It is nice to have the availability of a collet chuck
 when machining small diameter work pieces which demand accurate mounting in the lathe. Unfortunately, while these are available
commercially, they are quite expensive and some
 ready-made cheaper alternatives often fail to live
 up to the accuracy claimed thus making them useless.  What I wanted was something that
 allowed me to achieve the degree of accuracy I
required that was also relatively convenient to use.
As I already had an ER25 set of collets for my
Stent TC&G I decided my new collect chuck
would use these  rather than ER32 collets.  The
 main purpose of using an ER25 collet was both
accuracy and for holding small diameter work for which the standard 3 jaw was unsuitable on both
counts. One thing I always hate is having to
change the over chucks on my lathe (especially
 large heavy chucks such as the 160mm 3 jaw on
 the BH600G).  So it has been a long time desire to
have an alternative method to fit a collet chuck
without having to remove the 3 jaw chuck yet still
retain the associated accuracy that comes with
The conventional method with collets is to use a
 collet chuck fitted to a back plate suitable for the
 lathe spindle but this of course entails having to
 remove the existing 3 jaw chuck every time I
 wanted to mount a work piece in a collet!  The
 other problem with this type of mounting the
 ER25 collet is accuracy which may or may not be
 as one might expect.  While the collets I have are
 inexpensive they probably are not 100% accurate. 
The following idea is basic and simple yet it
 provides all the accuracy that one expects when using collets even if the collet has run-out!
The ER25 collets and chuck came from a Chinese
 supplier (web direct) and although low cost and of 
reasonable quality and accuracy, they probably would
 not compare well to established European
 manufactured collets/chucks. It is always a dilemma for
 the model engineer who strives to acquire accurate
 machines tools but is so often limited to a low budget
 and thus has to make compromises.  This has always
 been the situation for me but if care and time is taken
 then even cheaper tools can perform well in the home
 workshop environment, especially if they are used
The problem with such compromises is not
 only cost but time.  Industry cannot afford to spend
 time ‘fine-tuning’ and expects tools to always provide
 stated accuracy.  Fortunately, as a hobby user time is
 not a concern.


The main requirements for my new ER25 device
 no need to change the lathe 3-jaw chuck;
 any inaccuracy quickly fine-tuned; 
 simple design, and
 using existing collets.  The next consideration was the collet chuck and the
 supplier of the collets also supplied a variety of
chucks with different types of shank. 
Of course, a
 simple solution would be to buy a collet chuck with
 a 3 morse taper shank and fit directly to the spindle
 using, for the BH600G a 5 to 3 morse reduction
 sleeve.  While this approach might seem attractive it
 still requires removal of the 3-jaw chuck and
accuracy is then dependent on the collet chuck
 shank and collets.  What I wanted was to keep the 3-
jaw chuck on the lathe spindle and mount a collet
 chuck in it and have the ability to fine tune any
A few tests on my 160mm Chinese 3-jaw chuck
 revealed reasonable accuracy, actually better than
 expected and consistency not much different to my
 Pratt-Burnerd chuck which lives on the Myford ML7
 lathe, both of which are designated as workshop
 grade.  One thing I found, based on my initial tests
 with the Chinese 160mm chuck, was that it has a
varying runout (0.001" - 0.003") when mounting bar
 stock depending on the bar diameter and socket used
 to tighten the jaws.  As a matter of habit I always
 tighten using the same socket as I find this provides
 more consistency which is now important when it
 comes to mounting a collet chuck in the jaws. 

Fortunately, my tests also showed the 3-jaw to be
 very consistent when mounting the same size bar
 stock in the exact same position relative to a jaw on
 the chuck, in my case of variance was minimal,  no more than 0.0004" TIR so
 I concluded as long as the design used a one-size shaft
held in the 3-jaw chuck at a known position it would be
 sufficiently accurate and consistent and then have the
 ability to refine the accuracy further if required.

Making the new chuck

The collet supplier fortunately had available an ER25
 chuck with a parallel shank of 25mm diameter and
 100mm long so this was ordered post haste! The chuck 
assembly unit comprises of two main parts, one that fits to the 3-jaw chuck and the other the ER25 chuck
attached to it. 
Part one assembly
The first part comprises a 1.5" diameter shaft which fits
 in the lathe 3 jaw chuck; a 3" diameter x 0.5" thick
 flange plate is fixed at the outer end of this shaft using
 high strength Loctite and the the assembly bored with a
 generous clearance hole in which the ER25 chuck shaft
 can fit. 
With this part mounted in the lathe 3-jaw chuck
 a centre point mark is made on the shaft for alignment
 purposes to the centre of no.1 jaw thus providing the
same reference position each time the assembly is
 mounted.  Finally, the unit is mounted in the 3 jaw
 chuck and turned to face the outer face and the o/d
Part two assembly
The cost of the supplied ER 25chuck, which included a
 closing nut and C-spanner was very reasonable and
 came fully hardened and ground.  It makes absolute
 sense to buy this ready made since there is a lot of work
 needed to make one from bar stock plus the nut is quite
a tricky design and then hardening and finish grinding
 is rare in the home workshop environment.  



By Graham Howe