Custom Metal Parts Manufacturing: RFQ, Drawing & Supplier Guide

重要ポイント

• An effective RFQ includes 10 elements — CAD files in STEP format, exact material grades, 公差 per ISO 2768, quantity breaks, annual volume, surface finish specs, required quality documentation, 用途 context, target lead time, and compliance requirements. Missing any one can add days to your quote cycle.
• ISO 2768-m is sufficient for most industrial parts. Tightening to ISO 2768-f adds 1.5–2× to machining cost; ultra-precision 公差 (±0.001–0.005 mm) can increase cost by 10–24×. Call out tight 公差 only on functional surfaces.
• 设计 decisions determine 70–80% of manufacturing cost. 材料 selection, tolerance callouts, and part geometry lock in most of your cost before a supplier ever sees the drawing — a DFM review at the design stage is the highest-ROI step in custom part sourcing.
• 出荷前に問題を発見します。 and 板金 crossover at 250–500 pieces. Below that volume, CNC from billet is usually cheaper per part. Above it, 繰り返し注文の一貫性が保たれます。 (cut, bend, weld) becomes more economical due to lower material waste and faster cycle times.
• A structured supplier evaluation saves more than price shopping. ISO 9001 certification, 設備リスト, in-house 検査 capability, and communication responsiveness predict project outcomes better than the lowest quote.

Ordering custom metal parts — whether a one-off prototype bracket or 5,000 production enclosures — is fundamentally a communication exercise. The quality, cost, and lead time you get back depend almost entirely on the clarity of what you 送信 out. Yet most RFQs arrive at a machine shop missing at least one critical piece of information: no material grade, no tolerance standard, a PDF screenshot instead of a STEP file, or a quantity field that says “TBD.” Each missing detail triggers an email thread that adds hours or days to the quoting cycle.

This guide walks through how to specify custom metal parts so your RFQ returns accurate quotes the first time — covering RFQ structure, drawing preparation, tolerance selection, supplier evaluation, cost drivers, and process selection across CNC加工、板金 fabrication, and 溶接.

How to Write an RFQ That Gets Accurate Quotes

An RFQ (Request for Quotation) is not a purchase order — it is a question you are asking a supplier. The more precise the question, the more actionable the answer. These 10 elements belong in every custom metal parts RFQ:

1. 3D CAD files in STEP（.stp） or IGES (.igs) format. STEP is the universal standard for 出荷前に問題を発見します。 and 板金. STL files (mesh format) are for 3D printing — they lack the geometric precision needed for machining and will be rejected or require expensive reverse-engineering. See our STL vs STP format guide for the full explanation.
2. 2D drawings (PDF) with critical dimensions and 公差. The 3D model defines the nominal geometry; the 2D drawing defines what “good enough” means. Mark critical-to-function dimensions with explicit 公差 and reference a general tolerance standard (ISO 2768-m or -f) for everything else.
3. Exact material grade. “アルミニウム” → 6061-T6, 7075-T6, or 5052-H32? “ステンレス鋼” → 304, 316, or 17-4PH? The price and lead time can differ by 2–3× between grades. See our aluminum grade guide for help choosing.
4. Quantity and quantity breaks. “100 pieces” is a starting point. Also provide: “and pricing for 500 and 1,000 pieces.” The cost-per-part curve is steepest between 1–100 units; providing breakpoints lets the supplier optimize tooling and setup amortization.
5. Annual volume (even if estimated). A part ordered at 100 pieces per order, four times a year, is fundamentally different from a one-time 100-piece prototype run. Annual volume determines whether the shop invests in dedicated fixturing, which can reduce per-part cost by 20–40%.
6. Surface finish requirements. As-machined? Anodized Type II? Passivated? Powder coated? Each finish adds a separate process step and cost line. Be specific: “bead blast + clear anodize per MIL-A-8625 Type II, Class 1” is actionable; “nice finish” is not.
7. Required quality documentation. Do you need a 5. 最終検査と文書化 (FAI per AS9102)? 材料 certifications (mill test reports)? Certificate of conformance (CoC)? Each document adds 検査 time — specify only what your quality system actually requires.
8. 用途 context. A one-sentence description — “this bracket mounts an optical sensor in a 24/7 production line with ±5°C temperature swings” — tells the supplier things the drawing cannot: that vibration matters, that thermal expansion matters, and that a failure is expensive. Good shops use this information during DFM review.
9. Target lead time. “ASAP” means different things to different shops. “4 weeks” or “need delivery by August 15” lets the supplier tell you honestly whether they can hit the date — and whether a rush fee applies.
10. Compliance or certification requirements. ISO 9001? AS9100 (aerospace)? ISO 13485 (medical)? ITAR? RoHS/REACH? Not every shop holds every certification — filtering upfront saves everyone time.

