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Thread type for making worm gear??

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  • Thread type for making worm gear??

    Good Morning Guys,

    I plan to try making a worm gear and the matching gear. In general, what type thread is used, e.g., Acme, buttress, Brown & Sharp, etc? Also, what TPI is "normal"? In passing, this fabrication is for learning purposes only. I have nothing in mind for this project other than learning how to make a worm gear and corresponding gear.

    Thanks,
    Harold
    For those having fought for it, Freedom has a flavor the protected will never know.
    Freedom is only one generation away from extinction.

  • #2
    You answered your question within your question . Hence a worm gear uses Gear Teeth. Thus you want to make a gear not a screw. Look up diametrical pitch for English and module for metric sizes.

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    • #3
      Well, actually a worm gear drive is a screw form and the gear will be a helix form. The worm can be a straight shape with screw form teeth or an hour glass shape with a screw form. The driven gear will have to have a helix on the teeth to mate to the worm and the teeth will be a concave shape. For complete data do a search for, worm drive gear forms. You will find a lot of info there.
      It's only ink and paper

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      • #4
        Originally posted by ahidley View Post
        You answered your question within your question . Hence a worm gear uses Gear Teeth. Thus you want to make a gear not a screw. Look up diametrical pitch for English and module for metric sizes.
        I watched several You-Tube flicks and one guy used Acme threads for his worm while stating the Acme Thread was the normal thread used on worm gears while the other guy used Brown & Sharp threads citing this was the customary thread. So which thread is customary since there is a conflict of statements between the two individuals?

        Harold
        For those having fought for it, Freedom has a flavor the protected will never know.
        Freedom is only one generation away from extinction.

        Comment


        • #5
          Originally posted by Carld View Post
          Well, actually a worm gear drive is a screw form and the gear will be a helix form. The worm can be a straight shape with screw form teeth or an hour glass shape with a screw form. The driven gear will have to have a helix on the teeth to mate to the worm and the teeth will be a concave shape. For complete data do a search for, worm drive gear forms. You will find a lot of info there.
          Carld,

          We just passed each other as we both clicked "send" at the same time. Yes I knew about form for each member of the gear system as I watch each person hobb the driven gear. Both guys made their own hobber thus thread form was different e.g., Acme vs B & S.

          Harold
          For those having fought for it, Freedom has a flavor the protected will never know.
          Freedom is only one generation away from extinction.

          Comment


          • #6
            Acme is very close to a 14.5 PA worm. The TPI is an interesting problem, because a worm is not a screw, its a twisted gear. Since gears are measured by their diameter, and worms are on the tangent to the pitch circle, you normally have to use pi in your pitch.

            allan

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            • #7
              Allan,

              This is getting somewhat complicated. At second and third thought, I was getting the impression that most any thread form could be used as long as the self-fabricated hobber matched the "thread" used to cut the worm. Apparently this might not be the case ...... or is it?

              Harold
              For those having fought for it, Freedom has a flavor the protected will never know.
              Freedom is only one generation away from extinction.

              Comment


              • #8
                Hey Guys,

                This is an interesting You-Tube flick that I just watched regarding fabrication of a worm & worm-driven-gear.

                https://www.youtube.com/watch?v=-umcQtrn1CQ

                What are your thoughts? A great deal of information has been omitted leaving much to the imagination.

                Harold
                For those having fought for it, Freedom has a flavor the protected will never know.
                Freedom is only one generation away from extinction.

                Comment


                • #9
                  First, just what the heck is a "Brown & Sharp" thread? I have never heard of one. I can find no mention of it in Machinery's Handbook or on the internet. What is it?????

                  There is a BSW thread. It is a British Standard Whitworth thread. It is a Vee form thread with a 55 degree angle and a radius at the crests and roots. So, what are you referring to?


                  OK, there are some standards for worms and worm wheels and, of course, there are non standard ones. About the closest thing to a standard is Machinery's Handbook. A "standard" worm basically has the same profile as a standard rack for a involute form gear or a tooth face that is simply a straight line. That is for the WORM itself. So, it strongly resembles an ACME thread and the 20 degree pressure angle is normally used. Of course there are differences in the specifics. Machinery Handbook shows the following leads are considered standard: 0.030, 0.040, 0.050, 0.065, 0.080, 0.100, 0.130, and 0.160 inch. The working depth is 0.6366 times that pitch but the cutter for the worm and worm wheel needs to add something to that for clearance, at least in a production situation. For a single, matching worm and worm wheel, that clearance can be omitted. Those are the main parameters.

