CNC Lathe Machining

What is CNC lathe machining?

CNC lathe machining is the cutting of round parts — shafts, bushings, pins, threaded bodies — on a computer-controlled lathe, where the workpiece spins and the tool is fed into it. The lathe holds diameter and length to ±0.002 mm on this floor, verified on CMMs. For rotational parts it is usually the fastest and cheapest process, and its make-or-break spec is concentricity.

CNC machining is the subtractive manufacturing of parts on computer-numerically-controlled milling, turning, and grinding equipment. A cutter removes material from a workpiece along a programmed toolpath to produce a part that meets a drawing-defined geometry, tolerance, and surface finish. "CNC" means the tool path is numerically controlled rather than hand-operated.

The rest of this guide turns that definition into a buyer's checklist for this shop — what to verify before you commit, and how the numbers on the drawing actually reach the part.

The discipline separates into milling (the cutter rotates and the workpiece is fed; prismatic geometry), turning (the workpiece rotates and the tool is fed; rotational geometry), and precision grinding / wire EDM (the tightest tolerances on critical features). A plant that produces CNC machined parts end-to-end runs all three.

The output of CNC machining is a part that meets the drawing — geometry, tolerance, surface finish, and material certificate — and is documented to a quality plan (FAI for general work, PPAP for automotive-bound parts, IMDS for material data). That is the answer to "what is CNC machining": a numerically-controlled subtractive process that produces a part to a drawing, with the quality documentation that lets the part enter an assembly.

Concentricity is the spec that breaks turned parts

A turned shaft that spins at speed fails in service when the bore or an outer diameter is not concentric with the axis it rotates on. Concentricity is a tolerance of runout measured relative to the axis itself, so it cannot be checked with a simple two-point micrometer; a part can be in diameter tolerance on every section and still wobble. For a pump shaft, a spindle, or a bearing seat, that wobble becomes vibration, seal leakage, and premature bearing wear.

The buyer risk is specifying concentricity in words without a number, or assuming the lathe holds it by default. It holds it only as well as the spindle, the fixture, and the tool allow. The second-order risk is over-specifying: a print that calls concentricity where a cheaper control would do adds inspection cost without adding function. Both risks are controllable if you ask the right questions before you order — what the datum is, what the number is, and how the shop proves it.

The runout budget, and how we split it

Runout is a budget with three main contributors. First, the machine spindle: our turning centers are indexed and dressed so the live center and chuck run true. Second, the fixture and the stock: bar feeder and collet condition set how true the raw stock sits. Third, the tool and the cut: tool wear and deflection grow with overhang and depth of cut.

We split the budget by assigning the largest share to spindle and fixture (the systematic part we control with maintenance and setup) and keeping tool deflection small through short tool overhang and tuned parameters. On a critical diameter we prove the split with an SPC chart at Cpk ≥ 1.67, not with a one-off measurement. The split is the same logic you use when you buy the part: you pay for control where the error is controllable, and you inspect where the error actually comes from. Here is how the budget reads for a typical turned diameter:

Contributor	Error mode	How it is controlled	Evidence
Machine spindle	Axis runout, bearing play	Indexed and dressed live center, chuck runout check before setup	Setup inspection record
Fixture and stock	Collet runout, bar straightness	Dressed collets, bar feeder condition check	Setup inspection record
Tool and cut	Deflection, insert wear	Short overhang, tuned feed and depth of cut	SPC chart across the run

The split matters because the three contributors fail differently. Spindle and fixture error is systematic — it repeats part after part until you fix it. Tool error is drift — it grows over the run as the insert wears. A control plan that only checks the first piece catches systematic error and misses drift; an SPC chart catches both.

Holding ±0.002 mm on turned diameters, with Cpk

The headline turning tolerance is ±0.002 mm. That is the band we hold on turned diameters across the parts we run, from small turned pins to diameters up to Ø720 mm and lengths to 1280 mm. But ±0.002 mm is only meaningful with a process-capability number behind it, because a machine that cuts one good part can still drift over a 500-piece run.

