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Why can't the holes be too close to the edges in mechanical design ?

The hole feature is widely used in mechanical design for connecting bolts, positioning, and circulating media functions. These are not randomly located, and when considering functionality, we also need to consider the convenience of manufacturing. Some designs cannot be processed, or manufacturing defects and hidden dangers will inevitably occur after processing. Therefore, engineers need to avoid similar structures during design, either by modifying the size or modifying the design structure. The feasibility of manufacturing is always the top priority. Excellent engineers always combine functionality and manufacturing perfectly.

1、 Hole edge, structural strength is directly cut in half (the most core risk)

Insufficient wall thickness at the edge leads to direct tearing under stress

There is only a thin layer of material left between the hole and the outer edge of the part, equivalent to a thin sheet that is about to break. When the equipment is subjected to tension, vibration, and bending forces, the thin edge will be the first weak point for the entire part to crack and collapse, and heavy load conditions are prone to direct fracture and scrap.

Double stress superposition significantly shortens fatigue life. The pores themselves are natural stress concentration points; Once approaching the edge of the sheet, the edge boundary constraint will amplify the stress peak and the stress concentration factor will increase exponentially. Under long-term reciprocating vibration and alternating loads, fatigue cracks will appear in the parts in advance.

Bent parts are prone to tearing and opening holes. If the hole is too close to the bending line, the metal sheet will stretch and flow during bending, causing the circular hole to elongate and tear, and the finished product will be discarded.

2、 The entire processing process is overturned, and the scrap rate is soaring

Different processing techniques can cause problems if the hole edge distance is too small:

Drilling/punching process

The lack of material support at the edge makes the drill bit prone to deviation, stabbing, and creating elliptical holes; Punching can result in severe flanging, burrs, and even immediate cracking of the sheet metal during stamping.

Laser/wire cutting material cutting

The edge of the board has poor heat dissipation ability, and leaving the hole at close range can cause local high-temperature thermal deformation, exceeding the size tolerance of the parts, edge ablation and carbonization, and both appearance and accuracy are not up to standard.

Tapping machining (threaded hole)

The threaded hole is too close to the edge, and the material is prone to tooth breakage and thread slippage during tapping. The effective load-bearing length of the thread is greatly reduced, and the screw locking directly fails.

3、 Assembling lock attachments and continuous malfunctions during actual use

Many designs neglect margins in the early stages and only realize during the assembly stage that they cannot be assembled:

Fastener interference, uneven locking attachment

The outer diameter of nuts, flat washers, and spring washers is larger than the remaining edge distance, and fasteners will protrude from the side of the parts. This not only causes uneven installation and easy loosening during operation and vibration, but also scratches other surrounding components.

Locking and crushing the board, sealing failure

When tightening bolts, the thin edge material has extremely poor compressive strength and is easily crushed and deformed by washers; If the position needs to be sealed, the deformation gap will directly cause oil and gas leakage, greatly reducing the reliability of the whole machine.

4、 Universal standard margins, designed to be directly applied (industry standard experience)

Based on the general specifications for sheet metal and machining, the minimum safe distance for different hole types is:

Ordinary through-hole (bolt through-hole)

The distance from the center of the hole to the edge of the part is ≥ 1.5 times the diameter of the hole (as shown in the example figure for a Φ 12 hole, with a minimum edge distance of L=12 × 1.5=18mm); For heavy-duty and vibrating equipment, it is recommended to enlarge the aperture to 2 times.

The distance from the center of the internal thread bottom hole to the edge of the part should be ≥ 2 times the diameter to ensure the integrity of the thread and prevent thread breakage during tapping.

Compared to carbon steel and stainless steel, soft materials such as aluminum and plastic have an additional minimum margin of 20% to 50%, resulting in weaker tear and compression resistance.

5、 Design Summary

The opening closely adheres to the edge of the part, seemingly optimizing the size of the part, but in fact, there are four fatal defects:

✅  Reduced structural strength and susceptibility to fatigue fracture

✅  Processing deformation and scrapping lead to increased production costs

✅  Assembly interference and easy loosening of bolts

✅  Sealing failure leads to decreased overall reliability. Mechanical design prioritizes ensuring safe hole margins and does not blindly compress external dimensions. Follow the basic specifications of 1.5 to 2 times the aperture size to avoid structural failure and rework from the source, reducing the cost of later rectification!