Reaming is a critical finishing process in CNC machining that guarantees optimal diameter accuracy, dimensional tolerance, and superior surface finish for the holes in your workpiece. But what exactly is reaming, and why is it indispensable in manufacturing? This comprehensive article will answer these questions about CNC reaming definition, purposes, and potential challenges.

What is reaming?

Reaming is a method where a reamer tool removes a small amount of metal from the workpiece hole wall to improve its dimensional accuracy and hole surface quality. Reaming is one of the precision machining methods and finds widespread applications in manufacturing.

For smaller holes, it’s a cost-effective machining technique compared to internal cylindrical grinding and precision boring. Reaming processes require the use of a CNC machine to enhance workpiece precision. Common components such as bearings and gears often require reaming tools to complete the corresponding hole machining.

What Is the Purpose of Reaming?

Improve Hole Precision

Generally, the rough reaming allowance is set between 0.35mm to 0.15mm, and the finish reaming allowance is between 0.15mm to 0.05mm. Reamed hole dimensional accuracy typically falls within the range of IT9 to IT7.

Achieve Adequate Surface Roughness

The surface roughness of reaming, Ra, is usually in the range of 3.2 to 0.8. Reaming typically employs lower cutting speeds (when machining steel and cast iron with high-speed steel reamers, v < 8m/min) and the use of appropriate cutting fluids for cooling, lubrication, and chip removal to help prevent chip accumulation and ensures the attainment of adequate surface roughness.

How is reaming done step by step?

Step 1: Selecting the Right Reamer

  • Choosing the appropriate reamer based on the reaming allowance and precision requirements is crucial for hole quality.
  • The reaming allowance should not be too large or too small.
  • Too small an allowance makes it challenging to correct deformations from previous machining processes and may fail to remove existing tool marks, affecting hole quality.
  • Excessive allowance increases the cutting load per tooth, destabilizes the reaming process, generates more heat, causes reamer diameter expansion, and enlarges hole diameter.
Reaming Diameter<5mm5~20mm21~32mm33~50mm51~70mm
Reaming allowance0.1~0.2mm0.2~0.3mm0.3mm0.5mm0.8mm

Step 2: Secure Workpiece Clamping and Alignment

Ensure secure and precise clamping and positioning of workpieces to maintain reamer-to-hole coaxiality.

Step 3: Clean and Lubricate the Reamer

Clean the reamer tool body and apply an adequate amount of cutting fluid both inside the hole and on the reamer.

Step 4: Tool Change and Setting

  • Refer to the programming guide to replace the tool and establish the tool’s contact height with the reference surface.
  • Set the Z-axis zero point at a safe position and adjust the relative coordinates accordingly.

Step 5: Machining Parameters Setup

  • Determine the spindle speed (N) using the formula: N = 1000V / (3.14D), where V is the cutting speed (m/min) and D is the tool diameter (mm).
  • Set feed speed (F) as per: F = N × M × Fn, where M is the tool’s edge count, and Fn is the tool’s chip load (mm/rev).
  • Define chip load per edge (Fn) using Fn = Z × Fz, where Z is the tool’s edge count and Fz is the cutting amount per edge (mm/rev).

Step 6: Machining Process

  • The reaming process typically involves center drilling for initial positioning, followed by drilling with a bit slightly smaller (0.5-0.3mm) than the required hole size, and finally, reaming with the reamer tool.
  • Maintain spindle speeds between 70-180 rpm during reaming.

Step 7: Reamer Maintenance

After use, clean the reamer thoroughly and apply machine oil to prevent corrosion.

Protect the cutting edges when storing to prevent accidental damage.

What is a reamer?

A reamer is a type of rotary cutting tool used in metalworking for enlarging and finishing to accurate dimensions holes that have been drilled, bored, or cored.

Different Types of Reamers

Hand reamer

A hand reamer has a longer taper or lead in at the front than a machine reamer. This is to compensate for the difficulty of starting a hole by hand power alone. Hand reamers enlarge or finish an existing hole in a metal workpiece to a desired size.

Machine Reamers

These reamers are similar to hand reamers, except that the shank is tapered

Spiral-Fluted Reamers

Spiral-fluted reamers are special-purpose reamers that are ideal for hard-chipping materials like stainless steel. Thanks to their spiral-shaped flutes, these hole reamers evenly distribute cutting forces, enabling them to effectively cut through tough materials.

Chucking reamers

Chucking reamers are general-purpose cutting tools that provide superior wear and tear resistance at high temperatures. They come in a straight shank or a morse taper shank and are used for CNC lathes tools or drill presses.

Expanding Reamers

Expansion reamers are used to enlarge or finish an existing hole to a precise tolerance or size. it can be reground to its original size due to They have an adjustment screw that expands the size of the cutting head.

Adjustable Reamers

Adjustable reamers are designed to ream odd-sized holes. In these reamers, separate blades are inserted in the grooves provided in the body of the reamer. The blades can be moved up or down of the reamer.

Tapered Reamers

Taper pin reamers are used in metalworking applications to enlarge, finish, or shape holes for a precise fit when installing taper pins into the hole. These reamers are used to finish the taper holes for cutting the taper things used to secure the collars, and pulleys to the shaft.

