Lathe rigid tapping G33.1

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21 Mar 2017 14:37 #90007 by HalaszAttila
Hello,

I make a retrofit on lathe with Mesa 7i92 + 7i76.
I want to understand rigid tapping (G33.1), and with it I have some questions.

My feed drives gets STEP/DIR, the spindle is analog +/-5V with 800 PPR encoder.

What is the right method to control the spindle for rigid tapping?
Can Linuxcnc to control spindle like C axis?

I my program, the spindle speed is controlled by PID. But the depth of thread are changing according to PID settings (deceleration) and the spindle speed.
How can I control the spindle to make a precise depth thread?

Theoretically the spindle should be act as a rotary table, and every time rotate exactly the same number of rotation?

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21 Mar 2017 16:31 #90008 by andypugh
Replied by andypugh on topic Lathe rigid tapping G33.1

What is the right method to control the spindle for rigid tapping?


As long as LinuxCNC can start, stop and reverse the spindle then it can do rigid tapping (It doesn't even have to be able to control the speed).
What happens is that LinuxCNC waits at the stat position until it sees the encoder index, then drives the Z axis to a position that advances one thread-pitch per spindle revolution. You can even stop the spindle and turn it forwards and backwards and the motion will stay in synch. This video shows someone re-synching a thread with the spindle disconnected using this feature, though this is single point threading.

At the end of the programmed move LinuxCNC reverses the spindle rotation. The tap will then cut somewhat deeper (depending on spindle speed and motor power etc) untill the spindle stops and reverses. The Z-axis remains synched to the spindle all the way through.

Can Linuxcnc to control spindle like C axis?


It can, but that is not necessary for rigid-tapping or single-point threading. It also requires a low-backlash spindle drive system.
The following user(s) said Thank You: HalaszAttila

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21 Mar 2017 16:49 - 21 Mar 2017 16:58 #90012 by PCW
Replied by PCW on topic Lathe rigid tapping G33.1
I think what HalaszAttila is suggesting is not possible directly with Linuxcnc without using something
like a C axis and weird gcode (for precise depth controlled rigid tapping)

To do this you would need a rotary angle profile for the plunge so a angular position controlled
spindle could follow the rotary profile

(The Z axis might still be spindle synchronized with this rotary motion)

If you just need more consistent depth with G33.1, it might help to run the spindle speed command
through the limit component so you are less dependent on (possibly variable)
spindle accel/deccel characteristics
Last edit: 21 Mar 2017 16:58 by PCW.
The following user(s) said Thank You: HalaszAttila

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22 Mar 2017 11:26 #90057 by andypugh
Replied by andypugh on topic Lathe rigid tapping G33.1
I would think that a spindle with enough responsiveness to operate as a C-axis would have very low over-shoot in conventional G33.1 mode.

However, if the lathe is configured for C-axis moves then you would not use G33.1. C-axis threading becomes a simple
G1 Fnn C10 F-10 move, and then the reverse to extract the tap.

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