Spindle speed fedback?

[Katoh]

Colleagues
I am slowly acquiring parts for a lathe conversion, and the subject had arisen of spindle speed feedback.
Please excuse my ignorance here but is that a necessity?
I can see how it would be beneficial on threading, and if needed, can someone recommend what to look for and were to purchase, as I really have no idea and have never needed anything like that in the past.
Lastly the lathe is belt driven.
Cheers
Katoh

[Hood]

Normally your programme in feed per rev on a lathe, If you dont have the Use Feedback In Sync mode chosen Mach can still be programmed in feed per rev but will not go on the true spindle speed but rather the programmed, that could be a bit dodgy especially if you have a manual spindle and dont set the RPM right.

I now just use the encoder on my spindle motors as I use the SmoothStepper and it can read the very short index pulse, if however you are using the parallel port then you need a longer pulse.
Previously I used a slotted disc and opto to good effect. The Opto I used was an OpTek OPB916B or OPB917B. The only other thing you will need with that opto is a 220 ohm resistor to limit the current to the optos LED.

Hood

[Katoh]

Hi Hood
I will be using just a standard Break out Board. The motor is controlled with VFD  and speed through mach.
I'm just trying to get my head around this, with the Optek that will send the speed back to mack and will mach compensate for that?
IE: so I need to put spindle speed back into mack for things to work correctly?
Where would I find information or setup diagrams on how to make these work on our machines?
sorry If sound Silly, I am going into the unknown.

[Hood]

Yes ideally you need spindle feedback if commanding in feed per rev also you definitely have to if wanting to thread. As threading is a dream with CNC you would be mad not to.

As far as connecting to Mach, just a single input from the Opto to your BOB (well also a 5v and 0v supplied from BOB) and then tell Mach which port and pin the Index signal is connected to and thats it, piece of cake
Hood

[Katoh]

Many thanks Hood!
I will get organised as well.
Cheers
Katoh

[BluePinnacle]

This looks like the beginnings of turning the spindle into effectively a big servo. Doing this with my mill spindle would enable rigid tapping ... I like that idea. Distant dream, or not?

[Hood]

This looks like the beginnings of turning the spindle into effectively a big servo. Doing this with my mill spindle would enable rigid tapping ... I like that idea. Distant dream, or not?

If using the parallel port then its likely just going to be a dream.
Hood

[BluePinnacle]

Shame

Hope you're feeling better by the way.

[djc]

This looks like the beginnings of turning the spindle into effectively a big servo. Doing this with my mill spindle would enable rigid tapping

The way to work around this with Mach is to have a true servo motor physically turning the spindle. Most servos have three control modes: velocity, position and torque. You use it in velocity mode as a spindle and then have to change it to position mode and drive it as a C axis for your tapping. Even doing this, I would be happier to semi-rigid tap with a flating holder than go full-solid. Dare I say EMC?

[Katoh]

I maybe way off the mark here but is it not what your describing just basically spindle speed feedback? using a servo to drive a spindle will give you what advantage?
So wont the Optical sensor giving feedback back to mach control all other motors, eg feed speed, to compensate while threading, if you get slip or loose rpm.