© all rights reserved G W Howe 2017


Project to design an unconventional ER25 collet chuck

for the BH600G lathe

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, whilst 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 and 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 which the standard 3jaw 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 collets.  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!  The other problem with this type of mounting an ER25 collet is accuracy which may or may not be as one might expect.  Whilst 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! My 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 carefully.   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.

Requirements for the new ER25 Chuck

no need to change the lathe 3-jaw chuck; any inaccuracy quickly fine-tuned;  simple design; 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.  Whilst 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 inaccuracy.  A few tests on my 160mm Chinese made 3-jaw chuck revealed very acceptable accuracy, actually better than expected and consistency not much different to my Prat- Burnerd chuck which lives on the Myford ML7 lathe, both of which are designated as workshop grade.  One thing I discovered with the Chinese 160mm chuck, was that it has a varying run-out (0.001" - 0.003") when mounting bar stock depending on the bar diameter and using ther same socket 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 no. 1 jaw.  In fact the degree 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 1 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 od machined.

Part 2 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 unlikely in the home workshop environment.   The next task was to turn up another 3" diameter flange which is bored so that the ER25 shaft can be permanently fixed to it using Loctite again.  This flange is 1" thick and it is drilled on the front face with three radial attachment holes, each 7mm diameter and recessed using a 0.4375" diameter slot mill.  It is necessary for the holes and recesses to have clearance when an M6 cap screw is fitted.

Final Assembly

The two flanges are clamped together and a 7mm drill used to mark matching through-hole positions in the other flange.  These are then drilled 5mm diameter and tapped M6. The two flanges can now be fixed together using three M6 cap screws.  In order to ensure that the ER 25 assembled chuck and flange assembly is concentric a ground 0.5" bar (silver steel) is mounted into a collet in the collet chuck with sufficient bar protruding such that this 0.5” bar could be mounted in a 4-jaw chuck and accurately trued.  This approach ensures that the subsequent machining operations to the flange will achieve best alignment within the accuracy of the collet. With both flanges now fully faced and outer surfaces concentric they are assembled together.  A set of three holes were made in the ER 25 chuck flange perimeter used for a holding bar when tightening the collet nut.  Also, 4 further equally spaced M8 tapped holes were added to the periphery and these would be fitted with grub screws.  The purpose of these 4 grub screws is when making fine adjustments by way of tapping the appropriate screw (with an insert hex bit).  This being analogous to a four jaw chuck only in this case adjustments are made by tapping.  The reason grub screws are used is to provide a simple means of tapping at a specific point and not having any marked surfaces.

Using the completed ER 25 chuck.

In practice, the routine approach is to first mount the complete ER 25 chuck assembly into the lathe 3-jaw chuck making sure the alignment of the chuck marking is correct relative to no. 1 jaw.  This will provide generally good concentric working and is usually within 0.001" tir.  If a machining task requires better accuracy then a dial indicator reading off the work piece indicates the degree and position of run-out.  Fine tuning is then done by slightly slacking off the three flange attachment cap screws and tapping the flange on the periphery (using an insert hex bit in the appropriate grub screw).  With there being 4 such adjustment grub screws positions alignment is easily achieved by light tapping using a light hammer.  The degree of hammer blow weight applied when making adjustments and amount of resistance between the flanges is something that has to be guessed, but if the adjustment needed is very small then a light tap will usually achieve the required adjustment.  After setting is completed the 3 M6 cap flange attachment  screws are re-tightened so that there is no chance of further movement.  A change of collet should centre work within the limits of the accuracy of the collet but even if a collet shows an error with this chuck this can be corrected as described above. Purists will of course prefer to remove the 3-jaw chuck and mount a conventional ER chuck with its own integral back plate mounted to the lathe spindle or use a taper shafted chuck directly into the lathe spindle. Undoubtedly this is the normal standard approach but if the collet in the chuck is slightly out this results in the work piece also being out with no easy method of correcting this.  I am not a fan of over using the spindle taper for mounting chucks as there is a risk of movement or even worse scoring of the spindle ground taper.  The end result is an adjustable ER 25 chuck able to provide accurate TIR for a range of small diameters.

GWH Engineering

creative engineering in a home workshop