Tolerances by Process and Surface Finish
What each manufacturing process can hold, how roughness uses up tolerance, and what a tight tolerance costs.
Every manufacturing process makes parts within a band of sizes, and the band gets narrower as the process gets more precise and more expensive. Choosing a tolerance is therefore also choosing a process: a tolerance that one process holds easily may need another operation altogether on a different one. This page uses the Machinery’s Handbook tables that connect processes to tolerance grades and to surface roughness, adds the standards for casting, molding and welding, and explains the cost of tightening a tolerance. It is part of the Fits & Tolerances guide.
How tolerance grades step
Tolerance grades are numbered so that a bigger number is a looser tolerance. The steps are not even: from one grade to the next the tolerance widens by about 60% (the IT grades are built from multiples 10, 16, 25, 40, 64, 100 and so on of a tolerance unit; each is about 1.6 times the last). Five grades make about a tenfold change. So tightening a tolerance by two grades roughly cuts it to 40%, and the difference between IT6 and IT11 is a factor of 10.
| Grade (25 mm size) | Tolerance width | Ratio to the previous grade |
|---|---|---|
| IT5 | 9 µm | |
| IT6 | 13 µm | 1.44× |
| IT7 | 21 µm | 1.62× |
| IT8 | 33 µm | 1.57× |
| IT9 | 52 µm | 1.58× |
| IT10 | 84 µm | 1.62× |
| IT11 | 130 µm | 1.55× |
| IT12 | 210 µm | 1.62× |
Which process makes which grade
The Handbook’s Table 7 (from ANSI/ASME B4.1) is a general guide to the machining processes that will, under normal conditions, produce work within the tolerance grades indicated. The ANSI grade numbers 4 to 13 are the same numbers as the ISO IT grades.
| Process | Tolerance grades | Character |
|---|---|---|
| Lapping and honing | 4 to 5 | Fine to precision |
| Cylindrical grinding | 5 to 7 | Fine to precision |
| Surface grinding | 5 to 8 | Fine to precision |
| Diamond turning | 5 to 7 | Fine to precision |
| Diamond boring | 5 to 7 | Fine to precision |
| Broaching | 5 to 8 | Fine to precision |
| Reaming | 6 to 10 | Ordinary to fine |
| Turning | 7 to 13 | Ordinary to fine |
| Boring | 8 to 13 | Loose to ordinary |
| Milling | 10 to 13 | Loose to ordinary |
| Planing and shaping | 10 to 13 | Loose to ordinary |
| Drilling | 10 to 13 | Loose to ordinary |
- Read it as a range. A process can make the finer grades at its left end only with care: sharp tools, a stiff machine, good fixtures and temperature control. The right end is what it makes easily.
- Precision work needs finishing. Grades 4 to 7 are the province of grinding, honing, lapping and diamond tooling. A turned part that needs IT6 usually needs a grinding or honing operation after the turning.
- Drills and mills are rough tools. Drilling makes holes in grades 10 to 13; an H7 hole is drilled, then bored or reamed (grades 6 to 10 for reaming) or honed. This is why the standard fits are built around reamed holes and ground shafts.
- Machines differ. A modern CNC with a good tool can do better than the chart; an old worn one worse. Use the chart for planning and your own capability data for commitments (see Statistical Tolerancing and Process Capability).
What the grades mean in micrometers
The tolerance width depends on the size as well as the grade. The table gives the ISO IT values for four sizes, with the grades each process can reach. A process line shows the finest and the coarsest tolerance it can hold at that size.
