From concept to product
From prototyping to production, our production sites can handle all your manufacturing projects.
What level of precision can be achieved with custom CNC turning parts?
The short answer
Custom CNC turning holds ±0.01 to 0.05 mm to standard, tightens to 0.002 to 0.005 mm on a precision lathe, and reaches 0.001 to 0.002 mm on a Swiss-type machine. Roundness runs 0.5 to 3 micrometres and surface finish reaches Ra 0.4 micrometres. Geometry, not the machine alone, sets the limit.
Three tiers, and what separates them
Turning capability is usually quoted as a single number, which is misleading, because the same lathe that holds 0.05 mm on a long slender shaft will hold 0.005 mm on a short rigid collar in the same program. The published tiers are still a fair starting point. General industrial turning sits at 0.01 to 0.05 mm, precision turning at 0.002 to 0.005 mm, and Swiss-type turning at 0.0025 to 0.005 mm with repeatability in the 0.002 mm band on a high-end machine. At the extreme end, a purpose-built machine can reach 0.002 mm and a diamond-turned optical part tighter still, but that is a specialised process rather than a turning shop's normal output.
| Tier | Dimensional tolerance | Roundness | Finish Ra | Typical use |
|---|---|---|---|---|
| Standard lathe | plus or minus 0.010 to 0.050 mm | 2 to 3 micrometres | 0.8 to 1.6 micrometres | Brackets, spacers, general shafts |
| Precision lathe | plus or minus 0.002 to 0.005 mm | 0.5 to 1.0 micrometre | 0.4 to 0.8 micrometre | Bearing seats, hydraulic spools |
| Swiss-type lathe | plus or minus 0.0025 to 0.005 mm | 0.5 to 1.0 micrometre | 0.2 to 0.4 micrometres | Bone screws, pins, connectors |
The reason the two ends of the scale behave differently is the guide bushing. On a conventional lathe the bar hangs out of the chuck, so the unsupported length grows as the tool moves along it, and a long part deflects under cutting force. A Swiss-type lathe feeds the bar through a guide bushing a millimetre or two behind the cutting edge, so the supported span never changes and the effective overhang is constant whatever the part length. That is what lets it hold 0.005 mm on slender work and produce finishes down to Ra 0.2 to 0.4 micrometres in a single pass.
The geometric callouts nobody quotes, and everybody needs
Size and form are different things, and a part can be perfectly on diameter and still useless. A bearing seat that is 0.003 mm oversize but two micrometres out of round will transmit vibration; a spool that is round in every cross-section but bowed by five micrometres will not seal. The four callouts that carry that function are roundness, cylindricity, concentricity and runout, and they are quoted separately from the diameter for a good reason.
| Callout | What it controls | Typical achievable figure |
|---|---|---|
| Roundness | Whether every cross-section is a true circle | 2.5 micrometres standard, 0.5 to 1.0 micrometre controlled |
| Cylindricity | Roundness and straightness combined over the whole surface | 5.0 micrometres standard, 1.0 to 2.0 micrometres controlled |
| Concentricity | Whether inner and outer diameters share one axis | About 0.01 mm, or 0.005 mm with one-setting machining |
| Total runout | The total indicator reading as the part rotates | 0.010 mm standard, 0.003 to 0.005 mm controlled |
One worked example shows why these matter more than the diameter in hydraulics. On a dynamic seal seat the roundness figure should not exceed 0.002 mm, and a deviation above 0.005 mm is enough to make the seal snake and wear unevenly; on a piston rod 500 to 2,000 mm long, cylindricity has to stay under 0.01 mm over the whole length. Studies cited in that sector put the penalty bluntly: a cylindricity deviation beyond 0.003 mm on a dynamic seal cuts seal life by 30 to 50 percent.
Surface finish is a tolerance in its own right
Ra is the roughness average of the turned surface, and on a rotating part it decides how the part behaves at the interface rather than how it fits. As-machined turning typically lands between Ra 0.8 and 1.6 micrometres. A programmed finishing pass with an appropriate nose radius and a wiper insert takes the same part to 0.4 to 0.8 micrometres, and a Swiss-type machine with a rigid setup reaches 0.2 to 0.4 micrometres without grinding. Going finer than that normally means grinding or lapping rather than turning.
The counter-intuitive part is that smoother is not always better. Hydraulic piston rods specify Ra 0.2 to 0.4 micrometres after grinding and plating, and a surface below Ra 0.1 micrometres can hold too little oil to keep the seal lubricated, which increases friction rather than reducing it. Above Ra 0.8 micrometres the surface abrades the seal. At 350 bar, an Ra above 0.8 micrometres on a rod has been reported to cut seal life from a planned 2,000 operating hours to under 800. Corrosion resistance behaves differently again: anodising and plating amplify surface texture rather than hiding it, so a rough turned surface finishes rough.
What actually limits the precision you receive
Four factors decide whether a shop delivers the tier you asked for, and only the first one is bought rather than managed.
- Machine condition. A lathe is only as good as its current geometry. A machine with 0.002 mm positioning accuracy and closed-loop tool wear compensation is the entry condition for micron work; a worn spindle bearing is not fixed by slowing the feed.
