[jmccracken]

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!

[awemawson]

Excellent Jim,

I have a pair of Bosch servo cards from an Interact, a SEM servo motor and even the choke gizzmo used to provide the power to the servo drive. Not sure whether to interface them to a rotary table or use a 90 degree worm drive that I have. It's been a 'project in a box' for the last 6 years - I moved house and my Interact got trashed in the move , but I have a TNC355 driven Beaver Partsmaster that replaced it waiting for that 4th axis, so very interested in the minutiae of your set up as it will save me hours / days of head scratching to learn from your experiences. The TNC355 is very similar to the 151 interface and program wise.

Andrew Mawson

[email protected]

[RMARCH]

Awesome Jim !!

I would love to build one but i just don't have the time !!

[jmccracken]

Hi Andrew ... trying to send you a private message, it says your account won't accept PM's ...

[awemawson]

Jim, it's very odd, I've just been through all the settin gs for my account and they seem correct - very puzzling !

Can you please contact me on andrew <at > mawson <dot> co <dot> uk ([email protected])

Many thanks

Andrew

[jonesturf]

Awesome thread. I have been looking for info on the exact thing for my upcoming BP boss.

Does the RT have a through hole and if so any idea what the diameter is? I'm looking for a way to engrave caliber markings on finished rifle barrels and ideally would like at least 1.25" through the chuck since most barrels are 24" long. I know a super spacer has more than enough room but a RT is quite a bit cheaper. Not a total killer because I would definitely use it for other things. Plus I have a J Head retrofit with a 9x42 table I could always use but moving things back and forth would suck.

[awemawson]

John McCracken I salute you !!!!

Posting your success encouraged me to resurrect a project I started many years ago, and I'm glad to say as of last night I now also have a functioning A axis. I stole your idea of belt reduction of the encoder to get the right count, but I turned my encoder 180 degrees to shorten the whole assembly as my Beaver Partsmaster is fully enclosed and it would have fouled the doors otherwise. I just need to tidy up the wiring and fold up a cover to protect the motor & encoder from swarf and it'll be ready to use.

So again, a big thank you for your inspiration.

Andrew Mawson

[jmccracken]

Great Andrew! I'm delighted. Post some pictures sometime ... I'd like to see it.

[awemawson]

I will post some pictures when it's up on the machine - at the moment it's on one of those pump up trolley things for testing as it's a very heavy lump !

I may have been slightly premature in my excitement - I think I've screwed up my Bosch servo tuning, at the moment if I go too fast I get gross positioning errors, also if I test the drive using an external source max motor rpm at 9V servo drive is 1000 rpm, but I'm sure I selected resistors to set it to 3000. So I've made an error somewhere and will have to re-work it over the next few days. One thing that is working superbly is the pneumatic brake. The plc in my TNC355 is set to apply it unless an A axis move is called, whereupon it releases it (like the crack of a gun ) and re-applies it at the end of the move. I was searching for the relevant M codes, but they are not needed.

[jmccracken]

I noticed the same phenomenon, specifically that the X, Y, & Z axis motors are enabled all the time, but the A-axis motor only enables when a move is called ... thus using some sort of brake (pneumatic or solenoid) is easily doable as one could use the opposite of the enable bit. I wonder if that's what they intended ...

Further, I too have an issue if the A-axis tries to go too fast. I found that I cannot do a G00 move as it results in a gross-positioning-error, rather the maximum stable speed that my A-axis will traverse is about 2160-deg/min. Thus for any "rapid" move on the A-axis, I do a G01 Axxx F1800, where 1800-deg/min is a little slower than the maximum safe speed that I've found (through trial and error). I find its OK, and didn't require that I learn how to tune the A-axis servo for optimum performance ... I decided the reward just didn't justify the investment in time, effort, and risk.

[awemawson]

John, that's very interesting.

I did find that there was a slight fault with the first servo drive card I installed, one of the thyristors wasn't always turning off when it should giving an uneven drive. Fortunately I had a second to install. But I'm not out of the woods

I cannot get the drive to increase the current through the motor. R30 is supposed to set it on the little plug in personality board, but it seems to have zilch effect. I'm monitoring motor current by having a 0.1 ohm resistor in series with the grounded leg of the motor, and watching the voltage developed across it on an oscilloscope. As the motor runs in a steady state you see half sine waves (well not true sine waves as the start point is not at zero time wise), either +ve of -ve depending on direction. I assume that the firing point is the way current is varied, and instantaneous current at a point in time can easily be worked out from 'scope peak volts divided by the resistor value of 0.1 ohms. At the peak I'm getting 2.7 amps, so the rms value will be much less (actually presumably the 'area enclosed by the curve')

To get any reasonable speed without gross positioning error C's I've tweaking the acceleration machine parameter right down to a silly low value, and it shows as it starts slowly and decelerates slowly. Max speed I can get at the moment is 1024 degrees / min.

On the TNC 355 there is a parameter to set the 'rapid travel' speed per axis - if the TNC151 has the same you can perhaps regain the use of G01

My Nikken 4th axis came equipped with a pneumatic cylinder acting on a brake shoe (which was broken but I've re-made). As the axis is de-activated the PLC puts out a '4th axis brake' signal which I use to drive a solenoid valve that defaults to 'brake off' but engages with a crack like a pistol shot when movement is transferred from the A axis to one of the others.

I need to find someone who is conversant in setting up the Bosch servo card - I have a write up but some of it is defeating me at the moment

Meanwhile a couple of pictures:

[jmccracken]

Nice looking install!

Regarding the Bosch drives, I have the following document that I got from Bosch-Rexroth in their office somewhere in North Carolina. There was a gentlemen there I spoke to who had been with Bosch for 30-years or so and was able to dig this document up for me. This may or may not help you ...

The TNC151 also has a rapid speed; however, this parameter relates to a k-factor (or some such) that, when monkeying with rapid speed, makes the whole thing unstable. Bottom line - for the TNC151, rapid speed is not an independent variable as one would expect ... I have learned to live with not doing a G00 move.

[awemawson]

John,

Thanks for that. I do have a copy of that exact manual even down to the same persons handwriting on the front cover

It's been suggested that maybe my problem is due to the phasing of L1, L2 and U & V - the manual (page 12) calls for L1 & U to be in phase, which they are on my machine, and also for L2 & V to be in phase. now L2 is grounded so how the heck do you check its phase, however U&V are definitely anti phase.

I assume that you have just added your 4th servo board to the same transformer set up originally driving the existing three axis? In which case yours must be correct, however as mines not on an Interact, I've had a transformer wound whose secondary replicates the 180-0-180-380 of the Interact one and I'm pretty certain it's right, but it's one variable I can't be certain of.

Do you have a 'scope so we could compare results?

I put the big lump of a chuck on to see if it affected the acceleration negatively - no change perceptable

Andrew

[awemawson]

Well I'm delighted to say this is all now fixed and wrapped up. Turned out to be a tuning issue on the Bosch servo drive card. I'm now getting 2500 motor rpm corresponding to 2500 degrees per minute on the rotary axis. I had the good fortune to meet a fellow who used to install these drives, and after four hours of head scratching and tweaking we got the good result.

Not had the opportunity to use it much as we've now started lambing, but it works. Did one little test piece - mounted a 1" mild steel bar and did a square and a triangle, so not very demanding but it came out well.