We hold Cpk ≥ 1.67 on critical characteristics and ≥ 1.33 on general ones, which tells you the process sits comfortably inside the band with margin for normal variation. A Cpk of 1.67 means the process mean is at least five standard deviations from the nearest tolerance limit — far enough that ordinary drift does not push parts out of spec. If your drawing marks a diameter as critical, ask for the Cpk on that characteristic specifically, not the plant average.

Concentricity vs true position to datum A

Buyers often write concentricity when true position to datum A would serve them better, or vice versa. Concentricity controls the axis of one feature relative to another, with no datum surface reference, which makes it costly to inspect because it needs a full rotational scan. True position to a defined datum A (the primary locating diameter) is usually what a mating assembly actually needs, and it is cheaper to verify on a CMM.

Control	What it checks	Inspection cost	When to call it
Concentricity	Axis of one feature to the axis of a datum feature	High — full rotational scan, axis reconstruction	True axis-to-axis relationship, no reference surface
True position to datum A	Feature axis location relative to a defined datum	Lower — direct CMM evaluation	Mating assemblies that locate on a primary diameter
Circular runout	Surface deviation per cross-section while rotating	Moderate — dial or CMM sweep	Roundness plus off-center error on a rotating surface
Total runout	Surface deviation across the whole surface while rotating	Moderate — full sweep along the axis	Bearing and sealing diameters, taper plus roundness

We machine to either callout exactly as drawn. The point for your drawing release is to specify the control that matches the function; over-calling concentricity adds inspection cost without adding function, and we flag it during the print review rather than quote it blind.

Circular runout vs total runout: which callout applies where

Runout comes in two levels, and mixing them up is a common print error. Circular runout checks each cross-section of a surface independently while the part rotates, so it catches roundness and off-center error but not taper along the axis. Total runout sweeps the entire surface in one pass, so it also catches how the axis of the surface changes along its length.

For a sealing or bearing diameter, total runout is the correct call, because the mating surface contacts along a length and a taper is just as damaging as a wobble. Circular runout is enough for a surface that only has to clear a bore without touching. The routing follows the callout: total runout on a long hardened diameter usually routes through a grinder, while circular runout on a short journal can often hold on a lathe. Ask which level the assembly needs, and the quote stops over-paying for inspection the part does not use.

Hard turning vs grinding to ±0.002 mm

When a turned feature needs better than ±0.002 mm, hard turning can get close, but grinding is the dependable route to ±0.002 mm. We run surface, centerless, and cylindrical grinders for that band. The decision is geometry and material: a through-shaft diameter is a centerless-grind job; a shoulder or a face is cylindrical; a flat reference is surface.

Hardened steels (through-hardened to about 60 HRC) are ground after heat treatment because cutting them in the hard state loads the tool. For a sealing diameter on a hardened shaft, plan the grind into the routing up front so the part is not quoted as turned-only and then found short. The trade-off is a handling step and a queue: hard turning on a rigid lathe can hold a few microns on a simple revolved diameter and removes that step, but it cannot reach shoulders and faces the way a grinder can. State the real need and the routing lands on the cheapest process that still holds it.

Material effects: stainless work-hardening, titanium springback

Material changes how a turned diameter behaves after the tool passes. 303 and 17-4 PH stainless work-harden, so a dull insert or a slow feed smears the surface and hides true size; we keep inserts sharp and parameters controlled. Titanium Grade 5 springs back, so a thin-walled turned section grows after the cut and must be taken to a light finishing pass with allowance left for it.

