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  • Gear ratio changes and new thread chart???

    Evan and other mathmaticians, I have a problem. My lathe does not have some threads listed on the chart I would like to make. I have a 30t, 36t and 45t gear for setting up the quick change box as well as a 120/127 metric conversion gear. The 120t part of that gear is used as an idler between the 30t on the headstock output shaft and the 45t on the input to the quick change box shown in photo below. The input gear is turning 1/2 rev. to the spindles one rev. but I don't think that figures into my equations listed below.



    If I put the 36t gear in place of the 30t and figure the ratio of the 30t:45t and the ratio of the 36t:45t then figure the ratio of each to the other. Then use that figure to determine the change to the chart for each thread number would that work??? I want to use the 36t to 45t and the 45t to 45t and if I have to I will buy another gear to get what I want.

    I am not good at math but fair at logic. Are my thoughts logical?
    Last edited by Carld; 02-08-2009, 12:43 PM.
    It's only ink and paper

  • #2
    If you know the ratio of the desired thread to others you CAN make, then you can see if you can make up the correct turns per rev with what you have.

    27 tpi means it takes 27 spindle turns to advance one inch. Same with 16 tpi or any other. So you just need to change the ratio the correct amount.

    You have 2 spindle turns per idler turn, and everything else right now is just a transfer until the input shaft of the box. You would insert a ratio to change that.

    30t at 1/2 spindle speed...... to 45 t on input..... so as it is set up, the spindle is turning 2 *(45/30) times per input shaft rev, or evidently 3x per input shaft rev.

    if you needed to cut 25 tpi, and had a 20 tpi setting, you would need 25/20 more turns of the spindle per input shaft turn, or 1.25 x more.

    you could get that for instance by changing out the 30 tooth to one having 30/1.25 the number of teeth, or in that case 24 teeth.

    The 45 tooth doesn't let you have that ratio, you'd need to make it be a 56.25 tooth gear..........

    To base a different tpi off one you can already cut with your setup, you don't need to consider anything but the new ratio you need. lead screw etc doesn't come into it.

    With a lot of QC choices, you may have many different possible starting places, and can nearly always get there. But it helps to have a bunch of gears available to sub into the driving chain.
    1601

    Keep eye on ball.
    Hashim Khan

    Comment


    • #3
      Jtiers, if I changed the 30t for a 24t why would I have to change the 45t to a 56.25 gear? I don't understand that part.

      Also, if the ratio of the chart thread to new thread number is say 5:4 then would all the threads on the chart be the same ratio change?
      It's only ink and paper

      Comment


      • #4
        You should be able to get a great number of additional threads with what you have. You have four gears that can possibly be changed around: 30, 36, and 2 45s. At present you are using 30:45 and getting the threads marked on the QC box so that is your starting point. The ratio of 30:45 is 0.6667 or 2/3s. Make a chart with that as the first column header and list all of the threads on the QC box below.

        Now make columns for ALL the other possible positions of these four gears: 36:45, 45:45, 30:36, 45:36, 36:30, and 45:30. Starting with the 36:45 column, the ratio here is 0.8. Dividing the original ratio (2/3) by that ratio (0.8) you get 0.833333. So multiply all the values in the first column by 0.833333 and you will get the corresponding threads for the second column. Thus 24 TPI will become 20 TPI. As a double check you can reason that the 36 tooth gear will turn the 45 tooth gear faster than the 30 tooth would have so the lead screw will move faster and the thread will be coarser. 20 TPI is coarser than 24 TPI so we did it the right way.

        Now, not all the threads you will get here will be useful. Many will be odd values and can be just ignored unless you want to make some really hard to duplicate screw like some manufacturers do.

        Do likewise for the other columns, always using the ratio for that column vs. the one for the first column in your figures.

        This will give you a complete chart of what you can cut with the gears you have. A good way to do this would be using a spreadsheet program like Excel as you could make an additional column for experiments. The values for the gears could be entered separately and all the TPI values they would give would instantly appear. This can be very handy when looking for a particular thread. When looking for gears to produce a thread you presently do not have the gearing for, you need to consider the prime numbers in that thread. Thus, if you wanted a 50 TPI thread and could not cut it, 50 = 2 X 5 X 5 so you would consider gears with these primes in them, like 5 X 5 = 25 (you probably already have the 2 in your QC box). Or a 35 tooth (5 X 7) may also work if you have one of the 5s in the QC box and it would also give you a second prime (7) for additional use.

