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Metric threading with 120T/127T gears?

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  • Metric threading with 120T/127T gears?

    Anyone have hands-on experience using those change gears on a lathe with an Imperial leadscrew?

    I've read several places that this solves the Imperial/Metric conversion issue but have never seen any details or pictures as to where they go and how it works. I have a set coming and am wanting to give it a whirl when they arrive.

    I measured and found that I do have the clearance needed to fit a 5" gear onto the leadscrew without hitting the table.
    Milton

    "Accuracy is the sum total of your compensating mistakes."

    "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

  • #2
    I do metric threading on my South Bend using compound gearing made up using the standard change gears that came with the lathe. By compounding two of the standard gears you can thread nearly all the usual metric sizes with no more than a 2 threads per metre error.

    http://ixian.ca/gallery/metric/metric.htm
    Free software for calculating bolt circles and similar: Click Here

    Comment


    • #3
      Yes, adding a compound with 127 teeth and a second gear with some round number should allow you to cut metric threads.

      You will have to make a new chart to show the gear positions for them. Since inch threads are measured in TPI and metric are measured in actual lead in mm, the two sequences do not correspond well and you will have to calculate the metrics the hard way. I have an Excel spreadsheet that does this for a SB and if you send me your e-mail in a PM I will send you a copy. It was based on a 100:127 compound so you will need to do some changes, but it would be a starting point. You don't say if you use change gears or a quick change box and the the approach is different in each case, so let me know which.

      If you have the quick change box, you will be limited to the number of metric threads you can cut. When I did the calculations for my SB with an 8 TPI lead screw and manual change gears, I found that the 100:127 compound gave me more usable combinations for metric threads than either the 50:127 or the 120:127 would have. I hope you have some basis for choosing the 120 for your lathe.

      Also, if you are getting individual gears to be combined into a compound one, some threads may more easily be cut by using the 127 by itself with some of the existing change gears.

      Another problem is that some gear arrangements for specific threads that might work in theory are not physically possible on every lathe due to mounting constraints. This can be frustrating if you know the ratios work but the gears just don't fit.
      Paul A.
      SE Texas

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

      Comment


      • #4
        Thanks Paul, PM sent.

        Evan, my gear chart shows close combinations for metric threads as well but I thought heck, I'm already ordering an extra set of change-gear hardware & carrier from Grizzly for this project http://bbs.homeshopmachinist.net/showthread.php?t=37869 so I might as get the "proper" gears in the same shipping deal. The 120T is only $13.50 and the 127T is $15.00
        Milton

        "Accuracy is the sum total of your compensating mistakes."

        "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

        Comment


        • #5
          My CVA (10ee to the yanks ) uses 127 compounded to do metric threads.
          Just the day before yesterday I posted pictures over here showing the setup.

          CVA uses 32 DP gears for the metric train, and 16DP for imperial, the smaller size of the gears means the 127 tooth gear fits inside the endcovers, which is nice, as they are interlocked in the electrics

          One thing is that even tho you have a 127 tooth gear you need to leave the halfnuts engaged and reverse the lathe / leadscrew to get back to the begining of the thread for the next pass.

          Dave
          Just south of Sudspumpwater UK

          Comment


          • #6
            Thanks for the link Dave!

            My lathe doesn't have a thread dial anyway so no biggie on the leaving the halfnut engaged routine. Makes more sense to me that way anyhoo.

            How small are the planes your login name refers too?
            Milton

            "Accuracy is the sum total of your compensating mistakes."

            "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

            Comment


            • #7
              Depends on what you are making. If you want it to be accurate you will need 120/127.

              Comment


              • #8
                Unless you are making something like a lead screw the approximations will do fine in nearly all cases. The error using my setup is only 2 parts per thousand which means that for one centimetre of thread the error is only 20 microns.
                Free software for calculating bolt circles and similar: Click Here

                Comment


                • #9
                  The small planes is something I did back in 2000, about 30 grams, 12 - 15 inch wingspan. IR control, back when you had to build the stuff from scratch to get it light enough to fly. Powered by Nicads, before LiPo was available...
                  A vid here
                  Now Im into small helicopters, from about 200mm rotor diameter up.
                  A long running project is the worlds smallest fuel powered heli....