| Process (grades) | 10 mm: finest to coarsest | 25 mm: finest to coarsest | 50 mm: finest to coarsest | 100 mm: finest to coarsest |
|---|---|---|---|---|
| Lapping and honing (4 to 5) | 4 to 6 µm | 6 to 9 µm | 7 to 11 µm | 10 to 15 µm |
| Cylindrical grinding (5 to 7) | 6 to 15 µm | 9 to 21 µm | 11 to 25 µm | 15 to 35 µm |
| Surface grinding (5 to 8) | 6 to 22 µm | 9 to 33 µm | 11 to 39 µm | 15 to 54 µm |
| Diamond turning (5 to 7) | 6 to 15 µm | 9 to 21 µm | 11 to 25 µm | 15 to 35 µm |
| Diamond boring (5 to 7) | 6 to 15 µm | 9 to 21 µm | 11 to 25 µm | 15 to 35 µm |
| Broaching (5 to 8) | 6 to 22 µm | 9 to 33 µm | 11 to 39 µm | 15 to 54 µm |
| Reaming (6 to 10) | 9 to 58 µm | 13 to 84 µm | 16 to 100 µm | 22 to 140 µm |
| Turning (7 to 13) | 15 to 220 µm | 21 to 330 µm | 25 to 390 µm | 35 to 540 µm |
| Boring (8 to 13) | 22 to 220 µm | 33 to 330 µm | 39 to 390 µm | 54 to 540 µm |
| Milling (10 to 13) | 58 to 220 µm | 84 to 330 µm | 100 to 390 µm | 140 to 540 µm |
| Planing and shaping (10 to 13) | 58 to 220 µm | 84 to 330 µm | 100 to 390 µm | 140 to 540 µm |
| Drilling (10 to 13) | 58 to 220 µm | 84 to 330 µm | 100 to 390 µm | 140 to 540 µm |
These are total tolerance widths (the whole zone, not ±). Turning is shown from IT7 (a precision lathe with sharp tooling) to IT13; ordinary turning on a stable job is typically toward the middle of that range. The widths are the Handbook’s ISO table values (BS 4500).
Surface roughness by process
The Handbook’s Table 1 (“Surface roughness produced by common production methods”) is a bar chart of the average roughness Ra each process produces. It shows an average application range (black bars) and a less frequent range (gray bars). The values below are read from the chart; the Handbook cautions that the ranges are typical of the processes listed and that higher or lower values may be obtained under special conditions. Ra in micrometers (microinches are the same value times 40).
| Process | Average application: Ra (µm) | Full range: Ra (µm) |
|---|---|---|
| Flame cutting | 12.5 to 25 | 6.3 to 50 |
| Snagging | 6.3 to 25 | 3.2 to 50 |
| Sawing | 1.6 to 25 | 0.8 to 50 |
| Planing, shaping | 1.6 to 12.5 | 0.4 to 25 |
| Drilling | 1.6 to 6.3 | 0.8 to 12.5 |
| Chemical milling | 1.6 to 6.3 | 0.8 to 12.5 |
| Electrical discharge machining | about 1.6 to 4 | 0.8 to 12.5 |
| Milling | 0.8 to 6.3 | 0.3 to 25 |
| Broaching | 0.8 to 3.2 | 0.4 to 6.3 |
| Reaming | 0.8 to 3.2 | 0.4 to 6.3 |
| Electron beam | 0.8 to 6.3 | 0.2 to 6.3 |
| Laser | 0.8 to 6.3 | 0.2 to 6.3 |
| Electro-chemical | 0.2 to 3.2 | 0.05 to 12.5 |
| Boring, turning | 0.4 to 6.3 | 0.025 to 25 |
| Barrel finishing | 0.2 to 0.8 | 0.05 to 3.2 |
| Electrolytic grinding | about 0.2 to 0.6 | 0.1 to 0.8 |
| Roller burnishing | 0.2 to 0.4 | 0.1 to 0.8 |
| Grinding | 0.1 to 1.6 | 0.025 to 6.3 |
| Honing | 0.1 to 0.8 | 0.025 to 1.6 |
| Electro-polish | 0.1 to 0.8 | 0.012 to 1.6 |
| Polishing | 0.1 to 0.4 | 0.012 to 0.8 |
| Lapping | 0.05 to 0.4 | 0.012 to 0.8 |
| Superfinishing | 0.025 to 0.2 | 0.012 to 0.8 |
| Sand casting | 12.5 to 25 | 6.3 to 50 |
| Hot rolling | 12.5 to 25 | 6.3 to 50 |
| Forging | 3.2 to 12.5 | 1.6 to 25 |
| Permanent mold casting | 1.6 to 3.2 | 0.8 to 6.3 |
| Investment casting | 1.6 to 3.2 | 0.4 to 6.3 |
| Extruding | 0.8 to 3.2 | 0.4 to 12.5 |
| Cold rolling, drawing | 0.8 to 3.2 | 0.2 to 6.3 |
| Die casting | 0.8 to 1.6 | 0.4 to 3.2 |
Rough processes (flame cutting, sand casting, hot rolling) sit at Ra 6 to 50 µm; machining processes cover 0.4 to 6 µm; grinding, honing and lapping reach 0.025 to 1.6 µm. A cast surface will never reach a ground tolerance without machining. The link between roughness and tolerance (the Handbook’s one-eighth rule) is on the Temperature, Coatings and Fit Problems page.
