Atlas 618

When you release the pin, M6-256 on my lathe, you should be able to rotate the bull gear, M62-241X separate from the pulley, 2140-10. The pin has two detents for engaged and free. There is a ball and spring followed by a set screw which rides in the detents. If the set screw is turned too far in, you won't be able to move the pin to the free position. With the pin in the engaged position and the back gears engaged it will lock the spindle.

It is possible that the previous owner permanently pinned the bull gear to the pulley. The pin will work against the Zamak pulley and wear tyhe pulley to the point that you can no longer lock the pulley to the gear. Also using the pin and the back gear to lock the spindle to change the chick can shear the Zamak if too much force is applied. A solution, albeit a poor one, would be to pin the two together. This would permit direct drive but prevent not driving with the back gears.

If you can rotate the spindle and the gears in the gear train are properly engaged, you will be able to turn the lead screw. If the carriage is not moving with the lead screw turning and the half nuts engaged, the problem is the half nuts. Looking at your photos, the lead screw looks OK but the half nuts need cleaning. You can remove the half nut assembly by unscrewing the two screws on either side of the half nut lever. Clean the half nuts with a fine wire brush or a toothbrush and some WD-40. You can clean the lead screw in the same way.
 
mattthemuppet2 said:
with all due respect, whether or not the back gear is engage or not has no effect on leadscrew rotation. The lead screw is driven off the end of the spindle via a forward/ reverse idler. All the back gear does is to reduce spindle speed and increase torque, by driving the spindle via the back gear and bull gear.

Before you do anything to the leadscrew, check this out first:

1. with back gear engaged, does the spindle turn by hand?
2. If no, is the bull gear pin (hard to see, it's in the large gear next to the spindle pulley) engaged?
3. If yes, pull it out and go back to #1.
4. If no, check that the screw in the middle of the pulley isn't screwed all the way into the spindle (it's an oil screw)
5. If yes, unscrew it and go back to #1.
6. If no, sounds like your spindle pulley is seized to the spindle and you need to dissassemble the head stock.

I've had these lathes apart numerous times and have owned a 618 for 4 or 5 years. There is no way that the half nuts will engage and drive the carriage in direct drive but not in back gear.
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I will try to do better on my answers today. Yes, I can turn the spindle by hand with the back gear engaged. I can also turn the spindle by hand with the back gear disengaged as well. The oil screw is not tightened, but just covers the hole. Yesterday I was basically just putting my hand on the chuck and felt the "tension" and did not try any real pressure to move it. It works fine.
I still have the same problem. What I did find this morning was the front lead screw bracket screws were loose allowing the gear on the lead screw shaft to disengage. This would cause a slight noise. I tightened both screws and the noise is now gone. When I engage the back gear the thread dial still does not move and the carriage does not move when I engage the half nut lever.
I went back to the shed for another look. By putting "some pressure" on the half nut lever I could get the carriage to travel some in both directions. The dial still did not move. I am still inclined to believe the lead screw is worn.
 
RJSakowski said:
When you release the pin, M6-256 on my lathe, you should be able to rotate the bull gear, M62-241X separate from the pulley, 2140-10. The pin has two detents for engaged and free. There is a ball and spring followed by a set screw which rides in the detents. If the set screw is turned too far in, you won't be able to move the pin to the free position. With the pin in the engaged position and the back gears engaged it will lock the spindle.

It is possible that the previous owner permanently pinned the bull gear to the pulley. The pin will work against the Zamak pulley and wear tyhe pulley to the point that you can no longer lock the pulley to the gear. Also using the pin and the back gear to lock the spindle to change the chick can shear the Zamak if too much force is applied. A solution, albeit a poor one, would be to pin the two together. This would permit direct drive but prevent not driving with the back gears.

If you can rotate the spindle and the gears in the gear train are properly engaged, you will be able to turn the lead screw. If the carriage is not moving with the lead screw turning and the half nuts engaged, the problem is the half nuts. Looking at your photos, the lead screw looks OK but the half nuts need cleaning. You can remove the half nut assembly by unscrewing the two screws on either side of the half nut lever. Clean the half nuts with a fine wire brush or a toothbrush and some WD-40. You can clean the lead screw in the same way.
Click to expand...
Everything works as it should except while in back gear. I cleaned and oiled the half nuts and they don't appear to be as worn as is the lead screw. Read my response above. Thanks
 
It's not clear to me. Does the lead screw turn, with the back gears engaged and lathe running/spindle spinning?
 
So if I get this right, the lead screw turns, both in back gear and direct drive?

And the half nuts engage and drive the carriage, but only in direct drive and not in back gear?
 
hmm, sounds like your half nuts are popping open, maybe due to the lower speed in back gear. I think you mentioned that you can stop the problem by holding the half nut lever down, right? That would support that hypothesis. Couple of possibilities. One the ball detent that keeps the half nut lever in position isn't doing it's job (missing, stuck in its bore, ?) or two, the half nuts are so worn that they ride up and out of the leadscrew thread.

I very much doubt that your leadscrew is too worn, looks fine to me in the pics. Even if it was worn, the main effect would be thread pitch inaccuracy over long threads, not the half nuts popping out. The thread dial is not relevant here. It's a bugger to keep it engaged when tightening the bolt as doing so rotates the whole thing and the gear at the end out of engagement with the leadscrew.
 
chippermat said:
So if I get this right, the lead screw turns, both in back gear and direct drive?

And the half nuts engage and drive the carriage, but only in direct drive and not in back gear?
Click to expand...
Yes, except I did get the carriage to move by putting some pressure on the half nut lever while in back gear. I just said the heck with this and bought a lead screw from ebay for a really good price, if it is as described $29.00 shipped, and a NOS half nut set for $40.00 shipped. The wait is on. I plan to change the lead screw first and see if that works, since I am hard headed and think that is the main problem.
 
OK, you'll get'er figured out one way or another. Best of luck, it's all fun :encourage:
 
I had something similar and made a couple of small shims that went between the half nuts and the carriage. It seems like there was too much play in the half nuts when they were engaged. The shims tighter it up and solved my problem.
 
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