                  Back on topic, Evan is correct that there are approximations which will give you almost the correct pitch, and for most practical purposes (IE a few diameters as per normal bolts) it doesnt matter.
                  However you have a set coming, presumably with a 127 gear in it. Post some pics when you get it, these sort of things get glossed over in a theoretical manner to often.

                  Dave
                  Just south of Sudspumpwater UK

                  Comment


                  • #10
                    I have a Logan 10" lathe which can use either 127/100 transposing gears or 47/37 gears with a 1% error. The advantage of the smaller gears is both cost and they will fit under the gear cover. Here is a link to how the Logan setup wirks. The Logan has an 8 TPI lead screw so the gear formulas should work for your lathe if your lathe has the same.

                    http://www.lathe.com/metric_threading.htm

                    Comment


                    • #11
                      Thanks for the link! My lathe has a 16 tpi leadscrew and a 40t gear on the spindle so I'll have to do some cypherin' to convert that chart, as Jethro Bodine would say.
                      Milton

                      "Accuracy is the sum total of your compensating mistakes."

                      "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                      Comment


                      • #12
                        Screw it

                        Here is the OP:
                        Originally posted by DICKEYBIRD
                        Anyone have hands-on experience using those change gears on a lathe with an Imperial leadscrew?

                        I've read several places that this solves the Imperial/Metric conversion issue but have never seen any details or pictures as to where they go and how it works. I have a set coming and am wanting to give it a whirl when they arrive.

                        I measured and found that I do have the clearance needed to fit a 5" gear onto the leadscrew without hitting the table.
                        DICKEYBIRD.

                        The reason that a 127 tooth gear is used in conversions between and metric is that:

                        1" = 25.4mm

                        So, the ratio is 1:25.4 - but we need "full" teeth on our gears - not partial teeth.

                        So:
                        25.4 x 5 = 127 ("full") teeth.

                        It works whether cutting inch threads with a metric lead-screw or if you are cutting an inch thread on a metric lead-screw.

                        The "where they go" bit is usually given on a diagram on your lathe change-gear cover.


                        If your 127 gear is say 20DP its Pitch Circle Diameter (PCD) will be 127/20 = 6.35"

                        If it is 25DP the PCD will be 127/25 = 5.08"

                        If it is 30DP the PCD will be 127/30 = 4.2333"

                        Outside diameter (OD) = (PCD + (2 x 1/DP)) = (PCD + 2/DP)

                        If space is "tight" in the gear-train, there are other combinations -including "ganged" or "cluster" gears (2 or more gears acting as a single unit).

                        Comment


                        • #13
                          Originally posted by Evan
                          I do metric threading on my South Bend using compound gearing made up using the standard change gears that came with the lathe. By compounding two of the standard gears you can thread nearly all the usual metric sizes with no more than a 2 threads per metre error.

                          http://ixian.ca/gallery/metric/metric.htm
                          Looks like useful information for the yahoo south bend group .

                          Ive turned it into a pdf ..
                          is it OK if i upload it to the group .

                          i will credit you as the source

                          all the best.markj

                          Comment


                          • #14
                            Thanks 'Tiff. Unfortunately, my lathe's "Where they go" chart doesn't show a 127T gear or any metric combinations at all. Calculating the dia/teeth ratio on my other gears, I had guessed the 127T gear would be about 5.08" dia when it arrives which matches your 25DP figure perfectly. Now I know what my lathe's gear DP is.

                            It'll probably take a bit of experimenting when the gears arrive for me to get my head around some sort of system to calculate the gearing and where the gears need to go.

                            My goal is to procure an R8/ER32 collet chuck for the mill and an extra nut to use with a home made flange mount ER32 chuck for the lathe. Maritool shows the ER32 nut threads to be 40mm x 1.5.
                            Milton

                            "Accuracy is the sum total of your compensating mistakes."

                            "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                            Comment


                            • #15
                              Gear train

                              DickeyB.

                              These should help. They are from a "Metric" book published by one of our Universities in OZ - but it works in "inch" as well.

                              Just ask if you need any further info:







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