Thread Bar Project, nothing is going right and I need help

jlsmithseven

jlsmithseven

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Hi Guys
So it's my second semester at school. Our first project is a thread bar done on the lathe. It needs to be a 100% project or no credit.

I have my 2nd attempt at this turned down to size and grooved.

Basically this is what I ran into today...

My bar is like wobbling when it's between centers. I always notice when it's wobbling and on my practice piece, it did the same thing and only cut threads on one side of the bar. At one point it only cut threads on one side and didnt even touch the other side.

I've turned my dead center down to 60 degrees true to the chuck. I even indicated it as well in the chuck. I don't know why it's not turning true, can someone help?
 
By "thread bar" do you mean single pointing external threads?

If I understand correctly you first turned an OD between centers, when beginning the threading op there was much run out?

If so the only thing that can explain this is that something moved.
 
I'll take a shot at this. I'm not sure what a "thread bar" is, though. I am assuming you need an accurate OD and turned it between centers to do that. By grooved, I assume you mean a thread relief. So, things that might contribute to wobbling:
  • If you turned a dead center in a 3 jaw chuck then that's fine - it will still be accurate and on the spindle centerline - but you cannot remove it and then put it back in and expect it to be accurate. Once you remove the center from the chuck it will no longer run concentric with the spindle and the work will wobble.
  • Is your lathe leveled? Did you check tailstock alignment? These things, if off, will turn a taper.
  • Depending on how long the work piece is you may also have some deflection from the cutting tool, both the turning tool and the threading tool. This is less of a problem if you're using sharp HSS tooling. Carbide tools, especially when used at low speeds, will deflect more and potentially cause inaccuracies.
  • The accuracy of the center holes can cause issues as well. If you drilled the distal center hole using a steady rest then that hole is not going to be that accurate. If you chucked up the work so it just sticks out of the chuck, faced and then drilled each end then it will be more accurate. Also be sure you drilled deep enough to get almost the full conical section of the drill engaged but no more. If you go too deep your centers will ride on the shoulder of the hole and will not be supported by the 60 degree taper and the work will move.
  • The amount of pressure you apply with the tailstock ram can also cause the work to bend so be careful to only apply enough pressure to engage the tailstock center fully. As the work heats up it will expand and this will reduce clearances at the centers so check it from time to time and adjust tailstock pressure as needed; and keep the tips of the centers lubed.
  • Watch your cutting speed. Turning between centers can cause vibration at higher speeds because of the drive dog. This isn't a problem when threading but it can definitely be a problem when external turning. This affects finish and accuracy.
Without a clearer idea of what you're doing, I can't think of anything else. I'm sure if I missed something the other guys will figure it out.
 
Not much to go on here... I don't know how robust the bar is that you are trying to thread, but you have stated that you are turning between centers, meaning that you must be using a drive dog to rotate your part. Be sure that the drive dog is not pushing against your chuck and puts no strain on the part. Grab a different drive dog and see how the part reacts. Some times old machines have been damaged and another thing to watch for is a bent lead-screw that can apply strain on the half-nut and move a light weight saddle up and down and in and out on every revolution (of the lead-screw). This would only show up when threading because conventional feed rates are usually controlled by the feed-rod and not the lead-screw. However, some low price point machines may only have a lead-screw that does both feeds and threading. Try a different machine if that is possible.

Good Luck!

Gary
 
Sounds to me like your bar is not true.
If you need it accurate you would start with oversized stock. Put centers in each end, and then turn it to size between centers.
Then thread it between centers.
Till us what you're working with , like thread size and length of this rod.
If your cutting your own center that is held in the chuck...the only thing that needs indicating is the tool height and the compound angle. The center will then be correct until removed and can be skim cut to true it when installing to use later.
 
Wreck™Wreck said:
By "thread bar" do you mean single pointing external threads?

If I understand correctly you first turned an OD between centers, when beginning the threading op there was much run out?

If so the only thing that can explain this is that something moved.
Click to expand...

But what moved? Yes you got my scenario right but how am I supposed to fix this? Is there anyway? I turned it between centers and it looked fine. I've been moving the piece from machine to machine because I can't get on the same machine every day. Do you think this could be the issue? I didn't even start threading anything yet on it, I can just see it wobble while between centers.
 
A lot of helpful information here, thank you. I only have until Friday to finish this product, which is why it's urgent. SO many little things can go wrong and I think my biggest deal is applying too much pressure with the tail stock and my center drill holes are probably not right. Somehow we lost like all of our #3 center drills and they were the only ones I could get working for me. I've use a #4 for this project and had to go almost right up to the end of the taper. Is that too deep? How do I know how deep to go, I thought it was 2/3 of the way up the taper. I might have to start over on this project if it still wobbles I guess, right? THanks guys.
 
jlsmithseven said:
But what moved? Yes you got my scenario right but how am I supposed to fix this? Is there anyway? I turned it between centers and it looked fine. I've been moving the piece from machine to machine because I can't get on the same machine every day. Do you think this could be the issue? I didn't even start threading anything yet on it, I can just see it wobble while between centers.
Click to expand...
If you have drilled the centers then turned the OD then the centers and OD's will be concentric even if there is a butt load of taper. If moving the part between machines then the error is in the next machine attachments, IE the centers or spindle.

In an effort to help you out, ask yourself how is the instructor going to measure the threads that you make? Over wires or with a ring gauge, a ring gauge will tell someone that the Major Diameter is too large, measuring over wires only gives the Pitch Diameter.

Does your school not have bench centers?

http://cdn.mscdirect.com/global/images/ProductImages/0171874-11.jpg
 
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I'm thinking that you are tightening the tailstock too much like mikey said. That was my first guess as I read this.
Good luck, I'm assuming that you should be able to ask the teacher for opinions.

One more thing, if you center drilled and did not go deep enough (not too deep) to remove where the tip would ride, you could wobble.
 
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