Jet GHB 1340 thread\ half nut issues - beginner

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cattleman99
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Joined: Sat May 11, 2019 8:24 pm

Jet GHB 1340 thread\ half nut issues - beginner

Post by cattleman99 » Sat May 11, 2019 8:29 pm

Hello,

I am new to this forum, and machining in general. I am 20 years old and a full time college student. I recently purchased a Jet GHB 1340 lathe to do some gunsmithing on, and everything is going fine except one thing.

The threading system, be it half nuts or feed rod, it seems to have a binding issue, a pinch point if you will. It will turn smoothly for the majority of the rotation, but there is a point at which is slows down. I can both see it and feel it slow down. Ive yet to try to thread anything as I am awaiting shipment of my tooling, but do you happen to know what may be causing this?

Everything aside from that functions fine, smooth and precise.

pete
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Re: Jet GHB 1340 thread\ half nut issues - beginner

Post by pete » Sat May 11, 2019 9:56 pm

It's difficult to nail down a machine issue through these forums. Plus there's not quite enough information. But my best guess might be that both the feed rod and lead screw got bent when you moved it? You haven't had a serious crash on it have you? Running it as is will most definitely further damage other parts until you can get the problem figured out. It's somewhat possible the machine might have an internal gear box issue. Is the machine new or used?

cattleman99
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Re: Jet GHB 1340 thread\ half nut issues - beginner

Post by cattleman99 » Sat May 11, 2019 10:13 pm

It did tip over during transport into my truck rail, and it is possible the lead screw got bent. Machine is used. I can further dig into the threading gear box tomorrow and check it out.

pete
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Re: Jet GHB 1340 thread\ half nut issues - beginner

Post by pete » Sat May 11, 2019 10:39 pm

I don't know that particular machine at all, but if it's like almost all lathes there should be shear pins on both the lead screw and feed rod just outside the end where they enter the gear box. First move the carriage as far to the tail stocks end as you can. Support each rod in turn at the bottom with the correct sized piece of wood then "gently" drive the shear pins out. That will disconnect them both from the gear train in the gear box. Then try rotating each of them by hand. If either or both are bent enough to start to bind in use then seeing that should be easy enough. With a used machine there's no telling how it was used or even crashed by the previous owner. It might be worth pulling the gear box cover and spin those shafts over to check just how smoothly they rotate. Best hunch at this point is it's just the lead and feed rod that got damaged. One other possible cause is the pinion shaft that rotates the pinion that gives you the longitudinal hand feed on the rack got bent. But you should be able to tell that just by moving the carriage back and forth with the hand wheel. If that runs smooth then it's likely the previous problem.

cattleman99
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Joined: Sat May 11, 2019 8:24 pm

Re: Jet GHB 1340 thread\ half nut issues - beginner

Post by cattleman99 » Sat May 11, 2019 10:44 pm

I will pull it apart tomorrow and see what the issue may be. What would I risk damaging if they were bent and I ran them? I plan on cutting a test piece or two, and seeing just how much the hang up effects the accuracy of the threads before I go and buy a new lead screw.

If they are bent, is there any good way to straighten them without having to buy new ones?

pete
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Re: Jet GHB 1340 thread\ half nut issues - beginner

Post by pete » Sat May 11, 2019 11:01 pm

A bent lead screw will certainly tear up the half nuts since that binding is if it's caused by a bent screw slowly tearing up the half nut threads and probably wallowing out the bushings at each end. A bent feed rod won't be good for it's bushings or the keyed follower that picks up the drive from what's likely a slotted feed rod. Straightening them? That usually takes a great deal of experience and a light educated touch with some type of press for where to start straightening and how much pressure to apply. Any local job shops in your area that might be willing to straighten them for you?

