Something wrong...

cdhknives

cdhknives

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Here's the setup. Everything has been running fine for an hour. I finished cutting a 3/4" rod of mild steel down to create a 3/8" stub about 1" long in prep for threading 3/8x16. I changed the QC gear box off .048 feed rate to set for 16 TPI. (Yes I've used that setting recently...just last week threaded another exact same setup 3/8x16) I set the bit, get ready for a scratch pass to start threading, flip the power switch on...

...and the chuck spins once or twice and stalls...belt squeals...I hit the power off ASAP! No pop, snap, bang, etc...it just went from fee spinning to belt squealing. I never had time to push the belt tension lever forward to lock it in...just the loose belt friction is normally enough to turn the chuck freely and full speed when not cutting.

Disengage the leadscrew and everything rotates fine.

With leadscrew engaged, it is dragging badly. The gear train on the left of the headstock seems loose and free (rocking the chuck back and forth, within the confines of gear slop they rotate fine). Drop both QC levers down (disengaged) is no help. I reversed the lead screw direction and it would turn (with high effort) as I turn the chuck by hand. Go back forward, and get the same...high effort to turn the chuck by hand with leadscrew engages, spins freely when disengaged. I tried several settings on both QC levers, little effect.

Anyone with a suggestion on where to start, or do I start pulling the leadscrew and QCGB right away?
 
It's been a while since I owned a QC Atlas, but if I remember correctly, once you drop the QC handles down the lead screw is completely disengaged from the drive.

See if you can turn the lead screw by hand with everything disengaged. If you can, then there is no problem with the apron or the tail bearing.

If you put the F/R lead screw lever in in the neutral position, and the chuck turns on then the problem must be in the gear box. Maybe a chip caught in a gear tooth? Bearing?

I would start by removing the F/R selector so you can get to the gear box main input gear.

Here is a link to a parts break down

http://bridgeport.askmisterscience.com/atlas.htm

Hope this helps.
 
Maybe a scuffed bearing that is dragging on the leadscrew? An overheated bearing will tighten to a shaft, once cooled problem is gone. Check lubrication. You can try a light machine oil to see if it will loosen up, but this is not a fix just a try by hand test. Likely will have to take apart to inspect.
Pierre
 
The leadscrew turns freely by hand with both QC handles dropped (in neutral).

The problem persists with the lathe cold by an hour+.

I just popped the seal on my gallon Mobil Spindle oil today and started using it in place of the old non-detergent 20wt oil I had laying around. That is what I use everywhere except the ways and dovetails...and the little oil pots on the QCGB are filled to overflowing every time I use it. Coincidence I think that I changed oil today.

The lathe did sit for years in a garage. I didn't tear it down, but I did flush out the gunk with several cans of WD40 and some light toothbrushing before I started using it. I probably have 50 hours of operating time since it was put back into service.

From looking it seems like it would be dang near impossible to suck up chips under that apron over the QCGB...but maybe...??? I was getting some 6-12" springs while turning this evening as I tried to get a clean light final cut or 2.
 
Inside the apron, the lead screw runs through a round collar that has a gear on it. I believe it's called the mitre gear. Inside that collar, there is a flat, inverted keyway that rides in the slot inside the lead screw. Make sure that inverted keyway is not completely worn out or possibly mangled with swarf.


Ray
 
OK, Lets go through the logic here

1 Put the F/R lead screw selector in neutral, the chuck turns freely

2 Disengage the feed selectors, the lead screw turns freely

Therefore the problem must be in the feed selector gearbox.

Of course, the questions is; what is the problem?

The good news is that the gear box comes off pretty easy.
 
Select the first handle but leave the second handle out and see whether or not the chuck still turns. If the chuck does not turn then the problem is on the input side of the box, otherwise if it does then the problem is in the leadscrew side of the box. The two sides of the box are split by the handles.
Pierre
 
I got my big inspection mirror out and started poking around, didn't see anything obvious. I tried the levers individually and everything was loose until I had both locked in. So I was watching the gears, looking for debris as I cranked the chuck over by hand and after about 2 turns it popped free and because loose again.

I never saw anything, but I'm betting some debris is in there. Time to get out the carb cleaner and start flushing the gearbox!
 
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