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Gear question. How important is the diameter of the gear for thread cutting?

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  • Gear question. How important is the diameter of the gear for thread cutting?

    Hello again friends.. Long time no post.

    I have the opportunity to make use of some free design and EDM machine time, so I am going to have some missing gears made for my lathe.

    In my train of gears, I need 3 ( I dont have the chart infront of me), a 60T, 58T and a 54T. I have the 60T and it is about 4.25" in diameter.

    I cant find a calculator or website to make it easy for me to determine the diameter of the 2 gears. I believe its important, but not sure.

    Thoughts.. links? A Tiffie weblink???

    Thanks

  • #2
    First you need to determine the pitch and pressure angle.

    Could be module or diametral pitch, and likely to be either 14.5 or 20 degree pressure angle.

    In any case you need good measurements of one of the mating gears.

    in most cases count the teeth of a mating gear, add 2 then divide by the outside diameter.
    Last edited by 1200rpm; 10-17-2018, 11:34 AM.

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    • #3
      Here's a good basic gear tutorial:

      https://www.hobby-machinist.com/thre...orial-1.68592/
      It's all mind over matter.
      If you don't mind, it don't matter.

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      • #4
        You can calculate the circumference of a gear you have and you know that the circumferential length per tooth as a result. So gears within a reasonable diameter range will just add the distance per tooth up to find the new circumference and from that you can calculate back to the diameter. So using the OD of gears you have you can figure out the OD of the gears you need for gears that are within a few teeth. You can't do this for larger differences because the teeth are pitched to a circle partway down the teeth. But for examples such as finding the proper OD for the blank for a 62t gear using a 60t or a 48t from a 50t it should work just fine.

        The other option is that once you know the diametric pitch circle diameter for the size of teeth then you can find that same diameter for the other tooth sizes. That circle grows in circumference directly with the tooth spacing. And the OD of the gear is then the DP diameter plus the extra height to the top of the teeth. Again a value that stays constant.

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        • #5
          Originally posted by MrWhoopee View Post
          Where? if you're talking about the link in the discussion you linked to, you have to be signed in (a member) to see the PDFs.

          Comment


          • #6
            This should help...

            https://geargenerator.com/#200,200,1...0,0,0,0,2,-394
            Keith
            __________________________
            Just one project too many--that's what finally got him...

            Comment


            • #7
              Originally posted by cuemaker View Post
              Hello again friends.. Long time no post.

              I have the opportunity to make use of some free design and EDM machine time, so I am going to have some missing gears made for my lathe.

              In my train of gears, I need 3 ( I dont have the chart infront of me), a 60T, 58T and a 54T. I have the 60T and it is about 4.25" in diameter.

              I cant find a calculator or website to make it easy for me to determine the diameter of the 2 gears. I believe its important, but not sure.

              Thoughts.. links? A Tiffie weblink???

              Thanks
              What model lathe s it? Your 60T gear would appear to be 1.75 module (works out to 4.27").
              Peter - novice home machinist, modern motorcycle enthusiast.

              Denford Viceroy 280 Synchro (11 x 24)
              Herbert 0V adapted to R8 by 'Sir John'.
              Monarch 10EE 1942

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              • #8
                Here's another gear calculator which may be useful https://www.technobotsonline.com/gea...alculator.html

                Comment


                • #9
                  Originally posted by LKeithR View Post
                  GREAT link Keith; thanks!
                  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


                  • #10
                    For gears of a similar size it is a linear relationship. So the 58 is 58/60 x the dia of the 60. Simple as that. For larger differences there is a slight variation because of that "+2" mentioned in an earlier post above. However you do need to know the module/DP and pressure angle to get/make the cutter. If you can name the lathe mfr and model someone probably knows those details.

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                    • #11
                      You might state the make of machine....and maybe include a pic of the gear train.

                      If there are only 3 gears and you need two it might just make sense to replace all of them with new ones of a common pitch.
                      Of course that would only be possible if you are not dealing with fixed center distances....but it would be awful nice if,say, 16DP gears could be used...those are cheap and common....but without knowing what the machine is...

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                      • #12
                        Gearotic outputs gcode for gears as well as dxfs and stls. Fusion 360's spur gear add-in has a few extra settings for backlash and root radius.

                        p.s. oldtiffie isn't around, so you'll have to do the search for these sites yourself

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                        • #13
                          Originally posted by LKeithR View Post
                          I like this one. JR
                          My old yahoo group. Bridgeport Mill Group

                          https://groups.yahoo.com/neo/groups/...port_mill/info

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                          • #14
                            Originally posted by elf View Post
                            Gearotic outputs gcode for gears as well as dxfs and stls. Fusion 360's spur gear add-in has a few extra settings for backlash and root radius.
                            p.s. oldtiffie isn't around, so you'll have to do the search for these sites yourself
                            Originally posted by JRouche View Post
                            Originally posted by LKeithR View Post
                            I like this one. JR
                            That does not output anything that one might want to actually put in a machine tool they wish to keep running well, or depend upon, much less have them mesh correctdly with other gears they are unfamiliar with the design of.

                            The glaringly obvious lack of Root Radius or Trochoid would be ( should be ) fairly concerning. Drawing an Involute does not make something a "gear". There's a lot more to the task than one simple equation, repeated for tooth count. As well, Fusion360's plug in is severely lacking, too. Great for shiny pictures. Dismal for real life.

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                            • #15
                              Originally posted by cuemaker View Post
                              Hello again friends.. Long time no post.
                              I have the opportunity to make use of some free design and EDM machine time, so I am going to have some missing gears made for my lathe.
                              In my train of gears, I need 3 ( I dont have the chart infront of me), a 60T, 58T and a 54T. I have the 60T and it is about 4.25" in diameter.
                              I cant find a calculator or website to make it easy for me to determine the diameter of the 2 gears. I believe its important, but not sure.
                              Thoughts.. links? A Tiffie weblink???
                              Thanks
                              To actually answer your question - With certain very important caveats, you do have some leeway on diameters. That said, what you seek to find out is simple math. Your largest issue is making sure of the conditions that the mating gears exist within. If they are all bone standard, then Z+2/Pitch= your desired OD.

                              However, that is the least of the issues you face soon.

                              Largest of all is that you need to find out just exactly how these gears are to be drawn, and how they will be cut. The VAST MAJORITY of people with WEDM machines have not the single foggiest clue as to how to properly cut a gear with one. The end result being so poor that you are actually much better served to have Form Milled one in its stead. I have written and consulted on the subject extensively and don't have time to rewrite it all, here. Do a search on Practical Machinist for gears and WEDM along with my name and you can find much of it.

                              It can be done. We do it all the time. ( As do others. ) But various factors have to be taken into consideration and accommodated for.

                              Good luck. Most of all, have fun.

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