Writing a manufacturing specification: method and template

A workshop that receives a drawing on its own has to guess the rest: the function of the part, the inspection level expected, the date actually needed, the treatments to allow for. Every missing piece of information turns into either a question (and therefore delay) or an assumption (and therefore a risk of a gap between what you expected and what is delivered). A well-written specification cuts down the back-and-forth, makes quotes comparable and serves as a reference in the event of a disagreement on delivery.

It also saves you time: when you request quotes from several workshops, you give them all the same basis. The offers you receive then cover the same scope, and the price difference genuinely reflects differences in expertise, machines or workload, not misunderstandings.

This is not a forty-page document. For a machined part, a batch of sheet metalwork or a prototype, one or two pages are enough. The rule is simple: anything that can change the price, the lead time or the acceptance of the part must be written down.

Context and function of the part

Say what the part is for, what it is assembled with and in what environment it operates: temperature, humidity, splashing, impacts, friction, food or medical contact. An experienced workshop will work out what is critical and may suggest a cheaper alternative (different material, different tolerance, different process). If the part belongs to a regulated sector (aerospace, medical, automotive), say so from the outset.

Drawings and files

Supply a dimensioned PDF drawing and a neutral 3D file (STEP). Check that the revision index is the same on both, and state which prevails in the event of a discrepancy. For an assembly, add the bill of materials (list of parts, quantities, materials).

Material

Give a standardised designation rather than a generic term: “aluminium EN AW-6082 in T6 temper” or “stainless steel 1.4404 (316L)” are better than “alu” or “stainless”. Specify the delivery condition, any heat treatment and the certificate expected (for example a 3.1 material certificate under EN 10204). If you are unsure, state the properties you are looking for (strength, corrosion resistance, weight, machinability) and let the workshop make a proposal: the guide materials for machined parts will help you choose.

Tolerances

State the general tolerances in the title block (for example “ISO 2768-mK”) and dimension individually only the functional features. A tight tolerance is a cost: it is better to have few, justified ones than many out of caution. The guide machining tolerances: ISO 2768, fits and dimensioning details the method.

Treatments

Specify the heat or surface treatment (anodising, zinc plating, passivation, painting…), the standard or specification to meet, and any colour and thickness. Also say whether the drawing dimensions apply before or after treatment: a thick coating changes dimensions.

Inspection and quality documents

List the dimensions to be inspected, the frequency (100% or sampling), the measuring method if you have a requirement, and the documents expected: dimensional inspection report, certificate of conformity, material certificate, first article inspection report. These requirements have a cost: only ask for those that matter for the use of the part. For measurement needs, the page on inspection and metrology presents the suppliers concerned.

• Quantities. State the quantity of the first batch, but also the alternatives to be quoted (for example 1, 10 and 100) and your forecast annual requirement if you have one. A workshop does not price a prototype and a recurring series in the same way: see how a machining quote is calculated.
• Lead times. Give the delivery date you want and what is negotiable, as well as the date by which you want to receive quotes. For a series, specify the delivery rhythm.
• Packaging and delivery. Protection of functional surfaces, labelling, quantity per box or per pallet, address and transport terms.
• Confidentiality. State whether a non-disclosure agreement is required before the drawings are sent and who may access them. The guide drawing confidentiality and NDAs explains good practice.

The headings above apply to all processes. Each, however, calls for a few specific details.

• CNC machining: threads and their standard, acceptable internal corner radii, functional surfaces, edges to be broken, tolerated rework (clamping marks, for example). See CNC machining subcontracting.
• Sheet metalwork and cutting: sheet grade and thickness, surface finish and any protective film, bend radii and direction, visible side. The cutting file (DXF) is added to the drawing: see sheet metalwork and cutting.
• Welding and boilermaking: welding process, symbols and quality level of the welds, weld inspection, post-weld treatment. See welding and boilermaking.
• 3D printing: preferred technology or, failing that, the intended use of the part, orientation to respect, colour, post-processing (sanding, dyeing, painting) and areas to be finished by machining. See 3D printing.
• Electronics: circuit manufacturing files, bill of materials with manufacturer part numbers, number of layers, surface finish, electrical test expected. See electronics.

Copy this list, fill in what you know and write “to be defined” for the rest: a flagged open field is better than a silent assumption.

1. Project: project name, part or assembly reference, drawing revision index, date, contact person.
2. Context and function: what the part is for, in what environment, which parts it is assembled with.
3. Attached documents: dimensioned PDF drawing, 3D file (STEP), bill of materials for an assembly. Which document prevails.
4. Material: standardised designation, delivery condition, certificate requested (yes or no, type).
5. Quantities: first batch quantity, alternative quantities to quote, forecast annual requirement, delivery rhythm.
6. Tolerances: general tolerances, critical dimensions and fits, surface finishes.
7. Treatments: heat and surface treatments, applicable specification, colour, thickness, dimensions before or after treatment.
8. Inspection and quality documents: dimensions to inspect, method, frequency, report and certificates expected, sector-specific requirements.
9. Lead times: desired delivery date, deadline for replies, any milestones, flexibility margin.
10. Packaging and delivery: protection, labelling, quantity per box, delivery location and terms.
11. Confidentiality: non-disclosure agreement required or not, distribution of drawings.
12. Selection criteria: price, lead time, certifications, proximity, references; indicative budget if you wish to share it.

• A 3D file with no drawing. The 3D model gives the shape, not the tolerances or treatments: the workshop then quotes the standard case.
• An approximate material. “Steel” or “plastic” is not enough: the grade changes the price, the machinability and the procurement lead time.
• Tight tolerances everywhere. A drawing dimensioned entirely at ±0.01 mm makes the part more expensive without improving its function.
• A vague quantity. “A few parts” or “to be seen” makes it impossible to price the fixed costs and to offer a series price.
• An unrealistic or unjustified lead time. Apparent urgency can push up the price; an impossible date puts off serious workshops.
• Drawing versions in circulation. Every file must carry its revision index, and old versions must be removed from what you send.
• Forgetting the finishing. Edges to be broken, deburring, marking, protection: what is not written down is not priced.
• Contradictory requirements. A very fine roughness requested on an as-rolled surface, or a treatment incompatible with the material: re-read the file with fresh eyes before sending it.
• Forgetting the packaging. A polished or anodised part that is poorly protected arrives damaged even if it was compliant when it left the workshop.

Your specification is the best starting point for a clear request. On Meaflow, posting a request is free: you describe your need, attach your drawings (visible only to logged-in members), then several suppliers can send you their quote. You compare them, discuss through the messaging system and make your choice. The full process is explained in how it works.

If your drawing is not finalised, describe at least the functional need and state what is still to be defined: suppliers will be able to ask their questions before quoting. To understand the amounts you will receive, then read the guide on CNC machining prices.