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Logan 11 threading what am I doing wrong.

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  • Logan 11 threading what am I doing wrong.

    Admit its been a year since I cut threads on my Logan, no issues set the Left and Right hand levers according to the chart. Turned out fine. Today Need to clean up threads on a 5/8 inch 11 tpi shaft. Have never cut 11 tpi on it.

    From the chart set the left lever on A, right 4 holes over for 11 tpi. Its cutting a thread twice as fine 22 tpi. Never moved the stud gears, what could have changed ?

    Couple things come to mind.

    1 Bought the lathe 3 years ago from a guy that overhauled and painted . He was not a machinist and never ran it. Several things needed adjusting. Posable he put the gears in wrong Sequence ?

    2 Is it possible moving the lever is not moving the gear ?

    Never had the gear box apart, manual does not address inside the box. Any advise or comment appreciated.


  • #2
    I can't help with the settings, but before I cut an external thread, I just let the tool make a mark on the OD and check it before cutting metal. That it because once I misread the pitch numbers and produced a beautiful thread which didn't fit.


    • #3
      Look up the settings for 22 tpi & re-check your settings. It can be easy to get the wrong hole on the QCG.


      • #4
        Originally posted by old mart View Post
        I can't help with the settings, but before I cut an external thread, I just let the tool make a mark on the OD and check it before cutting metal. That it because once I misread the pitch numbers and produced a beautiful thread which didn't fit.
        the DSG thread cutting controls are bloody complicated, I always cut the first pass with a pencil or scriber on magic marker then check with a thread gauge. Saves the odd big mistake

        For your thread issue.....double check everything then start counting all the teeth on all the gears (well, not the idlers) between the spindle and leadscrew
        Last edited by Mcgyver; 07-03-2020, 06:50 PM.
        in Toronto Ontario - where are you?


        • #5
          I always check with a thread gage before starting. This job, freshen up existing thread, ran a test on scrap 50 MM Delrin rod first. That showed the error. Checked and re checked the pin positions too.

          It may be I never actually cut a thread on this lathe, kept my Atlas 12 set up for couple years after buying the Logan and I don’t thread often. In the manual it mentions moving 72 and 36 tooth gears from stud to idler position. 72 is for cutting 4 to 7 pi. 36 the rest, Wrong gear ? Can see how that could cause It and my cuts spacing are exactly half desired.

          Back after I look into that.



          • #6
            The gears in most QC gearboxes are USUALLY arranged in a sequence from smallest to largest. A quick look inside the box should confirm that they are in that sequence.

            If it is a QC box with two setting arms, then the one with the smaller number of holes, a range setting, may have an out of sequence gear or two due to changing the external gears for some ranges or for a fine range for power feed.

            Some photos may be helpful here. The first one I would want to see would be the table of settings. And then the gear train from spindle to the QC box. Count the gears on those external gears and post those numbers with the photo.
            Paul A.
            SE Texas

            And if you look REAL close at an analog signal,
            You will find that it has discrete steps.


            • #7
              A bit of advice.
              If you are a home hobbyist with a small machine and rarely single point threads, verify the lead before threading, you have all of the time in the world to do 1 job.

              Place a dial indicator in such a way that it will measure carriage travel, place the spindle in gear and engage the 1/2 nuts and rotate the chuck by hand measuring the Lead per Revolution.
              In this case .091 per rev.
              If the machine will not allow rotating the chuck by hand whilst in gear either because it is to large to rotate by hand or has a spindle brake take a skim cut first and measure the lead.


              • #8
                Thanks all, looking at photos is a kind offer. My old Atlas is in my sons shop now, will do the thread clean up on it. Finish up a couple pressing jobs then pull the gear box off the Logan. Meanwhile Gunsmith I know has a Logan like mine will run it by him too.



                • #9
                  Originally posted by boats View Post
                  Any advise or comment appreciated.

                  I think Jerold is our Resident Expert Re: a Logan Lathe.

                  Hit him up, I am sure he can help one way or anther. JR


                  • #10
                    Are there not some gears on the end of the lathe that can be switched around? It sounds like the stud gear is say a 16 tooth when it should be a 32 as it is cutting twice as many threads as the setting.


                    • #11
                      Originally posted by JRouche View Post

                      I think Jerold is our Resident Expert Re: a Logan Lathe.

                      Hit him up, I am sure he can help one way or anther. JR
                      Phooey, I have a change gear machine, not a QCGB. Plenty here have Logan machines just like the OP..

                      But I agree with the others, for what that is worth, double pitch suggests wrong gears.
                      CNC machines only go through the motions.

                      Ideas expressed may be mine, or from anyone else in the universe.
                      Not responsible for clerical errors. Or those made by lay people either.
                      Number formats and units may be chosen at random depending on what day it is.
                      I reserve the right to use a number system with any integer base without prior notice.
                      Generalizations are understood to be "often" true, but not true in every case.


                      • #12
                        Hey Boats, have been waiting to see how you made out. As I said in my other post I think your leadscrew is turning too slow by 1/2. If you double it's speed in relation to the spindle you will get 11 tpi. I think that there are usually two different stud gears available for small Logans and South Bends. Something like 16 and 32 or 20 and 40. The threading chart probably shows which one to use for different ranges of threads obtained by the quick change box. In any case if you can double the speed of the leadscrew you should get your 11 tpi.


                        • #13
                          I have been known to do something like you are doing on purpose. The museums Smart & Brown model A can only produce threads up to 8tpi as standard. To get 5tpi, I doubled the speed of the leadscrew by changing the gears and then set 10tpi on the QCGB.


                          • #14
                            The other thing that often screws up people when they seldom use a feature is extra optional controls. That lathe should have a selector for threading / fine feed. From another site with a similar problem I found this nugget.

                            "Jon in Tucson, was right! For some reason I was engaging the clutch instead of the half nut lever...."

                            Might be time to double check all the controls being used, both on apron and headstock.

                            At the end of the project, there is a profound difference between spare parts and left over parts.

                            Location: SF East Bay.


                            • #15
                              Well I cured the problem, operator error. On my old Atlas all feed was with the Half nut thread and between centers, did a lot of threading on it.

                              Logan feeds between centers with a separate clutch Thread engagement with lever engaging the half nut. Operated properly it cuts threads like it should. I was using the clutch not the thread lever.

                              Figured it out taking the garbage out. Last year cans lid hinge pin broke. Fixed it with a new pin from 50mm Scrap Delrin Ends threaded 3/4 - 10 & nuts . Looked at it and knew the job could only have been done on the Logan. Rod too large to pass through the Atlas Headstock. No doubt when the Logan was new to me followed the manual.

                              Went back to the Logan and error was obvious. Good thing I don't do this for a living.

                              Thanks all who responded & Dan gets the prize for figuring it out.