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G&L 340T Power Draw Bolt / Spindle Adjustment

Way Out

Plastic
Joined
Feb 20, 2021
Hello Everyone,

I have a 1957 G&L 340T with a NMTB50 taper and power draw bar.

I have two issues I hope someone can speak to....

1. The draw bar is not retaining the tool holders as well as it should. How difficult is it to adjust the retention pressure?

2. The backlash between the spindle and the turn style is causing me an issue trying to lock the spindle in place. When I engage the spindle clamp at the rear of the headstock it does indeed lock the turn style nice and tight from movement. However the backlash is still present in the spindle and I can move the spindle in and out approx. 1/2" even with the spindle lock on. Can anyone speak to this relationship between the spindle lock and it actually preventing the spindle from moving?

I have the manuals and have started to dig into them. I cannot have this machine down for very long at the moment as I have quite a few jobs lined up for it. If anyone has worked on these 2 issues and can give me an idea of what I may be in for with regard to downtime I would greatly appreciate it.

Thanks,

AaronIMG_0134.jpg
 
Does this machine have the "impact" style drawbar? How are you determining that it's not retaining the toolholders properly? Do you have a drawbar tension tester of some sort?

If it is this style, they're basically sort of like an outsized electric impact wrench at the rear of the spindle. Possible causes of poor drawbar holding tension are the motor running too slow, bad threads on the drawbar insert, bad threads in the toolholders, and I can't recall but there might have been a spring under a nut with adjustable tension that affected the impact hammer. Pull it apart and have a look, or look in the parts manual.

The rear spindle lock should lock the spindle tight. If it's still moving that much you may have a bearing nut or retainer or something that has come loose. The spindle rides in a large carrier that slides back and forth inside the head of the machine - this is moved by a gear rack that the "turnstile" runs against. If the handles are locked up tight with the spindle lock tight, the lock is functioning as it should, so the problem is between the spindle and the carrier.
 
Does this machine have the "impact" style drawbar? How are you determining that it's not retaining the toolholders properly? Do you have a drawbar tension tester of some sort?

If it is this style, they're basically sort of like an outsized electric impact wrench at the rear of the spindle. Possible causes of poor drawbar holding tension are the motor running too slow, bad threads on the drawbar insert, bad threads in the toolholders, and I can't recall but there might have been a spring under a nut with adjustable tension that affected the impact hammer. Pull it apart and have a look, or look in the parts manual.

The rear spindle lock should lock the spindle tight. If it's still moving that much you may have a bearing nut or retainer or something that has come loose. The spindle rides in a large carrier that slides back and forth inside the head of the machine - this is moved by a gear rack that the "turnstile" runs against. If the handles are locked up tight with the spindle lock tight, the lock is functioning as it should, so the problem is between the spindle and the carrier.
Yes, I would consider it an impact style drawbar. I can physically pull with just my hands the toolholders away from the spindle. There is evidently a spring mechanism involved as that spring/clutch system does not allow me to fully remove the toolholder. I do not have a tester for tension but it seems way less than it should be.

The drawbar motor seems to run well and fast. As I mentioned it seems there is a clutch system between the drawbar drive at the rear of the headstock and the threads engaging the tool. I can continually actuate the drawbar to tighten and though it will continually run as if it is tightening, it never gets the tool any tighter.

As you say there must a be a spring adjustment in there. I hope its not so deep I have to fully tear it apart.

The spindle lock does lock the carrier tight. I pulled off some inspection plates and can see the carrier locked tight from the spindle lock. However, this does not resolve the issue with the spindle having "slop" within the carrier. I hope to see what the connection between the spindle carrier and the spindle itself actually is. I fear something has broken, but hope that maybe something just came loose.

Thanks for the reply.
 
Yes, I would consider it an impact style drawbar. I can physically pull with just my hands the toolholders away from the spindle. There is evidently a spring mechanism involved as that spring/clutch system does not allow me to fully remove the toolholder. I do not have a tester for tension but it seems way less than it should be.

The drawbar motor seems to run well and fast. As I mentioned it seems there is a clutch system between the drawbar drive at the rear of the headstock and the threads engaging the tool. I can continually actuate the drawbar to tighten and though it will continually run as if it is tightening, it never gets the tool any tighter.

As you say there must a be a spring adjustment in there. I hope its not so deep I have to fully tear it apart.

