Hello Max,
The following picture is of a part machined on a client's Mori with Yasnac MX3, three axes machining centre that I set up with a Forth Axis controlled by a Galil Servo Controller. It was far from a true 4th axis, but it could do simple tasks such as the Helical Cam shown the pictured part and of course, indexing. The system achieved being able to what seemed to be true interpolation between the X and A axis via a feature of the Servo Card called Electronic Gearing. This works by having a Master and Slave encoder; the Slave keeping pace via the Gear Ratio specified.
In the case of this application, the X Axis of the Machine was the Master and the A Axis the Slave. The Encoder Count of the True X axis encoder of the machine could have been captured, but we opted to install a Stand Alone encoder at the opposite end of the X Axis Lead Screw as the Master. The MX3 didn't have User Macro, which could have been used to control Indexing and other features of the 4th Axis via DPRNT, so a unique X axis coordinate was used to signal certain tasks to be carried out by the 4th Axis.
View attachment 293348
Regards,
Bill