Hi Azalin,
AFAIK that all looks good and I don't know why its not working.


Tweakie

Mach requires one pulse per rev, an index, to calculate and synchronise a thread. Mach assumes that the speed is constant even when the tool engages the
material and starts cutting. We all know that is never actually the case, the spindle always slows a little. Even with closed loop control a feedback of one pulse
per rev is not enough for a controller to maintain spindle speed accurately enough for threading.

Sorry Craig but this is not quite true - there are many lathe users threading this way.

My understanding is:
Mach assumes constant speed. If the speed slowed over one revolution as a result of the cutting forces then Mach would attempt to cut the thread but do so
inaccurately. With spindle speed averaging it will take Mach several revolutions before a more accurate estimate of speed is available. If the spindle has significant
inertia and/or power then the speed change will be slight and the thread cut fine. A lightweight or underpowered spindle and the speed change could be significant
and the thread inaccurate.

For situations where the spindle is marginally powered, which is the same as saying taking heavy cuts relative to its power, then a higher bandwidth control
loop makes the most of the power available.

To my knowledge no-one has repealed Nyquist's sampling theorem and the control bandwidth is inextricably linked to the number of pulses per rev.

Several months ago I bought a 1.8kW Allen Bradley servo and drive for a spindle motor. Its encoder is 2000 line or 8000 count per rev with a loop update rate
of 20kHz. It beats any other spindle motor for speed/position accuracy that I've ever come across. Had I not bought one and tried it for myself I would not have
believed how much superior a servo could be over an index pulsed motor.

Craig

Hi Craig,

I really don't want to get into a discussion about this but...  

Even with closed loop control a feedback of one pulse per rev is not enough for a controller to maintain spindle speed accurately enough for threading.

It's perhaps not ideal but that's the way threading has been done with Mach3 (one pulse per rev) .

Tweakie.

Threading in Mach3 with a single pulse per revolution does rely on
the pulse to measure spindle speed as well as to sync the Z axis
motion at the start of a threading pass.

The spindle speed is not modified by the threading process when
it measures the spindle speed. Only the Z axis traverse rate is
modified.

The Z axis traverse rate is not a regular PID control loop. The new
measurement of the spindle speed is fully adjusted for the change
as soon as it is measured. This is effectively a "P" only control loop.
Only the acceleration limit of the Z axis inhibits instant changes in
the Z axis motion.

Adding a PID speed control loop to the spindle drive may improve
things, but it also runs the risk of the two feedback loops interacting
in difficult to analyze ways. A strong servo with direct PID feedback
control will likely improve theading without causing interactions. Using
the regular speed control input to a VFD and software in Mach making
speed change requests will likely be too slow and complicate things.

The Z axis threading control loop is not ideal, but as stated, with a
strong enough spindle motor and sufficient spindle inertia, it does work
for "hobby grade" theading.

For machines run by a motion controller in Mach3 or Mach4 and
utilizing more than one pulse per revolution, better control loops
are possible. The nature of the loop will depend on the motion
control vendor because they can filter the speed feedback as
desired.

Don't know what the difference is in Mach4 threading as compared to Mach3.

This is how the threading works in Mach 3 and all info below is from Threading On The Lathe writeup done
some 9 years ago.

One complete thread cycle or pass is basically composed of the following:

Trigger – index pulse is seen and activate start of movement
Accelerate – move to an exact Z location relative to the turning spindle
Threading – move / control the tool such that the feedrate is correct relative to spindle rpm
Pullout – the tool is removed at the end of the thread
Retract – the tool is moved back to a starting point for repeat of the cycle

During the threading the rpm is monitored by the controller for variations and Mach plans on how to modify the next threading pass such that the Z axis movement will maintain the lead of the screw. Testing has shown that the lead is tightly controlled to a fine tolerance such that a near perfect thread can be produced if the lathe system is capable of it. Should the spindle slow down, Mach will change the Z movement to try maintaining the lead. Spindle slowdown in the range of 10 to 75% may be the range, but, as of this writing has not been tested. Past testing of past Mach versions on spindle slowdown is relative but not definitive for the new threading version.

3.9 MOTOR - GENERAL  SLOWDOWN / POWER / EFFECT ON TREAD’G
WW:
The rpm stability and power delivered to the spindle will affect how Mach plans the Z motion
for threading. Motor rpm does change and in threading it can have a dramatic effect during the threading cycle. The horsepower required for making a cut can be calculated, and actual cutting tests by the Society of Manufacturing Engineers have provided practical ways of calculating the power. General formulas for horsepower are helpful for comparisons, ie; stepper hp delivered verses spindle hp, but, calculations are not “exact” / subjective, and frankly is beyond the average users understanding or application of them.

The stepper motor needs to have adequate power to move the Z axis during threading. Thus,
during threading, the combination is a “chain’ so to speak, and the application of the power is only as good as the weakest link in the chain. Changing gearing / belt ratio’s for either motor
along with driver setup ( ie; voltage / amperage , etc ) can improve the operating range of  the “system”. The stepper must be able to accelerate / decelerate within the parameters the user
defines in the Gcode. Experience gained by just cutting a range of threads, using different cut depths, rpm, cutting methods, etc is highly suggested.

See Section 4, page 22 of 51, Figure 4.4 for the basis of lead tolerance. See page 8 of 5, 3.6.1 for an example of testing.

Just remember this:
Threading will only be as good as "YOUR" lathe "SYSTEM".  A "perfect" software controller in all aspects
won't help a piece of poo lathe produce good threads.

RICH

Is this an answer for me or for @joeaverage?

Hi Azalin,
sounds my like general advice rather than to you specifically.

Are you using the lathe screen set or the mill set?. I was going to suggest setting up a profile in Mill to see if the
index/DRO True Speed function works.

Craig

Hi Craig,

I already did. I also created a new lathe profile. I have uploaded the profile folder if you (or anyone else wants to help) want to take a look.

Hi Azalin,
I'll try it tonite after work . The code for the speed estimation and synchronisation is built into Mach, beyond what
we can see or manipulate. If nothing obvious ocurrs to us then you may be best placed opening a support ticket at
NFS. The only couple of times I have done so worked out well, took a few days but it did happen.

Craig

Thanks Craig.