John Evans
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Location: Phoenix ,AZ

Re: Jet GHB 1340 thread\ half nut issues - beginner

Post by John Evans » Sat May 11, 2019 11:26 pm

OP please show your location ,city or even state . You may have someone local that could help you out.
www.chaski.com

cattleman99
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Re: Jet GHB 1340 thread\ half nut issues - beginner

Post by cattleman99 » Sun May 12, 2019 8:00 am

Lincoln, NE area

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rudd
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Re: Jet GHB 1340 thread\ half nut issues - beginner

Post by rudd » Sun May 12, 2019 10:55 am

OK, he is saying "lead screw OR feed rod". (OP: Feed rod is not used for threading. )
If both are causing issues, either both are bent, or the problem is further upstream in the gearing.
Here's a real rough test to see if they are bent - run carriage all the way to one end or the other. stack up "stuff" in the chip tray till it's real close to the rod. You could lay something horizontal on some wood blocking to get to the screw. Then turn the screw and or rod and watch the gap. This would be better with a dial indicator but I'm guessing you don't have one yet. A DI would be hard to use on the leadscrew anyway.
A better test would be to pull the shafts and roll them on a known flat surface.
If bent, they need to be replaced or straightened. Straightening will take someone with some experience and the known flat surface to test on.

cattleman99
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Re: Jet GHB 1340 thread\ half nut issues - beginner

Post by cattleman99 » Sun May 12, 2019 5:00 pm

Feed rod works fine, I misspoke. I threaded some pvc today and the threads are perfect, but I can still see and feel a little hiccup when the half nuts are engaged. The hiccup didn’t seem to effect the threading capabilities. Should I simply not worry about it?

pete
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Re: Jet GHB 1340 thread\ half nut issues - beginner

Post by pete » Sun May 12, 2019 5:42 pm

Threading pvc is obviously a whole lot different than steel. And how do you know the threads are perfect, how were they measured? If the lead screw is bent enough to feel and hear the lathe slowing and speeding up then that will directly affect the lead and lag errors along the threads pitch since there's variable loads being put on the machine along the length of the screw pitch being cut. During single point thread cutting with a straight lead screw and half nuts brand new or even worn there obviously has to be some clearance plus the increased back lash with a worn machine. But the lead screws pitch is still correct. Any clearance or back lash caused due to wear then that back lash get's taken up due to cutting pressures so the thread flanks of the half nut bear against the thread flanks of the lead screw and the carriage travels the correct amount per each spindle revolution to match the thread pitch selected on the gear box. A bent lead screw will as I mentioned tear up the half nuts threads and end bushings that the screw turns on as well as vastly increase the wear on those components. Plus is has a direct and measurable affect on the thread pitch that's being cut with it. Maybe for most average threads it might not matter that much, or it could since we still don't know just how serious the issue is. For a machine I was planning on using for gunsmithing as you said and I expected to do some decent quality work with? No one other than your self can judge if you should or shouldn't worry about the issue. And more so without knowing just how much it's bent, or even if that's the actual cause of the problem yet.

whateg0
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Re: Jet GHB 1340 thread\ half nut issues - beginner

Post by whateg0 » Mon May 13, 2019 11:09 am

cattleman99 wrote:
Sun May 12, 2019 5:00 pm
Feed rod works fine, I misspoke. I threaded some pvc today and the threads are perfect, but I can still see and feel a little hiccup when the half nuts are engaged. The hiccup didn’t seem to effect the threading capabilities. Should I simply not worry about it?
What do you mean by "little hiccup"? Is it a sudden thing like when a bandsaw is missing a tooth and the next one catches on the work? Or is it a sound that it makes?

The traverse handwheel on my lathe had a giant knob on it when I got it. There was enough play in the traverse gear and rack that when the knob went over the top, it would fall and when the gear made contact with the other side of the teeth, I could feel a little jolt in the carriage. I'm able to disengage the handwheel, and have since made a normal-sized knob for it, so that doesn't happen anymore. I never really saw any effect in the threads visually, but I can't help but think it was causing an irregularity in them when that happened.

Dave

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