Threading on a CNC lathe

[Mcgyver]

How does threading a work on a cnc lathe? Is the leadscrew completely independent of the spindle? ie spindle, crossfeed and leadscrew all know where they are and there is no gear train between spindle and leadscrew?

I can imagine that with a really accurate servo system you could thread metric/imperial/odd ball as well as say turn an ellipse or gash a hob.

[Geof]

For threading; yes times how many questions you had. The controller knows how fast the spindle is going and has an index feedback so it knows the angular position. The controller also knows where the axes X and Z are. It needs a bit of space to get everything moving in synchrony so the threading cut is started a short distance ahead of the workpiece. Tapered threads move X and Z, straight threads just Z. You can do any pitch because you define the feed rate. For large pitch threads it may be necessary to restrict the spindle speed to avoid running up to a feed rate limit for the machine. For small threads such as 3/8-16 it is easy to run at 1000 rpm maybe faster on some machines. When you compare between manual and CNC machines threading is the operation that is speeded up the most on CNC.

Regarding you other question about turning an ellipse (intentionally) the answer is yes in principle but not so simple in practice. For an ellipse with only a small difference between the major and minor axes it may be possible but to have any sort of decent peripheral speed on your workpiece the X axis has to whip in and out so fast it is impossible. You also run into clearance problems with the tool because when the cut is moving from the major axis of the ellipse down to the minor the workpiece 'underhangs' the tool so you need extra clearance. Conversely it overhangs going the other way so the effect is your top rake and tool clearance are both changing and both have to be increased resulting in a very delicate tool tip. I do not know of any standard CNC lathe that has ellipse turning capacity implemented.

[Mcgyver]

thanks Geof, don't really need to turn an ellipse, more that it was just a convient way of asking the question of controlling the axis in relation to the indexing of the spindle. A practical application is making hobs, but the ellipse was an easier way to to ask. in making a hob running at a slow rpm would be expected. The light kinda went off for me on the advantage of cnc lathe in this respect

[Geof]

Running through my tired old brain is a faint recollection of a hobbing, or would it be gashing lathe. The cross slide screw was mounted on a cam or pitman driven from the spindle so the cross slide went in and out with spindle rotation while you could put on feed as normal with the handwheel which remained staionary by means of a spline. These could be run at normal speeds (for manual machines). Also I seem to recall an arrangement where the primary drive went into the gashing cam drive with a reduction going to the main spindle.

[Mcgyver]

never seen a dedicated machine for it, only regular manual lathes with diy add-ons mechanically linking the spindle to a cam to the disconnect crossfeed. With a cam and no feedscrew, i can see how it would run at something close to regular speeds, but in the sense that for diy making this would a rare activitiy, running at 20 or 30 rpm so the y screw could keep up would be no big deal, in other words the cam action is replaced by the y axis servo action

making hobs isn't a burning issue for me, but it did seem to be something a cnc lathe would have the capability to readily do - whereas it takes extensive mechanical modifications to do on a manual lathe

I've also always wanted a belt drive lathe like the high end tool room models - a VF drive, encoders on spindle and axis and belt drive on the spindle has appeal

[HuFlungDung]

Making a cnc "form relieving lathe" might be most possible with a Galil (or other) motion control system, so long as your spindle motor is also a servo motor. You could write a special routine in Galil commands to accomplish this. The movement would then become a 3 axis interpolated movement.

This differs slightly from a rigid tapping or single point threading, in that both lathe slides and the spindle would move together in electronic cam mode.

But running a normal 3 phase spindle motor as the third axis would not give tight enough spindle control to give good results, IMO.

[Geof]

Originally Posted by HuFlungDung "form relieving lathe"

Thank you! I knew there was a correct name for it; gashing is something else isn't it? And hobbing is what you do with it after you have made it (in some cases).

I have a flawless memory; everything is in it I just forget where to find it.