Hole Tolerance Chart and Design Notes
Hole tolerances matter because holes often control assembly. A clearance hole can be loose, but a dowel hole, bearing seat, or alignment bore may require much tighter control.
Key Points for Buyers and Engineers
| Topic | What to know | Practical impact |
|---|---|---|
| Clearance hole | Allows screw or bolt passage | Usually less demanding. |
| Dowel hole | Locates parts accurately | Often needs reaming and positional control. |
| Press fit | Interference with mating part | Requires tight diameter and material review. |
| Bearing bore | Supports rotating or seated components | Needs roundness, diameter, and finish control. |
| Threaded hole | Accepts fasteners | Needs tap size, depth, and thread gauge checks. |
How This Affects CNC Machined Parts
Holes can be drilled, bored, reamed, tapped, or thread milled. For complex patterns, connect tolerances with inspection planning and datum control.
Design and Purchasing Checklist
- Use tight tolerances only on functional holes.
- Specify datum references for hole position.
- Do not expect drilled holes to behave like reamed holes.
- Confirm fit with mating components.
- Define inspection tools for critical holes.
Related MTSPi Resources
FAQ
What is a normal hole tolerance?
It depends on function. Clearance holes can be looser, while dowel holes, bearing bores, and press fits need tighter tolerances.
Is drilling enough for tight holes?
Usually not. Tight holes may need boring or reaming.
How are hole tolerances inspected?
Common tools include pin gauges, bore gauges, CMM, thread gauges, and depth gauges.
Need help with hole tolerance review?
Send drawings, material, quantity, finish, tolerance notes, and application requirements. MTSPi can review manufacturability before quoting.
Practical Buyer Notes
Hole tolerance chart pages should explain fit intent, not just dimensions. Clearance holes, dowel holes, bearing bores, press fits, tapped holes, and fluid passages have different tolerance needs.
This page should connect hole tolerance to process choice. Drilling, boring, reaming, and CMM inspection are not interchangeable; each provides a different level of size, position, straightness, and finish control.
How to Evaluate This Topic on a Real Drawing
| Decision point | Recommended approach | Why it matters |
|---|---|---|
| Clearance fit | Use practical tolerance | Allows fastener passage without extra cost |
| Dowel fit | Use reaming and position tolerance | Controls alignment |
| Bearing bore | Control diameter, roundness, and finish | Supports rotating or seated parts |
| Press fit | Define interference and material pairing | Avoids cracked parts or loose assemblies |
Common Mistakes to Avoid
- Using tight hole tolerances without explaining fit purpose.
- Expecting drill-only holes to meet reamed-hole requirements.
- Ignoring positional tolerance for hole patterns.
- Not defining inspection method for critical bores.
Recommended MTSPi Review Workflow
MTSPi should classify each hole by function before quoting. Critical holes may need boring, reaming, special gauges, or CMM checks, while simple clearance holes can stay economical.
- Confirm the part function, operating environment, and assembly risk.
- Review material, process, tolerance, surface finish, and inspection requirements together.
- Separate critical features from general dimensions so cost is not wasted on non-functional details.
- Use related resources such as CNC materials, surface treatment, and quality assurance to complete the specification.
- Send the final drawing package to MTSPi for quote review before production.
What to Send for a Faster Quote
Send hole diameter, tolerance, position requirement, depth, thread callout, mating part details, and whether inspection reporting is required.
- 3D CAD file and 2D drawing with revision number.
- Material grade or acceptable alternatives.
- Quantity, delivery target, and whether samples are required first.
- Critical dimensions, thread callouts, finish notes, and inspection requirements.
- Application notes when the part is used in heat, moisture, wear, vibration, or visible assemblies.
Deeper Engineering Notes
For hole tolerance selection, the strongest page is not just a definition. It should help a buyer decide what information belongs on the drawing, what questions to ask a supplier, and which details affect cost, quality, and delivery. In real RFQ work, the important details are usually clearance holes, dowel holes, bearing bores, press fits, true position, and measurement method. These details decide whether the supplier can quote confidently or has to return with repeated questions.
The common risk is tightening every hole instead of matching tolerance to function. That risk can make the content look complete on the surface but weak in a real manufacturing situation. A stronger specification connects the topic to material behavior, machine access, tolerance control, finishing, and inspection before the order is released.
Example Specification Review
| Review area | Question to answer | Good specification practice |
|---|---|---|
| Function | What does this feature or part need to do? | Describe the load, fit, motion, appearance, sealing, or assembly requirement. |
| Material | Is the selected material suitable for the environment? | Confirm strength, heat, corrosion, wear, chemical exposure, and cosmetic needs. |
| Geometry | Can the feature be reached by standard tools? | Review depth, corner radius, wall thickness, tool clearance, and setup direction. |
| Tolerance | Which dimensions are truly critical? | Use tight tolerances only where function requires them and keep general dimensions practical. |
| Verification | How will acceptance be checked? | Define gauges, CMM checks, roughness measurement, thread inspection, or visual standards when needed. |
How This Page Should Support Ranking
This topic can attract both beginner searches and buyer searches. The beginner wants a clear answer; the buyer wants enough detail to make a manufacturing decision. A page has a better chance to rank when it answers the simple question first, then adds examples, process limits, design notes, comparison tables, and practical next steps.
That is also why internal links should be helpful rather than forced. A reader should naturally move from this page to a process page, material page, surface finish page, inspection page, or quote page only when that next step solves a real question.
Conclusion
Hole tolerance should match the assembly function. Clear callouts for diameter, position, depth, and inspection help CNC suppliers make reliable parts.
