Application and design. Like going into a shoe store and pondering the
differences one finds there... ;-)
Your best bet would be to look at pictures, rather than trying to get it
all described textually here. The difference between a turret lathe and a
engine lathe might be comparable to a pneumatic nailer and a claw hammer.
Meaningless without a mental image of both, eh?
Can you imagine the difficulties in facing a 100 lb plate on a typical
lathe, as opposed to doing the same on a vertical lathe? Gravity will be
your friend or foe, depending on that choice.
A gap-bed lathe has a section of bed ways near the
headstock that is removable, for turning large
discs that would hit the bed otherwise.
An engine lathe is as opposed to a bench lathe,
where the bed is just bolted to the bench. The
engine lathe has some sort of a pedestal as part
of the machine, sometimes two completely separate
pedestals at either end of the bed. Or, it may
imply a geared headstock.
A turret lathe has a tool turret on the
carriage/cross slide, and generally has no
tailstock, as the turret is often tall and wide
enough to block material from passing over it.
There is usually some system like a turret stop on
each axis as well.
It is used for production work on relatively short
A knee mill would refer to machines like the
It has a "knee" that elevates the entire X-Y table
and the work.
The Bridgeport is called a "turret mill" by the
maker, due to the swivel arrangement at the top of
the main base casting. That swivel is not used
much at all in most cases, but can get you out of
a jam on a huge workpiece. So, the turret there
is totally different than the turret on a lathe.
Some other machines elevate the head by a variety
of means to compensate for fixture and workpiece
height, and tool length. There are benchtop knee
mills, like the one of the Burkes, I think. Many
benchtop mills elevate the head to adjust height.
Well ... knee only applies to mills. The X-Y table is on a knee
which is raised by a jackscrew to move the work closer to or farther
from the spindle.
As for the lathe names you used:
1) gap -- the bed of most lathes continues right up to (and under)
A gap-bed lathe has a section of the bed which can be removed
near the headstock to allow you to turn workpieces of a diameter
which would otherwise hit the bed. (Think making a flywheel or a
model locomotive wheel.)
The disadvantage is that once you remove the gap plug, is is
highly unlikely that you will get it back in precisely enough to
restore the originally accuracy. The plug is fitted, and the
ways are ground with it in place, and the factory never touches
it again, so it is up to you to make the tradeoff between
workpiece capacity and accuracy.
2) Engine -- pretty much any lathe with gear fed threading motions
for the carriage. (This then eliminates wood lathes, chucker
lathes and CNC lathes.
Some say that it is simply a lathe with its own motor instead of
being driven by overhead shafts and belts -- a really old way of
supplying power from a single big electric motor or steam engine
to many tools).
3) Turret (not turrent) -- instead of the tooling all being
mounted, one at a time, on the cross-slide, it has a moving and
rotating object which holds multiple tools (six in my machine),
and automatically rotates to bring the next tool into position as
the current one finishes its task and is withdrawn. These do
not have a tailstock and are designed to work on fairly short
workpieces on the end of bar stock fed through the spindle.
Finish one workpiece, part it off, and advance the bar through
the spindle to bring up stock for the next workpiece.
This is very good for small scale production. It takes some
work and knowledge to set it up for a specific job, while the
typical toolroom lathe has interchangeable tools on the
carriage, and a tailstock for supporting long workpieces. This
is a lot easier to use for one-off projects, but a bit slower at
production runs of identical parts.
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