Press Fit vs Slip Fit
A press fit has a shaft OD larger than the hole ID — the parts must be forced together and the joint transmits torque by friction alone. A slip fit has a guaranteed positive clearance, so parts slide together by hand and can be removed the same way. Choosing wrong costs you a seized bearing, a spinning hub, or a difficult field repair. This guide explains the physics, the ISO 286-1 code system, and exactly how to pick the right fit class for your application.
What a “Fit” Actually Means
When you assemble a shaft into a bore, the dimensional relationship between those two surfaces — the fit — determines whether your parts slide together freely, sit snugly in place, or must be forced together with a press or a heat gun.
The key number is the diametral difference: bore diameter minus shaft diameter. When that number is positive (bore is bigger), you have a clearance — the shaft can slide in. When that number is negative (shaft is bigger), you have an interference — the shaft has to be forced in, elastically deforming both parts. When the difference can be either positive or negative depending on where each individual part lands within its tolerance band, you have a transition fit.
This difference is called the algebraic fit, and it is what every ISO 286-1 designation encodes. The three families — clearance, transition, and interference — cover every shaft-bore assembly you will encounter. Let's look at each one before going deeper.
- Shaft OD < bore ID always — gap is guaranteed
- Positive clearance at every tolerance combination
- Hand or light-mallet assembly
- Allows rotation or sliding between parts
- Easy disassembly — no tools required
- May result in a small clearance OR a small interference
- Which outcome you get depends on part-to-part variation
- Locates parts accurately with minimal play
- Mallet or small arbor press typical
- Must add retention (circlip, set screw) — no torque transfer
- Shaft OD > bore ID always — overlap is guaranteed
- Both parts elastically deform at the interface
- Hydraulic press or thermal differential required
- Transmits torque and axial load purely by friction
- Surface damage typically occurs on disassembly
Why the Distinction Matters in Practice
Consider two failure modes that both trace back to choosing the wrong fit:
A gear hub specified with H7/h6 instead of H7/p6 has zero guaranteed interference. Under reversing torque, the hub spins on the shaft, fretting the surfaces within hours. The fix is a field press, which often means a machine tear-down.
A bearing outer race specified with H7/s6 instead of H7/p6 generates excess hoop stress in the outer ring during press-in, reducing the bearing's internal radial clearance below its design value. At speed, the bearing runs hot and fails early from fatigue.
The numbers in ISO 286-1 exist precisely because “press it in until it's snug” is not a repeatable specification. Standardized tolerance codes ensure that a machinist in any shop produces a part that assembles predictably every time.
How the ISO 286-1 Code System Works
When you write “H7/p6” on your drawing, you are using ISO 286-1 — the international standard that turns the vague concept of “press fit” into a precise, machinist-readable number. Understanding the code unlocks every tolerance table in every bearing, bushing, and coupling catalog you will ever use.
Decoding a Fit: H7/p6
Every fit designation is written as HoleTolerance / ShaftTolerance. The notation contains four separate pieces of information. Let's take H7/p6 apart:
- HHole fundamental deviation: zero lower bound
A capital letter means this is the hole. The letter H means the lower deviation of the tolerance zone is exactly zero — the smallest acceptable bore is exactly the nominal diameter. The tolerance zone only grows upward (toward a larger bore). This is the standard "hole-basis" system used for most machined assemblies because bores are harder to make to a specific deviation than shafts — so you fix the hole and vary the shaft.
- 7IT grade 7 — the size of the tolerance band
The number after the letter sets the International Tolerance (IT) grade, which defines how wide the tolerance band is. IT grades run from IT1 (tightest, ±1–3 µm, for gauge making) to IT18 (loosest, for rough castings). IT7 is the most common grade for bored housings — for a Ø25 mm bore, IT7 = 21 µm total band, achievable with standard reaming or finish boring.
- pShaft fundamental deviation: above zero (interference)
A lowercase letter means this is the shaft. The letter sets where the tolerance zone sits relative to the nominal diameter. Letters a–h sit below zero (the shaft is undersized → clearance). Letters j–n are near zero (transition zone). Letters p–z sit above zero (the shaft is oversized → interference). 'p' gives the lightest interference class.
- 6IT grade 6 — one grade tighter than the bore
It is conventional practice to make the shaft one IT grade tighter than the hole (e.g., h6 pairs with H7). A tighter shaft tolerance means less variation in the resulting fit, which is desirable since shafts are easier to grind or turn to a tighter band than bores are to hone.
