The Complete Guide to RFQs for CNC Machined Parts
An RFQ for custom CNC machined parts requires more than a 3D model and a quantity. This guide covers exactly what manufacturers need to quote accurately — drawings, GD&T, material callouts, inspection requirements, and how to structure your request so you get precise quotes, not padded estimates.
Your RFQ Determines Your Quote Accuracy
A well-structured RFQ for custom CNC machined parts includes a 3D model (STEP format), a 2D drawing with GD&T per ASME Y14.5-2018, a complete material callout (alloy + temper + governing spec), quantity tiers, surface finish requirements, and inspection expectations. Shops that receive incomplete RFQs typically add 15–25% to their quotes to cover ambiguity — or they miss requirements entirely and deliver parts that do not meet spec. This guide covers every element of a CNC RFQ so you get precise pricing on the first round.
What Makes a CNC RFQ Different
A CNC machining RFQ is not a generic procurement form. It is a technical document that communicates exactly what the shop needs to program, fixture, machine, inspect, and finish your parts. Missing any of these elements forces the shop to assume — and assumptions always cost you money.
Material Certifications (MTRs)
CNC parts often require traceable material. A mill test report (MTR) ties the raw stock to a specific heat lot with certified chemistry and mechanical properties. Medical device and critical structural applications require these — a generic RFQ does not prompt this.
Tolerance Classes
Standard CNC tolerance is ±0.005 in. (±0.13 mm) for machined features. Tighter tolerances (±0.001 in. / ±0.025 mm or below) require different fixturing, slower feed rates, and often CMM inspection — all of which affect the quote.
Surface Finish Specs (Ra per ISO 21920-2)
As-machined finish is typically Ra 125 µin. (3.2 µm). If your sealing surfaces need Ra 32 µin. (0.8 µm) or your bearing bores need Ra 16 µin. (0.4 µm), the shop must know at quote time — these require additional passes or grinding.
Inspection Requirements
FAI (First Article Inspection) adds $200–500 per part number. CMM reports for critical dimensions add $50–150 per part depending on feature count. CoC (Certificate of Conformance) is standard. Specify these upfront so the shop prices them in.
Why Incomplete RFQs Cost You
Shops add 15–25% markup when an RFQ is vague. They are pricing in the risk that missing information will require rework, additional setups, or material reorders. A complete RFQ removes that risk premium and gives you a quote that reflects actual manufacturing cost — not worst-case assumptions.
The Technical Package
The technical package is the foundation of your RFQ. It tells the shop exactly what to make and how to inspect it. Without a complete package, you are asking for an estimate, not a quote.
3D Model
STEP (.stp) is the universal standard — every CAM system imports it. Native CAD files (SolidWorks .sldprt, Inventor .ipt, Creo .prt) are also accepted by most shops. STEP is preferred because it avoids version-compatibility issues across different CAD platforms. The 3D model is used for CAM programming, toolpath generation, and fixture design.
2D Drawing
The 2D drawing is the contractual document. It must include GD&T per ASME Y14.5-2018, a title block with material callout, surface finish requirements, and tolerances on critical dimensions. The drawing governs — if the 3D model and drawing conflict, the drawing wins.
What to Include on the Drawing
Common Drawing Mistakes That Delay CNC Quotes
| Mistake | Impact | Fix |
|---|---|---|
| No material callout | Shop guesses or asks, adding 1–2 days | Specify alloy + temper: "6061-T6 per AMS-QQ-A-225/8" |
| Missing tolerances on critical dims | Shop quotes to general tolerance, parts don't fit | Add GD&T to mating surfaces and critical features |
| Surface finish not specified | As-machined Ra 125 µin. (3.2 µm) assumed, may not meet sealing reqs | Call out Ra values per ISO 21920-2 where needed |
| Threads not specified fully | Wrong thread class, parts don't assemble | Include thread size, pitch, class, and depth |
| No quantity specified | Shop can't optimize programming/fixturing | State exact quantity + anticipated annual volume |
Pro Tip
Always send both the 3D model and 2D drawing. Some shops accept 3D-only for quick-turn prototypes, but for production parts the 2D drawing is the contractual document that defines what "in spec" means. A 3D model alone cannot convey GD&T, thread classes, or surface finish requirements.
Material Specification
Always specify three things: alloy designation, temper or condition, and governing specification. Writing "aluminum" on an RFQ is the single most common mistake — there are over 500 wrought aluminum alloys, and the shop does not know which one you need.
Correct Material Callout Examples
6061-T6 per AMS-QQ-A-225/8Covers wrought aluminum bars, rods, and shapes. The AMS spec ensures certified mechanical properties.
