Both go into the same ER11 collet, but the difference between a 4mm and a 6mm shank is much bigger than "a bit thicker or thinner." At the same stickout and the same side cutting force, a 6mm shank is 5.06 times as stiff in bending as a 4mm shank. That number decides whether the outer profile of your carbon fiber frame comes off in one clean pass or ends up with a wavy, tapered edge.
The One-Minute Version
- ER11 tops out at 7.0mm (DIN 6499 / ISO 15488), so a 6mm shank is already near its limit. Anything larger means moving to ER16.
- 5.06x stiffness difference: the area moment of inertia scales with the fourth power of diameter, and (6/4)4 = 5.06. Operations with high side loads, such as outer profiles and roughing, always go on the 6mm shank.
- The smallest inside corner radius sets the tool diameter, not the sheet thickness. An R2 inside corner rules out a φ6 tool; you need φ4 or smaller.
- Stickout hurts more than shank diameter: deflection scales with the cube of stickout length. Going from 15mm to 25mm of stickout multiplies deflection by 4.6.
- Finish every feature that uses the same tool in one go, and a drone frame sheet drops from 6 tool changes to 2.
How Big a Tool Can ER11 Hold? Start with the Spec Limit
ER spring collets are defined by DIN 6499 / ISO 15488. The number after "ER" corresponds to the collet's outside diameter, and each size has its own clamping limit. The full clamping range of ER11 is 0.5mm to 7.0mm, with an outside diameter of about 11.5mm and a length of about 18.0mm.
Note that this range is not covered by a single collet. An ER collet clamps by collapsing its slotted body, and each collet has an effective collapse range of about 1mm (even less, about 0.5mm, for small bore sizes). So the 0.5–7.0mm range is actually covered by a whole set of collets, each handling its own slice. If you want to run both 4mm and 6mm shank tools, you need both an ER11-4.0 and an ER11-6.0 collet.
ER Collet Series: Clamping Limits by Size
Carbon-CNC uses ER11; the standard 4mm and 6mm shanks fall inside its clamping range
| Collet size | Max clamping diameter | Tightening torque (standard nut) | Typical machine | What it means for a desktop machine |
|---|---|---|---|---|
| ER8 | 5.0 mm | 6 N·m | Micro engravers, dental handpieces | Cannot hold a 6mm shank; too few roughing tool options |
| ER11 | 7.0 mm | 8 / 24 N·m | Desktop CNC, engraving/milling machines | Takes both 4mm and 6mm shanks; the mainstream choice |
| ER16 | 10.0 mm | 8–56 N·m | Small vertical machining centers | Collet and nut are too heavy for a desktop spindle to drive |
| ER20 | 13.0 mm | 16–80 N·m | Machining centers | Beyond the power and rigidity range of a desktop machine |
| Torques are for standard hex nuts. For ER11, 8 N·m applies to 1.0–2.9mm bores and 24 N·m to 3.0–7.0mm bores. Values differ for ball-bearing nuts; follow the tool holder manufacturer's specifications. | ||||
Why Desktop Machines Use ER11 Instead of the Larger ER16
The answer is not the collet itself but spindle power. The Carbon Pro BLDC spindle is 350W and the Carbon Lite is 200W, and that power puts a natural ceiling on the tool diameter it can drive. Force a φ8 tool in and you won't cut faster; the spindle will simply lose speed as soon as it takes a deep cut.
It is easier to see using the machine's own specs. Cutting speed vc = π × D × n ÷ 1000; plugging in the 20,000 rpm maximum gives:
| Tool diameter | Cutting speed vc | Feed per tooth, 2 flutes* | Shank size | Typical use |
|---|---|---|---|---|
| φ2.0 mm | 126 m/min | 0.063 mm | 4mm (reduced neck) | Narrow slots, small radii, antenna mount openings |
| φ3.175 mm (1/8″) | 199 m/min | 0.063 mm | 3.175mm | Strap slots, M3 clearance holes |
| φ4.0 mm | 251 m/min | 0.063 mm | 4mm | Inside radii, lightening holes, finishing |
| φ6.0 mm | 377 m/min | 0.063 mm | 6mm | Outer profiles, roughing, long straight edges |
| *Calculated at the 2,500 mm/min maximum feed rate, 20,000 rpm, and a two-flute tool: fz = F ÷ (n × z). When actually cutting carbon fiber, multi-flute diamond-pattern (corn-cob) end mills take a chip load per tooth far below this value. This column only shows where the machine's parameter ceiling sits. | ||||
ER11 is not a compromise that is "weaker because it is smaller." It is matched to the rest of the spec set: a 350W spindle, ±0.05mm accuracy, and 20,000 rpm. Carbon fiber machining is a high-speed, shallow-cut process to begin with, and φ6 is plenty for profiling 3mm CFRP sheet.
4mm vs 6mm: A 5.06x Difference in Stiffness
A tool shank's resistance to sideways bending is set by its area moment of inertia, I. For a round section, I = πd4 ÷ 64. The key is the fourth power: increase the diameter by just 50% and stiffness goes up by more than 5 times.
