Hi,
threading requires a stable spindle speed, the more stable the more accurate the pitch of the result.
The key word here is 'stable', is does not have to be accurate. For instance you might dial up 500 rpm
or so, but whatever non-linearaities results in a speed of 400rpm. Mach will read the actual speed, whatever it is
and then calculate the Z axis progression rate to make the thread. It does not matter to Mach particularly that
the speed is 500 OR 400, so long as it stays the same throughout the operation.
In truth Mach does try to recalculate the Z axis progression on the fly in order to get the right pitch despite spindle
speed variation, however the best result is till had when the spindle speed is constant.
You may be interested to know that Warp9's ESS has a fairly new (six months or so) feature of PID spindle control.
The spindle is fitted with an encoder of some kind, including an index signal, and the PWM output of the ESS is
adjusted under PID control to vary the analogue output with a view to control spindle speed in closed loop fashion.
This is a boon if your spindle is marginally powered or has very little inertia. It must be said that the closed loop
bandwidth is not high, certainly not as high as a position loop servo, but sufficent for an improved spindle speed control
for threading.
If you cannot control the spindle speed in a program or an MDI then threading is a joke. You must solve this lack of
spindle speed control. A PWM to analogue circuit can be made with a resistor and a capacitor (5 cents each) at the simplest,
and with an op amp and a handful of resistors an capacitors (less than $5.00) if you want something a bit flash.
Do you have or have access to an oscilloscope? If you do probe the PWM output of the ESS and see whats happening.
I'm suspicious that the BoB is not set up quite correctly.....everyone else has got PWM to work....why not you?
Craig