Short Answer:

Reaming is a finishing process used to slightly enlarge and smoothen a previously drilled hole. It is done using a tool called a reamer, which removes a small amount of material from the inner surface of the hole. This helps to improve the accuracy, roundness, and surface finish of the hole. Reaming is not used to create holes, but to finish them.

Reaming is done when high precision is required. A drilled hole may be slightly rough or not exactly the right size, so reaming is used to correct it. This ensures that parts like pins, shafts, or bolts fit properly and function smoothly without looseness or tightness.

Detailed Explanation:

Reaming and Why It Is Done

In mechanical engineering, making accurate holes is very important. Holes are needed for inserting fasteners, shafts, pins, bushings, or for aligning parts. A drill is usually used to make holes, but drilled holes may not be perfectly smooth or exactly the right size. That’s why an extra operation called reaming is used after drilling.

Reaming is a precise finishing process. It removes a very small amount of material (usually between 0.1 mm to 0.5 mm) to correct the size and shape of the hole. The tool used for this is called a reamer. A reamer has multiple cutting edges that cut evenly on all sides of the hole, making the hole rounder, smoother, and more accurate.

How Reaming Is Performed

Drilling the Initial Hole
First, a hole is drilled that is slightly smaller than the required final size.
Choosing the Right Reamer
A reamer with the exact final size is selected. The hole is not made larger by force, but carefully finished.
Mounting the Reamer
The reamer is fixed in a machine such as a drill press, lathe, or milling machine. It can also be used by hand for small jobs.
Feeding the Reamer into the Hole
The reamer rotates slowly and is fed gently into the drilled hole. The reamer cuts a thin layer of material as it moves.
Finishing the Hole
After reaching the required depth, the reamer is removed. The hole is now of accurate size and smooth finish.

Why Reaming Is Done

To Get Accurate Hole Size

A drilled hole is often slightly undersized or oversized. Reaming gives the exact dimension needed, especially when working with tight tolerances.

To Improve Surface Finish

Drilled holes may be rough inside. Reaming removes the roughness and creates a polished and smooth surface, which is important for moving parts.

To Ensure Proper Fit

In many assemblies, parts like shafts, pins, or fasteners go inside holes. Reaming ensures the hole is the right size so the part fits correctly and stays in place without too much force or looseness.

To Improve Roundness and Alignment

Reaming corrects small defects in roundness and straightness of drilled holes. It helps when accurate alignment is needed in machinery.

To Prepare for High-Precision Parts

In industries like automotive, aerospace, or tool-making, reaming is used to prepare holes for bearings, dowel pins, and bushings, which require tight fits.

Types of Reamers

Hand Reamer: Used manually with a handle. Good for small work.
Machine Reamer: Used with machines for faster and uniform finish.
Adjustable Reamer: Can be set for slightly different hole sizes.
Taper Reamer: Used for making tapered (angled) holes.
Shell Reamer: Used for large diameter holes with replaceable cutting heads.

Applications of Reaming

Inserting dowel pins in assembly
Preparing shaft holes in machine parts
Fitting bearings or bushings in automotive and industrial machines
Finishing holes in die and mold making
Achieving high-precision holes in aerospace and defense equipment

Conclusion

Reaming is a finishing process used to enlarge and improve an already drilled hole. It gives the hole a better surface finish, correct size, and perfect roundness. Reaming is very useful when high accuracy and smooth fit are required, especially in machine assemblies. Whether done manually or with a machine, reaming ensures that the final product is reliable and performs well in real-world use.