Plugging in 4mm and 6mm: I4 = π × 44 ÷ 64 = 12.57 mm4, and I6 = π × 64 ÷ 64 = 63.62 mm4. The ratio is 64 / 44 = 5.06. At the same side load and the same stickout, a 6mm shank deflects only 19.8% as much as a 4mm shank.
Shank Stiffness (Fourth Power of Diameter) vs. Cantilever Deflection
Left: area moment of inertia ratio Right: tool tip deflection under the same side cutting force (deflection exaggerated)
Stickout Hurts More Than Shank Diameter
The formula has one more term, L3: how far the tool sticks out of the collet. It is only a cube rather than a fourth power, but it causes damage more often, because stickout is a variable operators change every day, while shank diameter is not.
| Stickout L | φ4 shank relative deflection | φ6 shank relative deflection | Cutting behavior |
|---|---|---|---|
| 10 mm | 0.30 | 0.06 | Cleanest edges, but only enough for thin sheet |
| 15 mm | 1.00 | 0.20 | Typical setting for 3mm sheet |
| 20 mm | 2.37 | 0.47 | φ4 starts producing tapered edges; φ6 still acceptable |
| 25 mm | 4.63 | 0.91 | φ4 no longer suitable for outer profiles |
| 30 mm | 8.00 | 1.58 | Both chatter; reduce feed |
| Relative values calculated from δ ∝ L³ / d⁴, not absolute deflection. Actual values also depend on tool material, clamping depth, and cutting parameters. | |||
Set stickout to "sheet thickness + 3–5mm." For 3mm carbon fiber sheet, 8mm of stickout is enough; you don't need 20mm. When loading a tool, push it all the way into the collet first, then pull it out to the length you need. That is far more reliable than inserting it by feel.
Tool Assignment Table for Drone Parts
The first rule for choosing tool diameter is not sheet thickness but the smallest inside corner radius on the drawing. A tool cannot cut an inside corner tighter than its own radius: if you want an R2 inside fillet, the tool can be no larger than φ4. This rule alone moves certain features from the 6mm shank to the 4mm shank.
| Part / feature | Typical size | Recommended tool diameter | Shank | Why |
|---|---|---|---|---|
| Frame main plate, outer profile | 2–3 mm thick | φ6 | 6 mm | Long straight edges and high side load; stiffness keeps edges square |
| Arm outer profile | 4–5 mm thick | φ6 | 6 mm | Thick sheet means many step-down passes; stiffness first |
| Arm root inside fillet | R2–R3 | φ4 | 4 mm | Tool radius must be equal to or smaller than the fillet radius |
| Motor mounting holes (M3 clearance) | φ3.2 | φ3 helical interpolation | 4 mm (reduced neck) | Helical milling is less likely than drilling to delaminate the exit side |
| Battery strap slot | 20 W × 3 H | φ3.175 | 3.175 mm | Slot is much wider than the tool; side-mill in several passes |
| Lightening / cooling holes | φ8–φ20 | φ4 | 4 mm | Helical entry for round holes; φ4 is enough and clears chips well |
| Carbon tube clamp inner arc | φ16–φ30 tube | φ4 | 4 mm | Inner arc radius is 8–15mm, leaving φ4 plenty of margin |
| Antenna mount narrow slot | 2 mm wide | φ2 | 4 mm (reduced neck) | Slot width equals tool diameter; must be cut in a single pass |
| Edge chamfer | C0.3–C0.5 | φ6 chamfer tool | 6 mm | Low chamfering forces; prioritize fewer tool changes |
Reduced-neck tools (for example, a φ2 cutting edge on a 4mm shank) are especially useful on ER11: the cutting portion is small while the shank keeps the clamping rigidity of 4mm, so you don't have to swap collets just for one small feature.
Tool Order: Keep Tool Changes to a Minimum
A tool change costs more than the time to swap the tool. It also means re-setting tool length: every tool is a different length, so Z zero has to be reset. On thin sheet, this is a real risk. A 0.1mm error in tool setting either leaves the part uncut or cuts into the spoilboard along with it.
The right approach is to group by tool, not by part. Finish every φ6 feature on the whole sheet in one go, then switch to φ4 and finish all the small features.
Tool Change Sequence in a Single Setup
The same 5-inch frame sheet: tool changes under two sequencing approaches
| Step | Tool | Operations | Why it goes here |
|---|---|---|---|
| 1 | φ6 shank | Rough all internal openings (lightening holes, large windows) | All four sides of the sheet are still intact, so the part is most stable |
| 2 | φ6 shank | Edge chamfers | Done with the same tool, no extra tool change |
| 3 | φ4 shank | All inside radii, mounting holes, and fine finishing | Small tool, low cutting force, and the part is still attached |
| 4 | φ2 necked | Narrow slots (if any) | The most fragile tool goes last to limit breakage losses |
| 5 | Back to φ6 shank | Cut outer profile free (leaving tabs) | The part is released from the sheet only at the very end |
| Going back to φ6 in step 5 adds one tool change. If the inside radii on the outer profile allow it, you can instead cut the profile with tabs right after the first tool. The actual order depends on the drawing. | |||
4 Common ER11 Clamping Mistakes
1. Shank diameter does not match the collet size
The most common case is forcing a 3.175mm (1/8 in) tool into a collet rated 3.0mm. An ER collet is designed to collapse about 1mm, but that is inward; it does not expand outward. Even if the tool goes in, it is held only because the collet is being deformed, which destroys concentricity and introduces runout, so the slots you cut come out wider than the tool. Inch tools need inch collets.
