ER collets are a system that were developed for holding milling tools but are also suitable for holding round stock accurately. Unlike most collets they cover an 'extended range', typically 1mm per collet.

Most ER chucks are available with an MT shank that will fit many lathes and milling machines.

You can also get ER chucks that fit on the flange of many lathes or any lathe with a suitable backplate.

Neil

NW has put the facts very succinctly.

The single important advantage of NW’s last para is that this method allows round bar to be used without overhang – as long as the bar will feed through the spindle.

Why bother with collets if you already have 3-jaw and 4-jaw chucks you might wonder?

I use 3 and 4-jaw chucks for almost everything I turn, which mostly needs not be highly accurate. My problem with both chucks is resetting work. Say you turn a rod to size, part-off, and then have to reverse the rod to work on the other end. It's very unlikely that a 3-jaw will grip in the same way so the new cuts won't be coaxial with the original. A 4-jaw can be set to get the original axis back but it's a time-consuming fuss, especially if the workflow requires the  chuck to be reset repeatedly.

Collets may be a better solution whenever round work comes on and off the lathe. Provided the collet-chuck is running true, work can be removed and replaced without losing accuracy, even when fitted into a different sized collet. Also work held in a collet can be accurately transferred between collet chucks on different machines. Even if moving isn't a consideration, collets may be a better choice for accurate work because they have lower run-out than a 3-jaw chuck.

As always the use you would get from collets depends on the type of work you're doing. I don't use collets on my lathe very often, others use them all the time. In my workshop they're a luxury rather than essential, simply because of the limited range of work I do.

One disadvantage with ER series collets rather than, say 5C, is the ER clamping nut protrudes a few mm beyond the flat collet face making it awkward to get very close – if that matters. I already had ER collets on my mill and felt the benefit of sharing the same collet system with my lathe outweighed this slight disadvantage.

Dave

Edited By SillyOldDuffer on 21/10/2018 13:44:21

Not guilty of that particular mistake though I did exaggerate the overhang. Usefully, I've found how bad it is depends on the nut:

Mounted on Stevenson's Blocks for comparison:

• On the right, a plain nut. The nut protrudes 0.55mm above the face.
• On the left, the better ball-bearing type. The nut protrudes 0.2mm above the face.

Never occurred to me there might be a difference!

Dave

Freudian slip. I expect I was compensating for something…