Casting, forming, molding, additive
The Handbook’s ANSI table covers machining. Other processes have their own standards and their own scatter, and the numbers depend more on the supplier, the material and the part shape. The usual practice is to ask the supplier what it can hold and to design to its capability data.
| Process | Tolerance standard | Notes |
|---|---|---|
| Castings (sand, die, investment) | ISO 8062-3 (casting tolerance grades CT, and machining allowances) | Sand casting commonly falls in the coarser grades; die and investment casting hold tighter. Cast surfaces are rough (see the table above); machine any feature that mates or seals |
| Plastic moldings | ISO 20457 (tolerance groups for molded parts) | Plastics vary more than metals: shrinkage, warpage and processing conditions all matter, and metal-part methods do not transfer directly. Tolerance each feature to the material and the tool |
| Sheet metal (laser, punch, bend) | ISO 2768 for general dimensions; supplier capability data | Cut edges are tighter than bends; bend angle and flange position depend on springback. Give tight tolerances to cut features and loose ones to bends |
| Welded structures | ISO 13920 (general tolerances for welded constructions) | Weld shrinkage distorts the assembly; machine mating faces after welding |
| Additive manufacturing (3D printing) | Process- and machine-specific; check the supplier | Depends strongly on process (FDM, resin, powder), orientation, size and shrinkage. See the 3D Printing Tolerancing Guide for fits between printed parts |
ISO 2768-1 (see General Tolerances and Drawing Notes) suggests the usual accuracy of the workshop as the basis for picking a general tolerance class: m for general machining, c for sheet metal and fabrications.
Tolerance and cost
Cost does not rise smoothly as a tolerance tightens. It rises a little within a process’s normal range and then jumps when a different process is needed (turning to grinding, grinding to lapping). The Handbook’s chapter on tolerance analysis puts the point plainly: overly restrictive tolerances limit the choice of processes and drive up cost, and small differences in tolerance can change cost dramatically. The sensible approach is a cost-against-tolerance trade for each dimension:
- Find the loosest tolerance that works. Start from function (the fit or the stack-up), not from what the machine can do.
- Stay inside a process’s normal range. If a tolerance is just beyond a process, loosening it a grade may avoid a whole operation.
- Spend tolerance where it is cheap. In a stack-up, give the loose share to the dimension that is hard to hold (the weight factors on the stack-up page).
- Count inspection too. A tighter tolerance needs a more accurate gauge, a controlled environment and often 100% inspection.
- Ask for two quotes. The cheapest way to learn what a tolerance costs is to ask a supplier to price it at two values.
Worked examples
Example 1: how to make 25H7/g6
The hole is IT7 (21 µm) and the shaft IT6 (13 µm). Using the chart:
- Hole, IT7: reaming covers grades 6 to 10, so a reamed hole holds IT7; boring (8 to 13) does not, unless it is a fine boring operation; broaching (5 to 8) can; honing (4 to 5) is finer than needed. The usual route is drill, then bore or ream.
- Shaft, IT6: turning covers 7 to 13, so a turned shaft cannot reliably hold IT6. Cylindrical grinding covers 5 to 7. The shaft is turned, then ground.
- Surface finish: by the one-eighth rule the shaft (13 µm) needs Ra ≤ 1.63 µm. Grinding’s average range is 0.1 to 1.6 µm: it meets this; turning’s is 0.4 to 6.3: it usually does not.
- Fit: 7 to 41 µm of clearance, from the two tolerances (21 + 13 = 34 µm of spread, plus the minimum clearance).
Example 2: a ± 0.05 mm slot
A 25 mm slot is toleranced ± 0.05 mm: a total width of 0.100 mm = 100 µm. The loosest IT grade at 25 mm that fits inside 100 µm is IT10 (84 µm). The chart lists milling (10 to 13), boring (8 to 13), turning, drilling and planing as able to make grade 10 or looser. An ordinary milling operation is enough; grinding would be an unnecessary cost.
