Precision Matters: Tolerances in CNC Metal Fabrication Explained

13 November 2025

If you spend your days in a metal fabrication shop or on the floor of a machine shop, you learn quickly that tolerances are where projects live or die. Not on the glossy render, not in the quote, but in the final thousandths that decide whether a shaft slides, a gasket seals, or a jig locates a part on the first try. CNC metal fabrication gives us incredible control, yet that control only pays off if the specified tolerances match the purpose, the process, and the budget.

I have watched a bracket designed with ±0.001 inch tolerances across every feature rack up cost like a tax meter, while looser surfaces would have done the job with no impact on performance. I have also scrapped a run of parts because a seemingly generous ±0.010 inch callout let manufacturing shop https://myanimelist.net/profile/arthuswjap two hole centers drift just enough to bind during assembly. The craft lies in deciding where precision matters, how much, and why.
What tolerance actually means in a fabrication context
A tolerance sets the allowable variation from a nominal dimension. It is not a promise of perfection, it is a defined window in which parts will function and interchange. In CNC metal cutting and machining, we often speak in three bands of control. Coarse tolerances run around ±0.010 to ±0.020 inch and are common in structural steel fabrication, rough cut blanks, and many welded fabrications. General tolerances sit between ±0.003 and ±0.005 inch, the bread and butter of most machined features. Precision tolerances tighten to ±0.001 inch and below, entering the territory of precision bores, bearing fits, and calibrated metrology parts.

The number tells only part of the story. Tolerances apply to single features, but parts are systems. A true functional tolerance considers datum structure, stack-up, and how one feature relates to another under real loads and thermal change. This is why geometric dimensioning and tolerancing, or GD&T, exists. It is also why a drawing with simple plus/minus dimensions on every edge can still build a nonfunctional part.
Machining processes and what they can reasonably hold
CNC mills and lathes are capable machines, yet they cannot force steel, aluminum, or stainless to behave. Tools deflect, cutters heat, and workholding flexes. When you ask a machinist for ±0.0005 inch on a deep bore in 304 stainless, you should expect a conversation about toolpath strategy, coolant, and time.

A few practical ranges help align expectations. Laser cutting on a well-maintained system, with a flat sheet and proper focus, holds about ±0.005 inch on contour accuracy in mild steel up to a quarter inch thick. Waterjet is similar on thin material, and typically drifts slightly more as thickness increases. Plasma is less precise, with kerf variation that pushes tolerances into the ±0.010 to ±0.020 inch range unless you invest in high-definition setups and excellent process control. CNC metal cutting by saw often lands in ±0.015 to ±0.030 inch on cut length, depending on the saw and the setup.

Once you move to CNC machining, common milling operations on a rigid VMC with decent fixturing deliver ±0.002 to ±0.003 inch without heroic effort. Achieving ±0.001 inch is feasible on critical bores or bosses with tool compensation, probing, and thermal control, but everything must cooperate. Lathes excel at concentricity and diameters, so turning to ±0.0005 to ±0.001 inch is routine in a climate-controlled environment with stable tooling. Grinding brings you into the tenths and sub-tenths, but at meaningful cost.

Welding and post-weld machining add another layer. A welding company can weld within tight geometric control if fixtures are robust and procedures are disciplined, but heat input moves metal. A welded frame expecting ±0.005 inch flatness over 36 inches, prior to machine finishing, will probably disappoint. If you need those flats, plan to skim cut after stress relief or design in adjustability.
Material behavior and why it controls your reality
Metal does not hold dimension out of goodwill. It expands with heat, contracts when cold, and relaxes when you remove stock. Aluminum grows about 0.000012 inch per inch per degree Fahrenheit, mild steel about 0.0000065. A 20 inch aluminum plate warms 20 degrees during machining, and the free-state size shifts nearly five thousandths. If you clamp it aggressively and remove a lot of material from one side, you will release residual stress and watch it banana when you crack the vise.

Experienced machinists compensate. They rough, leave stock, let parts rest, then finish in a controlled pass. They choose cutters that minimize deflection, slow down on thin walls, and measure with the same temperature the part will see in service. If your contract manufacturing partner is quoting tight tolerances on thin sheets or long shafts, ask about these steps. If they are missing, the numbers on the drawing will not match your parts on the bench.

