalized?
Yes — for prototype-stage RFQs this is common. Flag clearly that the design may change and specify a shorter validity period on any early pricing. A good supplier will provide cost-saving suggestions during the DFM review process that can be incorporated before final production.
Conclusion
A CNC machining RFQ is not just a request — it is the foundation of your entire manufacturing process. Clear and complete RFQs lead to faster quotes, lower costs, better production outcomes, and stronger supplier relationships. Every missing specification is an opportunity for miscommunication, cost overruns, and production delays.
By following the comprehensive checklist outlined in this guide — including a 3D STEP model, fully dimensioned 2D drawing with GD&T, precise material callouts, clearly defined tolerances and surface finishes, quantity tiers, and realistic lead time expectations — you can transform your RFQ from a routine purchasing document into a powerful tool for manufacturing success.
Start your next project with confidence. Submit a complete RFQ to SOMI Custom Parts and experience the difference that engineering-led precision manufacturing makes. For more insights, explore our CNC machining blog or contact our team directly.