The spindle lock does lock the carrier tight. I pulled off some inspection plates and can see the carrier locked tight from the spindle lock. However, this does not resolve the issue with the spindle having "slop" within the carrier. I hope to see what the connection between the spindle carrier and the spindle itself actually is. I fear something has broken, but hope that maybe something just came loose.

Thanks for the reply.

The connection between the spindle and the rear carrier is the spindle bearings. As I recall, the rear carrier slides back and forth, and the spindle goes with it. It sounds maybe like you have a backed off locknut or something. I believe that locknut is at the rear of the spindle and can be accessed by removing the lower rear cover. I've never seen that happen, pretty unusual in my experience. Was the machine crashed? The upper rear cover houses the drawbar impact motor and hammer assembly I think, so the two areas you need to investigate are right next to each other.

Getting to the area of concern is a matter of an hour or two max. A time estimate to repair the issue without you determining what exactly is wrong first is impossible, I'm afraid. If it's just a matter of tightening the locknuts back down it won't take long at all, just be sure to set the bearing preload tight enough. That spec was in the manual as I recall, and was measured by hanging a weight at a set distance from the spindle centerline. You tighten the bearing adjustment until the weight is just held or barely moves, I forget. The rear bearing was usually the culprit if something loosened a little, in my experience. I never had to adjust the sleeve bearings up front, but they are adjustable as well. Hang the weight and see what you've got with the rear locknuts loose, then if that's good the rear nut is tightened to add just a smidge more. Should be a table in the manual about that.

Thinking on the drawbar a little further, I think the hammer mechanism is more like an over-riding clutch than anything. Once the drawbar torque gets to a certain level, the clutch dogs, which are cut at something like 45°, ride up over each other and it then slips past the dogs and re-engages afterward. This will repeat as long as you hold the switch down. Clack-clack-clack. I remember there being a spring in this setup/stack near the front but I don't think it was anywhere that should be giving you slack when the drawbar is tight, so something may have come loose there as well - or perhaps the slack in the spindle itself is removing your preload for the drawbar somehow, could be related. Or maybe the spring in the over-riding clutch isn't applying enough pressure to get you up to a good torque, something may have broken or come loose there as well.
 
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To provide an update. Attached are some pictures looking in from the back of the spindle.

In these photos I have removed the power draw bar motor and approx. 12" of casting to expose the spindle/carrier connection.

The problem is evident. The back of the spindle has a nut with double roller bearing that has backed off. Due to the fact that the spindle lock is actually on the larger carrier surrounding the 4" spindle, a backlash is created when this nut is not properly adjusted and tight.

The nut that backed off has 3 set screws (dog point screws) that are supposed to be positively secured by drilling through each screw hole into the spindle which results in the set screw positively securing into the spindle (in one of the pictures you can see the previous holes drilled into the back of the spindle from prior adjustments). A person drills these set screw locking holes once the spindle adjustment is finalized. The manual says to attach a bar to the front of the spindle and hang a certain poundage weight a precise distance on that bar from center. You are to tighten this nut with 3 set screws, preloading the internal roller bearings until the bar attached to the front of the spindle only rotates with the prescribed amount of weight hanging from that bar.

Seems like a simple enough fix. I have purchased all new bearings to replace every one I encountered to get to this point. Should be re-assembling the machine today.

Hope this helps someone.
 

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When this nut gets as loose as it did, the draw bar can fall back and not engage the back of the tool holder. Since I removed the drawbar the removable threaded tip seems fine. It is not stripped. The only thing I have not checked is if there is a keyed or pinned connection between the draw bar motor and gearing to the draw bar itself that has sheared. that is the only thing I can think of why it would not tighten very well. The drawbar in this machine is essentially one piece and so all of the torque from the draw bar motor would go to the tool holder.

Also, I am not getting that clack clack noise from the dogs slipping past one another. So I don't think I am getting the torque to that point. Must be poor connection between motor and the clutch dog.
 
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Yes the drawbar has a spring that holds it back until it starts to thread into the toolholders. This is so that you can engage the holder with the drive keys in the spindle before the thread is already pulling the holder in. After it starts to thread into the toolholders, the drawbar will pull itself forward against the spring pressure until it seats solid against a shoulder. The threaded tip section of the drawbar should be removable, as I recall it is keyed to the drawbar where it mounts and held in with a bolt. Check there but check the other end too.
 








 
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