Without wanting to sound a smart arse a boring head can only bore a
hole whereas a boring & facing head can also face a flange.
Essentially there's a mechanism that causes the boring tool to edge
out slightly every revolution when requires so that a flange could be
both bored parallel and faced up true.
Boring & facing heads usually have a knurled ring around the body.
If the cutting tool only sticks out at the side of the head then it can only
bore. If it sticks out at an angle that's adjustable for diameter while also
always being below the bottom surface of the head then it can both bore and
face. A true flycutter by contrast has a tool that only sticks out below the
head and not outwith its periphery so that can only face.
The advantage of a single purpose boring head is if the tool adjustment is
perpendicular to the bore it can be easily and accurately adjusted for
cutting diameter with a micrometer attachment. A tool moving in and out at
an angle can also face but is harder to set up for boring. You either need
to take trigonometry into account or have a more complex registering and
setting system. A true flycutter can be adjusted for diameter without losing
rigidity whereas a boring and facing head loses rigidity as the tool
protrudes more. Every compromise has its downsides. Horses for courses.
For high precision boring such as for engine blocks you'd always use a
single purpose boring head. For high precision skimming you'd ideally use a
single purpose flycutter. For roughing out material in either mode you have
your compromise boring/facing head.
Some confusion here, I think.
What most people think of as a boring & facing head is strictly a
boring and *automatic* facing head, such as the Wohlhaupter.
for what they can do.
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