Is there a way to cut THREAD without closed loop feedback?

[cjchands]

Is there a way to cut THREAD without closed loop feedback?

I am in process of connecting MACH 3 with Inverter drive to control spindle motor, but I just realized that the spindle drive has no FEEDBACK!!!!

If my understanding is correct, you cannot cut thread without ENCODER or some sort of feedback to the spindle drive??

Or perhaps, you can cut it, but the thread won't come out precised?

Anyone has any suggestion of what should I do???
Greatly appreciated any suggestion.

--building the first CNC lathe --
Jeff

[HuFlungDung]

Tap and die will do it

The encoder is most commonly used, or more exactly the index marker on the encoder is what is used. This is nothing more or less than a switch that gets 'made' once per revolution of the spindle. Now an encoder is a neat way to do this, because it doesn't have bouncy contacts, and has a very short pulse duration, making a clean signal.

The rest of what happens are simple repetitive mechanical sequences. If motion of the Z axis motor begins with always the same delay from whenever the pulse was detected, then the carriage should track along the same path as before.

Analog circuits are good for detecting change of state. Digital detection of the encoder pulse can be more difficult because of sampling rate. An encoder running at 500 rpm, with a pulse width of only .001 revolution, requires a very high sampling rate to catch the pulse going past the detector. From my own testing with a Galil controller, I found that although the pulse could be detected with a high speed latch, there were other things going on within the controller that produced a variable length of time between detection and initiation of motor motion. This spoils the tracking of the tool and can easly spoil the thread at any time a next cut is attempted.

Lathe type threading does not require feedback because it is not 'rigid tapping'. The spindle never stops, so the Z axis cannot be electronically geared to mesh with the rotation of the spindle encoder because the sync will be lost whenever the Z must stop and reverse. A lathe spindle has far too much momentum to stop on a dime and reverse instantly unless it is a very powerful servo itself.

[Al_The_Man]

Although, I think you will find that the commercial systems like Fanuc and Mits, do use the spindle marker to sync the start point, after which the Z is geared to the spindle encoder pulses, this allows for slight spindle speed variation if electronic gearing is used.
These machines usually have the canned cycles for variable pitch and taper threading as well as rigid tapping that uses this, (G84).
Al.

[HuFlungDung]

I stand corrected

I don't know about the newest Haas, but on my older one, the spindle gets a bit of an impulse, so its rotating relatively slowly when the encoder pulse is detected, and the electronic gearing takes place. Then it accelerates to speed and does the rigid tapping. I wonder if that was their solution for obtaining sync more easily? The older Haas had an open loop spindle drive.

[Al_The_Man]

I have been working on getting synchronized threading Using Galil's electronic gearing, but got side tracked because of paying projects!
Hopefully this next year will see it in place, I have used the principle with a flying shear, so I am hoping it will just be a case of fine tuning.
Al.