Stainless steels, nickel alloys, and quenched tool steels bring another wrinkle: they work harden and punish dull tools. Tighter tolerances multiply the challenge. The time it takes to hold ±0.0005 inch in 17-4 PH is not a small increment over ±0.002 inch, it can be double or more once you include tool changes, checks, and scrap risk. A knowledgeable machining manufacturer will signal this in the quote with lead time and price. If they do not, the lesson comes later.
The cost of chasing zeros
Precision is not free. Every additional decimal point influences tool selection, fixturing, cycle time, inspection, and scrap backstop. Here is a rule of thumb I share with industrial design teams and procurement at a Manufacturer or an industrial design company: if you tighten a general tolerance from ±0.005 to ±0.001 inch across multiple features, expect cost to rise by 20 to 50 percent and lead time to extend by days. If you ask for ±0.0005 inch on more than a couple of features, expect specialty processes or grinding, and treat the quote as a different part.

Equally important, tight tolerances often force process choices. Consider a steel fabricator building a large weldment that will interface with custom industrial equipment manufacturing downstream. If the drawing calls for ±0.002 inch hole position over six feet, they cannot rely on layout and drilling. They will need precision fixtures, line boring, or portable machining after weld. The welding itself must be controlled through sequence and heat input to avoid distortion that burns down the budget when the machinist tries to make the final pass.

I have seen teams specify a blanket ±0.001 inch on a cover plate because they associated small numbers with quality. The same plate, with ±0.005 inch on the edges, ±0.002 inch on the countersinks, and a single precision bore held to H7, would have cost half and assembled more reliably.
GD&T is not optional if you care about function
Plus/minus tolerances handle size. Function asks for flatness, parallelism, perpendicularity, concentricity, and position. That is the domain of GD&T. Many engineers avoid it because it looks abstract. In the shop, it is the language that tells us which faces matter, which direction to measure from, and how to distribute allowable error so your parts fit together.

A hole located with ±0.005 inch in X and Y can, in the worst case, be 0.007 inch off from the intended datum when those errors stack diagonally. A position tolerance set with a coherent datum scheme gives you a round zone that acknowledges reality and controls stack-up. For CNC metal fabrication, using true position callouts on patterns of holes, and flatness and parallelism on mating faces, usually reduces cost while improving fit, because it directs effort where it matters.

A real case: a machinery parts manufacturer produced a gearbox top plate with eight bolt holes. The original drawing used ±0.003 inch linear tolerances, which forced a lot of touch-off and cautious feeds on every hole. We revised the print with a position tolerance of 0.005 inch at maximum material condition, tied to three sensible datums. The shop ran a higher feed drill with a reamer for finish, probed one datum hole, and the CMM passed the lot without drama. Cost dropped by 18 percent.
Tolerances across the workflow: cutting, machining, forming, welding, finishing
Metal parts often cross multiple processes before delivery. Every handoff affects tolerance.

Cutting. Laser, waterjet, or plasma create blanks with their own accuracy and kerf. If you need a final contour to ±0.002 inch, treat the cut as rough stock and machine finish. For cosmetic parts, laser edges look crisp but hold slight taper. Waterjet reduces heat-affected zones but can leave striations, which are fine for subsequent machining but not for bearing surfaces.

Machining. The CNC phases bring features into spec. Pay attention to workholding. Soft jaws or vacuum fixtures can distort thin sections, which then relax when removed, pretending your parts moved after inspection. If the walls will be 0.040 inch thick, do not ask for ±0.001 inch without talking through staging.

Forming. Brake forming moves metal, particularly around bends. Hole distortion near a bend line is a known edge case. If you need holes tight to a bend, pierce later or oversize before bending and finish with a reamer. Grain direction in sheet also influences springback. A good metal fabrication shop will call this out or build it into their bend allowances, but tolerance stack-up may still require post-form machining.

Welding. Heat input drives distortion. Strong fixtures help, but each tack and pass adds cumulative change. If you need a welded frame to accept linear guides, plan to machine the pads after weld and stress relief. Powder coating or plating then adds thickness. A simple zinc plate can add 0.0002 to 0.0005 inch per side. Powder coat ranges from 0.002 to 0.006 inch, depending on the system. Tight bores should be masked or finish-machined after coating.