Incoming material is verified by XRF against the mill certificate before it is cut, so the alloy in the machine matches the alloy on your drawing. Here is how the common turned-part materials behave on a lathe:

Material	Machining behavior on a lathe	Runout / tolerance notes
6061-T6 aluminum	Cuts fast and clean; low cutting force	Holds tight diameters easily; watch thermal growth on long cuts
7075-T6 aluminum	Harder than 6061, still free-cutting	Good dimensional stability; slightly higher tool load
303 stainless	Free-machining; work-hardens if rubbed	Sharp insert and positive feed keep the surface true
304 / 316 stainless	Gummy, work-hardens readily	Needs controlled feed; avoid dwelling on a surface
17-4 PH stainless	Machines well in the solution-treated state	Machine before aging; hardens after heat treat
4140 alloy steel	Cuts clean, chips predictably	Common shaft stock; grinds well after hardening
Titanium Grade 5 (Ti-6Al-4V)	Springs back; low thermal conductivity	Leave allowance for a finishing pass; light depth of cut

The material table is the second half of a runout plan. A tolerance that holds in aluminum will not necessarily hold in 17-4 PH with the same tooling, because work-hardening changes the surface and springback changes the size. The print should name the alloy, and the shop should confirm it by composition, not by trusting the label.

Density and stiffness sit underneath those machining behaviors. Aluminum at roughly 2.7 g/cm³ and titanium at roughly 4.4 g/cm³ are light, so under the same cutting force they deflect differently from stainless at about 8.0 g/cm³. Light does not mean easy: titanium combines low density with low thermal conductivity, so the heat stays in the cut and the part springs back. A 4140 shaft at about 7.85 g/cm³ is stiff and predictable, which is why it is the default shaft stock. The density matters twice — once for the part’s inertia in service, once for how the stock deflects under the tool during the cut.

Build-to-print: the drawing is the contract

We are a build-to-print manufacturer. That is stated in our certificate scope: IATF 16949:2016 (131941/A/0001/SM/En) excludes product design under Clause 8.3. For you that means the drawing is the contract. We machine to the released print, inspect to it, and report against it. We review the print and flag a tolerance or a callout that fights the process, but we do not redesign the part.

Send datums, material, and the critical-characteristic list and you receive a part made to that print with the Cpk and CMM evidence on the characteristics you marked. The build-to-print boundary is also an honesty boundary: when a callout is ambiguous or over-tight, we say so during review, before the quote is final. A shop that agrees to every print silently is pricing a risk you will find later.

How we verify roundness and runout

Roundness and runout are verified, not assumed. Critical turned diameters are measured on Hexagon CMMs, and we use a KEYENCE laser microscope for fine features, an XRF spectrometer to confirm material, a micro-Vickers for hardness on hardened sections, a Fischer coating gauge for plating and anodize, and salt-spray and tensile testers for finished-part validation. A first-article report pairs the CMM roundness trace with the Cpk chart so you can see both the number and the stability.

Defect	Instrument	Why it matters
Concentricity / runout	Hexagon GLOBAL S CMM rotational scan	Measures the axis error a micrometer misses
Roundness vs size	Roundness trace on the CMM	Separates shape error from diameter error
Surface finish	SURFCOM 1800D / KEYENCE VK-X3000	Sealing and bearing surfaces depend on it
Material grade	XRF spectrometer vs mill certificate	Confirms the alloy before it is cut
Hardness	HVS-1000A micro-Vickers	Confirms heat treatment on hardened sections

The entity behind the work is Dongguan Zhuohang Ltd, trading as Zhuohang Precision Machining Factory and Zhuohang Precision Machining Factory, registration 91441900MA4UWT6X39, founded in 2005, at No. 8, Shuibian Songyuan Road, Hengli Town, Dongguan, Guangdong, China 523470. MOQ is 1 piece; lead time is 3 / 7 / 30 days for simple, standard, and complex turned parts.

What ISO 2768 general tolerances mean on a turned print

A drawing without individual tolerances on every feature usually falls back to a general tolerance standard, and the common one is ISO 2768. It defines tolerance classes — fine (f), medium (m), coarse (c), and very coarse (v) — that apply to linear dimensions, angular dimensions, and geometric features that are not individually toleranced. When a print states ISO 2768-m, every untoleranced dimension inherits the medium class.

