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Thread: Threading issue

  1. #41
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    Quote Originally Posted by johnoder View Post
    The diff between 12TPI and metric is......

    You can open half nuts on 12 tpi

    You cannot open half nuts on metric using your Imperial lead screw

    See post # 36
    You lost me.

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    YouTube
    If this link works go to 22 minutes. Adam shows a good way to thread metric using the thread dial and revering the machine. You must leave the nut engaged or do as Adam shows.

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    Quote Originally Posted by Derek Smalls View Post
    First, you can NOT release the half nut when cutting metric threads with an imperial leadscrew. This was causing a tracking problem which led to the torn up threads you were getting before. Easy fix, leave half nut engaged and reverse spindle at end of each pass.
    Your second problem, the incorrect lead, should be easy to solve as well. Start by rechecking the positions of all the end gears. They must be set up exactly like the diagram. Count the teeth, assume nothing. Also check that you have all the levers in the correct position. Unless someone has done something really funky to your lathe, like changed gears in the QCGB or sheared a pin causing the leadscrew to slip, you should be good.

    On edit: Glad you sorted it out, I was typing during the last couple of posts!
    Well, I do not have it sorted out. Both of you and jonother indicate I can not open half nut. Reversing the spindle is what I am unable to process in my head, as , isn't the play in gears going in opposite direction cause the insert to be in different place? Other member posted a link on YouTube where Adam is doing similar operation. I will watch it tomorrow, as I need to grasp this concept.

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    You HAVE to retract cutter from thread before reversing the spindle ! Back out cross slide a turn or so run spindle in reverse to get cutter a 1/2 inch or so past the beginning of the thread. Then move cross slide back in to original setting advance compound for the next cut. Start spindle and by the time cutter reaches the work any and all back lash will be taken up.

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    Quote Originally Posted by Ratz View Post
    Well, I do not have it sorted out. Both of you and jonother indicate I can not open half nut. Reversing the spindle is what I am unable to process in my head, as , isn't the play in gears going in opposite direction cause the insert to be in different place? Other member posted a link on YouTube where Adam is doing similar operation. I will watch it tomorrow, as I need to grasp this concept.
    Back off the cutting tool in the same time when you switch to reverse. Otherwise you are guaranteed to break the insert.
    Need two hands operating in synchronous so it might bit harder than just opening half-nut when you reach the end groove.
    Easy with fine threads, lots of action and hand-waving if you are cutting coarse thread like 4TPI on a old sloppy lathe.

    --
    HSS or carbide both work just fine even at low speeds but they need to be sharp. Coated carbide(mitsubishi VP15TF) is my favourite, less build-up than HSS and works at any speed just fine. I have used carbide down to M1.4x0.2 (127TPI) threads and turned pins to 0.014mm diameter (yes you read correct, 14 micrometers) so obviously it works also for reaaaally slow speed.

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    Quote Originally Posted by Ratz View Post
    Well, I do not have it sorted out. Both of you and jonother indicate I can not open half nut. Reversing the spindle is what I am unable to process in my head, as , isn't the play in gears going in opposite direction cause the insert to be in different place? Other member posted a link on YouTube where Adam is doing similar operation. I will watch it tomorrow, as I need to grasp this concept.
    Well, it seems you have the machine now cutting the correct lead of 1mm, so that problem is solved if I understand correctly.

    The reason your thread looked so torn up is because the half nut was not being engaged in the correct orientation to the work causing the tool to not follow the already cut thread perfectly. This is always a problem when cutting metric threads with an imperial leadscrew, because the two threads don't match up.
    The solution used in the linked video uses multiple position references(orientation of the spindle, thread dial, position of carriage)to give you a way to re-engage the half nut in exactly the same point as a previous pass. This method can be useful at times, but it's usually much easier to simply reverse the spindle to return the tool to the start position, having never released the half nut. As others have stated, and as your intuition has told you, the tool MUST first be retracted from the thread grove by backing out the cross slide, just as you do when threading normally.

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    Quote Originally Posted by Derek Smalls View Post
    Well, it seems you have the machine now cutting the correct lead of 1mm, so that problem is solved if I understand correctly.

