Ordering custom machined parts for the first time — or even the tenth time from a new supplier — involves more decisions than most buyers expect. File formats, tolerance specifications, material grades, surface finish requirements, lead times, and quote comparisons all need to align before a single chip is cut. When any one of these elements is unclear or missing, the result is a delayed quote, a misunderstood drawing, or parts that arrive dimensionally correct on paper but wrong for your application.
This guide walks through the entire ordering process in sequence — from preparing your files to approving your first article — so you can place a well-specified order, get accurate quotes, and receive parts that match your requirements the first time.
Getting Your Part File Ready Before You Contact Anyone
The single most common reason quotes are delayed or inaccurate is incomplete part documentation. Before you reach out to any supplier, make sure you have the right files in the right format.
3D model files are the foundation. For CNC machining, the preferred format is STEP (.step or .stp) — it is universally readable across CAM software platforms and preserves solid geometry accurately. IGES (.igs) is an acceptable alternative. STL files are not suitable for machining quotes because they represent surface meshes without precise dimensional data — they are adequate for 3D printing, not for a precision machine shop trying to program a CNC toolpath.
2D engineering drawings are equally important, even when you provide a 3D model. The drawing defines tolerances, surface finish requirements, material specification, thread callouts, and any notes that cannot be captured in a 3D geometry file. A STEP file tells a machinist the shape — the drawing tells them what matters and to what standard. Without a drawing, the supplier must assume tolerances (typically to ISO 2768 medium, which allows ±0.1mm on most features) and you lose the ability to hold them accountable to anything tighter.
If you are ordering a turned or milled part from a 2D design only, a DXF (.dxf) file is acceptable for profile geometry, but a PDF drawing with full dimensioning and tolerancing is still required alongside it.
Before you send anything, check:
- Is your STEP file exported from the final version of your model, not a draft revision?
- Does your drawing have a title block with part number, revision, material, and finish specified?
- Are all critical dimensions toleranced — or are you relying on general tolerance notes for features that actually matter?
- Are thread specifications called out with standard designations (M8×1.25, for example) rather than just a nominal diameter?
Getting these details right before the RFQ saves multiple rounds of back-and-forth with every supplier you contact.
Choosing and Specifying Your Material Correctly
Material specification is one of the most consequential decisions in the ordering process — and one of the most frequently underspecified. Writing “aluminum” on a drawing is not a material specification. Writing “aluminum 6061-T6” is.
The grade and temper of a material affect its machinability, strength, corrosion resistance, and surface finish capability. Here is a practical reference for common choices:
Aluminum:
- 6061-T6 — the workhorse grade. Good machinability, good strength, widely available, cost-effective. Right for most structural and enclosure applications.
- 7075-T6 — higher strength, used in aerospace and high-stress applications. More expensive and slightly harder to machine than 6061.
Stainless Steel:
- 304 — general purpose, good corrosion resistance, widely available.
- 316L — better corrosion resistance, required for marine, medical, and chemical environments. Slightly harder to machine than 304.
Titanium:
- Grade 5 (Ti-6Al-4V) — the standard engineering grade. High strength-to-weight ratio, biocompatible, used in aerospace and medical applications. Requires experienced machining — it work-hardens quickly and generates significant heat during cutting.
Engineering Plastics:
- PEEK — high-temperature resistance, chemical resistance, biocompatible. Used in medical and semiconductor applications.
- Delrin (POM) — easy to machine, dimensionally stable, good for gears, bushings, and structural components.
Brass:
- C360 (free-machining brass) — excellent machinability, used for fittings, connectors, and decorative components.
When you specify material on your drawing or RFQ, always include the alloy designation and condition (temper or hardness). If your application has specific requirements — biocompatibility, ITAR-controlled material sourcing, material certification (e.g. EN 10204 3.1 mill cert) — state them explicitly in your RFQ.
Writing a Tolerance Specification Suppliers Can Actually Quote
Vague tolerance specifications produce vague quotes — and parts that may not work. Before you send an RFQ, every dimension on your drawing should have a clear tolerance, either explicitly or through a general tolerance note.
General tolerance notes (referencing ISO 2768 medium or fine, for example) cover the majority of non-critical features on most parts. ISO 2768 medium allows ±0.1mm on features between 6mm and 30mm — acceptable for clearance fits, non-mating surfaces, and structural features.
Feature-specific tolerances are required wherever the dimension directly affects function — bore diameters that receive bearings or shafts, mating surfaces, sealing faces, and any feature where position or size affects assembly. These should be called out explicitly on the drawing with the actual tolerance value (±0.005mm, ±0.01mm) or as GD&T symbols (ISO 1101) for form, orientation, and position requirements.
A practical rule: if a dimension being wrong by 0.1mm would cause an assembly problem or functional failure, that dimension needs an explicit tolerance tighter than your general tolerance note. If being 0.1mm off wouldn’t matter, the general note is sufficient.
Communicating tolerance intent clearly means a precision machining company can accurately assess whether the job is within their capability, price the setup appropriately, and plan their inspection process around your critical features — rather than discovering tolerance conflicts after they have already started cutting.
Surface Finish — When to Specify It and What the Numbers Mean
Surface finish is the most commonly either over-specified or completely unspecified parameter in machined part orders. Both cause problems.
Ra (arithmetical mean roughness) measured in micrometers (µm) is the standard surface finish parameter. Practical reference values:
- Ra 3.2µm — standard as-machined finish from a milling or turning operation. Suitable for most non-sealing, non-sliding surfaces.
- Ra 1.6µm — fine machined finish, achievable with careful finishing passes. Suitable for general mating surfaces and light-duty sliding fits.