The point for a turned part is that ISO 2768 is a default, not a precision claim. A diameter left to ISO 2768-m may be looser than the ±0.002 mm your assembly actually needs, so the print must still call the critical features out individually. The standard matters because it is the gap between "not specified" and "controlled": if a runout-sensitive diameter is only covered by a general tolerance, no shop can be expected to hold it tighter than the class allows. State the general class on the print, then tolerance the features that matter, that is how a drawing releases cleanly.

There is a second reading discipline worth stating. When a print carries both a general class and individual callouts, the individual callouts win — the frame overrides the class, and the class covers everything the frame does not. A shop that reads the print correctly applies the frame to the critical features and the class to the rest; a shop that applies one number everywhere is not reading the drawing at all. The overlap is where cost hides, because the frame is the expensive part and the class is the cheap part.

What surface finish should a sealing diameter carry

Surface finish is a functional spec, not a cosmetic one. A sealing diameter that spins against a lip seal needs a controlled roughness band; too rough and the seal wears, too polished and it may not retain a lubricant film. The common callouts are Ra 0.8 µm for sealing and bearing surfaces, Ra 1.6 µm for general mating fits, and Ra 3.2 µm for non-critical surfaces.

Ra value	Typical surface	How it is produced
Ra 3.2 µm	Non-critical, clearance faces	Standard turning pass
Ra 1.6 µm	General mating fits, housings	Fine turning or a light grind
Ra 0.8 µm	Bearing seats, lip-seal diameters	Cylindrical / centerless grinding
Ra 0.4 µm and finer	High-pressure sealing, precision spindles	Fine grinding, lapping where specified

Finish and runout travel together on a turned part: a ground sealing diameter is typically ground for both size and finish in one operation, so the two callouts should be specified on the same feature. Ask for the finish as an Ra number with the tolerance, not as "smooth" — a word a supplier cannot measure and you cannot verify.

How length-to-diameter ratio shifts the runout budget

The length-to-diameter (L/D) ratio of a turned part is the single largest driver of runout risk after the machine itself. A short, stiff part carries its runout budget in the spindle and collet, where it is cheap to control. A long, slender part shifts the budget to tool deflection and stock straightness, because the part bends under the cutting force and the axis wanders with it.

As a working rule, once the unsupported length exceeds roughly eight to ten times the diameter, the part needs a center or a steady rest to hold a tight diameter along its length, and the finishing pass gets lighter. The deflection grows with the cube of the length, so a small increase in length produces a large increase in deflection — this is why a long shaft cannot be quoted the same as a short pin at the same tolerance. When you release a long, slender drawing, expect the quote to mention a center, a steady rest, or a grind step; that is the routing paying for the L/D, not padding the price.

The same ratio also decides whether the part can be bar-fed or must be chucked. A short, stiff part runs from a bar feeder through a collet at high throughput; a long part needs a chuck and a tailstock, which is a slower setup with a different runout profile. The stock form — bar versus cut billet — follows from the same geometry, and the quote should reflect it. When a part crosses from bar-fed to chucked, the cost steps up, and the step is the geometry talking, not the supplier.

Which DFM rules keep a turned part cheap and concentric

Design-for-manufacturing on a lathe is mostly about giving the tool and the fixture a fair shot. A few rules cover most of the cost and quality on a turned part:

• Keep the L/D ratio manageable. A slender part deflects; a stiff part does not. Add a center or relax the tolerance before the ratio forces a grind step.
• Leave a runout groove at a shoulder. A square shoulder with a tight diameter next to it needs a relief, or the tool cannot cut the diameter without digging into the shoulder.
• Specify the datum you actually locate on. The primary datum should be a real, accessible surface the fixture can grip, not a theoretical axis.
• Avoid thin walls on turned features. A wall under about 1 mm deflects under the tool and springs back, which fights a tight diameter.
• Mark tight only where the assembly loads. Every tight callout adds a finishing pass or a grind; a print with twenty tight diameters costs like twenty tight diameters.
• Name the material and its condition. Heat-treated, cold-drawn, and annealed stock all machine differently, and the routing depends on it.