Drawing Preparation: What Makes a Manufacturing-Ready Drawing

The 2D drawing is the legal definition of the part. The 3D model shows what the part looks like; the drawing defines what is acceptable. A manufacturing-ready drawing includes:

• Title block: part name, drawing number, revision, material, finish, general tolerance standard (ISO 2768-m or -f), units (mm or inch), scale.
• Orthographic views: at least three views (front, top, right) plus an isometric view. Section views for internal features.
• Critical dimensions with explicit 公差: mark the dimensions that affect fit, function, or assembly. Every other dimension falls under the general tolerance standard.
• Datum reference frame: three mutually perpendicular planes (A, B, C) from which all measurements are taken. Without datums, the inspector and machinist may measure from different reference surfaces and reach different conclusions.
• GD&T callouts where needed: flatness, parallelism, perpendicularity, position, and profile 公差 are the most commonly used symbols for metal parts. A position tolerance of Ø0.2 mm on a hole pattern says more than coordinate dimensions with ±0.1 mm on each axis — the position tolerance creates a cylindrical tolerance zone that is functionally larger.
• Notes section: “Remove all burrs and sharp edges,” “Passivate per ASTM A967,” “Dimensions apply after coating” — these three notes alone prevent the most common receiving-検査 rejections.

Tolerance Selection: When ISO 2768-m Is Enough

The single most expensive four characters on a metal part drawing are “±0.05” applied universally. Tolerance selection is a cost lever. Here is the real cost impact of each tolerance tier:

The practical rule: Apply ISO 2768-m as your drawing’s general tolerance. Then add explicit, tighter 公差 only on the 3–5 features that actually affect fit or function. A drawing with 50 dimensions and three tight callouts costs what it should. The same drawing with 50 tight callouts costs 3–10× more — and most of those tight dimensions are on surfaces that will never mate with anything. See our full tolerance standards guide for more detail.

Supplier Evaluation: Beyond the Lowest Quote

Price comparison is the starting point, not the finish line. A supplier evaluation for custom metal parts should weigh three categories:

Technical & Quality (Weight: ~50%)

• ISO 9001 certification — minimum baseline for any production supplier. AS9100 for aerospace, ISO 13485 for medical.
• 根本原因を修正し、影響を受けたロットを再検査し、変更内容を文書化 list with capacities — does the shop have the right machine size, axis count, and tolerance capability for your parts? A shop running 3-axis mills cannot efficiently produce 5-axis geometry.
• In-house 検査 equipment — CMM, vision system, surface roughness tester, hardness tester. If the shop outsources 検査, add time and risk to your supply chain.
• 材料 certifications — can they provide mill test reports (MTRs) for the specific heat/lot of material used in your parts? Critical for aerospace, medical, and pressure-containing 用途s.

Communication & Service (Weight: ~30%)

• Quote turnaround time — 24–48 hours is the industry norm for straightforward parts. Longer than 72 hours suggests the shop is overloaded or disorganized.
• DFM feedback quality — did they catch the impossible undercut? Did they suggest a material substitution that saves money? A supplier who asks engineering questions during quoting will save you problems during production.
• 英語 (or your language) proficiency — if technical discussions require a translator, miscommunication on 公差 or finishes is a question of when, not if.
• Reference checks — ask for a recent client in your industry. A five-minute call with a previous customer reveals more than any certification.

