Precision
Precision Machined Components
Precision machined components are metal and plastic parts with features toleranced tighter than the ±0.005 in typical of machined work: ±0.001 in on critical turned diameters and down to ±0.0002 in on wire-EDM features, and ground features are quoted to ±0.0001 in. Send the drawing with those features called out and a person at MI CNC Parts, a Michigan-based parts supplier, quotes it in 24-48 hours.
Updated September 2026
Example part, shown for illustration.
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Parts where a few features carry a tight fit or location.
What kinds of precision components get quoted?
Precision components quoted here include turned shafts and pins, bushings and sleeves, manifolds and fluid blocks, punches and die parts, and small, slender parts: parts where a bore, a diameter or a hole pattern has to fit something else. The small, slender ones go to Swiss-type turning, where the tools sit just in front of a guide bushing that supports the bar, so the part has little chance to deflect or vibrate (source 1). Anything else quoted from a drawing or a model is covered under custom CNC parts.
Which process does a tight feature go to?
Most of a part is milled or turned to the general tolerance, and a tight milled feature is quoted from the print. A fit or a location that needs more goes to turning, wire EDM or precision grinding, as the table below shows. Rush 1-2 weeks applies to machined parts with no secondary operations. Heat treat, grinding, EDM and finishing add time, and the quote gives the real date.
| Feature on the drawing | Process | Why that process |
|---|---|---|
| Shaft journal, bearing seat on a shaft, slip-fit diameter | CNC turning | The part rotates against a nearly stationary tool, so diameters and faces are cut as it spins (source 2). |
| Profile, slot or small inside corner in hardened tool steel | Wire EDM | Spark erosion through a thin wire with no high cutting force: it cuts hardened tool steel, leaves low residual stress, and reaches small inside corners (source 3). |
| Bore, pin diameter or flat face at the tightest size or a fine finish | Grinding | It takes very shallow cuts, on the order of half a thousandth off a diameter, leaves a very fine finish, and cuts hardened steel (source 4). |
What should the drawing say about a tight feature?
Put a tight tolerance only on the features that need it, and leave the rest to the default tolerance in the title block, which applies wherever a dimension carries none of its own (source 5); CNC machining tolerances covers how to set that default. The design review on your quote points out a callout that costs more than its function needs, and design choices that raise a quote lists the others worth checking first.
Where a feature locates something, say what it is measured from. ASME Y14.5-2018 sets the symbols and rules for GD&T on drawings and in model files (source 6). With it, a hole gets a position tolerance, a face a perpendicularity tolerance or a diameter a runout tolerance, each referenced to datums: theoretically exact planes, lines, points or axes identified on the part. Flatness is different: it controls a face on its own and references no datum. Written this way, the drawing says which dimensions matter and from where, and each tolerance can stay as large as the function allows (source 7). Position, perpendicularity and runout callouts are quoted from the print.
The common grades have their own pages, for example 17-4 PH stainless and titanium. Other alloys and plastics quoted on request. Name the grade on the print.
Sources
All accessed 27 September 2026. Where a figure here differs from your print, the print governs.
1. Wikipedia, "Swiss-style lathe": en.wikipedia.org/wiki/Swiss-style_lathe (tools in front of the guide bushing, collet behind it; suited to slender workpieces, held firmly with little chance of deflection or vibration).
2. Wikipedia, "Turning": en.wikipedia.org/wiki/Turning (a cutting tool held nearly stationary against a rotating workpiece; straight turning and facing).
3. Wikipedia, "Electrical discharge machining": en.wikipedia.org/wiki/Electrical_discharge_machining (wire-cut EDM for punches, tools and dies from hard metals; hardened tool steels; low residual stress because no high cutting forces are needed; internal corners among the listed advantages).
4. Wikipedia, "Grinding (abrasive cutting)": en.wikipedia.org/wiki/Grinding_(abrasive_cutting) (very fine finishes and very accurate dimensions; very shallow cuts, such as half a thousandth off a shaft diameter; hardened steels).
5. Wikipedia, "Engineering drawing": en.wikipedia.org/wiki/Engineering_drawing (the title block carries default tolerances for dimensions where no tolerance is specified).
6. ASME Y14.5-2018 (R2024), "Dimensioning and Tolerancing": asme.org (symbols, rules and definitions for GD&T on engineering drawings and in digital model files).
7. Wikipedia, "Geometric dimensioning and tolerancing": en.wikipedia.org/wiki/Geometric_dimensioning_and_tolerancing (datums as theoretically exact planes, lines, points or axes; position, perpendicularity and runout reference datums, flatness is a form control that does not; form, fit and function with the largest possible tolerances).
8. Wikipedia, "Anodizing": en.wikipedia.org/wiki/Anodizing (oxide grows into and out of the surface by equal amounts, so each surface grows by half the oxide thickness).
9. Wikipedia, "ISO 1": en.wikipedia.org/wiki/ISO_1 (20 °C, 68 °F, the reference temperature for geometrical product specification and verification).
10. Wikipedia, "Thermal expansion": en.wikipedia.org/wiki/Thermal_expansion (linear coefficients at 20 °C, in millionths per kelvin: aluminium 23.1, brass 19, steel 11.0-13.0, stainless steel 10.1-17.3, titanium 8.6).
Questions
Before you send a job.
01 What paperwork ships with the parts?
No C of C or material certifications ship with parts by default. If your PO needs them, say so before the quote. Inspection reports on critical dimensions aren’t included by default. If you need one, ask when you place the order.
02 Can a precision part be quoted from a STEP file alone?
Yes. With no drawing, it is quoted to a ±0.005 in general tolerance, or to the tolerance your notes name. The model gives the shape, so a tighter feature needs a drawing or a note that names the feature and its tolerance. Quoting from a STEP file covers when a drawing is asked for.
03 Can one quote price more than one quantity?
Yes. Ask for more than one quantity, such as 1, 25 and 100, and the quote shows each. Blanket orders can be quoted too.
04 When does the lead time start?
Lead time starts at order approval. Put your need date in the part description and the quote answers it. You pick Rush, Standard or Extended on the quote form, and the CNC machining FAQs explain what each tier is for.
05 Does anodizing change a tight dimension?
Yes. Anodic oxide grows into the surface and out of it by equal amounts, so each face gains half the coating thickness, and an outside diameter grows, or a bore shrinks, by the full thickness (source 8). Say on the drawing whether a dimension applies before or after finish; the finish is specified on the print and quoted with the part. Anodizing covers Type II and hardcoat thickness and which features get masked.
06 How much does temperature move a tight dimension?
The reference temperature for specifying and checking part geometry is 20 °C (68 °F), set by ISO 1 (source 9). Per degree C, aluminum grows 23.1 millionths of its length, brass 19, steel 11-13, stainless 10.1-17.3 and titanium 8.6 (source 10). A 4 in feature measured 10 °C (18 °F) above reference is about 0.0009 in longer in aluminum, 0.0005 in longer in steel and 0.0003 in longer in titanium, each more than a ±0.0002 in tolerance allows.
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