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Author Topic: Lathe machine G76 thread work problem (video attached)  (Read 1205 times)

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Lathe machine G76 thread work problem (video attached)
« on: September 19, 2022, 09:52:11 PM »
This is a lathe machine question using Mach 4 and smooth stepper.
Using the G76 command, there is a problem with the tapping operation.
The same is true for working with the job wizard.



The spindle starts rotating fine, but soon, it barely rotates, and neither the X-axis nor the Z-axis moves.

The spindle uses an AC servo spindle, and if you check the Smooth Stepper Diagnosis, the RPM is reported correctly.
The screen also displays the RPM correctly.

I currently set it to output 500 pulses per rotation.


Commands like M3 S1000 etc also work very well and have no issues at all.
There are no problems with other machining, but   tapping operations is problems.

We have prepared a YouTube video below.
What am I missing?

--G76 operation problem video--
https://youtu.be/VjyVQ6coxfQ
https://youtu.be/EnKnQ2SQlc0
------------------------------------

--Automatic turret tool change head test video--
https://youtu.be/wspTNS7x5Ns
Re: Lathe machine G76 thread work problem (video attached)
« Reply #1 on: September 20, 2022, 01:02:35 PM »
So I bet you are using step and direction to control you spindle motor . The documentation for the smooth stepper says you can but it also states that there could be issues with threading .
Re: Lathe machine G76 thread work problem (video attached)
« Reply #2 on: September 20, 2022, 09:12:16 PM »
So I bet you are using step and direction to control you spindle motor . The documentation for the smooth stepper says you can but it also states that there could be issues with threading .

Right. I'm using a step, direction control spindle


Does anyone have any experience?
Successful tap machining in smooth stepper?
Re: Lathe machine G76 thread work problem (video attached)
« Reply #3 on: September 21, 2022, 07:45:40 AM »
You need to contact warp9 cnc They make the smooth stepper ,he has great customer service .Andy can answer any questions you have . The servo spindle is controlled by the motion controller .Good luck I went this route with a Pokeys set up spindle worked great until I tried to thread. Once it saw G76 spindle would just stop . I am still using an AC servo motor . Using 0 to +10 volts to control speed .

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Re: Lathe machine G76 thread work problem (video attached)
« Reply #4 on: September 23, 2022, 03:29:23 AM »
Same problem exist with PoKeys 57 as well. Threading doesn't work when spindle is in step-dir mode. I had to switch to velocity mode.
Re: Lathe machine G76 thread work problem (video attached)
« Reply #5 on: September 23, 2022, 08:07:19 AM »
i know nothing about tapping and metal work
but i have question ,because taping its very precise spindle job,and its not regular lath job, i mean spindle not need now rotae one direction long time( can be flow memory)
( i belive you call the rotation spindle)why not use spindle as axiss? interpolation between axiss in mach work perfect
Re: Lathe machine G76 thread work problem (video attached)
« Reply #6 on: September 23, 2022, 02:55:50 PM »
To use spindle as axis you have to control it with step a direction and that is the problem . The motion dose not support this.
Re: Lathe machine G76 thread work problem (video attached)
« Reply #7 on: September 24, 2022, 12:38:04 PM »
I mean play real ,mean put servo on spindle ( lath)
Re: Lathe machine G76 thread work problem (video attached)
« Reply #8 on: September 26, 2022, 04:34:03 PM »
Hi,
g76 is the standard lathe single point threading cycle.

It does not require a position capable lathe spindle but does require a very stable spindle speed. For instance if you 'dial' up 500rpm for lathe threading, it does
not matter particularly if its 450 rpm or 550rpm, but whatever it is it should stay there. So lets say you spindle is doing 525rpm and then it starts cutting a thread.
The Z axis will advance exactly 1mm per rotation or 8.75mm/sec  for a 1mm pitch thread....so the spindle must rotate at its assumed speed irrespective of the cutting load.
If the spindle slows down to 490 rpm as a result of the load then the Z axis must also slow down to 8.166mm/sec to maintain the same pitch.

All g76 capable spindles are fitted with at least an index pulse (that is one pulse per revolution), and some controllers will accept an index pulse and/or an encoder pulses of many pulse
per revolution. Mach uses the index pulse to calculate exactly how fast the spindle is going and when to start the Z axis travel so that multiple passes are synchronised on top of each other.
Mach will adjust itself to small variations in spindle speed.....but not by much. If you want to have good and accurate pitch control you NEED a spindle with a very stable rpm under load.

