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How to Make and Use a Split Bush (Chucking Piece) for Concentric Turning

I’ve recently been making a Stuart 10V with the help of the Andrew Smith/Pengwern building guide. At one point they recommend the use of a split bush to afford a greater degree of concentricity in turning the piston rod (and both the piston and cross-head attached to this). A sketch of the split bush is provided with the throw away comment that its use requires no description. OK, that much I agree with but the making of the device led me astray.

The basic idea of the split bush is to get around the fact that the lathe chuck is likely to be a wee bit off truely concentric to its rotation axis. i.e. even perfectly cylindical rod in an unworn chuck will not rotate about its centre. The split bush is bored in situ, which does guarantee the hole is centred on the axis of rotation and the work is held in the hole. In the 10V, there is a 5/32″ piston rod and 1/8″ valve rod that need to be treated in this way. By the way, I use a Warco 918, a relatively cheap Chinese lathe and I have demonstrated that the chuck is about 0.001″ out.


(click to enlarge)

A split bush for 1/8″ diameter rod. The shoulder should sit againt the outer nose of the chuck, which is tightened to cause the bush to grip the work piece

The problem with being a self-taught novice is that you don’t know some simple stuff and don’t know which of various factors actually matter. Paranoia about defeating the whole point of the operation by removing the bush from the lathe between boring the hole and using it – would the hole now be true – led me to make the bush the wrong way. Part of the reason for this is that I had previously made a larger (1 1/4″) chucking ring where a boring bar had been used. The error I made was to drill and ream (actually I use home made D-bits) the hole after cutting the slot in the side. This doesn’t matter when using a boring bar but it does when drilling, especially as the slot size gets close to the hole size. I didn’t notice this problem with the 5/32″ piece bit it became obvious with the 1/8″ piece (see below):

I didn’t spot this until using the bush as (remember the paranoia) the bush wasn’t removed after boring and before using it. Its not like I didn’t know about drill wander but I was surprised it made so much difference and I was still paranoid. It seemed like there was no way ahead. Some Googling was done and eventually I found more than an assumption that everyone knew how to make/use these devices; I found enough to get me to have another go, the proper way. Actually I had tried to find out how to make split bushes before but not found anything so had a go… the wrong way. I decided to write this post in case anyone else needs a bit of help to do it the right way the first time.

How to Make a Split Bush Properly…

(this one did indeed seem to be right – I checked quite carefully)

  1. Face the end of some brass and turn down the 1/4″ section.
  2. Part off or remove and saw off
  3. Re-chuck, ensuring the shoulder seats on the chuck nose “nicely”
  4. Face off, centre drill, drill 3mm and ream/D-bit 1/8″ (I love mixing units)
  5. Identify one of the chuck jaws (I centre punched the chuck backplate) and mark the nascent bush to it may be precisely aligned with this jaw. A light centre-punch is nice and permanent.
  6. Remove the piece, insert a piece of rod into the centre and cut the slot in the opposite side to the marked point. The rod avoids hard-to-remove swarf buildup in the inside which will interfere with the workpiece. I’ve started using a Dremel tool for little jobs like this.
  7. Return to the lathe chuck, reseat carefully and lightly tighten the chuck, insert the work piece, tighten the chuck fully and there you are…

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