Inspection. Measurement deserves its own tolerance. Calipers are quick, but they are not a CMM. If your drawing calls for ±0.001 inch and you accept caliper checks, you will have arguments at assembly. Agree on inspection methods and gauges early. For custom metal fabrication with critical fits, I like to align the drawing with the inspection plan: call out CMM for position and true diameter checks on bearing bores, use go/no-go gauges for threaded features, and note surface finish where it impacts sealing or sliding.
Environmental and operational considerations
Parts rarely live on a climate-controlled granite table. They go into industrial machinery manufacturing environments that heat up, vibrate, and sometimes get sprayed. That reality should shape tolerances.

If a shaft runs in a bronze bushing and the ambient temperature rises 30 degrees during operation, both components grow. The interference fit you set at assembly may loosen to a light press or even clearance. Differential expansion between aluminum housings and steel shafts is common. Designing for a nominal 68 degree shop and installing into a plant at 90 degrees catches teams off guard. Tolerances and material choices must account for this. For example, if you need stable alignment across temperature swings, steel on steel, or aluminum on aluminum, reduces differential growth. If weight requires mixed materials, build in adjustability or choose fits that shift in your favor with heat.

Vibration loosens fasteners unless you provide stretch and locking features. Hole position tolerance alone does not keep a bolt from backing out. In dynamic assemblies, a stack of generous hole clearances can amplify vibration issues because it reduces contact area. In those cases, tighter positional tolerances and controlled surface finishes at interfaces help, but always pair them with fastener strategy.

Harsh environments also stress coatings. If a part will be hot-dip galvanized, expect substantial thickness and variable buildup. Do not design tight slip fits that will later be dipped. For powder coat, mask critical bores and threaded holes, or call out machining after coating. Your steel fabricator will appreciate the clarity.
Designing tolerances for manufacturability
There is no single recipe, but a few habits consistently produce parts that function and build economically.

First, start from function and assembly. Decide which faces locate, which holes drive alignment, and which dimensions are cosmetic. Assign tight control only to those items. Second, establish a coherent datum scheme. Pick datums that are physically accessible and stable through the process. A center bore as a primary datum is fine if it is machined early and used for locating later features. Using an edge created by laser cutting as a primary datum for a final precision pattern is risky unless you plan a machining pass on that edge.

Third, consider tolerance stack-up, not just single features. If you have a series of plates and spacers, each with ±0.005 inch thickness tolerance, and you stack ten of them, you have built in a potential fifty thousandths of variation. If the final assembly cannot absorb that, tighten the right dimensions or alter the stack.

Fourth, match tolerance to process capability. If the job will be done on a manual mill at a local shop, do not write a drawing that assumes a five-axis CMM. If the work goes to a machining manufacturer with climate control, probing, and a metrology lab, you can push tighter where needed, but use that precision sparingly. Cost is not linear.

Fifth, communicate inspection and acceptance criteria. Agree on sampling, number of critical dimensions to measure per lot, and rework expectations. For contract manufacturing, this avoids painful surprises at receiving. If the part is safety critical, spell out special characteristics and process controls.
The two places tolerances regularly go wrong
Designers often over-tighten nonfunctional surfaces and under-control assembled relationships. I have seen ±0.001 inch on the thickness of an aesthetic panel, while the hole pattern that aligns a robot arm to a base plate floats with ±0.010 inch. It looks precise on paper, but it builds headaches. Flip that thinking. Loosen the thickness to ±0.010 inch, and tighten the position of the bolt circle relative to the mounting face with a GD&T callout.

The other problem is letting default title block tolerances govern complex features. A note that says two-place decimals ±0.01 and three-place decimals ±0.005 can be helpful, but it does not replace specific control for a slot that is both alignment and airflow. If the slot matters, call it out.
Working with your fabricator or machine shop
Good outcomes follow from early, specific conversations. Share the functional intent. If a set of holes must align with a mating part made elsewhere, send that print too. If you need a sliding fit, specify the fit system, not a blind decimal tolerance. Ask your shop what they can hold in the material and thickness you chose. A steel fabricator with a high-definition plasma table can hit tighter than a general welding company with an older unit. A machine shop with thermal compensation and probing can hold tenths in a pocket a small job shop would struggle with.

