Bridgeport Series 1 CNC problems

"Whirrs" as in screams while stalled with the shaft not actually turning? Could be any number of issues from mechanical to drive problems to drive power supply to axis acceleration settings.

Low drive voltage will reduce acceleration capability, potentially causing stalls.

Mechanical drag will increase the problem.

Can you tell if stepper is rotating?

Wes

Whilst the one thing not checked during my visit today was X axis belt tension, from the sound and action I believe that the stepper motor stalls in rapid.

Richard

I have checked a hell of a lot of the settings but cannot find any reference anywhere to acceleration settings. Too high a value is one of my concerns.

Drive voltage and current are to spec as I see it.

Totally agree. Now we have the machine "running" I have asked that a program be entered to cycle the table back and forth whilst the way oil is forced into the system. I almost feel like removing the table to check the condition of the lube system. Especially to the nut.

Richard

OK, the way the BOSS motor drive worked is it uses a saturable reactor in the low-voltage AC input to each axis to modulate the supply voltage to the drive. When not moving, the reactor is allowed to impose inductance in the circuit, cutting the applied voltage down to about 8 V, I think. When the motor is commanded to move above a crawl, a transistor is turned on, sending current through the control winding of the saturable reactor, driving it into saturation. With the inductor effectively out of the supply circuit, I think the drive now gets about 60 V DC.

Other than a driver transistor popping, a common falure is the reactor control transistor shorts, applying full power to the drive all the time, either blowing a fuse or smoking the whole drive.

It sounds like in this case the reactor control transistor had failed open, or is no longer connected in some way, so the reactor can never be "turned off". That would run the motor on the low voltage setting all the time, not allowing it to move fast.

Jon

So which parameters are not to spec, and how far off are they?

Ball nuts don't require a lot of lubrication since they are recirculating ball bearings with minimal friction. Of course if it's been crashed and damaged, that's another matter entirely. Can you turn them smoothly by hand with the stepper disconnected?

Also, if the problem is just the X axis, try swapping stepper connections between the X and the Y and see whether the issue follows the power supply and drive, or the stepper and axis.

Surely you've got to have good lubrication or all bets are off, ballscrews or no.

Because ball screws have rolling elements instead of sliding elements they require little lube. In fact, too much oil in the ball nut can cause the balls to skid instead of roll, and this skidding will eventually damage the ball screw and/or nut. ERS

Correct that is close to what I measured when not running the axis.

Again I saw this so correct on this machine.

Unfortunately Jon it is not that simple as I discussed above.

I spent 3 or 4 hours with the machine (I have never seen one before). At the end of that at least we had the axis moving upto 40ipm. However "Rapid" always failed, 120ipm according to the manual.

I sort of homed in on the "RCK" card as I did not see the numbers quoted in the manual.(Section 6.5.2) Typically TP3 should be 8.0v it was over 9v tweaking the appropriate pot got it to about 8.3v min The square wave at TP5 should be 4Khz my scope indicated a much lower frequency. (A ?ms period instead of a 250usec period. I do not use the scope much (portable handheld) and did not have the manual with me, so did not play with P10. From memory the frequency was more like 400Hz

All system voltages read OK with a tad of tweaking on 5v, 12v, -12v to get them perfect

The fact that the frequency appears low makes no sense as according to the manual it should be 4khz on rapid and 2khz at 30 ipm. I am confused as this does not relate to 120ipm rapid and 30ipm at

Not sure what to do next

Richard

Comments interspersed

Rather than repeat suggest you see my answers to Jon

Not enough time on the visit to check that out

Good option Pete. Actually I did not see rapid checked out on Y or Z. Probably best to see that first. If failures then maybe RCK card problem. If no failure then worth swapping motor outputs. Richard

Well ... that depends on *which* BOSS. BOSS-3 through BOSS-6 were as you describe below. BOSS-8 and later (IIRC) use DC servo motors, not the big steppers used by the earlier machines.

BOSS-1 and BOSS-2 apparently never escaped the factory, and I don't know where BOSS-7 falls in the design spectrum.

That sounds about right.

And a common cause of the stepper driver transistors failing in these machines in a home shop is imbalance in the voltage output from a rotary converter. The transistors are being run at pretty close to the maximum voltage, and an imbalance will cause one or more of the three axes to be running close enough so the inductive kick can zap the transistors.

If you have to run the thing from a rotary converter, I would suggest that you use two converters -- one for the spindle motor, and the other for the drive electronics. This lets you balance the spindle motor for maximum torque, and the drive electronics to keep the phase voltages from going too high.

That makes sense. And the CPU ramps the speed up assuming that it is also controlling the voltage, so when it gets to a certain speed it can't ramp any higher and just sits there and buzzes.

Good Luck, DoN.

WHOA! I missed this earlier, even though I read it.

The BP BOSS machines I'm familiar with don't HAVE steppers. They have DC motors with shaft encoders. If the "stepper whirrs away until the end of the expected distance", something mechanical has come loose. Try the coupling between motor and lead screw.

LLoyd

"DoN. Nichols" fired this volley in news: snipped-for-privacy@Katana.d-and-d.com:

'Xac'ly, Don. There are lots of BOSS-8 and BOSS-9 machines around, but they're the only one's I'm experienced with (I've got some 'rehab time' on a 9)

He never did say what vintage it was.

LLoyd

It's not a coupling, stepper motors "whirr" or more accurately "scream" when stalled.

He didn't say *which* BOSS. BOSS-3 through BOSS-6 (at least) have steppers. BOSS-8 on have DC servo motors. I don't know what the BOSS-7 has, if it even exists. BOSS-1 and BOSS-2 don't appear to exist at all (except perhaps as early prototypes in the factory).

The BOSS-3 through BOSS-6 have the DEC LSI-11 quad wide CPU module. I don't know what the later ones have, and I would be interested in hearing from you what it does have. Maybe one of the later double-wide LSI-11 CPUs.

And given what he has been measuring (elsewhere in this thread), I'm pretty sure that he has one of the stepper motor versions.

Enjoy, DoN.

I only know that my BOSS-3 (serial number 103 or 108 or something close, and they started with 100) was made in 1975. Where that puts later machines I don't know -- other than later than 1975. :-)

Enjoy, DoN.

The machine is stepper with saturable reactors as discussed by Jon.

I believe it is a 4 or 5 the manual is December 1980 and covers

So what do we think based on my results so far? I will see the machine again Monday am. Hopefully we will run a program to excercise the machine a bit and get more lube around. What tests or monitoring should I try next?

Thanks for all your comments guys, no definite answers to the problem but I get that cosy feeling that I am not on my own

I did smile at the storage capacity of the system - 80 feet

Richard

Now I'm getting nostalgic for my youth.

Minimum-wage grad student in charge of $millions worth of computers.

He didn't say which BOSS, but his first post indicated "steppers", so we can presume it's an earlier BOSS if we presume he knows the difference between steppers and servos.