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cutting a worm gear

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  • cutting a worm gear

    First a little background. We use long ovens(40 ft) at work to dry the product. The conveyor belt inside the oven is driven by a gear reducer and motor. The reducer and motor are placed in such a way that the oven would require a complete redesign if we changed to a different gear reducer and motor.The gears are no longer available as an off the shelf item.
    Over the years the worm gears wear out. not suprising since they're bronze. The worms are steel and even after ten years the do not look worn.

    So I thought that I'd build up the old gears with bronze welding rod and turn one of the worms into a hobb and recut the gear on the lathe. The gear being where the toolpost would normally be and the worm between centers on the lathe.
    Anyone out there in HSMLand do this before?

  • #2
    W/W gears ?

    Yes, all the time. I have orders for 8 of them right now.
    I use a mill and a rotary table, but others have used the lathe. will get you going.

    OOPS, well, not add on material to the existing bronze gear, then recut it, but all of the other things are covered in the linky. As long as you don't warp the gear and get the OD right, and can make a suitable hob, you should do fine. Someone with a knowledge of what happens to bronze when you heat it, as far as "strength" goes, may chime in here.

    Last edited by lenord; 04-23-2009, 08:59 PM.


    • #3
      I was going to gash and old worm and use it as the hob.


      • #4
        How does that work, Lenord? Do you allow the aluminum disk to rotate to match the spinning cutter?


        • #5
          Originally posted by tony ennis
          How does that work, Lenord? Do you allow the aluminum disk to rotate to match the spinning cutter?
          yes but remember it is a worm gear so the free spinning disk rotates at only 1/n rpm of the cutter where n is the number of teeth on the gear being cut.

          (read: it rotates pretty slow if the gear you are hobbing is of any size)

          I have only done this with taps so far but it was not hard.
          Tom C
          ... nice weather eh?


          • #6
            The worm is turned

            I don't know what the worm and wheel sizes or the loads on them are.

            A lot of these "home made" efforts are usually small and have brass or aluminium being cut. Cutting bronze may well be an entirely different matter.

            I would use a carefully ground and honed single tooth HSS fly-cutter with lots of "top rake". I would shape it from an unworn part of the worm and perhaps "adjust" it to more nearly approximate an involute form (not fussy here either).

            The blank would be mounted on an arbor/spindle in my 3-jawed chuck on my rotary table which in turn was mounted on my tilting angle-plate/table and which in turn was tilted the same angle as the helix angle of the worm. (Helix angle can be measured with a simple protractor on either the worm or wheel - at the mid-point of depth of cut).

            Note: take care that you get the right "hand" of the "gear". Its all too easy to get wrong and is as embarrassing as a tooth too many or too few!! (That's possible too!!).

            Do remember to clamp your "Y" slide from start to finish as well as your rotary table during a cut.

            If I have a number if "teeth" or "cuts" that are divisible into 360, I use my rotary table dials as its easier than setting up the indexing plats and arms etc. I just write out a list of the "degrees" that I need to use - and away I go.

            Provided my cutter centre height was set to intersect the wheel/blank immediately at its axis and in the middle of its outer diameter, I would set my cutter to either partial or full depth and start milling.

            The cutter (in a vertical mill) would be cutting in a horizontal plane.

            This would in effect be a set of straight grooves that would be correct at the centre of the gear.

            That should be sufficient to take most loads that will be applied.

            I don't think I'd use "hobbing" as a term in any of these contexts.

            For "depth of thread", don't bother to "work it out" - just mount a worm in the lathe (3 or 4 jaw chuck or between centres - your call), put a "tool" (again - your call) in your tool-post and put the tool on the outer diameter of the worm, move the carriage 1/2 the pitch (by eye is more than good enough), set the cross-slide dial to zero, then use the cross-slide to move the "tool" until it "bottoms" in the worm thread. Now read off your cross-slide dial. You now have your depth of cut.

            If the worm and wheel don't slip or foul each other in operation its OK - no need for "micron accuracy" here. If they don't reverse at all, then its even better as the "fit" can be a lot "looser" and still do the job.

