ER40 8mm Precision CNC Spring Collets Straight Bore Collet For Tool Holder

Need an 8 mm shank to sit dead centre in an ER40 holder? This collet does that one job: a 8 mm straight bore, an alloy spring steel body, and an 8–7 stamp on the face that spells out the clamping window at a glance. It drops straight into the nut of any ER40 tool holder on a woodworking CNC machine — no adapter, no second nut, no shim. One part number covers router bits, straight bits and drills with an 8 mm shank. Other sizes in the same line are quoted on request.

A router bit that runs off centre leaves three marks: chatter along the wall of the cut, a slot that measures wide on one pass and tight on the next, and a shank that creeps out of the holder. Shops answer with three moves — tighten harder, buy a new nut, blame the spindle. None of them touches the part that grips the shank.

The ER40 8MM spring collet is that part. The bore is cut to 8 mm and runs parallel, so an 8 mm shank enters to the same depth on every change and is held along its full inserted length instead of pinched at one ring. The 8–7 stamp on the face is the spec sheet: the collet closes through 1 mm, from 8 mm down to 7 mm.

The body is alloy spring steel, slit from the face toward the base so the segments flex open when the nut comes off and close evenly when it goes back on. Parallel bore plus a 1 mm window: one collet takes nominal 8 mm shanks and shanks worn a fraction under.

It fits ER40 tool holders on woodworking CNC machines — router heads, drilling stations and pattern work running an 8 mm shank. Other sizes come from the same line on request; price is quoted per order.

What separates this collet from a generic 8 mm one is geometry: where the bore is cut, how far the fingers travel, and how the size is read.

• Parallel bore, not a taper. The bore measures 8 mm from the mouth down, so a shank seats at one depth and grip stays even along the inserted length.
• Clamping window on the face. The 8–7 stamp records the full finger travel: 1 mm of compression between open and closed.
• Alloy spring steel body. The fingers flex open and shut thousands of times — the property the material is specified for, not hardness alone.
• Slots run the full body. Each slit runs from face to base, so the segments close progressively instead of bending at one weak point.
• Reads without a caliper. Size and the 8–7 range sit on the face, so a mixed drawer sorts by eye in seconds.
• Direct ER40 fit. Seats in the eccentric ring of a standard ER40 nut: one nut, one holder, no adapter.
• Metric shank, so import bits fit. 8 mm is the common metric shank on European router and boring tools.
• Other sizes, same line. A different shank is quoted on the same enquiry — contact us for details.
• Held as a consumable. Bought in pairs and swapped in seconds, it keeps a spindle running while a spare is on the shelf.

There are only two ways to deal with a shank that will not sit true: keep buying collets until one happens to work, or stop the spindle and chase the problem. Both cost more than the part.

• One shank size, one part number.Order the ER40 8MM and the size decision is finished — no range chart, no guess between adjacent sizes, no wrong-size return.
• Spring steel is what makes it reusable.Alloy spring steel is picked for the open-and-close cycle, so the fingers return to shape after each release and the next tool seats the same way.
• No adapter in the stack.The collet goes straight from box to ER40 nut, which removes one joint that could add runout between spindle and cutting edge.
• A spare costs less than a scrapped batch.Two collets on the shelf let a seconds-long swap happen mid-shift instead of a wait for a delivery.
• The rest of the rack follows.Because other sizes come from the same line, a shop can standardise its whole 8 mm-and-up tooling set on one supplier and one quote format.

Yes — if the holder is a standard ER40. This collet is built to the ER40 series, so it snaps into the eccentric ring of an ER40 nut and threads onto the holder like the collet it replaces. If the nut is an ER32 or ER25, the part will not fit; check the marking on the nut first.

8 mm nominal, closing to 7 mm. The face is stamped 8–7, which is the full travel: 1 mm of compression. An 8 mm shank goes in at the open end of that window; a shank that has worn down to around 7.2 mm still clamps.

So the size is readable on the bench. A drawer of mixed collets is indistinguishable by eye, and the 8–7 stamp plus the size mark lets an operator sort them without a caliper or a gauge.

Alloy spring steel. The fingers have to flex open and closed repeatedly, and that is the property the material is specified for — it is what lets the collet release a tool cleanly and re-grip the next one at the same size.

The bore is parallel at 8 mm rather than tapered, so the shank seats against a straight wall for its full inserted length. That is why these are used in tool holders that expect a straight bore collet rather than a drill-chuck style taper.

It is specified for woodworking CNC tool holders, which is the application it is quoted for. If you plan to run it on another material, tell us the material and the spindle speed with the enquiry and we will answer for that case specifically.

Yes. Other sizes are available from the same line — send us the shank diameter you need with the enquiry and the matching collet is quoted with the same lead time. The ER40 8MM is the stocked 8 mm item.

Snap the collet into the nut until it clicks into the eccentric ring, then thread the nut onto the holder by hand before tightening — never seat the collet in the spindle first. Price is quoted per order: send the quantity and the sizes you want and the quote comes back with the same part numbers.