Each of these is a question a print review should raise before the quote, not after the first article. A shop that reads the drawing and flags the relief, the datum, or the wall thickness is engineering your part; a shop that quotes it silent is deferring the cost to the floor.

One DFM rule deserves emphasis because it touches runout directly: specify the datum as a surface you can actually grip. A datum that is a theoretical centerline forces the fixture to approximate an axis it cannot hold, and the approximation becomes runout in the finished part. A datum that is a real, accessible diameter gives the collet or chuck a surface to grip true, and the runout budget starts from a known point. The cleanest turned prints are the ones where the datum is obvious the moment the stock is loaded.

Where concentricity fails in service: pump, spindle, and bearing seats

Concentricity failures concentrate in the parts that rotate under load, because that is where a small axis error becomes a large service problem. A pump shaft carries the risk in its bearing seats and seal diameters: an off-axis seat loads one bearing harder than the other and wipes the seal. A machine spindle carries it in the taper and the tool seat, where runout becomes vibration that shows up in the finish of every part the spindle makes. A bearing journal carries it in the seat and shoulder, where the control that matters is often true position to the primary datum plus a total runout on the diameter.

The common thread is that the assembly, not the print, decides which control matters. A bearing seat wants total runout and roundness; a locating shoulder wants squareness to the axis; a pulley diameter wants balance, which is a mass property, not a geometric one. Buying by part family means asking which features the assembly loads and routing the tightness there — the same logic that separates a cheap pin from an expensive spindle is the logic that should be on your drawing.

Cross-industry turned parts: e-bike, medical, and robotics

Turned parts repeat across industries with different failure costs, which changes how tight the controls need to be. An e-bike drive shaft runs at moderate speed and high torque, so its runout tolerance is a balance and vibration trade, not a micron chase. A medical instrument shaft or bone screw is small and regulation-bound, so the material certificate and the surface finish matter as much as the diameter. A robot joint pin runs in a closed loop that amplifies backlash, so a few microns of runout become positioning error at the end effector.

Across all three, you question are the same: what is the datum, what is the number, and how is it proven. The difference is where the budget goes — an e-bike shaft budgets for balance and a clean surface, a medical part budgets for traceable material and a finish a cleaning process can sterilize, and a robot joint budgets for the tightest runout the loop can tolerate. Our turning cell runs all three families with the same SPC discipline, and the routing changes with the part, not with the pitch.

One more family worth separating out: fluid-handling parts. Valve stems, pump shafts, and seal carriers live or die by their sealing diameters, so their critical feature is usually a total runout on a ground surface, not a balance or a micron across the board. The part name changes, but the drawing discipline does not — name the datum, number the critical feature, and let the routing follow the load.

What signals a supplier is hiding cost in the tolerance

Tolerance is where a supplier hides cost when it does not want to show it on the line item. The signals are specific and checkable. A quote that lumps every tolerance into one "precision machining" line is hiding which features actually cost. A supplier that agrees to a ±0.002 mm callout without asking which process will hold it is pricing a risk, not a part. A Cpk that is quoted without the characteristic it belongs to — "we hold 1.67" with no run length, no feature, no chart — is a number with no evidence behind it.

The counter is to demand the routing and the evidence in the quote itself. A shop that can tell you which machine, which pass, and which instrument holds each critical feature is showing you its cost structure; a shop that cannot is asking you to discover it later.

How to read a turned-part quote for runout risk

A turned-part quote is a document, and the runout risk is readable in how it is structured. The quote should name the material grade, not just "aluminum" or "stainless." It should separate the turning from the grinding, because a ±0.002 mm diameter that is quoted as turned-only is a risk waiting to fail. It should state the finish as an Ra number, the inspection as a CMM report, and the capability as a Cpk on the specific feature.