2. Setting stickout by feel
As the table above shows, increasing stickout from 15mm to 25mm multiplies the deflection of a φ4 shank by 4.63. The opposite mistake is insufficient clamping depth: the shank should go into the collet at least 3 times its diameter (at least 12mm for a φ4 shank). Too shallow, and the cutting force can pull the tool out.
3. No standard for tightening torque
The recommended torque for a standard ER11 hex nut with a 3.0–7.0mm bore is 24 N·m. Tightening by feel fails in two ways: too loose and the tool slips and pulls out during cutting; too tight and the collet deforms plastically, and its concentricity never comes back. Use a torque wrench, not a cheater bar.
4. Not snapping the collet into the nut's eccentric ring first
An ER nut has an eccentric ring inside. The correct procedure is to insert the collet into the nut at an angle until it clicks into place, then thread the nut and collet onto the spindle as one assembly. If you push the collet into the spindle taper first and then thread on the nut, the collet will jam in the taper when you remove the tool, and clamping concentricity will be off as well.
Carbon fiber dust is made of fine, electrically conductive particles that work their way into the gaps between the spindle and the collet. Every time you change tools, wipe the collet, nut, and spindle taper clean before reassembling. A layer of carbon dust in the taper ruins concentricity, and the ±0.05mm accuracy spec assumes concentricity is correct.
FAQ
Can an ER11 collet hold a 6.35mm (1/4 in) tool?
Yes. 6.35mm is within the ER11 clamping range of 0.5–7.0mm, but you must use an inch-size ER11-6.35 collet, not force it into a metric 6.0mm collet. Keep in mind that 6.35mm is already close to the 7.0mm limit and the tool selection is limited, so in practice desktop machines still run mainly on 4mm and 6mm metric shanks.
Do I need to buy a full set of ER11 collets?
No. In practice, a 4.0mm and a 6.0mm collet cover about 90% of carbon fiber drone part work; add a 3.175mm collet if you use inch tools often. Each ER collet only collapses about 1mm, so on a desktop machine most of a full set (more than ten collets covering 0.5–7.0mm) would sit unused.
If a φ6 tool is stiffer, why not use φ6 for everything?
Because a tool cannot cut an inside corner radius smaller than its own radius. A φ6 tool can only produce an R3 inside corner, so any fillet of R2 or smaller on the drawing is impossible with it. A φ6 tool also has too little room to evacuate chips in small holes or narrow slots, which leads to chip packing, recutting, and edge fraying.
Do I have to re-set the tool length after every tool change?
Yes. Every tool has a different overall length and clamping depth, so the Z position of the tool tip always changes after a tool change. On sheet 1.5mm thick or thinner, a 0.1mm Z error is enough to leave the part uncut or cut through the spoilboard. An automatic tool setter turns this step from the manual paper-feeler method into a measurement that takes a few seconds.
The ER11 tightening torque of 24 N·m sounds high. Can I reach it by hand?
With a standard-length wrench tightened by feel, you usually will not reach 24 N·m, and that is the most common cause of insufficient clamping. Use an adjustable torque wrench. If you do not have one, at least always use the same wrench and the same technique so clamping conditions are consistent, rather than varying from loose to tight and making results impossible to reproduce.
Sources
- ER Collet Guide: ER11–ER50 Sizes, Capacity & Runout (clamping limits across the ER series). Accessed September 3, 2026. https://aimsindustrial.com.au/blogs/product-guides/er-collet-guide
- PreciseBits, ER Collet Torque Chart (tightening torque by collet size and nut type). Accessed September 3, 2026. https://www.precisebits.com/reference/ertorque
- PreciseBits, Ultra-precision ER11 Collets (ER11 clamping range of 0.5–7.0mm per DIN ISO 15488). Accessed September 3, 2026. https://www.precisebits.com/products/equipment/er11_collets.asp
- REGO-FIX, Recommended tightening torque for ER and MR clamping nuts. Accessed September 3, 2026. https://usa.rego-fix.com/rego_torque.pdf
- Carbon-CNC Carbon Pro / Carbon Lite machine specifications (ER11 collet, 4mm/6mm end mill shanks, 20,000 rpm spindle). https://www.carbon-cnc.com/en/carbon-pro/
The Right Tool Needs a Stable Machine
Carbon-CNC machines come standard with an ER11 collet and a 20,000 rpm BLDC brushless spindle, and both the Pro and Lite models accept 4mm and 6mm shank end mills. Want to know which tooling setup fits your part drawing? Send us your DXF and let's talk.
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