A blind hole in engineering refers to a hole that doesn't extend through the entire thickness of the material, relative to the axis of the hole. This type of hole is can be created by drilling, or milling to a specified depth without penetrating through to the opposite side of the workpiece.

How are blind holes used in manufacturing? 

Blind holes can serve many purposes in engineering, they can be used for locating, weight reduction, indexing, fixturing or many other things. Many machine types can create bling holes, from manual to CNC. 

Blind holes can also be threaded.  Common methods for threading blind holes include conventional tapping, single-point threading, and thread milling with helical interpolation.

What symbol is used to call out a blind hole?

The callout for a blind hole is the diameter symbol ( Ø ) with the hole size followed by a depth symbol ( ↧ ) and dimension. On engineering drawings, through holes are indicated by specifying the hole's diameter and depth. 

A through hole in machining is a hole that extends completely through the material. This type of hole passes entirely through the workpiece and is sometimes referred to as a "thru-hole."

How are through holes utilized? 

Through holes serve various purposes dependent upon the engineering needs of the components. They provide pathways for fasteners (bolt holes), electrical connections, and other functional requirements in mechanical and electronic assemblies.

Thru holes can also be threaded.  Common methods for threading thru holes include conventional tapping, single-point threading, and thread milling with helical interpolation.

What symbol is used to call out a through hole?

The callout symbol for a through hole is the diameter symbol 'Ø' followed by the hole size and usually the word 'thru' to indicate that the hole continues at the same diameter thru the entire thickness.

Counterbored holes are most commonly used for bolt holes so that the head of the bolt is below the surface of the component. Though there are many other use cases.  A counterbored hole typically consists of a thru hole and flat bottom blind hole. 

What symbol is used to call out counterbored holes?

The callout for a counterbored hole typically follows a hole callout.  The symbol ⌴ indicates that information that follows applies is for a counterbore.  Immediately following the ⌴ symbol will be the diameter of the counterbore which will then be followed by the depth ( ↧ ) of the counterbore.

How are counterbored holes utilized in machining?

As mentioned above counterbored holes find frequent use in conjunction with socket-head screws. They are employed in applications where bolts or screws need to be recessed below the surface for a flush finish.  They may also be used as a clearance or alignment feature.

Unlike a counterbore hole, a countersink hole features a conical shape. It consists of a simple hole with a tapered recess above it. Creating this cone-shaped recess demands greater calculation and precision compared to drilling a counterbore hole.

How are countersunk holes utilized in engineering?

Countersunk holes are employed in engineering applications where a flat head bolt or screw is necessary. Similar to counterbore holes, countersink holes allow bolts or screws to sit flush or below the surface, ensuring a smooth finish and secure fastening.

What symbol is used to call out a countersunk hole?

The callout for a countersunk hole typically follows a hole callout.  The downward carrot symbol ( ⌵ ) indicates that information that follows applies to a countersink.  The first number is the diameter of the countersink at the part face.  The second number is the included angle of the countersink.  

The most common angles used for fasteners is 82°, 90°, and 100°.  Used respectively for english screws, metrics screws, and wood screws. 

A counter drilled hole resembles a countersink hole but features a recess above it.

How are counter drilled holes utilized in engineering? 

Counter drilled holes, similar to countersunk holes, are employed in engineering applications requiring flat head bolts or screws. Additionally, they are frequently used in PCB manufacturing processes to accommodate shorter screw lengths.

What symbol is used to call out a countersunk hole?

The callout for a counter drilled hole typically is less standardized.  They may be manually dimensioned or use multiple hole callouts.  If in question is best to consult with the designer.