Line items map to operations, and operations map to cost. A quote with a grind line on the sealing diameter is a quote that read the drawing. A quote with one lump number and no breakdown is a quote that skimmed it. When you compare two quotes, compare the routing, not just the bottom line — the cheaper number often means a tolerance that will not be held, and the difference shows up as rework you pay for later.

The quantity column is part of the same reading. A turned part's setup cost — programming, fixturing, tooling, first-article inspection — is fixed and lands once per order. Spread it over ten parts and it is a large share of each; spread it over a thousand and it nearly disappears. A quote that shows the setup separately is honest about the volume effect; a quote that buries it in a unit price is asking you to guess where the break-even sits.

Incoterms and how freight enters a turned-part order

A turned part shipped from Dongguan crosses a border, and the Incoterm defines where the risk and the freight cost change hands. EXW means you own the part from the factory gate; FOB means the seller loads it on the vessel; CIF adds insurance and freight to the seller's side; DAP or DDP pushes delivery further toward your door. The term is not a detail — it sets who is responsible when a shipment of precision shafts is damaged in transit.

The term does not change the part's geometry, but it changes the landed cost and the risk. A tight-tolerance shaft that arrives bent or corroded is a loss the Incoterm allocates to one party or the other. State the term in the RFQ, and ask how the parts are packed — a turned shaft needs a way to prevent flex and surface damage in transit, and the packing is part of the tolerance surviving the trip. The supplier should confirm the term and the packing in the quote, not leave both to be discovered at customs.

The buyer due-diligence checklist for a turned-part supplier

Before you release a turned-part order, run these checks. Each one is answerable in writing, and the answers together separate a manufacturer from a middleman:

• Legal entity. Ask for the legal name and registration number, then verify against a public registry. Ours is Dongguan Zhuohang Ltd, 91441900MA4UWT6X39.
• Machine list. Ask for models and counts, then reconcile with the floor on a visit. A manufacturer names them; a trading desk recycles brochures.
• Tolerance evidence. Ask for a CMM report from a real job and an SPC chart with the run length and the Cpk per critical characteristic.
• Certificates. Ask for the numbers and verify with the registrar — ISO 9001:2015 (116024/A/0001/UK/En), IATF 16949:2016 (131941/A/0001/SM/En), ISO 14001:2015 (116024/B/0001/UK/En).
• Routing. Ask which process holds each tight feature — turning, mill-turn, or grinding — and who owns the secondary operations.

The checklist works because a middleman cannot produce a machine list, a measured report, or a verifiable certificate on demand. A manufacturer produces all of them in writing, and the answers arrive without hedging.

How do you run the audit on us

The audit is not a sales exercise; it is a set of documents you can request and verify. Email [email protected] and ask for the registration number, the machine list, a CMM report or SPC chart sample, and the certificate numbers. We provide all of them in writing, and we welcome a visit to reconcile the list against the floor at our plant in Hengli Town, Dongguan.

The turning cell runs 25+ lathes — Citizen, Star, Tsugami, and Mazak — with the collets dressed true and the live centers checked so a critical diameter holds ±0.002 mm with Cpk ≥ 1.67 through the run, not just on the first piece. The rest of the floor covers 3-, 4-, and 5-axis milling, mill-turn, and surface / centerless / cylindrical grinding, all under ISO 9001, IATF 16949, and ISO 14001. The question is not whether we can hold the tolerance — the documents answer that — but whether the supplier you are comparing us to can produce the same evidence.

How to read an SPC chart in one pass

An SPC chart looks busy until you know the three things to read. First, the control limits: they sit inside the tolerance band, so a point can be inside the band and still outside the limits, that is the chart flagging drift before it becomes a reject. Second, the center line: it shows where the process mean sits, and a mean that has crept toward one limit is the early warning of a tool wearing. Third, the run: capability is a claim over a run of consecutive parts, not a single measurement, so the chart's length and its stability are the evidence, not the headline Cpk.