What is a “Tolerance Zone”?
A tolerance zone is the band of acceptable sizes for a feature. For a Ø25 mm H7 bore, the zone is 25.000–25.021 mm. Any bore measured within that range is conforming. The position of the zone (where it sits relative to nominal) is set by the letter; the width of the zone is set by the number.
The diagram below shows the tolerance zones for the H7 hole and several common shaft designations, all drawn to scale for the Ø18–30 mm diameter range. Everything above the zero line is larger than nominal; everything below is smaller.
IT Grades and the Processes That Achieve Them
The IT grade number sets the total width of the tolerance band. IT1 is the tightest (used for gauge making); IT16 is the loosest (rough castings). Every manufacturing process has a natural capability range — you cannot demand IT5 tolerances from a drill press, and you don't need to grind a shaft just to achieve IT9. Spec the grade that matches the process capability you actually need.
| IT Grade | Band (Ø25 mm) | Typical Process |
|---|---|---|
| IT4–IT5 | 6–9 µm | Cylindrical grinding, honing |
| IT6 | 13 µm | Ground shafts, precision turning |
| IT7 | 21 µm | Reaming, finish boring |
| IT8 | 33 µm | Finish turning, milling |
| IT9–IT11 | 52–130 µm | Standard turning, drilling |
| IT12–IT14 | 210–520 µm | Rough machining, casting |
Per ISO 286-1:2010, Table 1. Ø18–30 mm range.
Hole-Basis vs Shaft-Basis Systems
ISO 286-1 supports two approaches. In the hole-basis system, the hole deviation is fixed (always H) and you vary the shaft letter to choose the fit type. In the shaft-basis system, the shaft deviation is fixed (always h) and you vary the hole letter.
Hole-basis is almost always used for machined assemblies for a practical reason: bores are made with fixed tools (reamers, boring bars) that produce a consistent diameter. It is far easier to vary the shaft OD on a lathe than to keep a separate reamer for every fit class. Shaft-basis is occasionally used when the same shaft must carry multiple components with different fits (e.g., a bearing at one end and a gear hub in the middle).
Mixing hole-basis and shaft-basis on the same assembly drawing without flagging it is a common source of out-of-spec parts at incoming inspection. If you deviate from the H hole convention, add a drawing note: “SHAFT-BASIS SYSTEM — shaft tolerance is fixed at h6”.
Anti-friction bearing catalogs (SKF, NSK, Timken) specify shaft fits as k5, m5, n6 without a hole letter because the bearing bore IS the hole — it is already ground to a known tolerance by the bearing manufacturer. Your job is to match the shaft to the bearing's internal fit recommendation, not to specify a new hole.
Slip Fits — How Much Clearance Do You Need?
When you specify a clearance fit on your drawing, more clearance is not always better. If you specify too much clearance, the shaft rattles in the bore, creating wear, runout, and noise. If you specify too little, the shaft seizes — especially when thermal expansion eats the gap at operating temperature. The right clearance is the minimum needed for the lubrication film, the surface peaks, and the thermal growth, with a reasonable margin on top.
Use this when the shaft rotates continuously inside the bore — journal bearings, pump impeller shafts, motor end caps.
The minimum clearance of 20 µm is large enough for a hydrodynamic oil film to form and sustain itself. At shaft speeds above ~500 RPM, the rotating shaft wedges oil into the converging gap between shaft and bore, creating a pressurized film that lifts the shaft off the bearing surface entirely. This is why a well-oiled journal bearing lasts longer than a rolling-element bearing — there is literally no metal-to-metal contact in normal operation.
If you open this up to, say, 150 µm (like H7/d9), the oil film cannot maintain pressure and the shaft floats and vibrates. On precision spindles, even 30 µm of extra radial clearance doubles the runout.
- Plain journal bearings in gearboxes
- Pump shaft in housing
- Conveyor roller stub shafts
Use this for precision location where the shaft must slide in and out (e.g., a locating pin that indexes a fixture) or where a shaft rotates slowly under light load with grease lubrication.
The 7 µm minimum clearance is barely above the peak-to-valley height of a Ra 1.6 µm ground surface (approximately 4× Ra ≈ 6.4 µm Rz). This means the shaft clears the bore surface even at the tightest material condition, preventing seizure — but there is almost no slop. Parts locate with essentially zero perceptible play.
