Complete Guide to CNC Tool Holders and Collets: ER20, ER25, ER32 and More

Your CNC router bit is only as good as the system holding it. A worn collet, an incorrect collet size, or a poorly maintained tool holder can cause runout, vibration, poor surface finish, and even dangerous tool ejection. This guide covers everything you need to know about CNC tool holders and ER collets — the most widely used clamping system in CNC machining.

What Is an ER Collet System?

ER collets are the industry-standard clamping system for CNC spindles, milling machines, and routers. The "ER" designation comes from the original manufacturer (Rego-Fix), and the number following it (ER11, ER16, ER20, ER25, ER32, etc.) refers to the outer diameter of the collet in millimeters.

ER collets work by compressing around the tool shank when tightened into a collet nut and tool holder. They provide excellent concentricity (low runout), high clamping force, and are available in a wide range of sizes to accommodate different shank diameters.

ER Collet Series: Which One Do You Need?

ER11 Collets

• Outer diameter: 11mm
• Shank range: 0.5mm – 7mm (up to approximately 1/4")
• Common applications: Small CNC routers, engraving machines, PCB routers, Dremel-style spindles
• Typical shank sizes held: 1/8" (3.175mm), 3/16" (4.76mm), 1/4" (6.35mm)
• Best for: Compact machines with small spindles, engraving, and fine detail work

ER16 Collets

• Outer diameter: 16mm
• Shank range: 0.5mm – 10mm
• Common applications: Mid-size CNC routers, small milling machines
• Typical shank sizes held: 1/8", 3/16", 1/4", 5/16", 3/8"
• Best for: Versatile mid-range machines that need to handle a variety of bit sizes

ER20 Collets

• Outer diameter: 20mm
• Shank range: 1mm – 13mm
• Common applications: Production CNC routers, professional woodworking machines, light metal cutting
• Typical shank sizes held: 1/8", 3/16", 1/4", 3/8", 1/2"
• Best for: Professional shops running a wide range of tooling, including larger bits for production work

ER25 Collets

• Outer diameter: 25mm
• Shank range: 1mm – 16mm
• Common applications: Heavy-duty CNC routers, milling machines, production environments
• Typical shank sizes held: Up to 5/8" shank
• Best for: Large-format machines and production routing with larger diameter tooling

ER32 Collets

• Outer diameter: 32mm
• Shank range: 2mm – 20mm
• Common applications: Industrial CNC machining centers, heavy milling operations
• Typical shank sizes held: Up to 3/4" shank
• Best for: Industrial machining centers and heavy-duty production environments

ER Collet Quick Reference

Series	Max Shank	Holds 1/4"?	Holds 1/2"?	Typical Machine
ER11	7mm	Yes	No	Engravers, small routers
ER16	10mm	Yes	No	Mid-size routers
ER20	13mm	Yes	Yes	Professional routers
ER25	16mm	Yes	Yes	Heavy-duty routers
ER32	20mm	Yes	Yes	Machining centers

How to Select the Right Collet Size

Each collet is designed to hold a specific range of shank diameters. A 1/4" ER20 collet will hold shanks from approximately 6.0mm to 6.35mm (1/4"). You cannot use a 1/4" collet to hold a 1/8" shank — you need a separate 1/8" collet.

Important rule: Always use a collet that matches the exact shank diameter of your tool. Never use a collet at the extreme end of its range, and never use shims or tape to make a smaller shank fit a larger collet — this is dangerous and will cause runout and tool ejection.

Understanding Runout and Why It Matters

Runout is the amount of wobble or eccentricity in a rotating tool. Even a small amount of runout — measured in thousandths of an inch — has significant effects:

• Poor surface finish: The tool cuts unevenly, leaving marks on the workpiece
• Reduced tool life: One cutting edge does more work than the others, wearing faster
• Vibration and chatter: Especially noticeable in harder materials
• Dimensional inaccuracy: Slots and pockets come out wider than programmed

Quality ER collets are manufactured to very tight tolerances — typically 0.0002" (5 microns) or better runout. Cheap collets can have 10–20x more runout, which is immediately noticeable in cut quality.

Tool Holder Types for CNC Routers

Beyond the collet itself, the tool holder body connects the collet to your spindle. Common types include:

• ER Collet Chuck: The most common type for CNC routers. The collet sits inside the chuck body and is clamped by a collet nut.
• ISO 30 / BT30 / BT40 Tool Holders: Used on machining centers with automatic tool changers. The taper connects to the spindle with a standardized interface.
• HSK Tool Holders: High-speed hollow shank taper system used in high-performance machining centers. Provides excellent rigidity at high RPM.
• Integrated Spindle Collet: Many CNC router spindles (especially Chinese water-cooled spindles) have the ER collet system built directly into the spindle shaft.

Collet Maintenance and Replacement

Collets wear out. The clamping surfaces develop wear marks, the collet loses its ability to compress evenly, and runout increases. Signs it's time to replace your collets:

• Visible wear marks or scoring on the collet bore
• Tools slipping during cutting (even when properly torqued)
• Noticeable increase in vibration or chatter
• Runout measurement exceeding 0.001" (0.025mm)

As a general guideline, replace collets every 6–12 months in production environments, or every 1–2 years in light use. Collets are inexpensive insurance against tool ejection and poor cut quality.

Proper Collet Installation

1. Clean the collet, collet nut, and tool holder bore — any debris causes runout
2. Snap the collet into the collet nut before inserting the tool (the collet should click into the nut)
3. Insert the tool shank to the correct depth — at least 3× the shank diameter engagement
4. Tighten the collet nut with a proper collet wrench to the specified torque — do not overtighten
5. Check runout with a dial indicator if precision is critical

Shop ER Collets and Tool Holders at Mintech Tooling

Mintech Tooling carries a full range of ER collets in ER11, ER16, ER20, ER25, and ER32 sizes, along with collet nuts, tool holders, and collet wrenches. Whether you're setting up a new spindle or replacing worn collets, we have the tooling accessories you need.