The chart answers the question a Cpk number alone cannot: whether the process is stable or merely lucky. A Cpk of 1.67 computed on a short, unstable run is a fiction; a Cpk of 1.67 computed on a long run that holds its center line is a process you can buy against. When you ask a supplier for the SPC chart, ask for the run length and the characteristic it covers — a chart without those is a decoration.

Single-setup mill-turn removes re-clamping runout

Every re-clamp is a chance to re-introduce runout, because the part has to be located a second time and the second location is never identical to the first. Mill-turn removes that chance: a turned profile and its milled features are cut in one clamping, on one machine, so a bore and the face that must be square to it share a single setup and a single datum.

The economics follow the geometry. A part that would otherwise travel from a lathe to a mill — with a fixture in between and a re-clamping error nobody measured — stays on one spindle in a mill-turn center. That is why a flange with a bore, or a shaft with a cross-hole, is a mill-turn job, and why the quote should say so. Single-setup machining does not eliminate runout; it eliminates the re-clamping contribution, which is the contribution that is hardest to inspect and easiest to overlook.

What tolerance should the print actually carry

Choosing a tolerance is a design decision before it is a cost decision, and the rule is simple: tolerance only what the assembly loads. Every feature with a tight callout adds a finishing pass, a control chart, or a grind step, and the cost compounds with every extra tight diameter on the same part. A print with twenty ±0.002 mm diameters costs like twenty tight diameters, even if the assembly only loads two of them.

The practical sequence is to find the features the mating part actually touches, tolerance those to what the function needs, and let the rest fall to a general tolerance such as ISO 2768-m. A bearing seat gets a number; a cosmetic face gets the general class. The discipline pays twice: the part routes to the cheapest process per feature, and the supplier can read exactly which characteristics the inspection must prove. Over-tolerancing is the most common way a turned part costs more than it should, and it is entirely in your control.

How to read a feature control frame on a turned print

A feature control frame is the small box on the drawing that does the real work, and reading it tells you what the shop will have to prove. It reads in four parts: the geometric characteristic symbol (runout, true position, concentricity, or others), the tolerance value, any modifiers such as a material condition, and the datum reference letters. For a turned part the datum is usually the primary diameter, lettered A, and the frame hangs the other features off it.

The frame is also the contract the inspection follows. A runout frame with a tolerance and a datum tells the CMM exactly what to measure and against what; a bare diameter with no frame falls back to the general tolerance and is inspected as a size, not a location. When the frame and the function disagree — a runout callout where the assembly really needs position — the print review is the place to catch it, and a good shop flags the mismatch before quoting. Read the frame, check it against the function, and the drawing releases clean.

Surface treatments that change the finish, not the geometry

Finishing operations sit on top of a machined surface and change how it wears, corrodes, and seals — without changing the geometry underneath by much, if they are controlled. Anodize grows an oxide layer on aluminum, so a tight bore must be machined with the coating thickness in mind; plating does the same on steel, adding a few microns that matter on a close fit; passivation cleans the free iron off stainless without building up.

The interaction with runout is indirect but real. A plated sealing diameter that is not masked or controlled in thickness ends up out of round or out of size after the coating, so the finished dimension — not the machined dimension — is what the drawing must state. State the treatment, the thickness, and whether the tolerance applies before or after finishing, and the routing will place the machining and the finishing in the right order. A print that leaves the finish and the tolerance ambiguous is a print that leaves a few microns to be argued about at inspection.

Guides

This guide goes deeper on one material decision than the overview above can:

• CNC Lathe Machining Cost in 2026 — your Cost Model Where Runout Sets the Price.
• Aluminum CNC lathe machining — turning round parts from bar, the alloys that behave on a lathe, chip control in bores, and what finish and tolerance a turned diameter can hold.

• CNC lathe machining cost in 2026 — your cost model where the runout budget sets the price.

Frequently Asked Questions