# CNC Hole Edge Distance: Leave Enough Material Around Drilled Features

Review CNC holes near part edges by checking the remaining ligament, largest feature diameter, depth, tolerance and load path before releasing a machined part.

A hole can be dimensionally correct and still be a poor machining feature because it sits too close to an outside edge.

The risk is not captured by one universal “minimum edge distance.” It depends on the finished hole feature, the material left beside it, the operation used to make it, and what the part has to do in service.

Protolabs' recent CNC hole-design guidance makes the core point: holes close to an edge can weaken the part and complicate machining, and edge-distance review should consider the largest diameter of the entire feature, including counterbores and countersinks. [Protolabs CNC hole guidance](https://www.protolabs.com/en-gb/resources/blog/cnc-hole-design-through-blind-tapped-and-more/).

The practical review question is therefore: **how much material remains at the thinnest ligament after the complete hole feature is machined?**

## Center-to-edge distance is not the same as remaining wall

Drawings often dimension a hole centerline to an outer edge. That can be convenient for layout, but manufacturing risk is driven more directly by the material remaining between the finished feature and the part boundary.

For a simple drilled hole, the remaining ligament is approximately the center-to-edge distance minus the hole radius.

For a counterbored or countersunk feature, use the largest local diameter that actually approaches the edge.

A hole whose center is 8 mm from an edge may leave very different material depending on whether the feature is Ø4, Ø10 or a Ø14 counterbore.

Do not compare center distance alone across different hole types.

## Why holes near an edge are harder

Several effects can matter at the same time.

**Breakout risk.** The drill or enlarged feature can intersect the outside wall, intentionally or accidentally.

**Thin-wall deflection.** The material beside the hole can flex during cutting, especially with deep features or soft or thin sections.

**Burr and edge quality.** A small ligament can leave a fragile edge that deforms during deburring or finishing.

**Thread strength.** A tapped hole near an edge may have enough thread depth but insufficient surrounding material for the expected load.

**Inspection difficulty.** Tight positional tolerance plus a thin remaining wall can make the feature expensive to hold and verify.

**Secondary operations.** Counterbores, countersinks, spotfaces or reaming can require more radial material than the pilot hole suggests.

The simplest hole diameter is not always the controlling geometry.

## There is no single safe ratio for every CNC part

Online design guides sometimes publish useful supplier-specific minimums. Treat those as process capability guidance, not universal mechanical-design laws.

Edge distance depends on material and condition, hole diameter, through versus blind geometry, counterbore or countersink size, thread size and engagement, part thickness, hole depth, positional tolerance, machining direction, tool access, expected structural load, and finishing requirements.

A generous aluminum clearance hole and a highly loaded tapped hole in a thin hardened-steel ear should not share one rule.

If the distance is functionally constrained, show the geometry and requirement to the supplier instead of silently applying a generic multiplier.

## Review the largest feature envelope

When checking a compound hole, record each diameter that removes material near the edge.

| Feature | Diameter | Depth | Edge concern |
|---|---:|---:|---|
| Pilot/through hole | Ø6 | Through | Basic ligament |
| Counterbore | Ø11 | 4 mm | Often controls top-edge material |
| Entry chamfer | Ø12 max | 0.5 mm | May become the largest local envelope |
| Thread | M8 | 14 mm | Load path matters, not only nominal bore |

The outermost radial cut may be shallow, but it can still create the smallest wall at the surface.

Protolabs explicitly recommends considering the largest diameter of the entire feature when checking edge distance. [Protolabs CNC hole guidance](https://www.protolabs.com/en-gb/resources/blog/cnc-hole-design-through-blind-tapped-and-more/).

## Tapped holes need both thread-depth and edge-distance checks

A blind tapped hole can fail the design review in two independent ways.

It may be too shallow to provide the required full thread, or it may be deep enough but too close to the edge for the surrounding material and load.

The [blind tapped hole depth guide](https://www.cadprops.com/guides/blind-tapped-hole-depth/index.md) separates drill depth from full thread depth. Add edge distance as a separate line item.

For loaded threads, the correct requirement depends on fastener size, material pair, preload and joint design. Do not infer thread strength from CAD hole diameter alone.

## Reamed holes add a positional question

A reamed hole may be used for a dowel, bearing, bushing or precision location.

That can make edge distance more sensitive because the feature may carry load or establish a datum relationship.

The [reamed versus drilled hole guide](https://www.cadprops.com/guides/reamed-hole-vs-drilled-hole/index.md) explains why reaming improves size and finish but is not a substitute for poor hole location. If the hole is close to an edge, evaluate both the remaining material and the positional requirement.

## What CADProps can help you check

CADProps does not certify a hole-edge-distance rule or calculate allowable structural load.

You can use the [STEP viewer](https://www.cadprops.com/tools/step-viewer/index.md) and [dimensions tool](https://www.cadprops.com/tools/cad-dimensions/index.md) to inspect the part, verify units and measure relevant geometry. The current [hole recognition workflow](https://www.cadprops.com/guides/step-hole-recognition/index.md) classifies only bounded cases such as simple full cylindrical through holes and flat-bottom blind holes; it does not generally recognize counterbores, countersinks or threaded intent.

For a manual review, measure the finished feature diameter that is closest to the edge, the shortest distance from that feature boundary to the external part boundary, part thickness in the affected region, hole depth or thread engagement, and any critical positional tolerance.

That evidence is enough to start a useful DFM conversation without pretending the software has approved the feature.

## Put edge-distance questions into the RFQ when they matter

If a hole is intentionally close to an edge because of packaging or interface constraints, call that out.

A useful supplier note might say:

> “The Ø8 H7 locating hole is constrained to this position. Minimum remaining wall is 2.2 mm. Please confirm the proposed process can hold the position and finish without edge breakout.”

That is much better than sending only the STEP model and waiting for the shop to discover the risk.

The [CNC quote preparation guide](https://www.cadprops.com/guides/prepare-step-file-for-cnc-quote/index.md) covers the rest of the handoff: material, quantity, revision, tolerances, finish, threads and inspection.

The engineering principle is simple: **review the remaining material around the complete hole feature, not just the centerline dimension, and let the actual process and load case determine whether the distance is acceptable.**

## Related reading

- [Autodesk Fusion AutoTimeline: Why CAD Feature Recognition Matters Again](https://www.cadprops.com/guides/autodesk-fusion-autotimeline-feature-recognition/index.md)
- [Blind Tapped Hole Depth: Separate Thread Depth from Drill Depth](https://www.cadprops.com/guides/blind-tapped-hole-depth/index.md)
- [CNC Internal Corner Radius: Design Pockets for Real End Mills](https://www.cadprops.com/guides/cnc-internal-corner-radius/index.md)
- [Minimum Wall Thickness for CNC Machining: No Single Safe Number](https://www.cadprops.com/guides/cnc-minimum-wall-thickness/index.md)
- [Deep Hole Drilling: Use Depth-to-Diameter Ratio Before You Quote](https://www.cadprops.com/guides/deep-hole-drilling-depth-to-diameter-ratio/index.md)
- [Prepare a STEP File for a CNC Quote: A Shop-Ready RFQ Checklist](https://www.cadprops.com/guides/prepare-step-file-for-cnc-quote/index.md)
- [Reamed Hole vs Drilled Hole: Use Precision Only Where the Fit Needs It](https://www.cadprops.com/guides/reamed-hole-vs-drilled-hole/index.md)
- [STEP Hole Recognition: Detect Through and Blind Holes](https://www.cadprops.com/guides/step-hole-recognition/index.md)

Original article: https://www.cadprops.com/guides/cnc-hole-edge-distance/

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