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Both are hexagonal, so what is the difference between the outer hexagon and the inner hexagon?
Here, I will talk about the appearance, fastening tools, cost, advantages and disadvantages, and applicable occasions of the two in detail.

Exterior

Hexagonal bolts/screws should be familiar to everyone, that is, the bolts/screws with hexagonal head sides and no concave head;
The outer edge of the head of the hexagon socket bolt is round, and the middle is a concave hexagon. The more common one is the cylindrical head hexagon, and there are pan head hexagon, countersunk head hexagon, flat head hexagon, headless screw, Stop screws, machine screws, etc. are called headless hexagon sockets.
fastening tool

The fastening tools for outer hexagonal bolts/screws are more common, that is, wrenches with equilateral hexagonal heads, such as adjustable wrenches, ring wrenches, open-end wrenches, etc.;

The shape of the wrench used for hexagon socket head bolts/screws is “L” shape, one side is long and the other side is short, and the short side is used for screwing, holding the long side can save effort and tighten the screws better.
cost

The cost of the external hex bolts/screws is lower, almost half of that of the socket head bolts/screws.

advantage

Hexagon bolts/screws:

Self-selling is good;

Large preload contact area and large preload force;

Wider range of full thread lengths;

There may be reamed holes, which can fix the position of the part and withstand the shear caused by lateral force;

The head is thinner than the inner hexagon, and the inner hexagon cannot be replaced in some places.
Hexagon socket bolts/screws:

Easy to fasten;

Not easy to disassemble;

not easy to slip angle;

Small footprint;

bears a large load;

It can be processed by sinking the head, and can be sunk into the inside of the workpiece, which is more delicate and beautiful, and will not hinder other parts.
shortcoming

Hexagon bolts/screws:

It takes up a lot of space and is not suitable for more delicate occasions;

Cannot be used with countersunk heads.
Hexagon socket bolts/screws:

Small contact area and small pre-tightening force;

No full thread beyond a certain length;

The fastening tool is not easy to match, it is easy to slip when twisting, and it is inconvenient to replace;

Use a professional wrench when disassembling, and it is not easy to disassemble at ordinary times.
Applications

Socket head cap bolts/screws are suitable for:

connection of large equipment;

Suitable for thin-walled parts or occasions subject to shock, vibration or alternating loads;

Where the thread requires a long length;

Mechanical connections with low cost, low dynamic strength and low precision requirements;

Where space is not considered.

Hexagon socket bolts/screws are suitable for:

connection of small devices;

Mechanical connections with high requirements for aesthetics and precision;

When sinking the head is required;

Narrow assembly occasions.
Although there are so many differences between outer hexagonal bolts/screws and inner hexagonal bolts/screws, in order to meet more usage needs, we do not only use a certain kind of bolts/screws, but need a variety of fastener screws Use together.