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  • a worm and spur gear question

    I have this thought of making a dividing head like they show in Harold Hall book of "dividing" or something similar and he shows using a spur gear with a worm and i thought that was a great idea and i happened to have a set of change gears that are 16 D.P. that would mean 5.0929 T.P.I for the worm my question is would just 5.T.P.I acme thread work? mabe machine the threads a little bit thinner? also is there any easy ways to machine worm and worm wheels Thanks for all & any input Bill.

  • #2
    I bought a worm/wormwheel pair from Boston Gear a few years ago. While not cheap, they were certainly affordable enough so I could do the project right.

    While it's undoubtedly possible to make-do with a regular spur gear, it does mean the worm shaft ought to be set at the helix angle of the thread to get more or less straight line contact between the worm thread and the straight spur gear teeth. That adds a design complication. Add to that the fact that with a dividing head precision of motion is important, and for me at least a commercial worm/wormwheel would look better and better.
    ----------
    Try to make a living, not a killing. -- Utah Phillips
    Don't believe everything you know. -- Bumper sticker
    Everybody is ignorant, only on different subjects. -- Will Rogers
    There are lots of people who mistake their imagination for their memory. - Josh Billings
    Law of Logical Argument - Anything is possible if you don't know what you are talking about.
    Don't own anything you have to feed or paint. - Hood River Blackie

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    • #3
      You are right SGW, but for me its a different area to learn something, truth be told I have a 40:1 worm gear set I won on ebay and I still my use it instead but its the challenge right? thanks for the reply Bill.

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      • #4
        Pitch error between a worm and gear pair have to be fairly small - less than 0.002" per foot I would say although there is an AGMA standard for it.

        In the case of your candidate gear and the worm you propose to make for it you have to consider that if the worm to mesh properly with the gear it has to be presented at the helix angle of the worm. Therefore you would ordinarily have to calculate the "normal pitch" of the worm and select a pitch diameter for the worm that would properly mesh the teeth reducing the tooth to tooth pitch error.

        In your case, working backwards from the gear's linear pitch of 0.1963 and the worm 0.200, the worm's pitch is too long for the gear if the worm axi i in the plane of rotaion. While you could get a kind of mesh the action would be lumpy (chordal) as tooth bumped into tooth. Your table would not feed moothly or index reliably. The worm axis has to be inclined to the helix angle (call this the "setting angle". And the setting angle has to be adjuted to stretch or shrink so the worm pitch is presented to equal to the gear's pitch. Also the worm teeth have to be re-proportoned slightly to achieve full engagement.

        The setting angle cannot differ much from the helx angle if tooth bearing problems are to be avoided. Only a few degrees would be my suggeted limit. Picking a worm pitch diamater to secure a setting angle as close as possible to the helix angle is all-important.

        It can be done but your rotary table's handwheel will stick up a definite angle (or down depending on the hand of the worm) to make it work and the worm itelf will have to have a funny profile to mesh properly. Critics will point out that the tooth contact wih the gear tooth will be point loaded and they are right but as the worm rotate the point of mesh follow he setting angle. It will wear faster but so what? This an experiment.

        Better get the calculation worked out early in this project. If you don't you'll be sorry.
        Last edited by Forrest Addy; 11-30-2009, 07:26 PM.

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        • #5
          I cheated a bit and got away with it----

          I needed a steering gear box for my 2" scale freelance steam wagon that i was trying to build at minimum expense. In a large coffee can of gears collected over 30 yrs I found a worm of suitable size, but without a matching wheel. I poured the spur gears into my faithful old baking tray and found some reasonable possibilities. I then made a little temporary adjustable frame and tried and tinkered until i found a spur gear that meshed reasonably with about 7 degrees if inclination. next I actually made a drawin, built a steering box and fitted it to the wagon.Ithas done a couple of years of hard running with no problems so far. Yes I know what I did was not " right" but this is " The Home Shop Machinist's site, not the " Precision to drive you crazy site". Regards David Powell.

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          • #6
            I have taken the Helix angle into account I think mine should be around 3.6426 deg with a 1" dia worm and that would be my Helix angle off set also the worm carrier would be adjustable up & down for the meshing backlash to take out any play my big concern is if 5 tpi will work in place of the correct 5.0929 tpi with the Helix angle corrected.

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            • #7
              David I know what your saying If I posted this question over at PM I would have immediately deemed a hazard to my self and others and to stay away from any powered machinery and to get a frontal lobotomy as fast as possible and to never ask a stupid question like that again,,lol I crack my self up,Bill.

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              • #8
                Yes, you can probably get a make do setup using the gear and a home made worm and if used on a steering box or something accuracy is not needed on it would work.

