I'll try for an omnibus reply.

A few commercial sources for those that think this is just old-tyme
stuff.

http://www.yorkmachine.com/
http://www.americanmachinetools.com/Line_Boring.htm
http://www.boremaster.us/
http://www.dlricci.com/drill2.htm

Yes, I'm (hopefully) redoing some worn out pivots - or eventually I am -
I may need to keep abusing them for another summer to get work done, but
if I can do a bit at a time and get the worst ones done, it would help.

Wes has the basic idea down, and one can buy exactly that from several
places, but I see no reason (thus far) I can't put it together myself
for about $1000 less.

Hole size ranges from an inch or so (too small for most commercial
versions) up to 3 inches. It may actually be that the pin in the hole is
an inch, which means that the bore for the bushings (MIA, or I'd just
drive out and replace with new) is bigger - the smallest ones are for
the steering linkage, and lord knows what some idiot did, but there's no
trace of the bushings on those parts. Of course the loose pins have beat
the heck out of the holes. The big ones are either a 2 or 2.5 inch pin
with a bushing for another half inch diameter. Those 1/4" thick hardened
steel bushings were worn right through and wear continued quite some
distance in to the base steel.

George's book link is good, and he's got the right spirit - complicated
and dangerous, that's us. The main difference with the newer systems is
that the moving cutterhead is generally replaced by moving the whole
boring bar through bearings, and setting the bit in a hole in the bar
itself. This would seem to allow a thicker bar relative to the size of
the hole, and that keeps deflection down. I don't think any of the
modern ones try to run a pair of bits, either - getting them matched
precisely must have been fun.

The mag drill by itself places a lot of dependence on getting its
mounting perfect. At least one of the commercial bars does use a mag
drill as its drive (and feed, perhaps...)

Bridgeport head as drive and feed is not a bad idea at all, though I'm
willing to trend cruder. It limits to driving from one end, but that is
not an issue for backhoe parts as far as I can tell. Some of the
commercial bars tout flexible drive options for getting into more
difficult situations, but the hoe (and loader bucket) are comparatively
accessible, on that basis.

Radial drill press - I've had one on my list since I first learned about
them, but haven't gotten one yet. I'll up my shopping effort.

Age and ugliness - Hoe is a ford 4500 industrial TLB, only 40-41 years
old. Many joints and pivots are severely ugly (in the more important
mechanical sense) through some decades of no grease, at a guess. I might
have continued shopping had I known better at the time, but this was
also less than half the price of the next contender when I was shopping,
and a considerably better machine in the sense that it's a hoe built on
a hoe platform (27gpm pump!), not a tiny hoe stuck on an ag tractor.
I've certainly made it pay for itself - I've also wished it was bigger
(it's a 13 foot hoe) which would have surprised the heck out of me when
I was shopping and thought it was a bit intimidating size-wize - that
was before I started stumping hardwoods in bony gravel. The paint is a
crude job, and is has scars and bad welds (many not mine, some mine -
mine are at least not the worst on it), plus some big plates that the
previous owner was fond of tossing over cracks. My old welding
instructor would have looked at those and yelled stress riser, and I
agree, but have not had time to go over them and "fix them right". I do
at least try to make my repairs somewhat better designed. The buckets
both need work - I have a strip of old road grader edge my brother
donated to fix up the severely worn front bucket - the hoe bucket needs
teeth - well it needs shanks, too, as the "teeth" it has are actually
terribly worn shanks that were dug a long time without teeth on them.
Actual hard, sharp teeth would make root breaking about 9 times easier,
and that's the key to stumping - but at this point I have most of the
stumps out - the road is built, etc. Several cylinders are scarred, and
most could stand to be repacked, but time is finite and work to be done
is infinite...

What I really need is some sort of tractor restoration nut in dire need
of a project, but that sort of thing doesn't happen, or they get
terribly upset when you take the newly refurbished, tight hoe out and
scratch the paint on it, or get it dirty.

--
Cats, coffee, chocolate...vices to live by