                  I have seen worms with other thread forms. They can use a truncated tooth height. They can use almost any other thread form, including the Vee, the BSW, and possibly this mysterious Brown & Sharp (if it exists); but there will be varying results. The ACME form has been fairly well optimized for the transmission of power. Some worm systems will not be concerned with power and the variations may not matter. Some need precise linear movement and the tooth form again becomes critical. For that the ACME thread form with the resulting involute gear form on the worm wheel will provide the best, smoothest linear movement. Due to interference issues, a square thread form would be a very poor choice unless the worm wheel has a lot of teeth: 100, 200, or more. Even the 14.5 degree pressure angle of older gears can be problematic for smaller worm wheels. I guess this is part, a big part, of the reason why the 20 degree PA is the normal.

                  The worm wheel should have the teeth set at an angle to match the helical angle of the worm. Like a helical gear. JUST like a helical gear (but more below). These things don't change very much or very fast.

                  Finally, the worm wheel can take one of two general forms. First it can have straight (helical) style gear teeth. This allows the worm to contact a tooth only along a small line of contact on the face of the tooth so the power transmission capability is limited. The other form is a wrap-around tooth form that makes contact with the worm for a significant percentage of it's circumference, perhaps 1/3 or 1/4 or 1/8 of the way around. This provides an area of contact between the worm and each tooth on the worm wheel and can transmit a lot more power. So these are used for applications where power transmission is important. This form is fairly easy to fabricate if you use a hobbing technique.

                  Get a copy of Machinery's Handbook, even an older version is very valuable.
                  Last edited by Paul Alciatore; 01-25-2015, 05:23 PM.
                  Paul A.

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

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                  • #10
                    If you just want to learn the principle, get piece of 3/8-16 all thread then make the gear like the video link below. There is no fixed "thread". The size and ratio (TPI) can be anything you want it to be.

                    Hobbing with a Tap: https://www.youtube.com/watch?v=XIzlg_uObwQ

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                    • #11
                      ACME thread (with "straight sides") should do the job nicely but the linear pitch of the "worm" should be the same as the "skewed" circular pitch of the worm wheel.

                      The circular pitch of the worm wheel will have a "pi" in it (~3.1416) and this can be used in conjunction with the "pi" pitch in your quick change gear-box (can also be done using only the lead-screw change gears if you have no quick change gear box).

                      The "skew"of the worm gear mentioned previously here equals the helix angle of the screw/worm.

                      In effect you may have to work from the worm gear to get the worm/screw pitch or alternatively from the pitch of the worm to get the pitch circle diameter of the worm wheel.

                      It is also possible that if you have specific worm pitch and worm-wheel diameter and number of teeth that you may need to consider working metric.

                      But first and foremost -if you don't have a copy of "Machinery's Hand Book" (I have edition 27) then I suggest you get one and read it thoroughly and understand it.

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                      • #12
                        Ooldtiffie. Your last paragraph takes a lifetime..... LOL

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                        • #13
                          As said a worm is a helical gear, therefore a screw must be a helical gear too, never really thought about it as a helical gear, but there we go, learn something new.
                          I'm thinking the most basic hobbing process, like the one where you stick a tap in the chuck and drive a freely rotating disk into it might be something to tinker with, ok it's not going to be the most efficient worm and wheel but I'm betting it wil work, in so saying it would probably be better than some of the pressure die cast rubbish I've seen.
                          To make one properly is slightly more involved, you need a hob, home made or bought, a dividing " method", could be a dividing head or spin index if there are the right index positions, seed it done with mdf!
                          You gash slots in the gear blank so the hob can drive the gear and away you go, the blank diameter is usually calculated as is the amount of tooth depth( btw teeth have no depth, they have a bit of tooth above the pitch circle, the addendum, and a bit below the pitch circle called the dedendum before I'm corrected, no pun!)
                          Honestly have a go, it's not rocket chemistry.
                          It is a useful skill to have though, I have a gearbox to do next week, if the shop warms up, there's a stripped cast gear in it, I'm not looking foreword to it as it has to be the right gear to work, we shall see, worst case , I mess up, best case I don't mess up as badly.
                          Regards
                          Mark
                          Btw, machinerys is a hell of a book to have around

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                          • #14
                            Originally posted by ahidley View Post
                            Ooldtiffie. Your last paragraph takes a lifetime..... LOL
                            Do you mean what I wrote or that I suggested the OP read (Machinery' Hand Book 27) - or both?

                            This is a sort of a "primer" (for spur gears - "Inch" and "Metric") that has most of the basics in it and should be understood before proceeding to both or either "spurs" and/or "spiral" and "helical" gears too.

                            http://i200.photobucket.com/albums/a.../BB_50-51A.jpg

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                            • #15
                              Nah, if you are going to make the worm and wheel yourself start by turning the worm from silver steel/drill rod and make it twice as long as you need then cut it in half keeping one half for your worm and relieve and harden the other to be your tap.

                              Cut the wheel using your tap and you will have a wheel that matches your worm.

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