    The reason your thread looked so torn up is because the half nut was not being engaged in the correct orientation to the work causing the tool to not follow the already cut thread perfectly. This is always a problem when cutting metric threads with an imperial leadscrew, because the two threads don't match up.
    The solution used in the linked video uses multiple position references(orientation of the spindle, thread dial, position of carriage)to give you a way to re-engage the half nut in exactly the same point as a previous pass. This method can be useful at times, but it's usually much easier to simply reverse the spindle to return the tool to the start position, having never released the half nut. As others have stated, and as your intuition has told you, the tool MUST first be retracted from the thread grove by backing out the cross slide, just as you do when threading normally.
    Thanks, the way you explained it, I think I am getting the picture. I got to clean up, fix few problems and start again.

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    Quote Originally Posted by Derek Smalls View Post
    Well, it seems you have the machine now cutting the correct lead of 1mm, so that problem is solved if I understand correctly.

    The reason your thread looked so torn up is because the half nut was not being engaged in the correct orientation to the work causing the tool to not follow the already cut thread perfectly. This is always a problem when cutting metric threads with an imperial leadscrew, because the two threads don't match up.
    The solution used in the linked video uses multiple position references(orientation of the spindle, thread dial, position of carriage)to give you a way to re-engage the half nut in exactly the same point as a previous pass. This method can be useful at times, but it's usually much easier to simply reverse the spindle to return the tool to the start position, having never released the half nut. As others have stated, and as your intuition has told you, the tool MUST first be retracted from the thread grove by backing out the cross slide, just as you do when threading normally.
    Thank you very much to all who participated. Could not be easier, once the light came on (thanks to you all)

    dscn1383.jpg

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    Quote Originally Posted by JohnEvans View Post
    You HAVE to retract cutter from thread before reversing the spindle ! Back out cross slide a turn or so run spindle in reverse to get cutter a 1/2 inch or so past the beginning of the thread. Then move cross slide back in to original setting advance compound for the next cut. Start spindle and by the time cutter reaches the work any and all back lash will be taken up.
    I was re-reading a post and it makes perfect sense this morning. I guess I was too tired last night.
    Thank You for the input.

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    I have worked on hydraulic cylinders for many years, and the most common thread pitch pretty much regardless of size is 12 tpi, and that can be in V, stub acme, and even buttress.
    I picked up a tip here 15yrs ago that has proved useful on several machines.

    Yes You can use the thread dial and halfnuts to thread up to shoulders, or to the bottom of blind holes when cutting metric threads! Or, if the machine has one, use a metric thread dial and lead screw to cut imperial threads. Very simple and useful on big machines that are hard to stop, or little machines like Southbend that are not fast coming to a stop.

    With the tool at the beginning of the thread-engage the halfnuts on a thread dial number-say No1

    At the end of the thread disengage half nut,like normal, but then, at about the same time shut off spindle.

    Return carriage with handwheel back to the beginning of the thread.

    Now reverse spindle, watching thread dial return to No1, or a little past so you can engage again for the repeat passes.

    The above is really much the same as trying to stop, and running the carriage back under power. The difference is the halfnuts are disengaged, but the carriage is returned by hand, and then the spindle has to be reversed returning the threading dial to No1, and the cycle repeated.
    One lathe I use, the carriage travels 4" per one turn of the thread dial. so on threads that are really long, I have to keep track of dial turns, but for short threads its hard to get lost.

    Depending on the work, some shops hardly if ever single point screw threads. A machine that does not regularly cut threads will get sticky in the gearing, the mechanical part of the halfnut system, and other mechanical parts like leadscrew reverse on tool room lathes.

    If a machine hasn't cut threads in a while, I clean the screw, and using plenty of oil, run the carriage back and forth enough times, that the half nuts will function smoothly, engage fully, and very important disengage.

    On larger machines, the leadscrews are coarse deep threads, you cant stab the half nut lever in, and expect the thread to track, its best to try to engage the halnuts a little early, feeling the halfnuts slide down the back of the leadscrews thread flank on down to the bottom-fully engaged!
    The situation above explains the lead screw reverse feature on toolroom lathes, where the halfnuts are left engaged the full cycle. That way, any halfnut camming in error is settled out by the time the thread is finished. Otherwise the best is be consistent engaging the halfnuts.

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