Hi Gordon
If you clamp it on the x slide you need to be able to clock the O/D true ie raise or lower it into the correct position to bore it  true 
Nobby

Edited By Nobby on 23/01/2015 18:17:43

Edited By Nobby on 23/01/2015 18:21:21

Gordon,
A couple of other considerations for you.

A between centres set up needs a bar twice the length of the job and then some more. At 150mm long you would finish up with about 330mm bar and that would be twice as long as a conventional set up, so what you gain one way you loose on the other. You would also need a good tailstock centre set up, live centre etc to ensure the good finish.
The other thing to consider before choosing which route to go. What are you going to measure hole size with and how?. A boring bar in the job hampers easy checking and it may prove difficult to temporarily remove the centre bar, size check, and then replace the bar sufficiently accurately to guarantee taking a fine cut to finish. There is less clutter around if you use a boring bar.
Additionally, What concentricity do you need between O/D and I/D, Are you going to mandrel mount and machine the O/D after doing the bore?

Bob D

After frustration with a 'simple' boring bar, I made one to the Geo Thomas design. It was well worth the effort, especially when using a micrometer to set an incremental increase in cutter radius. As the old boy wrote ~ the force on the cutting tool is not at 90 deg to the bar which aids producing a parallel bore.

I have tried both methods when reboring Blackstone stationary engine cylinders ,I bored a 4 hp cylinder on a roundhead master,using a large long between centres boring bar and using the T slots for mounting the steadies to secure the cylinder to the saddle,and a 2 hp cylinder mounted on the faceplate of a Colchester triumph via a tool post mounted boring bar, it was easier and quicker to bore the faceplate mounted cylinder,particularly measuring the bore,though in both cases the bore dimension was not critical as a new piston was cast and machined to suit the bore.In cases where the lathe is worn and not particularly accurate,the between centres boring bar has the advantage that tip of the boring tool describes a constant circle(ignoring tool wear) whereas a cylinder held in chuck or faceplate relies on how true the lathe bores.Setting up the work is far more tricky using the b/centres bar to ensure the new bore is square to the existing machined faces, there is also the problem that due to the length of the bar the tool will tend to chatter.When machining the cylinder on the faceplate I had to make a bored out sub plate between the face plate and cylinder so that the boring tool did not run into the faceplate.The 5 inch bore on the 4 hp cylinder was 17 inches long and over its working life the bore had worn conical by 2mm the bore at the crank end was near nominal ,the extreme wear was at the head end,this is due to the paraffin not completely vaporising in the hot bulb and diluting the oil,though it was 80 years old at the time,the piston also had .5mm wear.