Why Do CNC Quotes for the Same Part Come Back So Different?
You send a STEP file to three suppliers and receive three prices that are far apart — one quote half the price of another, one supplier asking for a drawing, another not. Usually, each supplier quoted a different part, because the request left the important decisions open.
A comparable CNC quotation needs a complete request package: a controlled 3D model and a dimensioned 2D drawing at a stated revision, a datum and tolerance scheme that separates critical features from the rest, the material grade and stock condition, the quantity and repeat pattern, the surface finish for each surface that matters, and the inspection evidence you expect. Send that package, and the quotes you receive will describe the same job. Skip parts of it, and every supplier will fill the gaps with different assumptions.
What Information Does a CNC Quotation Actually Need?
A CNC supplier prices five things: programming and setup, material, machine time, finishing, and inspection. Every missing item forces a guess, and guesses differ. The table below is the full checklist. If an item does not apply to your part, say so — a stated "not required" is information, while silence is a gap.
| # | Item | What to provide | Why it changes the quote |
|---|---|---|---|
| 1 | 3D CAD model | STEP file, stated revision, units (mm or inch) | Defines geometry; revision prevents quoting an old design |
| 2 | 2D drawing | Dimensioned PDF at the same revision as the model | The contract record for tolerances, datums, finish and notes |
| 3 | General tolerance standard | e.g. ISO 2768-m or ISO 2768-f in the title block | Sets the default for every dimension without its own tolerance |
| 4 | Datum scheme | Datum features A, B, C marked on the drawing | Defines where measurement starts; without it, inspection is guesswork |
| 5 | Critical dimensions | Explicit tolerances only on features that control fit or function | Tight tolerances drive setup, speed and inspection cost |
| 6 | Material and stock condition | Exact grade and temper, e.g. 6061-T6, POM-C, 316L | Material price, machinability and stability all differ by grade |
| 7 | Quantity and repeat pattern | First order, expected repeat batches, annual estimate | Setup cost is spread across the batch; repeat work changes fixturing |
| 8 | Surface finish | Ra value or finish callout per surface, not one blanket value | Finishing steps add time and can change dimensions |
| 9 | Threads and hardware | Thread standard and class, inserts, press-fit parts | Threads follow their own standards and need specific tooling |
| 10 | Inspection and evidence | Method, sample size, report format, first-article scope | A dimensional report and first-article inspection are scoped work |
| 11 | Cosmetic and appearance zones | Which surfaces are visible, defect limits, reference sample | Cosmetic requirements change finishing and visual inspection |
| 12 | Delivery and timing | Destination, target date, packaging, export or compliance needs | Logistics, packaging and documentation are part of the price |
Work through the list before you send the RFQ. A supplier who has to ask for half of these items will take longer to quote, and one who does not ask may have assumed the cheapest version of each.
Why Does the 2D Drawing Still Matter When You Have a 3D Model?
A 3D model shows the shape. It does not say how accurately that shape must be made, which surfaces control the assembly, or how the part will be checked. That is the drawing’s job.
The drawing is also the record that resolves disagreements. If the model and the drawing conflict, state in the RFQ which one governs — for example, "In case of conflict, the 2D drawing at revision C governs." Keep the revision letter identical on the model filename, the drawing title block and the RFQ text, and say what changed with each new revision.
How Should You Specify Tolerances Without Overpaying?
The most expensive sentence in a CNC RFQ is a tight tolerance applied to every dimension. Tight tolerances demand more careful setups, slower cutting, more inspection and more scrap. The goal is not the tightest possible tolerance, but the tightest necessary one, applied only where function depends on it.