At operating temperatures above ambient, this fit can become a slip fit borderline. A steel shaft in a steel bore expands essentially the same rate (Δα ≈ 0), so thermal growth is not an issue. But an aluminum shaft in a steel bore at +80 °C can lose its entire clearance — see the material pairing notes in the Selection Guide below.
- Precision locating pins in fixtures
- Shoulder bolt shanks through aluminum housings
- Reamer bores for accurate hole location
Use this when you need the most accurate location a slip fit can provide — zero nominal clearance — but still want to be able to disassemble the joint by hand without damage.
The minimum clearance at Maximum Material Condition (MMC — the tolerance extreme where each feature contains the most material, i.e., largest shaft and smallest bore) is 0 µm — both parts at their largest and smallest allowable size, respectively, just touch. In practice, most assemblies will have a small positive clearance because both parts will not be simultaneously at MMC. The result is the most accurately located removable joint available without moving into the transition fit territory.
Do not use this for parts that rotate at any meaningful speed without lubrication — 0 minimum clearance with surface roughness will cause fretting on the first rotation. Also avoid for mixed-CTE material pairs at elevated temperature.
- Tooling keys and spigot joints
- Precision machine tool fixtures
- Collet body shanks in spindles
Why Surface Finish Sets a Clearance Floor
Here is something many junior engineers miss: your specified minimum clearance must be larger than the combined surface peak height of both mating surfaces. Otherwise, at the tight condition, the peaks on the shaft and bore will contact each other even though the nominal size says there's a gap.
The relationship is: minimum clearance ≥ Rz_shaft + Rz_bore, where Rz is the 10-point peak-valley height (approximately 4× Ra for typical machined surfaces). For a Ra 1.6 µm finish: Rz ≈ 6.4 µm. Two mating surfaces at Ra 1.6 µm → combined peaks ≈ 12.8 µm. H7/g6's minimum clearance of 7 µm would be insufficient for Ra 1.6 µm surfaces if both parts hit MMC simultaneously. In practice, simultaneous MMC is rare — but for critical fits, use H7/g6 only with surface finishes ≤ Ra 0.8 µm (Ra 32 µin.), or upgrade to H7/f7 for Ra 1.6 µm surfaces.
Press Fits — The Physics of Elastic Interference
When you force an oversized shaft into a bore, both parts deform elastically — and that elastic deformation is what holds your assembly together. The shaft is compressed slightly; the hub expands slightly. Think of it like fitting a rubber stopper into a bottle neck — except both the stopper and the neck are steel, so the deformations are measured in microns, not millimeters.
Those elastic deformations create a radial contact pressure at the interface. That pressure, multiplied by the coefficient of friction and the interface area, is what transmits torque and axial load. The higher the interference, the higher the contact pressure, and the more load the joint can carry — up to the point where the hub hoop stress approaches yield.
Step-by-Step: How Contact Pressure Forms
The shaft OD is larger than the bore ID by δ (the diametral interference). For H7/p6 at Ø25 mm, δ ranges from 0.001 to 0.035 mm depending on where each part lands in its tolerance band.
The hydraulic press forces the shaft in. As the shaft enters the bore, both parts deform: the shaft OD compresses by δ/2 and the bore ID expands by δ/2 (assuming same material and similar wall thickness).
At rest, the elastic deformation is locked in. The shaft is in compression; the hub is in tension (hoop stress). Both want to return to their free-body dimensions, but the interface prevents them.
Applied torque or axial force tries to slide the shaft relative to the hub. The contact pressure generates a friction force that resists sliding. As long as the applied load is below μ × p × A_interface, the joint holds.
The Contact Pressure Formula — And Why It Has That Shape
The contact pressure at the mating interface comes from the Lamé thick-walled cylinder solution. For a solid steel shaft pressed into a steel hub:
Why does the hub outer diameter D appear? The term (D² − d²) / D² is the “wall factor.” A thick hub (large D relative to d) is stiffer and resists expansion more — resulting in higher contact pressure for the same interference. A very thin-walled hub (D ≈ d) expands easily, reducing contact pressure. This is why thin bosses on castings cannot carry the same press fit as solid blocks.
Why is δ/d the ratio that matters? Interference δ has to be normalized by the diameter because 0.020 mm on a Ø10 mm shaft is a large relative deformation (0.2%), whereas 0.020 mm on a Ø100 mm shaft is tiny (0.02%). The ratio δ/d captures the actual strain.
