What This ER Collet Square Bore Does Differently This is a CNC machined ER collet with a square internal bore, designed for gripping square shank tools, electrodes, and inserts without slipping. The s
This is a CNC machined ER collet with a square internal bore, designed for gripping square shank tools, electrodes, and inserts without slipping. The square bore geometry transfers torque directly to the flat faces of the tool, eliminating the need for set screws or keyways. You get runout of ≤0.005 mm at 3×D from the nose, hardness of HRC 58-60, and a surface finish of Ra 0.4 µm on the bore. Lead time is 10 days for standard sizes, 15 days for custom bores. This is precision manufacturing from our Dongguan factory, made in China, with no MOQ.
Round bores rely on friction alone. Friction fails when coolant enters the interface or when cutting forces spike. The square bore mechanically locks the tool shank on four flat faces. This is a positive drive system. The transition from round to square inside the collet is a gradual curve, not a sharp corner. Sharp corners create stress risers. We machine that transition with a 0.2 mm radius to keep the collet elastic. You can clamp a 6 mm square shank with the same ER25 body you already use. No chuck change needed.
We grind the square bore after heat treatment. Grinding, not milling, is what holds the ±0.002 mm tolerance on the flat-to-flat dimension. Milling leaves scallops. Grinding leaves a mirror finish. Each flat is parallel to the opposite flat within 0.003 mm. When you tighten the collet nut, the bore collapses evenly on all four faces. The tool does not tilt. That is how we hold runout below 5 microns.
The collet body is 65Mn spring steel. We chose 65Mn because it has a fatigue limit of 480 MPa after proper heat treatment. Softer steels like 45# will lose their spring memory after 10,000 cycles. 65Mn holds its elastic range for over 100,000 cycles. We harden to HRC 58-60, then temper at 380°C to relieve internal stress. The result is a collet that springs back to its original bore size after every release.
For corrosive environments or medical applications, we offer 17-4PH stainless steel. That version is HRC 40-44, slightly softer but fully rust-proof. The square bore is ground the same way. We also offer a TiN coating option (2-3 µm thickness) for the outer cone. This reduces friction against the collet nut and prevents galling on high-speed spindles above 20,000 RPM. If you need a special bore size, we wire EDM the square shape from solid, then grind. No skiving, no broaching. That keeps the grain structure intact.
Every single collet gets a final inspection, not a sample batch. We use a Zeiss CONTURA CMM for the square bore geometry and a TIR gauge with a 0.001 mm dial indicator for runout. The inspection report is included in the box. It lists the actual measured runout at the nose and at 10 mm depth. We also measure the clamping force on a hydraulic torque wrench at 50 N·m. The report shows the force required to pull the tool out axially. For a 6 mm square shank, that pull-out force is typically 2,800 N.
We keep a serial number on each collet body. That number traces back to the batch heat treatment certificate. If you find a problem, we replace it within 72 hours, no questions asked. We have been doing this since 2005. The rejection rate at our internal QC gate is 1.2%. The field return rate from customers is 0.15%. Those numbers are from our 2024 annual report.
| Specification | Value | Notes |
|---|---|---|
| Material (standard) | 65Mn spring steel | Fatigue limit 480 MPa |
| Material (optional) | 17-4PH stainless | For corrosive/cleanroom use |
| Hardness (65Mn) | HRC 58-60 | After tempering at 380°C |
| Hardness (17-4PH) | HRC 40-44 | Precipitation hardened |
| Square bore tolerance | ±0.002 mm | Flat-to-flat, ground finish |
| Runout (at 3×D) | ≤0.005 mm | Measured with TIR gauge |
| Surface finish (bore) | Ra 0.4 µm | Mirror ground |
| Bore size range | 2 mm to 20 mm square | Custom sizes via wire EDM |
| Standard lead time | 10 days | For standard sizes |
| Custom lead time | 15 days | For wire EDM bores |
| MOQ | No MOQ | Single piece accepted |
| Coating option | TiN 2-3 µm | For high RPM >20k |
The cone angle is 8° with a 30° nose angle, exactly to DIN 6499. That means you can use your existing ER collet nut and wrench. We do not make proprietary threads. The body diameter tolerance is h6 per ISO 286. This is a drop-in replacement for any standard ER chuck holder. We also machine the slots: 12 slots for ER25, 16 slots for ER32 and ER40. The slot width is 1.6 mm with a 0.8 mm radius at the base. This distributes the collapse force evenly.
We check the slot depth with a pin gauge. The slot must not extend into the square bore area. If it did, the square corners would crack under repeated clamping. Our slot depth stops 2 mm before the bore start. This is a detail most generic collet factories ignore. It matters because a cracked collet is a scrapped workpiece and a damaged spindle.
Each collet is individually wrapped in VCI (vapor corrosion inhibitor) paper, then placed in a rigid plastic tube. The tube is labeled with the part number, batch code, and inspection date. We do not use foam trays that trap moisture. VCI paper protects for 24 months in standard warehouse conditions. The tube prevents the collet from rolling and hitting other parts during shipping. We ship via DHL Express or FedEx, depending on your location. For a 100-piece order, we use a partitioned wooden box with individual slots.
We photograph every collet before packing. The photo is stored in your order file. If you need a certificate of conformity or material test report, we include it at no extra cost. Just add a note in your RFQ.
No MOQ — we accept a single piece for custom wire EDM bores, but the tooling setup fee of $45 applies for the first piece.
Yes, we machine hex bores (3, 4, 5, 6 mm across flats) using the same grinding process, with the same ±0.002 mm tolerance.
We insert a precision ground square test bar, clamp to 50 N·m, and measure TIR at the nose and at 10 mm depth with a 0.001 mm dial indicator.
Standard sizes are 10 days for up to 500 pieces; for 200 pieces we typically ship in 8 days because we keep raw 65Mn bar stock in-house.
Send your square bore size, ER type, material preference, and quantity. We will reply within 12 hours with a firm price and delivery date. For custom bores, include a PDF or STEP file. We will check the geometry against our grinding capabilities and suggest any minor changes for manufacturability. Our engineers are available for a technical call via WhatsApp or email. We handle precision manufacturing daily — from single prototypes to 10,000-piece runs. Before you buy, read our CNC cutting tool selection guide to see how collets fit into your toolholding strategy. For dimensional limits, check the CNC machining tolerances guide for what we can hold on the collet body itself.
Email your RFQ to sc@bquq.com or message us on WhatsApp at +86 13713157787. Send your drawing. Get a firm quote in 12 hours. Made in China, factory direct, no MOQ.
| Parameter | Capability |
|---|---|
| Materials | AL6061/7075, SUS303/304/316, 45# steel, T2 copper, H59 brass, POM, PEEK, nylon |
| Tolerance | ±0.005mm standard, ±0.003mm on request |
| Machines | 3/4/5-axis CNC, turn-mill, Swiss lathe |
| Surface | Anodizing, nickel, zinc, passivation, sandblast, polish |
| Size Range | Max 400 x 300 x 200 mm |
| Prototype | 72 hours, no MOQ on samples |
| Inspection | Full report per batch, CMM data on request |