Shell Reamers:

High-speed steel Shell Reamers are multi-fluted, end-cutting tools used to enlarge previously formed holes to a precise diameter with a smoother finish.  These reamers are however not very rigid and accurately inserted teeth or plates in shells further reducing the cost of reamers that can tip with cemented carbides.

Common Reaming Challenges

  • Inconsistent Hole Size: The reamer may create holes that are either undersized with a bell-mouth shape or oversized, often due to misalignment and incorrect feed and speed settings.
  • Reamer Seizure and Breakage: Problems such as high surface roughness, unsuitable tool materials, or inadequate cutting fluid supply can lead to reamer seizure and breakage.
  • Surface Finish Compromises: The reamer might affect the hole’s surface finish negatively, resulting from uneven chamfers, chatter, excessive spindle run-out, or insufficient cutting action.
  • Tool Wear or Breakage: Excessive tool wear or breakage may occur due to misalignment or excessive reaming pressure.

Guidelines for reaming operations

Feeds and Speeds for Reaming

  • Adjust feeds and speeds based on material type, machine specifications, and desired finish and precision. Typically, reaming is performed at about two-thirds of the feeds and speeds used for drilling in the same material.
  • Metal reaming usually requires higher feeds compared to drilling, often running at 200% to 300% of drill feeds. Avoid excessively low feeds to prevent reamer wear; it should cut rather than rub or burnish.

Precise Alignment

  • Perfect alignment of the spindle, bushing, reamer, and the hole to be reamed is critical. Ensure the helix angle aligns correctly with other parameters to minimize reamer wear and maintain hole accuracy.
  • For oversized, tapered, or irregularly shaped holes, verify alignment parameters. Adjustable holders can help mitigate alignment errors.

Reamer Regrinding

  • Maintain cutting tools to prevent excessive dullness before sharpening or replacement. Regularly regrind the chamfer on a reamer before it becomes too worn or ineffective.
  • Focus regrinding efforts on the chamfer or entering taper. Ensure even sharpening of each flute to avoid oversizing. Hand sharpening is not recommended due to the challenge of maintaining uniform cutting edges.

Stock Allowance

Proper stock allowance is crucial for reamer machining. Typically, it’s about .010″ for a ¼” hole, .015″ for a ½” hole, and up to .025″ for a 1½” hole. Hand reaming requires a smaller allowance, typically .001″ to .003″, owing to difficulties in handling larger stock.

Chatter Prevention

Chatter during reaming operations can compromise hole finish and reamer effectiveness. To minimize or prevent chatter:

  • Ensure machine rigidity.
  • Reduce reamer clearance.
  • Avoid excessive reamer overhang.
  • Use an appropriate feed rate.
  • Lower cutting speed while increasing the feed rate to minimize vibrations and maintain quality hole finishes.

Reaming Vs. Boring Vs. Drilling: What Are the Differences?

Reaming, boring, and drilling are distinct machining operations used to either create new holes or modify existing ones in a workpiece. To grasp the differences between reaming vs. boring and reaming vs. drilling.


CNC Reaming, often accomplished through CNC reaming, employs a rotary cutter called a reamer to refine or create holes with smooth interior surfaces in a workpiece. It necessitates a pre-existing hole, typically drilled in a prior operation.

  • Reaming refines existing holes.
  • Reaming extracts less material compared to drilling.
  • This operation enhances the smoothness of hole walls.
  • Reaming is conducted using a drill press or milling machine.
  • Tolerance in reaming is approximately IT9⁓IT6, with a surface roughness of Ra 3.2⁓0.2µm.


CNC Boring enlarges existing holes in a workpiece using a single-point cutter or boring head. It is distinct from drilling, which initiates new holes.

  • Boring widens pre-existing holes.
  • Boring is not associated with creating initial holes.
  • Milling machines, lathes, or horizontal boring mills are used for boring.
  • Boring accommodates various hole diameters.
  • Boring offers error correction and high positioning accuracy.
  • Precision in boring is typically IT9⁓IT7, with surface roughness at Ra 3.2⁓0.8µm.


CNC drilling is a standard process to create circular holes in workpieces using drill bits. The drill bit is pressed against the workpiece to initiate the drilling process, resulting in circular holes.

  • Drilling forms initial holes.
  • Drilling prepares holes for threading and subsequent processing.
  • It is suitable for drilling holes in hard materials like metal.
  • Drilling accuracy is lower than reaming and boring, usually IT13⁓IT11.
  • Surface roughness in drilling is Ra 50⁓12.5µm.

In summary, these processes have distinct purposes: reaming refines interior hole walls, boring increases hole diameter, and drilling initiates holes in the workpiece. Understanding these differences is crucial for efficient machining operations

Precision Hole Machining Services

Achieving the perfect hole in your machined parts is essential, but it can also be a complex task. With over two decades of machining expertise, KUSLA excels in various hole machining techniques, including reaming, drilling, boring, and tapping. Our commitment to quality ensures that you receive meticulously crafted parts with precision-engineered holes.

If your project demands top-notch hole machining, don’t hesitate to reach out to us. Contact us today, and we’ll offer you efficient solutions and competitive pricing for your machining needs.

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