Once you have contact pressure, the rest follows:
The axial force to press the shaft in. μ is the coefficient of friction during sliding (0.10–0.16 dry steel-steel). Applying light machine oil before pressing reduces μ to ~0.08–0.10, cutting insertion force by ~30%, without meaningfully reducing the static friction after assembly (oil is squeezed out under radial pressure).
The torque the joint can transmit before the shaft slips in the hub. Note it scales with d² (not d) — doubling the shaft diameter quadruples torque capacity for the same interference ratio and engagement length.
The hoop (circumferential tensile) stress in the hub. This is the stress that can crack or yield the hub. Always check this against the hub material yield strength before specifying the interference. For a D/d = 2.0 hub: σ_hoop = p × 5/3 = 1.67p. For D/d = 1.5: σ_hoop = p × 13/5 = 2.6p — thin walls amplify stress significantly.
Worked Example: H7/p6, Ø25 mm Steel Shaft in Steel Hub
- 1Contact pressurep = (200,000 × 0.022) / (2 × 25) × [(50² − 25²) / 50²]= 88 × 0.75 = 66 MPa
The wall factor for D/d = 2.0 is 0.75. A thinner hub (D/d = 1.5) would give a wall factor of only 0.56, reducing contact pressure to 49 MPa for the same interference.
- 2Hub hoop stress checkσ_hoop = 66 × [(50/25)² + 1] / [(50/25)² − 1] = 66 × 5/3= 110 MPa ≪ 250 MPa yield (hot-rolled 1018 steel) ✓
Safe. If you were using 6061-T6 aluminum hub (σ_y = 276 MPa), you would still be safe — but 6061 creeps under sustained stress at elevated temperature, which can relax the interference over time.
- 3Assembly forceF = 0.12 × 66 × π × 25 × 30= 18,600 N ≈ 18.6 kN (4,180 lbf)
This is the force your hydraulic press must supply. If you apply light machine oil (μ drops to 0.09), insertion force falls to ~14 kN. At maximum interference (δ = 0.035 mm), force rises to ~30 kN — confirm your press capacity covers worst-case.
- 4Torque capacityT = 0.12 × 66 × π × 25² × 30 / 2 / 1000≈ 233 N·m at nominal δ
This exceeds a standard 5 mm × 5 mm parallel key in the same engagement (~150 N·m). A H7/s6 fit (δ_nom = 0.031 mm) in the same geometry would reach ~330 N·m without a key.
Choosing the Right Press Fit Grade
Three interference grades cover the vast majority of CNC machined assemblies. Each step up roughly doubles the torque capacity but also doubles the assembly force and hub stress.
The workhorse press fit. Use this for anti-friction bearing outer races, bronze bushing installation in cast iron, and dowel pins that must be pressed in but not glued. The low end of the range (1 µm) is almost zero — it relies on friction only at the tightest material condition. For applications where you need reliable torque transfer, prefer the nominal mid-range interference (about 18 µm for Ø25 mm), which corresponds to both parts at mid-tolerance.
- Anti-friction bearing outer races
- Bronze bushings in cast iron
- Dowel pins (permanent)
For applications where the joint must transmit significant torque without a key. The 14 µm minimum interference guarantees contact pressure even at the loosest material condition. At maximum interference (48 µm), the hub hoop stress in a D/d = 2.0 steel hub reaches ~240 MPa — approaching yield for low-carbon steel. Always verify the hub material and wall ratio before specifying H7/s6 or higher.
- Keyless gear hubs on shafts
- Sprocket hubs
- Coupling flanges
Reserved for large-diameter drive applications where no keyway is practical. Assembly by mechanical press alone often risks yielding the hub — heating the hub to 150–200 °C expands the bore by CTE × ΔT × d = 12e-6 × 175 × 100 ≈ 210 µm for a Ø100 mm hub at 175 °C above ambient, easily accommodating the interference with no force required. After cooling, the hub grips the shaft with enormous contact pressure.
