A quotation can only be as accurate as the information it is based on. When a request for quotation (RFQ) arrives with a 3D model but no drawing, a drawing with no revision letter, or a finish note that says only "anodize," the supplier either quotes on assumptions or sends back a list of questions. Both cost time, and assumptions that turn out to be wrong cost more later. This article sets out what a complete drawing package for machined parts should contain and why each item matters. Where a standard governs the notation, it is named for context.
1. Send both the 2D drawing and the 3D model
The 3D model and the 2D drawing do different jobs. The model defines the nominal geometry and is what the programmer uses to generate tool paths. The drawing carries what the model usually does not: tolerances, datums, surface texture requirements, material, finish, notes and inspection requirements.
Model format
A neutral format avoids translation problems between CAD systems. STEP (ISO 10303) is the common choice; its Part 242 is the "Application protocol: Managed model-based 3D engineering" 1. Supply the native CAD file as well if your system allows it, but do not rely on it alone.
Which document governs
State on the drawing or in the RFQ which document governs if the model and drawing disagree. If you work in a model-based definition environment, where the tolerances are carried in the 3D model itself, say so explicitly, because the supplier must then inspect to the annotated model.
2. Control the revision
Every drawing and model in the package should carry a revision identifier, and the revisions should match.
ISO 7200 specifies the data fields used in title blocks, with the purpose of ensuring compatibility when documents are exchanged 2. It lists the legal owner, identification number, date of issue, segment/sheet number, title, document type, approval person and creator as mandatory fields, and the revision index as an optional field 3. The standard uses the revision index to identify the revision status of the document and recommends sequential letters or figures, avoiding the letters I and O 3. In the United States, ASME Y14.35 sets out practices for revising product definition datasets and associated documents, including methods for identifying and recording revisions 4.
Practical points for an RFQ:
- Use the same part number and revision on the drawing, the model file name and the RFQ line.
- If a revision is issued during quotation, send the full updated package and state what changed.
- Keep a record of which revision was quoted. The purchase order should reference the same revision.
3. Specify tolerances, including general tolerances
Individual tolerances
Dimensions that control fit or function should carry individual tolerances on the drawing. Geometric tolerances (flatness, position, perpendicularity and so on) need a clearly defined datum system. Two standard systems are in common use: the ISO geometrical product specification (GPS) standards, and ASME Y14.5, which establishes the symbols, rules, definitions, requirements, defaults and recommended practices for dimensioning and tolerancing 5. State on the drawing which system it follows, so that it is interpreted under the intended rules and defaults.
General tolerances: ISO 2768-1 and ISO 22081
Most dimensions on a machined part do not need an individual tolerance. A general tolerance note covers them.
ISO 2768-1 is intended to simplify drawing indications and specifies general tolerances for linear and angular dimensions without individual tolerance indications 6. It defines four tolerance classes: f (fine), m (medium), c (coarse) and v (very coarse) 6. The class is indicated in or near the title block, for example "ISO 2768-m" 6. ISO 2768-1 also states that, unless otherwise stated, workpieces exceeding the general tolerance "shall not lead to automatic rejection" provided the ability of the workpiece to function is not impaired 6. If you require strict rejection, say so.
The companion standard for geometrical tolerances, ISO 2768-2:1989, has been cancelled and replaced by ISO 22081:2021 7. ISO 22081 adds tools for specifying a general geometrical specification and a general size specification 7. Drawings that still reference ISO 2768-2 refer to a withdrawn standard; for new drawings, consider referencing the current one.
A general tolerance note is a decision, not a formality. If the note is missing, the supplier has no basis for deciding whether an untoleranced dimension is acceptable.
Tolerances that are tighter than they need to be
Each tight tolerance adds machining time, inspection time or both. Before sending the RFQ, check that every tight tolerance is functional. Where a feature is difficult to hold, the factory's production engineers will raise it. It is better to have that discussion before the quotation than after the first articles.
4. Specify surface texture correctly
Surface texture requirements should be shown with the standard symbol, the parameter and the limit value. ISO 1302:2002, the long-standing standard for surface texture indication, was withdrawn on 20 December 2021 and replaced by ISO 21920-1:2021 8.
ISO 21920-1 sets the rules for indicating surface texture by profile methods in technical product documentation 9. It changes some defaults that engineers used under ISO 1302:
- The maximum tolerance acceptance rule is now the default. The former "16 % rule" must be called up explicitly with the "T16 %" indication 9.