304 stainless per ASTM A276ASTM A276 covers stainless steel bars and shapes for general engineering use. Specify condition (annealed, cold-finished).
Ti-6Al-4V per AMS 4928AMS 4928 covers Ti-6Al-4V bars, forging stock, and rings in the annealed condition.
When to Request Material Test Reports (MTRs)
- Medical device components — traceability required by FDA 21 CFR 820
- Critical structural parts where material failure is a safety risk
- Customer-facing traceability requirements (OEM contracts, quality audits)
MTR Cost Impact
Material certifications typically add $50–150 per lot but provide complete traceability from mill to finished part. The cost covers the administrative overhead of sourcing certified stock, maintaining lot segregation during machining, and packaging the MTR with the shipment. For production quantities, the per-part cost impact is minimal.
Pro Tip
If you are unsure which governing spec to use, specify the alloy and temper and add "or equivalent." For example: "6061-T6 aluminum per AMS-QQ-A-225/8 or equivalent." This gives the shop flexibility to source from their standard inventory while ensuring the correct alloy and temper.
Tolerance & Inspection Requirements
Tolerances drive machining time, fixturing complexity, and inspection cost. Specifying them correctly on the drawing — not in the RFQ email — ensures the shop quotes to your actual requirements.
Standard CNC Tolerance
±0.005 in. (±0.13 mm)This is the default for CNC-machined features in aluminum and steel alloys. Most shops hold this without special fixturing or additional operations. If your drawing does not call out a tolerance on a feature, the shop applies their general tolerance — which is typically ±0.005 in. (±0.13 mm).
When to Call Out Tighter
- Mating surfaces: ±0.001–0.002 in. (±0.025–0.05 mm) for press fits and slip fits per ASME B4.2 (preferred metric limits and fits)
- Bearing bores: ±0.0005 in. (±0.013 mm) typical for press-fit bearing housings
- Seal grooves: O-ring groove width and depth per AS568 / ISO 3601 with tolerances to ±0.002 in. (±0.05 mm)
Specify the tolerance on the drawing, not in the RFQ email. The drawing is the contractual document.
Inspection Cost Impact
| Inspection Type | Typical Cost | When Required |
|---|---|---|
| CoC (Certificate of Conformance) | Standard (no extra charge) | Every shipment — confirms parts were manufactured to drawing |
| FAI (First Article Inspection) | $200–500 per part number | First production run — full dimensional report against drawing |
| CMM Inspection Report | $50–150 per part | Critical dimensions requiring 3D coordinate measurement |
| Material Certification (MTR) | $50–150 per lot | Medical, structural, or customer-required traceability |
Pro Tip
Request FAI on the first production run of every new part number. The $200–500 cost is small compared to the risk of shipping 100 parts that are out of spec. Once the FAI passes, subsequent production runs only need CoC and in-process inspection.
Get Engineer-Reviewed DFM Feedback with Your Quote
MakerStage provides engineer-reviewed DFM feedback and fast quoting for CNC machined parts. Upload your STEP file and get a quote with DFM analysis typically within one business day. Every quote includes tolerance review, material confirmation, and manufacturability feedback at no additional cost.
Upload STEP File for CNC Quote with Free DFMQuantity, Delivery & Packaging
Quantity drives the cost per part more than any other single variable in CNC machining. Setup cost (programming, fixturing, first-article) is amortized across the run — so a 100-piece order can cost 40–60% less per part than a single prototype.
Quote in Quantity Tiers
Request pricing at 1, 5, 10, 25, 50, and 100 units. This reveals cost-volume trade-offs and helps you determine the optimal order quantity. Setup/NRE amortization is the primary driver of per-unit cost reduction.
Lead Time Expectations
Prototypes: 5–10 business days. Production runs: 10–20 business days. Expedite services are typically available at 30–50% premium. State your required delivery date upfront — lead time affects shop scheduling and pricing.
Packaging Requirements
Specify if parts need individual bagging, VCI paper for corrosion protection (critical for carbon steel), foam inserts, or custom packaging. Unspecified packaging defaults to bulk-packed in a box — acceptable for prototypes, not for finished production parts.
Delivery & Logistics
Specify ship-to location (affects freight cost), preferred carrier, and any import/export restrictions. For international shipments, confirm Incoterms (FOB, DDP, etc.) and whether the shop handles customs documentation.
Pro Tip
Include your anticipated annual volume even if you are only ordering prototypes now. This helps the shop recommend fixturing investments, raw material blanket orders, or process changes that reduce per-part cost at production volume.
