Need Help!- SEM X Axis Motor on Interact Mk2

[88incher]

Hello all,

Please bare with me, If I have left out any details please let me know.

I recently purchased a Bridgeport Interact 1 MK2,1988, s/n 660600788C with Heidenhain 151b control. The mechanics of the machine are superb, however the mill had a 'crispy' KTK spindle motor driver. I was rather pleased with myself as I have managed to repair the spindle driver myself.

After the driver had been repaired and refitted I thought 'great plug it in and watch it go' Hmm, life's never that simple.

To cut a long story short, the x axis drive motor (SEM Motor - type MT30R4-58 - Serial Number F04420) had a shot armature (Gone to ground and was blowing the drives supply fuse (MF 1 in the control cabinet) as soon as the 'drive arm' button was pressed) so I had our motor rebuilders fit an armature from another SEM motor that we had on the shelf that had a broken end cap (Same type numbers). The motor now is basically fine (i.e it spins up on a DC supply) and generates a good Tach voltage.

I should point out at this stage that all three axis drives are good as are the Y & Z motors and the X, Y & Z machine wiring. These have all been carefully checked by substitution.

The problem we now have is the "new" motor is still blowing MF1 fuse and making a terrible noise, arcing and juddering as soon as the 'drive arm' button is pressed.

My main question is this..... When the motor rebuilders stripped the motor the encoder would have been removed (and the disc taken off the shaft). If the disc had not been positioned back exactly as it should have been would there be any chance that the control is trying to 'park' the motor in an impossible position? Or can any one please shed any light on these symptoms?

Generally I like to try and sort these problems out myself (more of an interest thing than a cost saving one) but I must admit after a scary number of hours and a diminishing number of ideas It would be much appreciated if someone could spare a minute to answer with any ideas?

All the best, and thanks for your time.....

James

[gus]

if the motor is arcing as soon as the enable button is pushed[as opposed to the x axis trying to home] them it is not the motor, as there is no voltage on the motor at this point.

and no, the machine does not care where the encoder is, except for the end of travel settings

[88incher]

Hello Gus,

Many thanks for your prompt reply....

If it is not the x motor, that could you explain why the problem follows the motor? I.e when the suspect (x) motor is moved upto the z axis it does the same thing?

Conversely, when the good (z) motor is wired up to the x axis that will function as it should?

Any other thoughts are really gratefully received.

All the best,

James

[gus]

if it follows the motor it is the motor, but here should still not be any voltage on the motor until the homing sequence is started

wait it occurs to me that what if the tach wires are mixed the drive wires or somesuch....that would follow the motor

[Al_The_Man]

Bad things can happen to a through hole encoder if it is not replaced correctly, I had a very similar condition on a machine where the encoder had been refitted and the positioning tab had not been used to gap the disk properly, this allowed the disc to rub slightly on the reading head and cause a mark on the disk.
The motor was very erratic and would go into overload very quickly as it was obviously getting faulty feedback.
Remember the control is looking at the encoder, even when stationary.
Al.

[machintek]

If I remember correctly, when the drives are enabled, the control position monitoring if functioning as well as the drives ON and voltage (minimal to maintain position) is present at all motors. I would double check all internal motor connections, comparing resistances to a known good motor.
Those old encoders used a metal disc with perforations. if assembled correctly, there is no timing necessary only clearances maintained to the top and bottom of the light source and reader.
Reversing a tach would create a runaway condition. Swapping tach and armature wires is uncommon due to the difference in size of the wires.
Motor may be a do-over.

George

[88incher]

Ok,

Motor wires are not mixed.... I hate to be so cock sure but, the internal tach wires, wires to the brushes and also the internal thermal trip are all quite different sizes and also colour coded. The tach wires are very small and in shielded sleeve, the thermal trip has two brown wires (again quite small) and the armature wires are large red and black wires. I'm absolutely sure that the rebuilders have not wired the thing up wrong internally. Also the tach output is around 9.5v at around 1000 RPM.

The armature resistances are the same (near as dam it) across all three motors, and the motor will run quite happily on a separate DC supply no arcing no funny noises or smells!

George, yes.... I'm sure that this is the case. I was slightly confused at first as I metered the DC side of the drives (to the motor) and found no voltage when enabled, but if the meter is in place as the drives are enabled the motors all show a voltage for a fraction of a second (presumably until one hole in the encoder opens or closes?).

I take it a runaway condition would only result in the axis motor spinning up to full speed in an uncontrolled way?

Just a theory (and I know that you guys have already said that the encoder disc doesn't need to be timed up) but.....

If on drive enable the control does 'kick' the motors until it sees a hole / or lack of hole in the encoder and the disc was placed in such a way that in this 'desired' position two of the motor commutators were bridged by each brush, would that give the symptoms I'm describing?

As always, everyones help, comments and suggestions are very much welcome and appreciated.

James

[gus]

Unless your machine is radically different from all the earlier ones that I have owned, there is no purposeful voltage to the motors until you press the cycle start buttons to home the axis. you can turn any motor, and while it may cause a flashing error it will move quite freely. The control makes no attempt to maintain axis position until any axis is homed and that axis is then locked after it sees the trip dog and the reference pulse.

There can be some level of stray voltage present I am sure

[Al_The_Man]

Originally Posted by gus The control makes no attempt to maintain axis position until any axis is homed and that axis is then locked after it sees the trip dog and the reference pulse.

That is radically different from my experiences .

IF the controller has loop control from the outset, on many of the older velocity drives, there can be a slight drive error due to drift (balance), when the PID loop is tuned properly the controller attempts to hold it in position often imperceptible to the naked eye.
Then of course, any encoder error would be detected.
I would have expected a system such as Heidenhain to use this method?
Al.

[88incher]

Originally Posted by gus There can be some level of stray voltage present I am sure

Hmm, but not enough to fry an egg on! As already mentioned the motor will 'flash up' on any of the axis drives when the drives are enabled, but I know the drives are functioning properly as they will all happily home the good motors.

Out of interest, would incorrect drive tuning (as described in the Bridgeport manual) cause this problem that I'm experiencing?

James

[gus]

AFAIK there is no control loop present prior to homing, only after the second move off of the trip dog. If you set the parameter for homing speed to a rapid number, yeah it would crash.

Just checked ZERO volts on output when power enabled but not homed, there is around 6 volts AC that is probably just noise

There is however 120 volts ac to the thermal switch, telling me that that is the only possible source of spark prior to the control loop being closed or the drive outputting homing voltage

[88incher]

Evening all....

I get no DC voltage at the motor side after enabling the drives (while the motors are stationary) However as already explained all the drives show a voltage momentarily when the drives are enabled.

I can have all wires connected to the motor (Therm, Tach) apart from the armature wires and have no problem.... Drives enable and can home the other two axis, I can hook up one or the other armature connections and again no problems, but as soon as both are hooked on, big noise rapid twitching back and forth and blown MF1.

I genuinely don't think that there is a motor wiring issue, the drives wouldn't enable if the therm loop wasn't intact.