Depending on exactly which flex shaft you've got, it's dirt simple,
Step 0: order flex shaft cable grease. (In a pinch, white lithium
grease will work.)
Step 0.5: unplug flex shaft.
Step 1: remove your handpiece.
Step 2: remove the sheathe. (The heavy black thing that covers the
Depending on which machine you've got, they either screw off (look
for a knurled or lined 'cup" looking thing on the motor end of the
sheathe) or some of them go into a socket that also screws off the
motor. Either way, you eventually end up unscrewing the sheathe.
Step 3: once the sheathe's unscrewed, it's completely loose. Gather
paper towels, and slide it off the inner cable by pulling away from
the motor. The inner cable is covered in grease. Since you're
asking this question, I'm guessing you've never lubed the shaft.
Which means the grease will be black and nasty. Do this somewhere
you can clean easily, and have plenty of paper towels. Be aware that
the inner cable is very flexible. (surprisingly..) Being flexible
and covered with grease, it becomes most ingenious at finding things
to drape itself across. It'll want to go exploring as you get the
sheathe off. I suggest dropping the sheathe straight down while
holding the motor. That way you'll have 30 inches of greasy snake,
but mostly wanting to hang straight down. Don't wear good clothes.
They beckon it.
Step 4: Once the sheathe's off, it's really simple: there's a bronze
socket at the motor end of the cable. It'll have either one or two
set-screws on it. Loosen them, and remove the cable.
Step 5: put the new cable on, and tighten the setscrews, making sure
they bear against the flat side of the motor shaft. (If it has one.
Not all do.) If it has no flat-side, make really sure the screws
are locked down tight.
Step 6: run a piece of cotton rope or something down the sheathe to
clean the crud out of it. Throw the rope out.
Step 7: slather the new cable in grease, and slide the sheathe back
Reverse whatever you had to do to get the sheathe off, re-attach
your handpiece, and you should be good to go.
This should only take about 15 minutes or so. It's not a big deal,