        Oh, and a similar chart could be worked out for metric threads using whatever setup you use for them. LOTS of threads here.
        Paul A.

        Make it fit.
        You can't win and there is a penalty for trying!

        Comment


        • #5
          Post a picture of your gearbox chart, the standard input train [ 30 - 100- 45 ? ] if I'm reading the first post correctly, number of extra gears you have and what you need to cut.

          .
          .

          Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



          Comment


          • #6
            Originally posted by Carld
            Jtiers, if I changed the 30t for a 24t why would I have to change the 45t to a 56.25 gear? I don't understand that part.
            no. you would change EITHER one OR the other, whichever was available and OK. Obviously a 0.25 tooth is not happening, so you'd use the 24!
            Also, if the ratio of the chart thread to new thread number is say 5:4 then would all the threads on the chart be the same ratio change?
            yes, the whole works shifts by whatever ratio change you make. Some may be useful others probably not, but whatever gets you what you need.
            1601

            Keep eye on ball.
            Hashim Khan

            Comment


            • #7
              Pikz plizz

              I'd prefer to wait for the items required by John Stevenson - as short of that its either shooting or pissing in the dark.
              Originally posted by John Stevenson
              Post a picture of your gearbox chart, the standard input train [ 30 - 100- 45 ? ] if I'm reading the first post correctly, number of extra gears you have and what you need to cut.

              .
              I'd be interested to see if there were any "transitional" threads (with a factor of "pi" ~ 3.1416 which approximates to a gear ration of 22/7 = 22:7 ~ 3.1429 - which for practical purposes is as near as "damn it" is to swearing).

              So lets wait for the OP (Carld) to post the pics. I'd like to see pics of all the levers/settings on that lathe as there is quite a few handles on it:

              Comment


              • #8




                I want to thank everyone for the help with this problem. I have been wanting to be able to get 11.5 tpi for pipe threads as well as some other threads I don't have.

                Here's the thread chart and the thread shift levers. The threads are controled by the A-B and C-D levers and the knob at the bottom. I don't know how the thread chart and lever photo's will help but here they are.

                It looks like J Tiers and Paul gave me something to make the calculations with so tomorrow I will do some math and with a spread sheet I will make charts for each gear set combinations. I will do as Paul and J said to determine what gear I may need to get the thread I want. This may not be as hard as I thought it would be.

                I will do the charts for the metric threads also. Tiffle, what is a transitional thread?
                Last edited by Carld; 02-08-2009, 11:31 PM.
                It's only ink and paper

                Comment


                • #9
                  Originally posted by Carld
                  Tiffle, what is a transitional thread?
                  He means a transcendental thread. A thread that's a multiple of Pi, as used on gear hobs.
                  "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

                  Comment


                  • #10
                    Gone to (smoking?) "pot"?

                    I think I need to consult my Oracle/Guru and get in some transcendental meditation (no "Woodstock" thanks!!) - or should I see my Dentist?

                    Comment


                    • #11
                      Just a quickie but if you want to do 11 1/2 tpi for pipe threads and given what I'm seeing in the pictures if you set for 4.5 metric this works out to 4.5mm per rev, obviously,

                      11.5 divided by 2.54 equals 4.527 tpi so over a short thread, like most pipe threads are this will get you going.

                      .
                      .

                      Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



                      Comment


                      • #12
                        Deleted- incorrect information

                        Deleted- incorrect information
                        Last edited by oldtiffie; 02-09-2009, 06:45 AM. Reason: Deleted- incorrect information

                        Comment


                        • #13
                          Originally posted by oldtiffie
                          Yep -
                          Alternatively for an exact match of 11.5 tpi use the setting for 22 tpi and change the final driven gear (to the QC gear-box) from 45 teeth to 90 which double the tpi indicated ie to 11.5 x 2 = 22 tpi
                          11.5 X 2 = 23 in my book but then again what do i know ?
                          I can't even program an Ipod
                          .

                          Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



                          Comment


                          • #14
                            Oops

                            Oops.

                            Thanks John.

                            Silly me - post deleted.

                            Comment


                            • #15
                              Thanks John, I didn't think about using the metric threads. Mostly because I would have to leave the half nuts locked and reverse the headstock. However, what you said gave me an idea. After if figure out all the threads I can get even if I buy a few more gears then I will see which imperial threads will convert very close to metric and WA LA I can cut metric without keeping the halfnuts closed, maybe.

                              This may be more fun than a barrel full of Beagle puppies. dogs, dogs everywhere, watch out! don't step on them, just hug them.
                              It's only ink and paper

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