- Tooling and insert grade. A sharp edge with the right nose radius and coating cuts consistently; a dull insert pushes the material instead of shearing it and the diameter drifts through the batch.
- Thermal stability. A one degree change in shop temperature moves a steel part by roughly 11 micrometres per metre of length, so a temperature-controlled cell is a real requirement above the precision tier rather than a marketing line.
- The gauging loop. Precision comes from measuring and compensating, not from hoping. Machines with thermal compensation and automatic tool offsets hold about 0.003 mm through a whole shift without an operator rescuing the dimension.
Material belongs on the same list. Aluminium and brass are geometrically stable and cut cleanly; austenitic stainless work-hardens and moves, and titanium generates heat, work-hardens quickly and springs back. A tolerance that is routine in 6061 may need a changed cutting strategy in Ti-6Al-4V, and a shop quoting the same figure for both is quoting a catalog rather than a process.
Capability versus assured capability
A single good part proves nothing about a batch. The statistic that settles it is process capability: a Cpk above 1.33 means the natural spread of the process sits inside the tolerance band and corresponds statistically to roughly 66 non-conforming parts per million, while high-pressure hydraulic work commonly requires Cpk above 1.67. This is the number to ask for when the part rotates, because it is measured on your feature, on your material, at your batch size, rather than on the machine specification sheet.
Annual quantity also shapes what is achievable. Conventional turning setup runs roughly USD 200 to 600, while a Swiss-type machine runs USD 400 to 1,200 plus guide bushing preparation, for a machine hour rate of about USD 90 to 180 against USD 60 to 120 for a conventional lathe. Swiss-type work becomes the right answer above roughly 3,000 parts a year, and a conventional lathe with a tailstock or steady rest is the sensible route below that, even when the tolerance could be met either way.
Where tight turning tolerances are the wrong thing to buy
Three situations call for a discussion before an order. First, when a tight callout is on a feature nobody measures functionally: every extra decimal place adds inspection time and scrap risk. Second, when the tight dimension spans a long unsupported length: ask instead whether a stepped diameter, a shoulder, or a change of process would remove the need. Third, when the part is plastic. POM, nylon and PEEK move with moisture and temperature, so holding turning tolerances tighter than the functional need on a plastic part buys a measurement that will not survive the first week in service.
How to specify a turning tolerance
Send the STEP model and a drawing that carries the general tolerance block plus explicit callouts on the functional features, the material grade and condition, the quantity and annual forecast, the finish per surface, and the specific diameters whose roundness or runout actually matters. Five items, and an engineer can then tell you honestly whether the figure is routine on their machines, achievable with a changed strategy, or impossible. See CNC machining for the process, surface finishing for what follows turning, and SOMI 3D printing when the feature you need cannot be cut from solid at all.
Scope and sources. Tier figures, geometric callouts and the industry tolerance table come from a CNC turning precision reference (standard 0.01 to 0.05 mm, high precision 0.002 to 0.005 mm, Swiss tighter still, roundness within 0.001 mm, Ra 0.4 to 1.6 micrometres, repeatability within 0.002 mm, and 0.002 mm positioning accuracy for medical work) and from a precision turning capability table (geometric tolerances 0.005 mm standard against 0.001 to 0.002 mm controlled, Ra 0.8 against 0.2 to 0.4 micrometres, roundness 2.5 against 0.5 to 1.0 micrometres, cylindricity 5.0 against 1.0 to 2.0 micrometres, total runout 0.010 against 0.003 to 0.005 mm). Geometric tolerance ranges also draw on a CNC turning accuracy specification (roundness within 0.005 mm, cylindricity within 0.01 mm, straightness within 0.005 mm, concentricity within 0.01 mm, perpendicularity within 0.005 mm, Ra 0.8 to 1.6 micrometres, tolerances per ISO 2768 or as designed). Hydraulic sealing figures and process capability come from a hydraulic turning reference (roundness under 0.002 mm for dynamic seal seats, cylindricity under 0.01 mm over 500 to 2,000 mm, Ra 0.2 to 0.4 micrometres after grinding, seal life cut 30 to 50 percent by a cylindricity deviation above 0.003 mm, Ra above 0.8 micrometres at 350 bar cutting rod seal life from 2,000 hours to under 800, Cpk above 1.33 equal to about 66 parts per million, Cpk above 1.67 for high-pressure hydraulic work, plus or minus 0.003 mm held through a shift with thermal compensation). Swiss-type capability, cycle structure and setup economics come from a Swiss turning comparison (Swiss plus or minus 0.005 mm against conventional plus or minus 0.02 mm, Ra 0.2 to 0.4 against 0.8 to 1.6 micrometres, spindle 6,000 to 12,000 against 3,000 to 6,000 revolutions per minute, setup USD 400 to 1,200 against USD 200 to 600, machine hour USD 90 to 180 against USD 60 to 120, break-even around 3,000 parts). Figures are planning ranges from published sources and not a quotation; confirm them against your material, geometry and quantity.
Are you looking for a reliable manufacturer of down home textile products?
We can quickly provide customers with market analysis, technical support and customized services.From concept to product
From prototyping to production, our production sites can handle all your manufacturing projects.
subscription