Use two layers. First, put a general tolerance standard in the title block, so every ordinary dimension has a defined default. Second, add explicit tolerances only on the few features that control fit, sealing, alignment or motion. ISO 2768-1 is the general tolerance standard most commonly used for machined parts, with four classes: fine (f), medium (m), coarse (c) and very coarse (v). The table below shows its default values for linear dimensions, as published by Protolabs Network, which machines metals to ISO 2768-f and plastics to ISO 2768-m by default unless the drawing says otherwise.
| Nominal size range | Plastics (ISO 2768-m) | Metals (ISO 2768-f) |
|---|---|---|
| 0.5 to 3 mm | ±0.1 mm | ±0.05 mm |
| Over 3 to 6 mm | ±0.1 mm | ±0.05 mm |
| Over 6 to 30 mm | ±0.2 mm | ±0.1 mm |
| Over 30 to 120 mm | ±0.3 mm | ±0.15 mm |
| Over 120 to 400 mm | ±0.5 mm | ±0.2 mm |
| Over 400 to 1000 mm | ±0.8 mm | ±0.3 mm |
| Over 1000 to 2000 mm | ±1.2 mm | ±0.5 mm |
Metals are more dimensionally stable during machining, so a fine class is practical. Plastics are more sensitive to heat, moisture and fixturing stress, and can warp or creep after machining, so a tight tolerance in plastic costs more. Protolabs Network’s guide to ISO-based CNC tolerances and finishes explains when ISO defaults are enough and when to call out tighter limits.
The table below gives a starting point for the second layer. Before you send the drawing, ask about every tight tolerance: what fails if this feature is at the loose end of the band? If the answer is "nothing we can name," relaxing it is the cheapest cost reduction available.
| Feature | Suggested approach | Reason |
|---|---|---|
| Overall length, non-mating outline | General tolerance (ISO 2768-m or -f) | No function depends on it; extra precision adds cost without value |
| Bore for a bearing or shaft | Explicit tolerance, e.g. Ø20 H7 or ±0.02 mm | Controls fit and running clearance directly |
| Bolt-hole pattern to a mating part | Positional tolerance from the assembly datum | Location matters more than the hole size alone |
| Sealing face | Explicit flatness and Ra requirement | A flatness error leaks; a size tolerance alone does not control it |
| Cosmetic outer surface | General tolerance, finish callout only | Appearance is a finish question, not a dimensional one |
| Thread | Thread standard and class (e.g. M6 × 1, 6H) | Threads follow their own ISO standards, not ISO 2768 |
| Deep pocket or thin wall | Discuss with the supplier before tightening | Tool reach and deflection limit what is practical at any price |
How Do Datums and GD&T Change the Quote?
A tolerance without a datum is an incomplete instruction. A dimension of 50 mm ±0.05 mm means little until the drawing says measured from where: datum A might be the flat mounting face, datum B the main bore, datum C a side face. Every critical dimension is then measured from those features, in that order.
This matters most for parts that assemble to something else: a hole pattern positioned from a cosmetic edge will not line up with a mating part that locates from a bore. Suppliers who receive a drawing without datums will either ask, which delays the quote, or assume a scheme that may not match your assembly.
Geometric dimensioning and tolerancing, or GD&T, controls the relationship between features instead of each dimension separately: the true position of a hole pattern relative to a datum, the flatness of a sealing face, the perpendicularity of a shoulder to a bore axis. These controls are defined in ASME Y14.5. Protolabs’ guide to fine-tuning tolerances for CNC machined parts explains the common GD&T callouts with worked examples. Use GD&T where the relationship between features is what matters, not where a simple size tolerance would do.
What Should You Say About Material, Stock Condition and Quantity?
Material is not a single word. "Aluminum" is not a specification: 6061-T6, 7075-T6 and 6082-T6 differ in strength, machinability and price, and the temper matters as much as the alloy. For plastics the same applies — POM-C and POM-H, or PA6 and PA66, are not interchangeable. State the exact grade, the temper or condition, and any requirement such as a material certificate. If the grade is still open, say which properties drive the choice, so the supplier can confirm the grade rather than guess it.
Stock condition deserves a line of its own. Machining starts from a solid block, bar or plate, and the stock form affects price and result. Thin-walled plastic parts are particularly sensitive: heat from cutting and stress from clamping can move a feature after the part leaves the fixture. If a part has thin walls or tight tolerances in a plastic grade, mention it and ask the supplier how they will hold it.