When a shop questions a tolerance, listen. They are not dodging quality, they are protecting your schedule. I once pushed a tight true position on a tall, thin bracket without adding machining after bend. The fabrication lead raised a flag. We ignored it, and three weeks later we were arguing about why the holes were oval after forming. We added post-form machining to the process and updated the drawing. The second run installed cleanly.
Practical numbers from the field
Every part and process differ, but these ranges are a fair starting point when you talk to a CNC metal fabrication partner.

Laser cut contour accuracy: ±0.005 inch on thin to medium sheet, a touch looser on thick plate. Hole sizes near the lower limit of the laser spot will drift, so avoid laser-cutting small precision holes; instead pierce undersized and ream.

Waterjet contour: ±0.005 to ±0.010 inch, influenced by thickness. Little heat but consider taper compensation.

Plasma cut edges: ±0.010 to ±0.020 inch typical. Holes are rough. Good for roughing and welded structures where edges will be machined later.

Saw cut length: ±0.015 to ±0.030 inch unless you invest in stops and post-processing.

Milling general features: ±0.002 to ±0.003 inch with standard setups.

Critical bores on a VMC: ±0.0005 to ±0.001 inch with boring heads or reaming, stable temperatures, and probing.

Turning diameters: ±0.0005 to ±0.001 inch routinely, tighter with grinding.

Flatness over large surfaces: depends on thickness and process. A 24 inch plate milled on both sides can hit 0.002 to 0.004 inch flatness if the stock is stress relieved and machining is staged. A welded frame will need post-weld machining to approach that.

Surface finish: 63 microinch Ra is comfortable for most machined faces. Bearing fits often want 32 Ra or better. Laser cut edges vary widely and may not suit sealing or sliding.

Fit systems: Use H7/g6 for a common slip fit on metric shafts and bores. For inch, a nominal 0.0005 to 0.0015 inch clearance on a 1 inch sliding fit feels right in a clean environment. Interference fits for press may range 0.0005 to 0.002 inch per inch of diameter, depending on materials and installation method.

These are not commandments. They are where most quotes start and where headaches diminish.
The economic logic of selective precision
For custom metal fabrication that feeds into custom industrial equipment manufacturing, the best projects trim precision where it adds nothing and invest it where it unlocks reliability. Put your tightest tolerances on datums, bearing seats, sealing faces, and hole patterns that define kinematics. Let covers, non-locating edges, and generous slots breathe.

A good machinery parts manufacturer will design fixtures only for the critical features. They will rough quickly, then slow down on the last ten thousandths. They will align inspection resources where failure hurts. If a Machining manufacturer tells you a part will run in two setups to control a relationship properly, that is money well spent. If they offer to do it in one setup but suggest a looser datum, ask what shifts. The cheapest path on paper is not the cheapest path when you count rework and assembly labor.

If you are a Manufacturer buying parts to assemble into a line, you have a bigger lever. Harmonize tolerances across vendors. The steel fabricator building the base frame, the machine shop making guide rails, and the welding company producing brackets should all reference the same datum scheme and tolerance logic. A tenth tighter on a rail may allow a bracket to be looser, and the frame to rely on shims and dowels for alignment. Cross-vendor tolerance architecture is where contract manufacturing either shines or bleeds.
When to relax, when to tighten
Relax tolerances when:
A feature does not locate or seal, and a looser range will not change assembly or appearance. The process cannot hit the number without special setups that add cost and risk. Thermal or coating effects will erase the benefit of the tighter spec.
Tighten tolerances when:
Multiple parts are meant to interchange without match-marking or hand fitting. The feature is a datum or affects motion, sealing, or load transfer. The downstream process depends on this feature to position its own work.
Notice the symmetry. Every tolerance choice sits in a web of function, process, and economics.
Common traps and how to avoid them
Templates and CAD defaults mislead. Many CAD systems export drawings with blanket tolerances that look tidy but do not match the reality on the shop floor. Before releasing a print, read it like a machinist. Imagine which faces touch the vise, which hole is probed first, which side is up during a pass. If your title block sets three-place decimals to ±0.005 inch and your drawing shows 0.250 in three decimals for a cosmetic edge, you just tightened a nonfunctional dimension. Either change the default or add a note that clarifies the intent.

Another trap: dimensioning to nonfunctional corners that vanish during deburring. A chamfer note is cleaner than a tight linear dimension to an edge that will be eased. If you care about clearance at a corner, specify the minimum distance after break, not an exact sharp that cannot exist.

