interim project: making a boring bar longer

 My lathe came with a small amount of tooling, most of it in poor shape.  Some of it - the Dorian QC tool holders - is quite good tho.  A mixed bag.

 One of these tools was a 0.75 x 6in boring bar, which someone had turned down so the end would fit in a 0.5in Jacobs chuck.  So when it is mounted in a proper 0.75in Dorian boring bar holder, there's only about 3in available now.

 I have a need to reach at least six inches into my lathe's spindle, so I've been thinking about how to stick some more boring bar onto the end, to make it 10 or 12in OAL.  I have a piece of 0.75in rod which was formerly a rail for linear ball bearings.  It has a few transverse threaded holes in it, but that shouldn't matter too much. 
(he said, optimistically)

It appears to be mildly heat treated, which is desirable because it makes my rod stiffer.  What?

The question then is how to stick the two bars together.  Since one has been turned down, the most obvious answer is to make a hole in the end of the new rod to leverage that.  Since I'm only interested in joining them securely and permanently, I won't explore set screws.  That leaves:

• making a sliding fit hole in the new extension and brazing the two together
or
• making an interference-fit hole and heat-shrinking the two rods together

Both are fiddly and require some planning.  Brazing is only semi-permanent; if I really wanted to, I could separate the two pieces again by heating.  If I shrink-fit the parts, they become one piece forever and ever amen.

 I am leaning heavily toward the shrink-fit option mainly because I don't have any brazing flux on hand (I'd like to try something new) and because a foot-long bar can always be pulled back in the mount to stick out however much one wants.

 I have had limited success with heat-shrinks.  It's easy to wing it, and hard to get good results that way.  The best results are obtained using the shrink-fit allowances found in Machinery's Handbook, and a bit of simple math.

This would not be do-able at all, if I had not recently bought a small boring bar that takes inserts.  So I can just barely get away with making a hole with a drill bit (which hole will just barely admit my little boring bar) to then bore a more precise hole for the "male part" on the part with the business end.  I haven't run the formula yet, nor looked things up in the allowance table, but my guess is the hole needs  to be between two thou and three thou smaller than the "pin".  Tolerances typically go to four decimal places, but I do not own a micrometer, only a pair of calipers.

On an aside: if the pin and socket locations were reversed, ie; if I needed to make the hole into the existing boring bar, I would use that opportunity to put a dampener inside it.  This consists of a small cylindrical mass of a higher-density material like lead or tungsten, either cemented rigidly into the hole with a filled epoxy, or suspended in the hole within silicone rubber.  Factory bars with this feature cost at least 4X the price of a non-damped bar, but it's a lot harder to get them to chatter!

More on this story as it happens...