Hi … I wanted to share that I’ve succeeded in building a 4th axis for my Bridgeport Series I Interact. I’ve been using it for about 6-months now without any problems – primarily as an indexer, but often in coordinated motion. I’m very satisfied with it, and it cost far less than buying true A-axis iron and mounting a Bridgeport compatible servo on it.
I started with a Phase II 8” rotary table and modified the worm-gear assembly to be shorter, have roller bearings (for pre-loading against backlash), precision sleeve bearings for better guiding the worm, precision turning of the worm shaft to mount in the sleeve bearings, threading the worm shaft for a pre-load nut, and for mounting the servo. I also installed some o-rings to better keep oil in the system and in the places where I want it – primarily on the worm and gear interface, and on/in the bearings.
My X, Y, and Z servos on my Series I Interact are SEM MT30R4-58’s driven via Bosch Z15-1A-240V amplifiers … so I used the same motor and amplifier (that way, in the event one of my other primary axes failed, I have a backup).
The trick was that the Heidenhain 151B controller requires a specific number of encoder counts per revolution – specifically it needs 18,000 or 36,000 counts per revolution of the rotary axis. The encoders on my Series I Interact are Heidenahin ERO 115 125 meaning that they’re 11µA sinusoidal encoders having 125 “lines” per revolution; and Phase II rotary tables have a 90:1 gear ratio. Thus 125 x 90 = 11250 … no good. The solution would be to find a 200-count per revolution 11µA sinusoidal encoder (I found these to be rare as hen’s teeth), get a direct-drive 18,000 or 36,000 count per revolution encoder (available but very, very pricey), or step up the drive ratio of the 125-count per revolution encoder by 8/5’s (125 x 90 x 8/5 = 18000). I did the latter …
Note that the Heidenhain 151B has a “signal evaluation” parameter (MP12 – 15), but that these parameters are binary (either 10 or 20). If Heidenhain had allowed for a user-settable signal evaluation (set this parameter to 10 x 8/5 = 16), I wouldn’t have had to make hardware to step up the drive ratio of the encoder …
I have an Omron IP67-rated snap-acting switch as the home switch. This is OK, but probably isn’t as accurate & repeatable as it needs to be – manifesting as a return-to-zero error (when the machine is shut down and then started back up) of as much as ±0.025°. Not much really, but results in an offset of 0.0017” at a 4” radius …
Regarding operation, hysteresis or its ability to go CW then CCW and return to a specified position is awesome! I measured zero or near zero backlash thus setting the backlash parameter in the Heidenhain controller for the IV-axis to 0.005° (MP39 for the Heidenhain 151B) and, over 6-months of (intermittent and non-exhaustive) usage, this value hasn’t appeared to grow or require adjustment. This manifests as ability to machine a deep through-pocket from two sides of object and having a near-imperceptible discontinuity where they meet.
I’ve also operated the 4th axis when machining with the Interact’s on-board flood coolant system – the system has had an opportunity to get wet (I use water-based lubricants), and I’ve been very happy that the oil has stayed golden – though I have taken care to not let the coolant directly impinge on the interface between rotary table and its stationary base. The step-up encoder mount has shaft seals and gaskets, so there hasn't been issue there (coupled with the fact that its far away from the cutting action).
Long story made short - I'm real happy, saved some money over buying real A-axis iron or a 3rd party indexer (though it still cost a good bit) ... and it's been a fun project.
Thanks!