Example 3: a casting with a machined bore
A sand-cast housing has a bore that must be 50H7 (IT7 = 25 µm). A sand casting’s surface has Ra of about 12.5 to 25 µm and a size tolerance of a millimeter or more; no casting process will hold IT7. The bore is cast undersize with a machining allowance, then bored and reamed (IT6 to IT10) to size. Giving the casting itself an ISO 8062-3 grade and the machined bore its own IT7 tolerance is the normal way to draw it.
Example 4: what a tolerance costs, qualitatively
A turned 25 mm shaft is toleranced IT9 (52 µm): easily within turning’s range. Tighten it to IT7 (21 µm): at the edge of turning, needing a good machine and care. Tighten it to IT6 (13 µm): beyond turning, so a grinding operation, fixtures, and a gauge capable of measuring a 13 µm band are added. The tolerance fell by 4.0 times; the process changed.
Troubleshooting
| Symptom | Likely cause | Try |
|---|---|---|
| The supplier cannot hold the tolerance | The tolerance is finer than the process’s range | Check the chart; loosen one or two grades, or add a finishing operation |
| A tolerance costs far more than expected | It forced a change of process (e.g. to grinding) | Re-examine function; widen it to the cheaper process’s range |
| The size is right but the part will not mate | Roughness uses up the tolerance, or form error | Check Ra against the one-eighth rule; check roundness |
| Castings vary from lot to lot | Cast tolerances are coarse and vary with the tool and the process | Specify an ISO 8062-3 grade, and machine mating features |
| Plastic parts drift out of tolerance after molding | Shrinkage and warpage continue after ejection | Condition before measuring; tolerance to ISO 20457 for the material |
Related
- Fits & Tolerances: the overview and quick reference.
- Fits – Metric (ISO System): the IT grades in detail.
- General Tolerances and Drawing Notes: ISO 2768 and workshop accuracy.
- Temperature, Coatings and Fit Problems: surface finish and fit.
- Statistical Tolerancing and Process Capability: using your own process data.
FAQ
Is the Handbook’s chart a guarantee?
No. It is a general guide to what each process will produce under normal conditions. Use it for planning, and use your own capability data (or a supplier’s) when you make a commitment.
Are ANSI grades and ISO IT grades the same?
The grade numbers 4 to 13 correspond. In the inch Table 6, a 1.19 to 1.97 inch size at grade 7 has a tolerance of 1.0 thousandths of an inch (25 µm), the same as IT7 for 30 to 50 mm.
What does Ra mean?
Roughness average: the arithmetic average of the deviation of a surface from its mean line, in micrometers (or microinches). The Handbook’s rule of thumb is that the peak-to-valley height is about four times Ra.
Why does IT6 need grinding?
Because ordinary turning and milling cannot reliably hold a band of 13 µm on a 25 mm size, and cannot reach the 1.6 µm Ra the one-eighth rule asks for. Grinding, honing or lapping can.
Where do I find tolerances for 3D printing?
They depend on the process, material, orientation and size, and on the printer. Use the manufacturer’s or print service’s data, and the 3D Printing Tolerancing Guide on this site for designing fits.
Should I always choose the cheapest process?
Choose the cheapest process that meets the function with the margin you want. A process running at the edge of its range gives low capability and many rejects, so a slightly more capable process can be cheaper overall.
References
- Machinery’s Handbook, 31st edition, Industrial Press: “ANSI/ASME Standard Limits and Fits” (Table 6, standard tolerances; Table 7, relation of machining processes to tolerance grades), “Surface Texture” (Table 1, surface roughness produced by common production methods; relation of surface roughness to tolerances) and “Tolerance Analysis and Assignment”.
- ISO 8062-3, Dimensional and geometrical tolerances for molded parts: general tolerances for castings; ISO 20457, Plastics molded parts: tolerances and acceptance conditions; ISO 13920, Welding: general tolerances for welded constructions; ISO 2768-1.
Last reviewed: October 2026. The grade ranges and roughness ranges are the Handbook’s typical values, not guarantees.