Post-Processing & Finish
Surface finish and coating specs are among the most frequently underspecified elements in CNC RFQs. The difference between "anodize" and "Anodize Type II, Class 2 (dyed black) per MIL-A-8625" determines whether you get what you need or what the shop assumes.
Common CNC Finishes
| Finish | Spec / Standard | Ra Value | Typical Use |
|---|---|---|---|
| As-machined | No additional processing | Ra 125 µin. (3.2 µm) | Prototypes, internal features |
| Bead blast | Specify media size (e.g., #120 glass bead) | Ra 63–125 µin. (1.6–3.2 µm) | Uniform matte appearance, hides tool marks |
| Anodize Type II | MIL-A-8625, Type II, Class 2 | N/A (coating over machined surface) | Corrosion protection, cosmetic color (aluminum) |
| Anodize Type III (hard) | MIL-A-8625, Type III | N/A (adds 0.001–0.002 in. / 0.025–0.05 mm per side) | Wear-critical surfaces, bearing races (aluminum) |
| Powder coat | Specify color (RAL or Pantone) + thickness | N/A | Cosmetic exterior parts, corrosion protection |
| Nickel plating | ASTM B733 (electroless) or AMS 2404 | N/A (mirrors substrate finish) | Corrosion + wear resistance, uniform coating |
| Passivation | ASTM A967 / Citric acid per ASTM A380 | N/A | Stainless steel — removes free iron, restores Cr₂O₃ layer |
Surface Finish Callout Reference
Standard tool-mark finish. Acceptable for most non-cosmetic, non-sealing surfaces.
Light finishing pass. Common for visible surfaces and O-ring groove flanks.
Requires slow feed rate or secondary grinding. Used for sealing surfaces.
Requires grinding or lapping. Bearing bores, optical mounting surfaces.
Masking Requirements
When specifying coatings (anodize, powder coat, plating), identify which surfaces must be masked to remain bare metal. Common masking callouts: threaded holes (to preserve thread class), mating surfaces (to maintain dimensional accuracy), and grounding points (for electrical contact). Specify masking on the drawing with a note: "Mask per areas indicated. Do not coat threaded features."
Pro Tip
Specify the finish in the RFQ with the full spec callout. "Anodize Type II, Class 2 (dyed black) per MIL-A-8625" is unambiguous. "Black anodize" leaves the type, class, and dye specification open to interpretation — and the shop may default to the lowest-cost option.
RFQ Template & Checklist
Use this checklist before submitting your next CNC RFQ. Every item on this list corresponds to information the shop needs to generate an accurate quote. Missing items will either delay the quote or result in padded pricing.
- 3D model in STEP (.stp) format
- 2D drawing with GD&T per ASME Y14.5-2018
- Title block: part number, revision, material, finish
- Critical dimensions with tolerances called out
- Datum references identified
- Thread callouts: size, pitch, class, depth
- Edge break / deburr requirements
- Alloy designation (e.g., 6061-T6, 304, Ti-6Al-4V)
- Temper / condition (T6, annealed, pre-hardened)
- Governing spec (AMS, ASTM, SAE)
- MTR required? (Yes / No)
- Approved substitutes? (If applicable)
- As-machined Ra requirement (if applicable)
- Coating type + full spec (e.g., MIL-A-8625 Type II)
- Color (RAL / Pantone number if applicable)
- Masking areas identified on drawing
- Passivation required? (For stainless steel)
- Quantity (with tiers: 1 / 10 / 50 / 100)
- Anticipated annual volume
- Required delivery date
- Ship-to location
- Packaging requirements (individual bag, VCI paper, etc.)
- Inspection requirements (FAI, CMM, CoC)
Pro Tip
Save this checklist as a template in your procurement system. Sending a consistent, complete RFQ package every time reduces quote turnaround time, improves accuracy, and makes it easier to compare quotes across suppliers on an apples-to-apples basis.
Further Reading
- RFQ Checklist for Custom Parts — a broader checklist covering CNC, 3D printing, and sheet metal.
- CNC Tolerances Guide — tolerance tables by material and process with cost impact data.
- GD&T Guide for Engineers — how to apply geometric dimensioning and tolerancing per ASME Y14.5-2018.
- CNC Machining Services — 3-axis and 5-axis milling, turning, and multi-axis capabilities.
- Surface Finishes Guide — Ra values, coating specs, and finish selection for CNC parts.
Frequently Asked Questions
Ready to Submit Your CNC RFQ?
Upload your STEP file and 2D drawing for an engineer-reviewed CNC quote typically within one business day. Include the drawing, material, quantities, finish, and inspection notes so the quote can match the actual part requirements.
Get CNC Machining Quote