Originally Posted by
handlewanker
Hi, just came across this thread and some time back I thought about the problem of synchroing the spindle with the leadscrew for screw cutting with CNC, and as I'm new to CNC it appeared to be a too hard basket situation, as in why go to CNC turning if screw cutting is so complicated.
Unless screw cutting is the prime object of making a lathe , any lathe, CNC or manual, straight turning is just an X and Y movement at all times.......and an A movement if you have a turret tool changer etc.
So, if screw cutting is to be done simply, what would be the problem of adding change wheels to the spindle and leadscrew, as in a manual lathe, but control the saddle and cross slide movement with the steppers, servos...... whatever.
OK, so you need to manually change the gears or throw a lever or two at the start to get the pitch when required somewhere along the way, usually when the job is finished with the turning part, but as screw cutting is about 1/50th of the time spent in turning, to my way of thinking, the CNC mode for synchroing the spindle and leadscrew is a complicated one.
In a manual lathe the headstock spindle drives the leadscrew when screw cutting, so if the lathe is CNC'd with a geared spindle and leadscrew and screw cutting is needed, the stepper, servo motor....whatever....driving the leadscrew just freewheels.
It would be a simple matter to have a toothed clutch and solenoid on the gear train to disengage the gears so that high speed turning under CNC control can occur unobstructed.
It could mean that the cheap small lathes on EBAY could do CNC turning with geared spindles and leadscrew without having to go to great lengths with encoders on the spindle for the odd amount of screw cutting that might occur occasionally, but would need to be present for that operation.
I doubt that anyone would want to create an odd pitch size like .70mm pitch.......change wheels will do it anyway if you have enough of them with the right tooth count.
It's just a thought......can anyone see a problem with such a set-up?......I'd be interested to find out, as I can't see any.
Ian.