- The new graphical symbols carry a bar above them to distinguish them from the ISO 1302 symbols 9.
- The basic symbol has three variants: any manufacturing process permitted, material removal required, and material removal not permitted 9.
Older drawings using ISO 1302 notation remain common and can be read correctly, but the drawing should make clear which standard applies, because the acceptance rule differs.
Apply surface texture requirements only where they matter: sealing faces, bearing bores, sliding surfaces and cosmetic faces. A blanket fine finish over the whole part adds cost without benefit. If the part is to be anodized, painted or plated, state whether the surface texture requirement applies before or after the finish.
5. Call out material and finish completely
Material
State the material by alloy or grade and condition, for example an aluminum alloy with its temper, or a steel grade with its heat-treatment condition. A grade without its condition is incomplete. If an equivalent material under another national designation is acceptable, say so; if it is not, say that too.
Material documentation
If you need material certificates, specify the type. EN 10204 defines the types of inspection documents supplied to the purchaser 10. The table below is our own short summary, not the standard's wording:
| Type | Name | Content |
|---|---|---|
| 2.1 | Declaration of compliance | Manufacturer's statement of compliance, without test results 10 |
| 2.2 | Test report | Compliance statement with results of non-specific inspection 10 |
| 3.1 | Inspection certificate 3.1 | Results of specific inspection, validated by the manufacturer's authorized inspection representative independent of manufacturing 10 |
| 3.2 | Inspection certificate 3.2 | Results of specific inspection, also validated by the purchaser's authorized inspector or an inspector designated by official regulation 10 |
Finish
Finish callouts need the same precision as dimensions. For anodizing, for example, ISO 7599 lists as essential information the intended service use, the coating thickness class, whether the finish is clear or colored, and whether and how the coating is sealed 11. It also states that significant surfaces should preferably be indicated on drawings or by suitably marked test specimens 11. Show masked surfaces, threads to be protected and acceptable contact points on the drawing, and state whether dimensions apply before or after finishing.
6. State quantities and call-off schedule
Quantity changes the manufacturing method and the price. An RFQ should state:
- the quantity for first articles or prototypes, if required;
- the first production order quantity;
- the expected annual quantity, and whether it will be called off in scheduled batches.
A price quoted for 50 pieces and a price quoted for 5,000 pieces are based on different set-ups, fixtures and sometimes different processes. Quoting several quantity breaks in one RFQ is common and useful.
7. Define inspection and documentation requirements
The drawing defines what is acceptable. The RFQ should define how acceptance is demonstrated:
- Critical characteristics. Mark the dimensions that are critical to function, so that they receive particular attention in inspection.
- First articles. State whether first-article parts and a dimensional report are required before production, and how many pieces.
- Inspection reports. State which characteristics must be reported with measured values, or provide your own QC sheet.
- 100 % inspection. If you require every part to be inspected for specified characteristics, state it in the RFQ, because it affects time and cost.
- Cosmetic standard. For visible surfaces, define what is acceptable (scratches, tool marks, color variation) and how it is judged.
- Packaging. State any requirement for individual protection, cleanliness or labeling.
RFQ checklist
| Item | Included |
|---|---|
| 2D drawing (PDF) with title block and revision | ☐ |
| 3D model (STEP), same revision | ☐ |
| Statement of which document governs | ☐ |
| General tolerance note | ☐ |
| Datum system and geometric tolerances where needed | ☐ |
| Surface texture requirements and governing standard | ☐ |
| Material with condition or temper | ☐ |
| Material certificate type, if required | ☐ |
| Finish callout with thickness, color, masking, sealing | ☐ |
| Quantities: first articles, first order, annual | ☐ |
| Inspection, reporting and cosmetic requirements | ☐ |
| Packaging requirements | ☐ |
How PAXIS handles drawing packages
PAXIS receives RFQs and drawings by email, to protect the confidentiality of the drawing package, and signs customer NDAs on request. Drawings are reviewed with the production engineers of the selected partner factory. Questions on tolerances, materials or finishes are raised before quoting, and any drawing change is confirmed and approved by the customer in writing before production. Quotations are issued within two to three working days of receiving complete drawings and specifications; complex projects may require additional engineering review. Production is to the drawing, and inspection reports are supplied to the drawing or to the customer's QC sheet. To send a drawing package, request a quote.
