PP = Parallel Port.
Hood

santos,
I am having difficulty due the translations so bear with me.

You need to have the index working properly to do threading. When working properly the displayed RPM  is accurate. There can be some change in the rpm and Mach will adjust the threading cycles based on the rpm.
If the rpm drops too much, say 50%, the threading cycle "may" recover. If the actual threading stops then probably the thread is ruined. Have a read of the Threading on The Lathe write up in Members Docs (page 4) .  

From your replies you say using a  proximetry sensor, 2hp motor,  and trying to do a 5mm pitch x 500 length thread at 229 rpm. So will comment on the above:
- proximetry sensor- don't care what you use as long as the displayed rpm is correct AND can provide an accurate  consistant index pulse  for the feedrate you are using during threading. If you use the threading wizard to generate the code it will provide the feedrate and required accel based on RPM and pitch.
 
-500 length - like Hood said, he has no problem cutting long threads. You should scribe test and see what you can do. Have a look at the Threading wirte up on what to do. Also eliminates motor slow down as a factor when testing.

- 5mm pitch  & 2hp motor - pitch is the distance between same points along the thread parallel to the axis.
So 5mm ( approx .197" imperial) between points. How much hp you need to cut that kind of thread will depend on type of thread and depth of cut and how it is being cut. So 2hp may or may not be adequate.
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threading stop for one second in first depth of cut and continue withine second same cycle because of that our threading pitch shift in second depth of cut of cycle.

Are you meaning ......during the actual cutting of the thread the spindle rpm drops a lot / almost comes to a halt.
If that is the case, then Mach "may" try to fix the thread. Any "fixing" would be applied to the next threading cycle. Chances are, the thread will not be correct. You may want to consider doing the thread using alterante
flank infeed method ( see page 44) in the wirte up on how to set.

RICH

How about posting the code you are using to do the threading? Maybe it is just a GCode problem?

Hi Hood,

I have some help to write my posts as English is not my first
language. I hope that my communication is clearer from now onwards.

I have checked my index pulses using an oscilloscope and they are clean.

The RPM display previously showed some momentary variation. After
changing the noise filtering value from 0 to 0.75 in the smooth
stepper configuration, this variation has gone away and the RPM
display is now solid.

This has changed the behaviour during threading and there is no more
stopping of the Z axis for 1-2 seconds as it used to previously during
threading. All the cutting is smooth for all the passes.

I still have the problem of pitch variation. After 130mm the

i still have the problem of pitch variation.After 130 mm the length of job.

santosh,
Can you define the amount of pitch variation?
Does  the pitch increase or decrease?
How accurate is the pitch from the Z=0 to  50, 100, 150 distance?

The pitch will only be as accurate as YOUR lathes mechanical "system". If the spindle maintains an accurate  rpm then the axis  must move at  a defined feedrate. The pitch accuracy is the result of the feedrate. Feedrate in terms of linear motion is the result of steps per unit, thus over a long distance if the steps per unit are not exact then the pitch will suffer and all other movements can also affect the pitch ie; backlash, acceleration, and quality of axis components, including the actual cutting action of the thread by the tool.

RICH



 

I think your issue may well be your spindle motor is too small for 6mm pitch threads but it may be worth connecting your index pulse to Port 3 on the SmoothStepper.
Hood

Hi  hood/Rich
Now i am using  5 hp spindle motor, 115 RPM , 9 inch centre hight of lathe machine (Heavy Duty Lathe).

Pitch Variation= 0.4 mm

Pitch is increasing.

pitch accurate distance= 0 to 110mm distance is getting accurate.

I am using new spindle motor.

Did you try the index in on port 3?
Does this happen on the first pass, in other words if you take a very light first pass and then stop the thread do you see the pitch varying?
Hood

yes i tried the index pulse on port 3 but i dint get any difference.

In first pass my pitch getting correct.

from second pass it start shifting after 110 mm .

And i have taken scribe test according to rich manual.

And i have taken very light first pass also.