                The thing is your making a precision rotary table and a make do gear set won't give you any precision if you use a hand wheel with marks to break down the degrees the table turns. If you only mark the circumference of the table and don't use the hand wheel to divide the degrees to minutes then it may work.

                I have the feeling you won't like the finished product if you do what your planning but it's your project and you can make it right the second time. It's not like your making them to sell and it's just a project.

                Is there a way you could make it so you could put the right gear set in after your experiment?
                It's only ink and paper

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                • #9
                  All geared up

                  Originally posted by BillC
                  I have this thought of making a dividing head like they show in Harold Hall book of "dividing" or something similar and he shows using a spur gear with a worm and i thought that was a great idea and i happened to have a set of change gears that are 16 D.P. that would mean 5.0929 T.P.I for the worm my question is would just 5.T.P.I acme thread work? mabe machine the threads a little bit thinner? also is there any easy ways to machine worm and worm wheels Thanks for all & any input Bill.
                  Bill,

                  I've checked your math and it seems to be OK to me.

                  So, I will step through it for any one else who is interested.

                  The gear circular pitch and the worm pitch need to be equal.

                  The gear circular pitch is the gear pitch circle circumference divided by the number of teeth.

                  Lets say the 16DP gear has 32 teeth. The Pitch Circle Diameter = 32/16 = 2"

                  The Pitch Circle Circumference = 2 x pi = 2 x 3.1416 = 6.2832"

                  The Circular pitch = Pitch Circle Circumference/number of teeth
                  = 6.2832/32 = 0.19635.

                  That is the pitch of the "worm" required the TPI of which is 1/0.19635 = 5.0929 TPI

                  As 16/pi = 16/3.1416 = 5.0929 you have a "transition" pitch worm which will require "pi" to be included in your gear train.

                  The ratio usually used is 22:7 but there are others that I can't find as I've lost the link I had. I hope that others will post it here.

                  The "slope" the sides/flanks of the worm must be equal to the gear pressure angle -probably 17.5 or 20 degrees - not a normal acme thread. The thread form to be correct must be normal to the helix angle of the worm (ie "tilted") and is not the same as a normal screw thread which is normal to the screw axis (ie "flat"). The tool can be made with sides "sloped" as for the gear pressure angle but it must be "tilted" to be the same angle as the helix angle.

                  The worm helix angle is ATAN worm lead/worm pitch circle circumference.

                  The worm and wheel centre distances will half the sum of the gear and worm pitch circle diameters.

                  The worm will have to be "tilted" by the worm helix angle for it to mesh with the gear.

                  I hope this helps.

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                  • #10
                    Carld I would test this set up first before I committed to building the rest of the rig as you stated if it wasn't going to work I can go to plan B.
                    I guess all that i would be out is some time making up the worm and seeing if it works I was hoping someone here maybe has tried it or steer me the right direction and it maybe a train wreck!!!.

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                    • #11
                      oldtiffie thanks for the reply but going over everything and as you stated it wont work correctly with a 5 tpi in order to make this work the gear train in the lathe would have to be altered to come up with the 5.0929 tpi and the Helix angle would have to be factored in as well.

                      Inquiring minds want to know,,,

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                      • #12
                        I would look at the spur gear as an already gashed blank ready for hobbing. Take a section of the acme rod and turn it into a hob. Use it to hob the gear in a fixture like this:



                        The fixture is designed for gashing and hobbing. A change gear is used to index the blank for gashing. The blank is then allowed to turn freely for hobbing by retracting the index pawl.

                        In this instance I made the hob from scratch using 1040 steel which was then hardened. This is the hob with the finished worm wheel.

                        Free software for calculating bolt circles and similar: Click Here

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                        • #13
                          I don't have the calculations handy but I think 5 MM is closer to 16 DP than 5 TPI is. I seem to recall seeing some photos of at least one dividing system on a lathe spindle where a shop made screw drove the bull gear. I don't recall the pitches involved. It would be interesting to know what kind of accuracy was achieved but the owner seemed happy with it.
                          Don Young

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                          • #14
                            Evan that is a very nice setup and the gear looks great, I think that maybe the way to go, how big is the gear? and the worm? pics are deceiving.
                            The radius on top of the gear did the hobb cut that as well? thanks for your reply.Bill

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                            • #15
                              The base of the fixture is 4 inches wide. The worm is a singly enveloped worm meaning that the worm matches the inward curve of the teeth as was cut by the hob. With a double enveloped worm it would also match the curve of the wheel. I was making a rotary table but abandoned the project as it was unnecessary when I built the 4th axis for my mill.

                              Free software for calculating bolt circles and similar: Click Here

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