            The key thing here is that the pitch circle diameter of the "gear" "worm thread form" has to be as close as you can get it to have a circumference equal to the number of teeth required x the axial pitch of the worm. Chances are that using the parts you have as "models" or "masters" that you will be OK.

            I have deliberately avoided using umpteen pages of "tables" and references to the Bibular text and holy writ of old gurus as well as calculations here in the hope that more people can get a good result with what they have in their shop.

            I would use this method to make any "thread chaser" gears I required - but there is a better, faster easier way there too - that will "do the job". That's for another time.

            This might be a "big ask" for a "Spindexer" but with time and care it might just be OK.

            Before you say "no - utter bull-$hit" (which it isn't) and/or that "Old Tiffie is a wanker" (which I clearly was but which I'd like to think that could still be true - but alack and alas .............. ) - give it some thought or try it.


            • #7
              Keep making bronze worms, its good, not like wimpy brass.
              bronze is the right metal.
              just make the damn things or you can pay a 14 year old Chinese to do it for you.

              Ya got it?

              Im not kidding.


              • #8
                Here is a gear I made for a fella's tiller. It's bronze.... what I did was I made a hob that was a tad bigger then the worm. For the blank size I measured for the root of the worm to the center of the shaft then took of a few thousand for the clearance. Gashed the blank placed it between centers on the mill shop made cutter in a collet and ran at low speed feed table in when I had a good cut started I upped the speed and cut.

                I would make the hob instead of slashing the old worm... drill rod works nice and can be hardened in the shop.
                Wow... where did the time go. I could of swore I was only out there for an hour.


                • #9
                  Top job

                  Thanks Tinkerer.

                  Top job.


                  • #10
                    Thank you Old Tiffie for that description. I did a similar method using a horizontal mill and it worked very well. That was only after disastrous results from attempts at hobbing, all I made was a smooth groove around the perimeter. I was obviously doing something wrong, but you method worked for me.


                    • #11
                      Universal machinging

                      Thanks Allan (Jackary).

                      I first used that method on a horizontal mill and then on a vertical mill, so your comment brought a wry smile.

                      It is really a variation of spiral gears and relies on a contact at the centre on the worm-wheel/gear. There will be some side-thrust on the worm which will tend to "bend" the worm/gear but it works and it works well - within limits. It is also useful in making "poor man's" or "make-do" helical/spiral gears without hobbing, CNC or differential indexing and/or universal milling machines etc.

                      Even more to the point, heavy hobbed wheels with deep so-called full-arc contact eventually evolve as single line contacts over time with wear.

                      I try to eliminate as much reference to high-priced books as well as try to keep to simple tools and methods so far as I can. This is a challenge for me and it may help some who are hesitant from all the myths and legends and talk of difficulties in gearing generally and worm-and-wheel in this instance to 'give it a go" and not only put their toe in but either put their whole bloody foot in or just dive right in.

                      I fully acknowledge and see the need for design and accuracy on sometime difficulty or expensive tools and machines, but its surprising often - but not always - how things can be made or done when really needed. If some elect to work at a "higher level" I have no problems with that either as that is their choice.

                      But back to the "helical worm".

                      There is a somewhat similar method of making a pseudo helical dove-tail cutter that gets over the "hammering" of straight teeth and is made and ground on a lathe. It has many of the advantages of helical teeth on some of the better side-and-face cutters as well as spiral T-slot cutters. It was made from "drill rod" (US) or "silver-steel" (UK and OZ and NZ). It was published in an OZ magazine as it was made here. That "super lathe" of yours and many others would "eat" that job as it required turning, milling and machine (T&C) grinding - as well as some hand grinding and honing to finish it off. Many here could do it as well with basic tools. A "Spindexer" would be ideal. I thought you had it recently in your thread about T&C grinding an dove-tail cutter on your machine. I was very impressed with that lathe of yours and the ways you use it as that is a universal machine if ever there was.


                      • #12
                        but its surprising often ... how things can be made or done when really needed.
                        You see the ingenuity of men in the 3rd world. They seem to be able to do a lot with nothing. I saw a beautiful turned wooden item once. The guy that made it had little more than a bow lathe made from a few boards with nails for centers. He used a file. He held the lathe secure to the ground with his feet as he sat before it.