And for the Warco WM 180 would a 4 or 6mm be best?

You employ the largest and shortest boring bar you can fit in the hole (subject to the machine toolholder). Rigidity is the name of the game where boring bars are concerned.

Utilising the lathe power feed is, by far, the preferable way to bore holes.  Th tailstock is generally used for drilling, not boring.

Edited By not done it yet on 10/07/2021 13:02:03

The biggest problems with holding a boring bar in the tailstock chuck is that you can't adjust the size and the hand feed does not lend itself well to a nice smooth bore.

Most of the time a round, square or round with flats shank boring bar is held in the toolpost so you can use the lathe's power feed to make the cut along the bore and the cross slide to put on a cut.

The tools you link to would generally be use din a boring head and be used in the mill. If using HSS then something like these though you may not find a use for all of the ones in the set for some time, other suppliers do singles. Or you could go for an insert type such as these .

4mm is a bit small for general work, 6mm would be a minimum but depends on the size of hole being bored. Generally you don't want to go much deeper than 4-5 x diameter of shank

The thing to remember with boring tools is that,. being relatively small, they deflect.

hence as the bore approaches finished size, you start to take spring cuts.

You will be surprised just how much a 6 or 8 mm bar deflects under cutting forces..

Often it will take more than one spring cut before the tool stops cutting (Hence the need to withdraw the tool with the work stationary, to avoid a helical back cut.

Again, stiffness in tooling and work increases accuracy.

Howard

It is how you make the hole to bore to size….