Internal Adjustment
Tightening a tapered screw or plug inside the hollow body pushes the slotted cutting section outward.
Reamers / Wear-compensating finishing reamers
Expansion Reamers
What Are Expansion Reamers and How Do They Work?
Expansion reamers are precision hole-sizing tools that compensate for cutting-edge wear or reach a closely controlled non-standard diameter. Their internal adjustment mechanism permits a small increase in cutting size.
Diameter Increase
The cutting edges expand by a small amount, typically a few thousandths of an inch, toward the target size.
Rotational Cutting
The adjusted edges shear a small amount of material from the hole wall as the tool rotates.
Sizing Control
The cutting diameter is checked with a micrometer before the finishing cut.
Key Features and Benefits of Expansion Reamers
Adjustable Core Screw
Allows small diameter changes for wear compensation and can extend useful tool life.
Slotted Body
Lets the cutting section flex outward while retaining the solid-body form.
Precision Margins
Support accurate sizing after the tool is adjusted and measured.
Wear Compensation
Can reduce replacement frequency compared with a fixed reamer that cannot be resized.
Multiple Flute Geometries
Straight and spiral flute options address different chip-evacuation requirements.
Rigid Shank
Provides secure holding in compatible CNC collets, chucks, or manual drivers.
Expansion Reamers Specifications
| Specification | Range / Details |
|---|---|
| Diameter Range | 1/4" to 2" in standard sizes |
| Expansion Capacity | Up to 0.006" to 0.010", depending on base size |
| Flute Type | Straight or spiral flute |
| Shank Type | Straight, Morse taper, or square end |
| Adjustment Mechanism | Internal tapered screw plug |
| Application Type | Through-holes and shallow blind holes |
Materials and Coatings Available for Expansion Reamers
| Material / Coating | Best For | Key Advantage |
|---|---|---|
| HSS (High-Speed Steel) | Mild steel, aluminum, and general resizing. | Provides toughness and the body flexibility required for expansion. |
| Carbide-Tipped | High-volume production and abrasive materials. | Adds edge wear resistance while retaining a flexible steel core. |
| Titanium Nitride (TiN) | General applications requiring added wear protection. | Adds surface lubricity and can reduce friction and heat buildup. |
HSS provides the body flexibility needed for expansion, carbide-tipped versions add cutting-edge wear resistance, and TiN can reduce friction and heat buildup.
Which Machines Use Expansion Reamers?
CNC Machining Centers
Use rigid holders for automated sizing and diameter wear compensation.
Manual Lathes
Mount the tool in the tailstock to finish holes in cylindrical workpieces.
Drill Presses
Provide controlled vertical finishing with a steady feed rate.
Applications and Industries for Expansion Reamers
Wear Compensation
Restores the cutting diameter after edge wear from repeated use.
Custom Tolerances
Produces non-standard diametrical fits that are not available from a selected fixed-size tool.
Abrasive Machining
Allows the cutting size to be readjusted as wear develops when machining abrasive metals.
Deep Hole Finishing
Can be combined with an extended-length profile for deep, difficult-to-reach cavities.
Frequently Asked Questions About Expansion Reamers
What is the difference between expansion reamers and adjustable hand reamers?
An adjustable hand reamer uses separate sliding blades to cover a relatively broad range, often up to 1/16 inch. An expansion reamer has a slotted solid body and expands only a few thousandths of an inch, primarily for wear compensation.
Can expansion reamers be used for roughing?
No. They are finishing tools designed to remove a small amount of material for final size and surface quality, not for bulk stock removal.
How much can an expansion reamer be enlarged?
The stated expansion limit is generally 0.006" to 0.010", depending on the base diameter. Exceeding the tool limit can crack or permanently deform the slotted body.
Are extended-length expansion reamers available?
Custom extended-length versions can combine added reach with an expansion mechanism for deep or difficult-to-access cavities.
How is an expansion reamer adjusted?
Turn the internal tapered screw with the appropriate hex key or wrench so the plug pushes the slotted cutting section outward. Check the final cutting diameter with a micrometer after every adjustment.
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Design and Engineering Support
Bauron provides design and engineering support for custom expansion ranges, extended lengths, flute geometries, shanks, cutting materials, and coatings. Share the target size and hole geometry for technical review.
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