Note that this technique is a very common lathe operation and many tens of thousands of users make nicely accurate threads every day....but does have that requirement...ie stable spindle speed.

The technique that Katz has proposed is to have a POSITION controlled spindle, ie Step/Dir, and then you can program a rotational move, ie the spindle with a linear move, ie the Z axis.
This is how I do rigid tapping with my vertical mill. My tapping spindle is a servo motor and is position controlled, and so I can program a Gcode move of:

g1 z-10 c 3600 f14400
g1 z0 c 0 f14400

and the machine will rotate 10 revolutions (3600 degrees) while the z axis descends 10mm for a pitch of 1mm per revolution in about 15secs, whereon it will back out out the hole
by 10 revolutions and the Z axis rise by 10mm, again in 15 secs.

You can see that I do not require a g76 move, I'm relying on Mach being able to calculate a coordinated trajectory between a rotating axis and a linear axis.

This technique works fine....but note that it requires a powerful spindle, otherwise the lag of the spindle will screw up the synchronisation with the linear axis. In fact BOTH
threading techniques require a superbly powerful and stable spindle or the result will be crap.

This is the take-away that 'threading requires a very stable spindle speed......if you are trying to do lathe threading with a sub-standard spindle you will come a cropper bigtime!'

Craig
« Last Edit: September 26, 2022, 04:43:01 PM by joeaverage »
'I enjoy sex at 73.....I live at 71 so its not too far to walk.'
Re: Lathe machine G76 thread work problem (video attached)
« Reply #9 on: September 27, 2022, 11:41:14 AM »
Hi,
g76 is the standard lathe single point threading cycle.

It does not require a position capable lathe spindle but does require a very stable spindle speed. For instance if you 'dial' up 500rpm for lathe threading, it does
not matter particularly if its 450 rpm or 550rpm, but whatever it is it should stay there. So lets say you spindle is doing 525rpm and then it starts cutting a thread.
The Z axis will advance exactly 1mm per rotation or 8.75mm/sec  for a 1mm pitch thread....so the spindle must rotate at its assumed speed irrespective of the cutting load.
If the spindle slows down to 490 rpm as a result of the load then the Z axis must also slow down to 8.166mm/sec to maintain the same pitch.

All g76 capable spindles are fitted with at least an index pulse (that is one pulse per revolution), and some controllers will accept an index pulse and/or an encoder pulses of many pulse
per revolution. Mach uses the index pulse to calculate exactly how fast the spindle is going and when to start the Z axis travel so that multiple passes are synchronised on top of each other.
Mach will adjust itself to small variations in spindle speed.....but not by much. If you want to have good and accurate pitch control you NEED a spindle with a very stable rpm under load.

Note that this technique is a very common lathe operation and many tens of thousands of users make nicely accurate threads every day....but does have that requirement...ie stable spindle speed.

The technique that Katz has proposed is to have a POSITION controlled spindle, ie Step/Dir, and then you can program a rotational move, ie the spindle with a linear move, ie the Z axis.
This is how I do rigid tapping with my vertical mill. My tapping spindle is a servo motor and is position controlled, and so I can program a Gcode move of:

g1 z-10 c 3600 f14400
g1 z0 c 0 f14400

and the machine will rotate 10 revolutions (3600 degrees) while the z axis descends 10mm for a pitch of 1mm per revolution in about 15secs, whereon it will back out out the hole
by 10 revolutions and the Z axis rise by 10mm, again in 15 secs.

You can see that I do not require a g76 move, I'm relying on Mach being able to calculate a coordinated trajectory between a rotating axis and a linear axis.

This technique works fine....but note that it requires a powerful spindle, otherwise the lag of the spindle will screw up the synchronisation with the linear axis. In fact BOTH
threading techniques require a superbly powerful and stable spindle or the result will be crap.

This is the take-away that 'threading requires a very stable spindle speed......if you are trying to do lathe threading with a sub-standard spindle you will come a cropper bigtime!'

Craig


Thanks for the advice.

Here is my question.

I used an AC servo motor with good power of 3 kilowatts.
I thought it would allow for more precise control.

Installing an encoder in a general motor and

Controls expensive high-end AC servo motors and receives precise positioning pulses from the motor
to feed the motion board

I thought it would be better to use a high-end Ac servo motor.
It's a pretty expensive motor, and the output is quite high.

How can I fix this now?

My dream is to make great tap machining with my machine.