Re: [CAD_CAM_EDM_DRO] CNC Lathe question

vrsculptor@... wrote:

>
> I have no experience with CNC lathes.

Ya gotta start somewhere.

> How do control the long axis on a CNC lathe? The Harrison I have
> access to has a gear rack and a lead screw.

Most manual lathes have that setup (not including the miniatures
which tend not to have the rack, but instead a handwheel directly
on the lead screw).

> When you convert a lathe to CNC do you replace the lead screw and
> half nuts with a motor driven ballscrew assembly or do you just drive
> the existing leadscrew and keep the half nuts? If you get rid of the
> half nuts how do you run the machine manually?

Thats the trick. On bigger lathes there is quite a compromise to
being able to retrofit to running both manually and by cnc.

I am assuming you want to still occasionally run the lathe in manual
mode, which means leaving the lead screw.

What you can do (there are several ways, here is the first):
- If there is room, install a ballscrew underneath the lead screw.

- The ballscrew goes to the far right of the lathe, where the
motor drives it directly (preferable in a cnc/manual mode, because
there is less resistance in 'backdriving it' with the rack) or by
pulley.

- If the ballscrew/motor can't move the carriage (with the halfnut
unclamped, obviously) because of resistance caused by the rack,
you have to <permanently> disengage the rack somehow. This is bad
because when you are running in manual, you have to move the carriage
backwards by hand (or run the lathe & leadscrew backwards, but thats
too much trouble and wear on the motor).

- So obviously when you are running in CNC mode, the halfnut is
unclamped.
When you are running in manual mode, the CNC motors are unpowered.

- Perhaps you can set it up (in manual) so when cutting the CNC motors
freewheel, but you have a 'dumb 555 timer-stepper' to engage the CNC
motor to haul the carriage back.

- I hope this makes sense...

The other simpler, cheaper, but less accurate way, is to simply CNC
motorize the pinion (of the rack and pinion). This is done all the
time on the miniature lathes. It is cheaper, because you don't have to
buy a ballscrew, but it is less accurate because the manual positioning
rack (&pinion) are quite crude and were never meant for accurate
positioning.

On the third hand, you could do the above, but put a linear scale on the
lathe (long axis == Z axis) - in that case the inaccuracy of the rack
is irrelevant. But this might be the most expensive way, because long
linear scales tend to cost a lot more than long ballscrews....

On the fourth hand, maybe you can improvise a linear scale with
USDigitals
mylar encoder tape ($30 for the encoder, $2/inch for the tape), and come
out ahead.

There ya go, clear as mud.

Doug Fortune
http://www.cncKITS.com