Attachment methods for building a two-piece gear blank
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  1. #1
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    Default Attachment methods for building a two-piece gear blank

    I am planning on building a two-piece gear, and I'm looking for some alternative methods for attaching the two pieces.

    This is a rack gear / pinion from an old Hendey lathe. The original Hendey parts are cut from a single piece of ductile iron (right in pic). The damaged one on my lathe (left) looks non-original, it has been made from two pieces, a disc about 4-1/2" OD, pressed onto a rod, about 1-1/4" OD. The two pieces are secured by three pins (set screws?) installed axially.

    I plan to machine a replacement gear, and I'm planning on using a two-piece construction with 4140HT. My question is, are the three axial set screws the best method of securing these two pieces? Are there any other alternatives I should consider? Obviously one-piece would be ideal, but that's expensive and I already have an appropriately sized disc and rod in my scrap bin.
    Attached Thumbnails Attached Thumbnails img_3128.jpg   img_3129.jpg  

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    I would consider welding, hot shrink, or loctite. Does it spin in opposite directions where threading would unscrew? The original was probably a forged one piece blank.
    Bill D

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    The larger the "bolt circle" the less the stress the "Dutchmen" have to put up with. So maybe a larger piece than 1 1/4"

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    Weld or press fit with a key . Press with a key would be the way I would do it with that material.

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    The dowel pin method is tried and true, a keyway would work but I feel like its not as durable as three dowel pins spaced 120 degrees apart. I would stay away from welding since its 4140 and is tricky to do correctly and becomes crack prone if not preheated and slowly cooled afterwords.

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    Do you know the hardness/material of the mating gears? It could be the cast iron gear was meant to be the sacrificial part.

    Are the teeth going to be cut before assemble or after? If you are making a blank, assembling, and then finish machining and tooth cutting I would weld the two parts (with proper procedure for Rc30 4140). If the gears are made separately and then assembled I would use a shrink fit with a single blind Dutchman.

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    Thanks for the suggestions everyone. Even just learning the terminology is helpful, I had never heard of "dutchmen".

    If I move forward with the dowel pins:
    I assume normal, straight dowel pins are not preferred. Should I be using taper pins? Or thread the holes and use screws? If I use screws, won't the clearance in the threads make for a slightly loose fit?

    Quote Originally Posted by Bill D View Post
    Does it spin in opposite directions where threading would unscrew?
    Yes, it turns both directions.

    Quote Originally Posted by tylersteez View Post
    I would stay away from welding since its 4140 and is tricky to do correctly and becomes crack prone if not preheated and slowly cooled afterwords.
    That's exactly what I was afraid of. I had considered welding, but prefer to avoid it because of my lack of experience with this material.

    Quote Originally Posted by gbent View Post
    Do you know the hardness/material of the mating gears? It could be the cast iron gear was meant to be the sacrificial part.
    The other gears and the rack are some fairly hard alloy steel, not unlike 4140HT. I was surprised when Hendeyman told me the originals are ductile iron, you could be right about it being an engineered weak link. But I don't really want to change course now, and I don't feel right about using a softer material for the pinion.

    Are the teeth going to be cut before assemble or after? If you are making a blank, assembling, and then finish machining and tooth cutting I would weld the two parts (with proper procedure for Rc30 4140). If the gears are made separately and then assembled I would use a shrink fit with a single blind Dutchman.
    Just for ease of workholding: I'm planning on cutting the pinion first, then assembling the blank, then cutting the large gear.

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    Quote Originally Posted by jwearing View Post
    . . .Just for ease of workholding: I'm planning on cutting the pinion first, then assembling the blank, then cutting the large gear.
    If you do it that way, you've got to be perfect with alignment after assembly. Might be better to join blanks, finish turn and then cut teeth (unless you plan on finishing the large blank after joining but before cutting teeth).

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    Another way to do this, which is a little more work but I think ends up nicer, is to put a shoulder/flange on your pinion shaft. If the back side of the bull gear has to be flush, you can counterbore for the flange. Then put a circle of bolts clearanced though the large gear, threaded into the flange. Again, depending on clearances you can use several different types of fasteners. Or you could run the bolts in the other way, depends on what's gonna hit which.

    Anyway, easy to do on just a mill, makes the part replaceable if it breaks again in future, stronger than a key, easier to do, and no welding distortion or aftereffects to deal with.

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    Quote Originally Posted by Gordon Heaton View Post
    If you do it that way, you've got to be perfect with alignment after assembly. Might be better to join blanks, finish turn and then cut teeth (unless you plan on finishing the large blank after joining but before cutting teeth).
    Yes I was going to finish the large blank after assembly. By cutting the pinion teeth before assembly I can do the whole job without cutting a separate mandrel. I understand it will take some care to keep everything concentric.

    You're probably right though. Maybe I should just do it the normal way.

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    I would first look at just replacing the pinion in the existing gear.

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    Quote Originally Posted by gbent View Post
    I would first look at just replacing the pinion in the existing gear.
    It’s hard too see in the photos but the large gear is damaged too. It got mangled by the mating gear.

    The lathe was poorly maintained. The interlock was removed, probably by the same genius that removed the lube fittings.

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    I have been making similar gears sets with press/shrink fits (about 0.003" at 1.3" diameter if memory serves). What I have found is that the bore collapses somewhat so I end up honing the hole back to size after pressing the parts together. Determining weather the shrink fit joint will e strong enough requires knowing what the peak loading is going to be.

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    I would use a force (or shrink) fit of the two pieces which would provide a joint without any play. Then you can drill and tap for two or three set screws which would be held in place with Loctite. The force or shrink fit would provide the basic joint and the threaded fasteners are just insurance against vibration and excessive torque.

    I am not a big fan of taper pins but they have been used in many assemblies. If you use them, do add some Loctite.

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    For what its worth, they make internally threaded dowel pins that aide in removing them if you ever had to pull the gear off again
    Last edited by tylersteez; 06-22-2021 at 01:58 PM. Reason: added the word "internal" for clarification


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