ER11 vs ER16 vs ER20 Collets: Size and Bit Shank Guide

ER11: The Smallest

ER11 collets hold up to 7mm shank diameter. In practical terms:

• 1/8" shank bits (3.175mm) — plenty of room
• Small engraving bits (2mm, 3mm)
• Tiny cutters for PCB work

Which spindles use ER11:

• 500–800W spindles (small machines, 3D printer spindles)
• Some trim router adapters

Drawback: The limiting factor for most hobby work. You can't use 1/4" end mills, which are the standard for wood and soft plastic cutting. 1/4" shanks are 6.35mm, and while they technically fit (just barely), the collet can slip.

Runout tolerance: Cheap ER11 collets can have 0.05mm+ TIR (total indicated runout). At 24,000 RPM, this vibration is noticeable as chatter.

Use ER11 for: Engraving, PCB drilling, 3D printer spindle mounts, laser cutter optics head. Not for production CNC work.

ER16: The Middle Option

ER16 collets hold up to 10mm shank diameter. In practical terms:

• 1/4" shank bits (6.35mm) — comfortable fit
• 3/8" shank bits (9.525mm) — tight but acceptable
• Small carbide end mills (2mm, 3mm, 4mm)

Which spindles use ER16:

• Mid-tier spindles (1.0–1.5kW)
• Aftermarket upgrades for 800W spindles

Advantage: Opens up 1/4" shank bits, which are the standard for small CNC work. Cheaper than ER20 spindles.

Drawback: Can't hold 1/2" shank bits (12.7mm). You're limited to 1/4" and smaller, which are less rigid under load.

Runout tolerance: Good quality ER16 collets run 0.01mm TIR. Cheap ones drift toward 0.05mm.

Use ER16 for: Hobby routers under 600mm, wood and soft plastic cutting, light aluminum work. Most MPCNC builds use ER16.

ER20: The Production Standard

ER20 collets hold up to 13mm shank diameter. In practical terms:

• 1/4" shank bits (6.35mm) — loose fit, not recommended
• 3/8" shank bits (9.525mm) — comfortable fit
• 1/2" shank bits (12.7mm) — primary use, perfect fit

Which spindles use ER20:

• Large spindles (1.5kW, 2.2kW+)
• Premium 1.5kW spindles

Advantage: 1/2" shank bits are the real upgrade. These are:

• More rigid (less deflection under load)
• Faster (more flute area for chip evacuation)
• More available (1/2" is the professional standard)
• Better for aluminum and dense materials

A 1/2" shank 3-flute end mill outperforms a 1/4" shank 2-flute equivalent dramatically.

Drawback: Spindle weight increases. Cost increases. Your Z-axis needs to be strong enough.

Runout tolerance: Quality ER20 collets run 0.008–0.01mm TIR. This is the precision you want for aluminum.

Use ER20 for: Serious aluminum work, production routers, large machines, any build where chatter is a limiting factor.

Runout: The Silent Quality Killer

Runout is measured with a dial indicator on the bit shank while the collet rotates. A sloppy collet (0.05mm runout) creates a wobbling bit that looks like the bit is broken. You get:

• Chatter marks on the surface (fine ripple pattern)
• Tool wear acceleration (vibration stresses the flutes)
• Deflection under load (the bit moves, cuts are oversized)

Budget collets: Often 0.03–0.05mm TIR. Adequate for soft materials, not great for aluminum.

Quality collets (Maritool, Schunk, even mid-tier Chinese): 0.008–0.015mm TIR. This is where chatter disappears.

Test before buying: Many sellers won't mention runout spec. Reviews that say "very true" or "no chatter" suggest good quality.

Bit Shank Size Reference Table

Collet Quality and Longevity

A quality collet costs $3–8 each. A cheap collet costs $1–2. Over the lifetime of a spindle (5+ years), you might buy 10–20 collets (as backups or replacements). Total investment: $30–160.

Cheap collets have looser tolerances, may slip on larger shanks, and develop play after 100+ clamping cycles.

Quality collets (Maritool, Schunk, quality Chinese clones) maintain runout specs for thousands of clamping cycles. They're worth it.

Collet Cleaning and Maintenance

This is critical and often overlooked. A dirty collet taper causes runout problems:

1. Before each bit change: Visually inspect the collet bore and taper for chips or dust
2. Compressed air: Blow out the collet chamber and spindle taper
3. Collet wrench: Don't over-tighten the collet nut. Snug (full contact) is enough. Over-tightening wears the threads
4. Monthly: Disassemble the collet nut and clean the taper with a soft brush and shop cloth
5. Quarterly: If you run heavy cutting, disassemble the collet assembly and clean thoroughly

A clean collet and taper maintain 0.01mm runout. A dirty one can drift to 0.05mm+ without you realizing it.

Choosing Your Collet Size by Spindle

If you have an 800W spindle (likely ER11 or ER16): Start with ER16 if the spindle supports it. 1/4" shank bits are standard. If you're stuck with ER11, you can still cut but you'll be limited to small bits.

If you're buying a 1.5kW spindle: Specify ER20 as a decision factor. The bit selection advantage is significant. Some sellers offer ER16 on 1.5kW spindles to save cost—avoid those.

If you're buying a 2.2kW spindle: Default is ER20. This is the right choice.

Quick Conversion: Shank Size Equivalents

• 1/8" = 3.175mm
• 1/4" = 6.35mm
• 3/8" = 9.525mm
• 1/2" = 12.7mm
• 5/8" = 15.875mm
• 3/4" = 19.05mm

Professional machinists work in 1/4" increments. That's why 1/4", 3/8", and 1/2" shanks are standard. Metric-named bits (1mm, 2mm, 3mm) are for tiny work (engraving, PCB).

Matched Spindle/Collet Recommendation

What We'd Buy

For a hobby spindle you already have: Buy a matched ER collet set (usually 5–6 collets covering common sizes). Expect to pay $15–30 for quality collets. Cheap aliexpress 10-packs are tempting but often have runout issues.

When choosing a spindle: Pick the wattage you need first. Then check what collet it comes with. If it's ER16 and you want to cut aluminum seriously, find a 1.5kW with ER20 instead (usually a $30 difference from the seller).

Pro move: Buy a set of quality collets (Maritool, Schunk, or reputable Chinese) even if your spindle came with cheap ones. The runout improvement is immediate and noticeable.

Shop This Guide

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