Cost & Business (Weight: ~20%)

• Payment terms — standard for first orders is 30–50% deposit, balance before shipment. Established relationships may move to Net 30.
• Shipping capability — can they handle export documentation, Incoterms, and your preferred freight forwarder? International shipping logistics are a supplier capability unto themselves.
• Minimum order quantity (MOQ) — some shops will not touch orders under $500. Others specialize in prototype-to-low-volume work. Find the shop whose sweet spot matches your order profile.

What Drives Custom Metal Part Costs

Most buyers focus on the unit price line. The real cost structure of a custom metal part looks like this:

Quantity breaks are real and steep. A prototype bracket that costs $85 each at 10 pieces might cost $22 each at 500 pieces. The setup cost (CAM programming, fixture, first-article 検査) is amortized across the entire order. For production quantities, always request pricing at 2–3 volume breakpoints — the difference between 100 and 500 pieces can halve the unit cost.

CNC加工 vs 板金 vs 溶接: Which 工程 for Your Part?

The first decision in custom metal manufacturing is process selection. 送信 a 板金 part to a CNC-only shop and you will pay for a billet-machined version at 5× the intended cost. Here is a practical decision framework:

The hybrid approach: many production parts use 板金 for the main structure (low cost, low waste) with secondary 出荷前に問題を発見します。 on critical features like bearing bores, dowel pin holes, or seal surfaces. This “fabricate then machine” workflow captures the cost advantage of 板金 at volume while maintaining precision where it matters. If you are unsure which process fits your part, include both the 3D model and 用途 context in your RFQ — a good supplier will propose the optimal process mix during DFM review.

Common RFQ Mistakes That Delay Your Quote

1. 送信ing a PDF screenshot instead of a CAD file. A screenshot of a 3D model is not a manufacturing file. The supplier cannot extract dimensions, check draft angles, or generate toolpaths from an image. They will reply “please 送信 STEP file” and your quote clock resets. Always attach the native CAD file — STEP（.stp） is the universal format for metal manufacturing.

2. Leaving the quantity field blank or “TBD.” Quantity determines whether the shop quotes a prototype setup (one-off fixturing, manual programming) or a production setup (dedicated fixture, optimized toolpaths, automated 検査). Without a quantity, the supplier must guess — and most will quote the higher prototype price to protect themselves. Even a range (“50–200 pieces”) is better than nothing.

3. Applying blanket tight 公差. A drawing with “+/-0.05 mm” in the title block on every dimension forces the machinist to treat every feature as critical. Most features on a typical bracket do not need ±0.05 mm — they need ISO 2768-m (±0.2 mm for dimensions under 30 mm). Call out tight 公差 only where function demands them, and your quote will reflect the difference.

4. No surface finish specification. “I want it to look good” means different things in different shops. As-machined? Bead blasted? Anodized? Each finish is a separate process, often at a separate facility, with its own cost and lead time. Specify the finish on the drawing or in the RFQ body — if you are unsure which finish is appropriate for your 用途, ask the supplier for a recommendation during quoting.

5. Choosing the supplier based on price alone. The lowest quote often reflects what was left out, not what was included: no 不動態化処理, no 検査 report, no material certification, no shipping. Compare quotes line-by-line, not bottom-line to bottom-line. A $50 part with included 検査 and certification is often cheaper in total cost than a $35 part that arrives non-conforming and needs replacement.

Get Your Custom Metal Parts Quoted Right

The difference between a smooth カスタム部品 order and three weeks of email threads comes down to the quality of information in your initial RFQ. 送信 a STEP file, a marked-up drawing with explicit 公差, a material grade, and a quantity — and a good supplier will return an accurate quote within 24–48 hours, often with DFM suggestions that save you money. BravoFabsは提供します custom metal parts manufacturing across CNC加工、板金 fabrication, and 溶接 for industrial clients in Europe and North America. Every RFQ includes a free engineering review — 送信 us your drawings and we will help you get the specification right before production starts.