G76 internal thread clarification - 0i TB

[cdspk]

G76 internal thread clarification - 0i TB
Two line format
New to lathes

Got an external thread working fine, took a bit of playing but also got an internal going OK.

Want to clarify in more detail what it is the controller is doing. As I understand it:

G0 X22 Z5
G76 P001060 Q50 R.02
G76 X18.2 Z-20 R0 P900 Q500 F1.5

This will give me a 20 OD external thread retracting out to 22 on each pass

G0 X16 Z5
G76 P001060 Q50 R.02
G76 X18.2 Z-20 R0 P900 Q500 F1.5

This will give me an 18.2 OD internal thread retracting inward to 16 on each pass

By changing the initial G0 position the controller decides internal or external? At what point does the controller decide which add each cut and which way to retract the tool? I'm presuming it's whether the initial G0 X is above or below the thread range in this case 18.2 to 20, what if the initial G0 is within that range?

(BTW this is on a flatbed so the tool is forward of the axis, but I don't think that makes a difference?)


Cheers, Sean

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[sinha_nsit]

Initial X level with respect to the G76-second block X determines internal/external threads.
For more details, read the Operator's Manual or this book.

[mikiemus]

G76 internal thread clarification - 0i TB
Two line format
New to lathes

Got an external thread working fine, took a bit of playing but also got an internal going OK.

Want to clarify in more detail what it is the controller is doing. As I understand it:

G0 X22 Z5
G76 P001060 Q50 R.02
G76 X18.2 Z-20 R0 P900 Q500 F1.5

This will give me a 20 OD external thread retracting out to 22 on each pass

G0 X16 Z5
G76 P001060 Q50 R.02
G76 X18.2 Z-20 R0 P900 Q500 F1.5

This will give me an 18.2 OD internal thread retracting inward to 16 on each pass

By changing the initial G0 position the controller decides internal or external? At what point does the controller decide which add each cut and which way to retract the tool? I'm presuming it's whether the initial G0 X is above or below the thread range in this case 18.2 to 20, what if the initial G0 is within that range?

(BTW this is on a flatbed so the tool is forward of the axis, but I don't think that makes a difference?)


Cheers, Sean

Hi Sean,

You asked: 'what if the initial G0 is within that range?'

The internal thread is cut from Dia. 16.4 to 18.2 (= 2 * P900), The outer thread from 18.2 to 20, so far so good.

Now, if you get into the area where the thread is cut, either you get an alarm (usual), or if the control
accept it, it takes either the lower or higher part first (which is closest) and run the threadcycle.
However, what will happen is that by the rectraction to Z+, the tool will retract to the starting point,
via your starting X, dragging material with it on the way out. Your tools will not like it.
So, avoid go in between starting and finnishing diameters. We all like some good clearance in this world

Cheers,
Mikie