Short Answer:

Reaming is a finishing process used in machining to slightly enlarge and smoothen a previously drilled hole. It is done using a tool called a reamer, which has multiple cutting edges and removes a very small amount of material to improve the size accuracy and surface finish of the hole.

Reaming is important when high precision is needed, such as in fitting pins, bolts, or shafts into holes. It ensures the hole is round, straight, and smooth, and helps in maintaining exact tolerances. Drilling alone cannot give the required finish or size accuracy, so reaming is used after drilling.

Detailed Explanation:

Reaming and Its Purpose

In mechanical engineering, many components require accurate and smooth holes to fit parts like shafts, pins, or fasteners. Drilling, which is commonly used to make holes, often leaves a rough surface and may not give the exact size needed. That’s where reaming comes in.

Reaming is a secondary machining operation that follows drilling or boring. It uses a tool called a reamer to slightly enlarge the drilled hole and improve its roundness, straightness, and surface quality. The amount of material removed in reaming is very small — usually between 0.1 mm to 0.5 mm.

Reaming helps ensure that components fit perfectly into holes and work smoothly without unnecessary looseness or tightness.

How Reaming is Done

1. Initial Hole Preparation:
   First, a hole is made using a drill bit. The hole must be slightly smaller than the final required size.
2. Selection of Reamer:
   A reamer of the exact final diameter is selected. Reamers come in fixed sizes and must match the required tolerance.
3. Mounting the Reamer:
   The reamer is fixed in a drilling machine, lathe, or reaming machine depending on the part and setup.
4. Reaming Operation:
   The reamer rotates slowly (at a lower speed than drilling) and is fed into the hole. It removes a thin layer of material from the inner surface.
5. Finishing:
   The reamer is carefully withdrawn after reaching the desired depth. The result is a high-quality hole with an accurate size and a smooth internal surface.

Why Reaming is Done

Reaming is done for the following main reasons:

1. To Improve Size Accuracy

Drilled holes are not always perfect in size. Reaming makes sure the hole matches the exact required diameter, especially when tight tolerances are needed.

2. To Enhance Surface Finish

Drilled holes often have rough internal surfaces. Reaming produces a smooth and polished finish, which helps in reducing friction and wear between moving parts.

3. To Ensure Roundness and Straightness

Reaming corrects slight errors in the shape of the hole made during drilling. It improves roundness, straightness, and alignment.

4. To Provide a Good Fit

When two parts need to be joined using a pin, shaft, or bolt, reaming helps in creating the perfect hole for a tight or loose fit depending on the application.

5. To Prepare for Precision Assembly

In mechanical assemblies, holes for bearings, bushings, dowels, or fasteners require precise sizes. Reaming ensures that these parts fit securely and function correctly.

Types of Reamers

• Hand Reamers: Used manually with a wrench for light work.
• Machine Reamers: Used with machines for faster and more accurate work.
• Adjustable Reamers: Can be adjusted for different hole sizes.
• Taper Reamers: Used to finish tapered holes.
• Shell Reamers: Hollow tools used for large hole finishing.

Applications of Reaming

• Fitting dowel pins in machine parts
• Preparing holes for bushes and bearings
• Finishing holes in automotive parts
• Used in toolmaking and die industries
• Precision parts assembly in aerospace and defense

Conclusion

Reaming is a precision machining operation used to improve the size and finish of drilled holes. It is done after drilling to ensure the hole is accurate, smooth, round, and perfectly sized. Reaming is widely used in mechanical engineering to achieve tight tolerances and high-quality assembly. It plays a key role in ensuring that machine parts fit correctly and perform efficiently without errors.