- Heavy industrial drive hubs
- Large machine tool spindles
- Railway wheel hubs
All Fit Classes Side by Side
Use this table to compare your fit options at a glance — from free-running clearance to heavy press. Tolerance values for the Ø18–30 mm diameter range per ISO 286-1:2010. Assembly force and torque for Ø25 mm, L = 30 mm, steel–steel, μ = 0.12. Negative clearance values indicate interference.
| Fit | Type | Clearance / Interference | Assembly | Torque (Ø25 mm) | Disassembly | Typical Use |
|---|---|---|---|---|---|---|
| H7/f7 | Free-running | +20 to +62 µm | Hand | None | Always easy | Journal bearings, pump shafts |
| H7/g6 | Close-running | +7 to +41 µm | Hand / light push | None | Always easy | Locating pins, reamer bores |
| H7/h6 | Sliding | 0 to +34 µm | Hand | None | Always possible | Spigot joints, tooling keys |
| H7/k6 | Transition | −15 to +19 µm | Mallet / light press | Negligible alone | Arbor press | Gears with key, hubs |
| H7/n6 | Transition (tight) | −28 to +6 µm | Hydraulic press | Low — needs key/pin | Press + mild heat | Pulleys with set screw retention |
| H7/p6 | Light press | −1 to −35 µm | Hydraulic press | 5–233 N·m | Press, likely damage | Bearing races, bronze bushings |
| H7/s6 | Medium press | −14 to −48 µm | Press or heat hub | 80–520 N·m | Destructive typically | Keyless gear hubs, couplings |
| H7/u6 | Heavy press | −27 to −61 µm | Thermal (≥ 150 °C) | 250–1,000+ N·m | Destructive | Heavy-duty drive hubs |
Clearance/interference for Ø18–30 mm range, ISO 286-1:2010. Torque for Ø25 mm, L = 30 mm, steel–steel, μ = 0.12. Ref: Shigley's ME Design, 10th Ed., §9-4.
Need H7/p6 or H7/g6 bores held to spec on your CNC parts?
MakerStage's vetted CNC machining network holds IT7 bores (H7, +0.000/+0.021 mm for Ø25 mm) and IT6 shafts on standard turning and reaming operations — no grinding surcharge for standard fit classes. Upload your drawing with the ISO 286-1 fit callout and our free DFM review flags any interference stack-up issues before quoting.
Upload Drawing & Get QuoteHow to Pick the Right Fit Class
Work through these five questions in order — each one eliminates categories until you arrive at a specific designation for your assembly. The explanations tell you the engineering reason behind each decision — not just the rule.
1. Must the joint transmit torque or axial load by friction alone — with no key, pin, or fastener?
2. Will this joint be disassembled during the product's service life?
3. Will there be relative motion (rotation or sliding) between the shaft and bore?
4. Do the shaft and bore materials have significantly different coefficients of thermal expansion (CTE)?
5. Is high location accuracy required (runout < 25 µm, no perceptible play)?
CTE Mismatch: The Most Common Overlooked Factor
Junior engineers often specify fits at room temperature and forget that the assembly operates at a different temperature. Here's the arithmetic for three common material combinations at a 60 °C rise above assembly temperature, for a Ø25 mm fit:
No correction needed. ISO 286-1 values apply directly.
H7/g6 minimum clearance (7 µm) is consumed entirely. Shaft seizes. Use H7/f7 or account for thermal growth in clearance budget.
No correction needed for clearance fits. For press fits: verify hub yield strength at temperature — aluminum yield drops ~20% at 150 °C.
How to Call Out Fits on a Drawing
There are two acceptable formats under ISO 286-1 and ASME Y14.5-2018 (ISO 1101 / ISO 14405 internationally). Both convey the same information to the machinist.
Preferred because any machinist with access to ISO 286-1 tables can look up the limits. Cleaner drawings, fewer numbers.
Use this for overseas suppliers or shops that may not have ISO 286-1 charts readily available. Eliminates any lookup ambiguity but adds numbers to the drawing.
Always add to the drawing notes: surface finish requirement (e.g., Ra 1.6 µm on bore and shaft mating surfaces), and lubrication instruction if applicable (e.g., “apply light machine oil to shaft before pressing” or “assemble dry”). The fit code alone does not specify these — and the omission causes field issues.
Frequently Asked Questions
What is the difference between a press fit and a slip fit?
A press fit (interference fit) has a shaft OD larger than the hole ID, creating a diametral interference of 0.001–0.060 in. (0.025–1.52 mm) depending on size and material. Assembly requires a hydraulic press or thermal differential (heating the hub or chilling the shaft). A slip fit (clearance fit) has a shaft OD smaller than the hole ID, leaving a guaranteed positive clearance so parts assemble by hand or with light tapping.
The ISO 286-1 standard codifies both: H7/p6 is a light press fit; H7/g6 is a free-running slip fit. The correct choice depends on load type, disassembly requirement, and whether relative motion is intended.