Quantity changes the price structure, not just the total. Programming, fixturing and first-article inspection are largely fixed costs: roughly the same for 10 parts or 500. Material, machine time and per-part inspection scale with quantity, which is why the unit price falls as the batch grows. Give the supplier the first order quantity, the expected repeat pattern and an annual estimate, even as a range. Also separate samples from volume: a first order of five parts for fit checking is a different job from a production batch of 500, even if the drawing is identical.
How Does Surface Finish Affect Price and Inspection?
Surface finish is usually specified as roughness average, Ra, in micrometres (µm) or microinches (µin). A standard as-machined finish is around Ra 3.2 µm (about 126 µin); Protolabs’ standard offering, for example, is 63 µin Ra for flat surfaces and 125 µin Ra or better for curved surfaces. A smoother finish — polishing, lapping, or a fine bead blast — is an extra operation with its own time and cost.
The point buyers most often miss is that finishing changes dimensions. Finishes that remove material, such as polishing, make a feature slightly smaller; finishes that add material, such as anodizing, plating and powder coating, make it slightly larger. A bore toleranced to ±0.02 mm before anodizing may be out of tolerance after it. So the drawing should state whether each tolerance applies before or after finishing — for example, "Ø20.00 ±0.02 mm after anodizing" — and the RFQ should list the finish per surface, not one blanket requirement.
Cosmetic requirements belong in the same conversation. If a surface is visible in the final product, say so, and state what counts as a defect: visible tool marks, scratches, dents, discoloration, and how the part will be judged. An appearance standard agreed before machining is far cheaper than a dispute over a mark that one side considers normal machining evidence and the other considers a reject.
How Do You Compare Two CNC Quotes on the Same Basis?
Two quotes can only be compared if they describe the same scope: the same revision, material grade, quantity, finish, and inspection evidence. A quote that omits first-article inspection will always look cheaper than one that includes it. Ask each supplier to break the price down into setup and programming, material, machining, finishing, and inspection. If one supplier’s inspection line is zero, ask what inspection is actually included.
The worked example below shows how quantity changes the unit price through setup cost alone. The numbers are illustrative, with stated assumptions, and are not a CKMOLD quotation or a price promise for any real part.
Illustrative example — assumptions: a small aluminum bracket; one-time programming and setup USD 480; material plus machining USD 14 per part; inspection USD 2 per part; same drawing, material and finish at every quantity.
| Quantity | One-time setup | Per-part cost | Total | Unit price |
|---|---|---|---|---|
| 10 parts | USD 480 | USD 16 | USD 640 | USD 64.00 |
| 100 parts | USD 480 | USD 16 | USD 2,080 | USD 20.80 |
| 500 parts | USD 480 | USD 16 | USD 8,480 | USD 16.96 |
At 10 parts the setup is USD 48 per part; at 500 parts it is under USD 1 per part. Nothing about the part changed — only the quantity over which the fixed cost is spread. That is why a quote without a stated quantity is not a comparable quote.
What Inspection Evidence Should You Ask For?
Inspection is not a single activity. It ranges from a quick check of a few dimensions with calipers to a full first-article report with CMM data for every critical feature, material certificates and a capability study. Each level costs a different amount and proves a different thing.
For most custom machined parts, a sensible starting point is: first-article inspection on the first batch, with measured values (not just pass/fail) for every critical dimension; a dimensional report for a stated sample of each production batch; and a material certificate for the grade ordered. If your product needs more — PPAP documentation, a capability study, or your own report format — say so in the RFQ, because those packages require extra samples, measurement and administration.
Agree the measurement method as well as the result. A bore measured with a micrometer and one measured on a CMM can give different numbers, and a plastic part measured immediately after machining can differ from the same part measured after 24 hours of conditioning. State the equipment where it matters, the conditioning time for plastic parts, and the datum scheme the inspection follows — the same datums as the drawing.
