Accurate CNC Parts Quotation Guide for Engineers

An accurate CNC parts quotation is not a single black-box figure. It is the sum of setup, programming, machine time, inspection, material and outside processing. Change one of those and the price moves. When a quote arrives with no breakdown at all, you cannot tell whether the supplier missed a feature or priced it deliberately.

Geometry is the first driver. A part that can be reached from three orthogonal directions is fast to program and fast to cut. A part with undercuts, deep pockets, thin walls or features on five faces needs more setups or a simultaneous 5-axis machine, and the programming hours climb with it. GreatLight runs 16 simultaneous 5-axis machining centers and 16 mill-turn centers, which handle these shapes in one or two setups instead of five.

Tolerance is the second. General machining holds ±0.005 mm on critical features, but that band is not free across every dimension on a drawing. A few controlled features are normal. A drawing where every dimension carries the same tight band forces slower feeds, more passes and CMM verification time. Give the tight tolerance to the features that actually locate, seal or mate.

Material and stock form come next. Aluminium 6061 cuts quickly and is cheap to buy. Stainless 316 and 17-4PH work-harden, so feed rates drop and tool wear rises. Titanium TC4 and Inconel push it further, and a part cut from solid bar costs more than the same part cut from near-net stock. Plastics machine fast but need care on clamping, because PEEK and POM deflect under light pressure.

Finishing, quantity and inspection close the list. Anodizing, plating, powder coating or heat treatment add outside processing and a second handling cycle. Quantity spreads the fixed setup across more units, so unit price falls. Inspection depth scales with the drawing: a first-article report, material certificates and 100% dimensional checks all take time that has to appear in the number.

Most inaccurate quotes trace back to an incomplete RFQ, not to a careless supplier. Send the same package to three shops and you should get three numbers within a narrow band. Send a STEP file and a one-line email, and the spread widens because each shop guesses differently about material, finish, quantity and inspection.

Step one is a native 3D model plus a 2D drawing for anything with a tolerance, thread callout or surface requirement. STEP and IGES files carry shape, not intent. If a hole is Ø8 H7, say so on the drawing; do not leave it to the model. Step two is the material and temper, written as a standard grade such as 6061-T6 or 17-4PH, not as aluminium or stainless.

Step three is quantity plus the split you want: prototype only, or prototype plus a production estimate. Step four is finishing and marking, including color, thickness class and laser marking text. Step five is inspection: what reports you need, whether first article is required, and whether material certificates must travel with the parts.

Add the deadline you are working to, and mark which tolerances are functional. A short note such as these two bores are bearing seats explains more than a page of general notes. If a feature is a reference dimension, label it that way so the shop does not price a tight band you never needed.

We answer with a quotation and a free DFM analysis within 12 hours, and production can start within 24 hours of approval. If your drawing has an issue that will raise cost, that analysis flags it before the chips fly rather than after.

Material selection is often made for mechanical reasons, then quietly paid for in machining time. Aluminium 6061-T6 is the default for housings, brackets and fixtures: good strength, easy chips, wide stock availability in 6061, 2024, 5052, 6082 and 7075. If the part does not need the strength of 7075, choosing it adds cost with no benefit.

Stainless 303 machines cleanly and is the friendly choice for shafts and fittings. Grade 304 and 316 resist corrosion better but gum up tools, and 17-4PH needs a heat-treat step to reach its final strength. Budget for that cycle and for the dimensional shift that follows heat treatment.

Titanium TC4 (Ti-6Al-4V) and Inconel are for high-temperature or high-strength aerospace work. Cutting speeds are low, tool life is short and cycle times can be several times longer than the same part in aluminium. Use them when the service condition demands it, not as a default.

Plastics are a different trade. ABS, PC, PMMA and POM are inexpensive and fast, but thin walls flex and deep pockets need light passes. PEEK holds up at temperature and costs more per kilogram than many metals. Carbon fibre composites machine to a fine dust that needs extraction, so expect a handling charge.

Quote accuracy depends on whether the shop can actually make what it prices. Ask what machines will run the part, how many setups the plan uses, and how the critical features get measured. A shop that says 5-axis but plans three setups on a 3-axis mill is telling you the quote and the process do not match.

Inspection is the other half. A tolerance of ±0.005 mm is meaningless without a way to verify it. CMM time, gauge blocks and first-article reports are real costs, and a supplier who leaves them out is either not measuring or not telling you. We inspect 100% before shipment, covering incoming material, in-process checks and final inspection, with reports on request.

Certification tells you which industries a shop is set up to serve. GreatLight holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022, which map to general industrial, automotive, medical device and information security requirements. Those audits cover process control, traceability and document handling, not just a wall certificate.

Capacity also matters for accuracy over time. With 127 high-precision CNC machines across 3 wholly-owned plants and 7,600 m² of floor space, a job can be scheduled onto the right machine rather than squeezed onto whatever is free. Parts ship in 3–5 days, with historical late delivery below 2%.