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.
one more
Actually, I'm trying to make a lathe as a hobby.
There are no plans to machine steel or aluminum.
My goal is to process mostly acetal plastics.
So the spindle speed is steel. It won't be as slow as aluminum machining.
Since my goal is plastic processing, the power and capability of my 3 kilowatt AC servo spindle will be sufficient.
The problem is that it doesn't work.
I want to solve this problem.
Any ideas?
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