Ask one more question on any part that will later be molded: what does this machined part prove, and what does it not prove? At CKMOLD we use CNC machining for accurate prototypes, mold inserts, cavities, cores and precision parts, and a machined prototype validates fit, assembly and general function in a real material. But machined parts do not reproduce molded behaviour — weld lines, shrinkage, flow orientation and residual stress are molding phenomena. Our comparison of injection molding and CNC machining explains where each process is the right choice, and our prototype injection molding service covers the next step when a design needs validation in the production resin.
What Are the Most Common CNC RFQ Mistakes?
- Sending only a STEP file. A model without a drawing leaves tolerances, datums, finish and inspection undefined.
- Tightening every tolerance. A blanket ±0.02 mm raises machining and inspection cost where nothing depends on it.
- Leaving the revision unstated. Quoting revision B while the team discusses revision C produces parts to the wrong design.
- Naming a material family instead of a grade. "Aluminum" or "nylon" is not a specification; the wrong grade can fail in service even if every dimension is correct.
- Stating quantity as "to be confirmed." Without a quantity, the supplier cannot spread setup cost and will quote the highest unit price.
- Asking for inspection after the order. First-article scope, report format and sample size are priced work; agree them in the RFQ.
- Using a machined prototype as proof of a molded part. A machined part validates geometry and fit, not molded behaviour, so do not treat prototype approval as production approval.
How Do You Send a CNC RFQ to CKMOLD?
If you are ready to request a quotation, send the package described in this guide rather than a single file. The CKMOLD request-a-quote page captures the structured information — project type, material, quantities, tolerances, finish, target date and destination — and your CAD model and 2D drawing can be emailed separately to jerry@ckmold.com. If the design is controlled, ask for an NDA before sending the files.
What helps most is the combination: a STEP file and a dimensioned drawing at the same revision, the material grade, the first quantity and repeat pattern, the surface finish for the surfaces that matter, and the inspection evidence you need. With that package, our engineers can review the geometry and requirements together and quote the job you described. If your project also involves injection molding, our guide to CNC machining benefits for injection mold projects explains how machining, DFM review and mold trials connect, and our mold testing and validation service describes the trial evidence we provide.
A good RFQ is the cheapest quality control you will ever buy: a few hours of clear specification before machining replaces weeks of rework, disputed parts and requoting after it. Send the complete package, compare the quotes on the same basis, and choose the supplier whose questions — not just whose price — show they understood the part.
Frequently Asked Questions
Can I get a CNC quote from a STEP file alone?
Sometimes, for a simple part at a general tolerance. But a STEP file alone leaves tolerances, datums, finish and inspection undefined, so the quote rests on the supplier’s assumptions. For critical fits, cosmetic surfaces or inspection requirements, send the dimensioned drawing too.
Do I need GD&T on my drawing?
Only where the geometric relationship between features matters — hole-pattern position relative to a datum, sealing-face flatness, shoulder perpendicularity to a bore. Simple size dimensions are usually better served by an explicit tolerance or the general ISO 2768 standard in the title block.
What tolerance can CNC machining hold?
There is no single number. It depends on the feature, material, size, setup and supplier process control. As a reference, Protolabs Network publishes ISO 2768-f as its default for metals (±0.05 mm on small features) and ISO 2768-m for plastics (±0.1 mm). Treat any published figure as a reference, not a guarantee — confirm the achievable tolerance for your features during quotation.
Can a CNC prototype prove that my molded part will work?
It can prove geometry, fit, assembly and general function in a real material. It cannot prove molded behaviour such as weld-line strength, shrinkage, flow orientation or residual stress. Validate fit and function with machined samples, then validate material and process behaviour with molded samples in the production resin.
Should I order samples first, or go straight to a production batch?
Order samples first when the design is new, the tolerances are tight, the material is unfamiliar, or the part assembles to other components. A small sample order with first-article inspection is cheaper than a production batch that does not fit. Go straight to volume only when the design is proven and the drawing is stable.