- Ra 0.8µm — precision finish, requires dedicated finishing operations or fine tooling. Suitable for sealing surfaces, bearing bores, and close-fit mating faces.
- Ra 0.4µm and finer — precision ground or superfinished surface. Required for high-load bearing surfaces, precision hydraulic components, and implant surfaces in medical applications.
The mistake most buyers make is specifying Ra 0.8µm or finer across the entire part when only one or two surfaces actually require it. Tighter surface finish on non-critical surfaces adds machining time and cost without any functional benefit. Specify finish where it matters — call it out on the specific surface in your drawing — and leave everything else to the standard as-machined finish.
If you do not specify surface finish at all, the supplier will deliver as-machined Ra 1.6µm to Ra 3.2µm. For many applications, that is perfectly adequate. If you are unsure, ask your supplier what finish their standard process produces on the material and operation you are ordering — a good precision machine shop will answer that question specifically.
Quantity and Lead Time — How Your Numbers Affect Your Options
The quantity you order affects almost every aspect of the quote — unit price, setup cost allocation, lead time, and which suppliers are realistically a good fit for your program.
For prototype quantities (1–10 pieces), the dominant cost is setup — programming, fixturing, and proving out the process. Unit price is high relative to production, but lead times can be short (3–7 days for simple parts, 1–2 weeks for complex geometry) from shops equipped for low-volume high-mix work.
For low-to-mid production (10–500 pieces), setup costs amortize across the run, unit prices drop significantly, and lead times extend slightly as the shop plans the job into their scheduling queue. This is where DFM (design for manufacturability) feedback from your supplier becomes particularly valuable — small design changes at this stage can meaningfully reduce unit cost at volume.
For production volumes (500+ pieces), dedicated tooling and fixtures, optimized cutting parameters, and process control systems (SPC, Cpk monitoring) become economically justified. Suppliers at this level should be able to quote consistent lead times with delivery reliability data.
When you are placing your RFQ, be honest about your actual quantity and your realistic production forecast. A supplier who knows you are ordering 10 prototypes today with a likely production order of 500 next quarter will invest differently in your relationship than one who thinks the 10-piece order is your total program.
RFQ Checklist — Everything to Include in Your Quote Request
A well-prepared RFQ gets accurate quotes faster and reduces the chance of surprises when parts arrive. Include the following every time:
Files and drawings:
- STEP file (3D model, final revision)
- PDF drawing with full dimensions, tolerances, and notes
- DXF if 2D profile work is involved
Material specification:
- Full alloy designation and temper (e.g. aluminum 6061-T6, not just “aluminum”)
- Material certification requirement if applicable (EN 10204 3.1 mill cert)
Tolerance and finish:
- General tolerance standard (ISO 2768 medium or fine)
- Feature-specific tolerances called out on the drawing
- Surface finish specified per surface where it matters (Ra in µm)
Quantity and delivery:
- Exact quantity required
- Required delivery date or lead time
- Whether this is prototype, pilot, or production intent
Quality requirements:
- First article inspection (FAI) required? Dimensional report to all callouts?
- Material certification to be included with shipment?
- Any industry certification requirements (ISO 9001, AS9100, ISO 13485)?
Packaging and delivery:
- Any specific packaging requirements (individual bagging, corrosion protection)?
- Delivery address and preferred freight method
Reading and Comparing Quotes — What the Numbers Actually Mean
When quotes come back, price-per-part is the most visible number — and the least informative one on its own. Here is what to look for beyond the unit price:
Setup charge vs unit price split. For low-volume orders, a high setup charge with a low unit price signals a shop optimized for repeat production — their economics work better if you reorder. A shop with lower setup and higher unit price is better structured for one-time or infrequent orders.
Lead time reliability, not just lead time. A three-day quote means nothing if the shop has a track record of slipping by a week. Ask for their average on-time delivery rate, not just the quoted number.
What’s included in the quote. Does the price include first article inspection? Dimensional report? Material certification? Some shops quote a bare price and charge separately for documentation — which matters if your quality requirements include paperwork.
DFM feedback. Did the supplier come back with any design for manufacturability comments? A shop that identified a potential issue with your design before quoting is a shop that reviewed your drawing carefully — and that behavior predicts how they will behave throughout the production relationship.
When comparing two quotes that are close in price, the one that came back with relevant technical questions is usually the better supplier.
First Article Approval — What to Check Before You Release Production
The first article is your most important quality gate. Before you approve production or release payment for a prototype batch, verify the following:
- Dimensional report against drawing. Every toleranced dimension on your drawing should appear in the report with an actual measured value. Spot-checking is not a first article.
- Material certification. Confirm the alloy and temper match your specification. For critical applications, verify the certificate traces to a mill source.
- Surface finish. If Ra was specified, confirm it was measured and recorded — not just described as “as machined.”
- Visual inspection. Check for burrs, tool marks on critical surfaces, sharp edges where radii were specified, and any cosmetic issues that the dimensional report would not capture.
- Fit check if possible. If the part mates with another component, do a physical fit check at first article. Dimensional reports verify individual features; fit checks verify that the combination of tolerances produces a working assembly.
Do not release a production order based on a first article you haven’t personally reviewed against these criteria. Discovering a recurring problem at 200 pieces costs dramatically more than catching it at piece one.
Ready to Place Your Order With Confidence
Ordering custom machined parts does not need to be complicated — but it does need to be specific. Clear files, correct material specification, explicit tolerances, and a complete RFQ package are what separate a smooth order from one that requires three rounds of clarification and still arrives wrong.
If you are ready to place an order or request a quote, Chiheng Hardware is an ISO 9001-certified precision machining company delivering custom parts to ±0.005mm tolerance — with a team that reviews every drawing, asks the right questions upfront, and provides full dimensional reports with first article inspection as standard.
