[sinha_nsit]

For a VMC to have rigid-tapping and helical-interpolation capability, is it necessary to have a servo motor for the spindle, or any other type such as AC induction motor would also work.

[andy55]

For a VMC to have rigid-tapping and helical-interpolation capability, is it necessary to have a servo motor for the spindle, or any other type such as AC induction motor would also work.

if you drive an AC induction motor with a vector type VFD which is able to keep torque up even at low rpm, and you put an encoder on the spindle, then rigid tapping is possible.

[sinha_nsit]

I guess, helical interpolation also should be possible.

What does Fanuc supply with its controls: servo or induction-type or both (is it MTB choice?)

[Al_The_Man]

Helical interpolation does not need an encoder as it is circular interpolation of 2 axis with the 3 axis, typically Z, at a set feed rate, the circular interpolation axis can be changed with the plane selection, X-Y, Z-X, or Y-Z.
Fanuc spindles are typically Induction motors.
Al.

[sinha_nsit]

I see.
The spindle motor has nothing to do with helical interpolation (I had thread milling in my mind, which caused this confusion. The synchronization for a constant pitch is ensured by axes motors, not spindle motor!).

Is the VFD needed with spindle induction motor supplied by Fanuc as a complete package, or does the MTB procure it separately, not necessarily from Fanuc?
Is induction motor accurate enough for rigid tapping?

[andy55]

Is induction motor accurate enough for rigid tapping?

The way rigid tapping is done is the Z-axis is "slaved" to the spindle encoder with a certain ratio (to match the feed/rev of the tap).
So it does not matter very much if the spindle has a certain acceleration time up to the tapping rpm, then some deceleration time as the tap comes to a stop at the bottom of the hole, and then acceleration in reverse as the tap comes out of the hole.
The Z-axis just has to be able to follow so that it is in sync with the spindle encoder at all times. I guess if you have a really capable super-spindle you will have to slow down it's accel/deccel phases, and limit max rpm, so that the Z-axis can follow and doesn't become "jumpy".

[sinha_nsit]

Thanks for useful information.
So, while purchasing a machine we do not have to worry about whether the spindle has induction motor or servo motor.

[Al_The_Man]

Very few machines are fitted with servo spindle, DC motors were often used with tach feed back to the drive and now AC induction style, both were/are fairly tightly controlled however, as most high end spindles now have a pulse wheel or encoder feedback from motor to drive, this is apart from any spindle shaft encoder required for rigid tapping etc.
Al.

[sinha_nsit]

I asked the same question from an MTB, and this is what they said:

We had already tried using AC induction motor and VFD drive.
For any threading by cutting in a lathe or tapping in alathe or mill, the spindle speed and axeis travel need be coordinated to achieve the correct pitch movement per spindle rotation.The coordination can be achieved by providing spindle encoder.But we find the vfd drive is not able to sensitively and precisely control the rpm with ref.to axis feed..Besides the spindle positioning is also important every time the tap to start thelead ,to stop precisel and return.Otherwise the threads of the job gets spoiled.
Hence we founf A servo motor directly coupled is able to give good rigid tapping.