Finally, tolerances that rely on magic numbers from suppliers can haunt you. If a bearing seat calls for a press fit designed around a nominal shaft from a vendor, check the tolerance on that shaft. Do not assume their center of tolerance equals yours. Industrial supply catalogs list fit classes and actual tolerance bands; align your drawing accordingly.
Collaboration as process control
The best results come from early collaboration between design, the machine shop, and the metal fabrication shop. At project kickoff, walk the print together. Decide where to hold tight, where to leave room, and how to measure. If a part crosses multiple suppliers, appoint a tolerance owner. For complex assemblies, build a master datum map that all vendors use.

A seasoned Steel fabricator can tell you when a weld sequence will distort a plate and suggest cutouts or order of operations to minimize it. A Machinery parts manufacturer can flag a tall thin wall that will chatter and propose a rib or a staged cut. An Industrial design company can translate functional intent into specific GD&T that the shop can execute.

I like to run a first article review around the first build, not just a paperwork approval. Put the part on the bench with the mating components, check the feel of a sliding fit, confirm hole alignment, and look at surface finish and edge breaks. Update the drawing based on what you learn. The revision that tightens one positional tolerance and loosens three cosmetics pays for itself within a week.
Where technology helps, and where it does not
Modern CNC machines with probing, thermal compensation, and adaptive toolpaths have expanded what is possible. They do not change physics. They do allow us to hold a consistent ±0.001 inch on a pattern of bores, inspecting each part on-machine and correcting drift before scrap piles up. They also enable smart fixtures that combine rough and finish operations in fewer setups. For high-mix work typical of custom metal fabrication, this matters.

Metrology has leaped forward too. Portable CMM arms and scanners let a welding company verify large frames without wrestling them onto a granite table. Software can compare a scan to CAD and highlight deviation maps. The danger is mistaking colorful plots for functional acceptance. A heatmap showing a plate wavy by 0.020 inch might be irrelevant if you machine the pads that actually interface. Focus measurement on the features that determine performance.
A brief anecdote: two decimals, two outcomes
We built a set of aluminum plates for a contract manufacturing client in the semiconductor space. The first set of prints carried blanket ±0.002 inch tolerances and a few tightly called bores. Parts looked beautiful on the bench but installed with a midday fight. The culprit was a pattern of mounting holes tied to an edge created by waterjet. The holes were within ±0.002 inch of the edge, yet the edge varied by far more because it was only a rough cut prior to anodizing.

We revised the datum scheme to use two reamed dowel holes as primary and secondary, then set the bolt pattern position relative to those datums at 0.004 inch at maximum material condition. We loosened nonfunctional edges to ±0.010 inch and masked only the critical bores for anodize. The second run slid into place with a gentle wiggle. The shop ran faster, the CMM report got shorter, and the line started on time.

The lesson was not that tighter is better or looser is cheaper. The lesson was to point precision where it carries the load.
Final thoughts that belong on your next drawing
Tolerances are an investment. Spend them where they earn a return in reliability, interchangeability, and ease of assembly. Save them where they feed ego or habit. Use GD&T to control relationships, not just sizes. Match the numbers to the process capability of your CNC metal fabrication partner. Remember coatings and temperature. Write inspection into the narrative, not as an afterthought.

If you work with a metal fabrication shop, a steel fabricator, or a machinery parts manufacturer regularly, build a shared tolerance playbook with real numbers from your equipment. Keep notes on which materials move, which operations bite, and which combinations make heroes or headaches. The next time a drawing crosses your desk, read the tolerances as if they were a budget. Because they are.

Waycon Manufacturing Ltd
275 Waterloo Ave, Penticton, BC V2A 7N1
(250) 492-7718
FCM3+36 Penticton, British Columbia
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Manufacturer, Industrial design company, Machine shop, Machinery parts manufacturer, Machining manufacturer, Steel fabricator

Since 1987, Waycon Manufacturing has been a trusted Canadian partner in OEM manufacturing and custom metal fabrication. Proudly Canadian-owned and operated, we specialize in delivering high-performance, Canadian-made solutions for industrial clients. Our turnkey approach includes engineering support, CNC machining, fabrication, finishing, and assembly—all handled in-house. This full-service model allows us to deliver seamless, start-to-finish manufacturing experiences for every project.