What tolerance class should I use for a press fit in CNC machined steel parts?
For CNC machined steel assemblies, H7/p6 is the standard light-to-medium press fit (diametral interference of +0.001 to +0.035 mm for Ø18–30 mm). For higher torque or axial load transfer without a key, use H7/s6 (interference +0.014 to +0.048 mm).
Heavy press fits (H7/u6, H7/x6) are reserved for large-diameter fits or when assembly by thermal differential is acceptable. Match bearing inner ring fits (ISO 286-1 k5, m5, n5) to shaft tolerance classes k5 or m5 — not p6 — per the bearing manufacturer's specification.
How do I calculate press fit assembly force?
Assembly force F = μ × p_contact × π × d × L, where μ is the axial coefficient of friction (0.10–0.16 for dry steel-steel), p_contact is contact pressure (MPa), d is bore diameter (mm), and L is engagement length (mm).
Contact pressure for a steel hub on a solid steel shaft: p = (E × δ) / (2d) × [(D² − d²) / D²], where E = 200 GPa, δ = diametral interference (mm), d = shaft diameter (mm), D = hub outer diameter (mm). Example: d = 25 mm, D = 50 mm, δ = 0.022 mm → p = 66 MPa. With L = 30 mm: F = 0.12 × 66 × π × 25 × 30 ≈ 18.6 kN (4,180 lbf).
What slip fit tolerance gives a smooth running clearance for a rotating shaft?
For a shaft rotating inside a bushing or plain bearing, ISO 286-1 H7/f7 (free-running fit) provides a diametral clearance of +0.020 to +0.062 mm for Ø18–30 mm — adequate for oil-film lubrication at moderate speeds. H7/g6 (close-running fit) gives +0.007 to +0.041 mm for the same range.
For precision sliding-fit applications (locating pins, shoulder bolts, reamer bores), H7/g6 is the standard choice. For high-speed spindles or precision linear guides, tighten to H6/g5 or use a ground journal and measure actual clearance with an air gauge.
Can a press fit replace a keyway for torque transmission?
Yes, for moderate torque loads. A H7/s6 press fit on a 25 mm steel shaft with 30 mm engagement and D/d = 2.0 hub transmits roughly 300–600 N·m of torque before slip, depending on surface roughness and lubrication during assembly.
This exceeds the capacity of a standard Ø5 mm parallel key (typically 150–200 N·m) in the same engagement length. However, press fits are sensitive to repeated disassembly — each press cycle degrades the mating surface Ra by roughly 0.2–0.4 µm, reducing grip over time.
What surface finish is required for a reliable press fit?
For steel-to-steel press fits, both mating surfaces should be ground or finish-turned to Ra 0.8–1.6 µm (32–63 µin. Ra). Surfaces rougher than Ra 3.2 µm (125 µin.) cause the interference peaks to shear during pressing, reducing effective contact area by 30–50%.
Apply a light film of machine oil immediately before assembly to reduce the insertion force by ~20% without materially affecting the frictional grip (oil is squeezed out under pressure). Avoid anti-seize compounds on intentional press fits — they reduce the friction coefficient below design assumptions.
How do I specify a press fit or slip fit on an engineering drawing?
Use ISO 286-1 designation on the drawing: specify the nominal dimension followed by the fit code, e.g., "Ø25 H7/p6" on an assembly view, or split it into "Ø25 H7 (+0.000/+0.021)" on the hole detail and "Ø25 p6 (+0.022/+0.035)" on the shaft detail (values for Ø18–30 mm range).
Per ASME Y14.5-2018 (ISO 1101 internationally), you can alternatively call out bilateral tolerances directly. Include a note specifying surface finish (e.g., Ra 1.6 µm) and any required lubrication during assembly. For CNC machined parts, H7 is achievable with standard boring or reaming; tighter than H6 typically requires cylindrical grinding.
What is a transition fit and when should I use it?
A transition fit occupies the zone between clearance and interference — the assembly may result in either a small clearance or a small interference depending on where individual parts land within their tolerance bands. ISO 286-1 H7/k6 and H7/n6 are common transition fits.
Use a transition fit when you need accurate location (low runout, no slop) but also need to be able to disassemble without a press — for example, a gear mounted on a shaft that must be replaced during field service. Transition fits require positive retention (set screw, locking washer, or circlip) since they do not provide reliable torque transmission by friction alone.
Related Resources
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