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  1. #1
    Milacron's Avatar
    Milacron is offline Diamond
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    Somehow in decades of messing with machines I've never needed to repair a gear...replaced a few that were stripped, but never had one exactly like this where out of 38 teeth only 3 are missing. But to replace the whole assembly, if it's even available, would cost megabucks because this gear integral section a more complex assembly with another gear and internal spline, etc.

    Gear is spur type, 58mm diameter pitch, 10mm width, 38 teeth. Unhardened, slow speed, non critical application. Should I try and replace the 3 teeth and if so, how...drill holes, drive pins and weld build up..or ?

    And if your wondering, the gear this one engages was stripped of all teeth and must be replaced...but being much simplier, not very expensive, so will just order a new one.

  2. #2
    tools is offline Hot Rolled
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    Cast iron? If so, I'd install drive pins and build up around them with brazing, then either grind or file to shape, or make a form cutter for use in a boring bar installed in a horizontal mill.

    If it's steel, just weld and otherwise the same.

    I've seen cases where just roll pins were enough to make it work alright.

    Tools

  3. #3
    Milacron's Avatar
    Milacron is offline Diamond
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    Either iron or steel...forgot to check...most likely cast iron.

  4. #4
    Mike C. is online now Diamond
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    You probably already know my answer to this... drill for light drive fit of pins just shy of flush to the top of the tooth. Insert steel pins and pre-heat entire blank to about 300 degrees or so. Braze pins into place. Keep gear at temp or re-heat if required. Begin to build braze on teeth and build reasonable tooth substitute. Mount on arbor and turn to OD dimension. Put on a dividing head and either on a shaper with form cutter or on a horizontal mill with fly cutter or involute cutter to cut new teeth. Go back and fill any bad gaps on tooth flanks or edges with brazing, true OD again if required and re-cut teeth. Repeat until it looks like it should. Mikey's custom gear dentures. Spur gears are cake.

    Reason for brazing instead of welding is less likely to distort the gear.

  5. #5
    stephen thomas is offline Diamond
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    I've done it like Mike describes, but i don't use pins in small CI gears. I don't think it adds anything, and provides a place for the (small) CI gear to crack around the roots of the holes that were drilled. The braze generally won't run in there, so it is just a weak spot unless you silver solder it up first, and build the braze up over that.

    Either way, CI is a pain, because it wants to chill and get hard spots that a file just skates over, and cutting tools deflect and dull more quickly. So use all due preheat/post heat precautions.

    Never done it, but for 3 teeth, I might try the trick of milling a pocket and silver soldering in a close fitting chunk of similar iron, then profiling and milling that. Again, preheat/post heat precautions are critically important or the entire block will be a glassy hard obstacle. A mild steel chunk could be siversoldered in and would be the most foolproof approach to easy machining substrate. But it might wear differently over time and affect the sound or vibration characterisitics.

    I once had a Hendey gear head lathe that had a number of gear tooth repairs with just pins driven in across and filed to gear shape, not brazing or other. But that was in the headstock bull gears.

    smt

  6. #6
    mobile_bob is offline Stainless
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    a couple of thoughts

    is there a gear available with the right specs?

    if so why not turn down your broken gear, then bore out the new and press the new one over the old core?

    how much power is transmitted thru this gear? is it a high stress gear or low stress/torque application?

    i have been working with some polymer cast mixes that with pins would make short work of replacing teeth, and machining them afterwards.
    you just make up a plaster mold, to cast the teeth in place over the pinned gear. pretty tough stuff, and wears like,, well you know "iron".

    just some alternatives from a guy that has done the gear tooth thing a few times, but not on machine tools [img]smile.gif[/img]

    bob g

  7. #7
    doc
    doc is offline Hot Rolled
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    i have repaired several cast iron 16 pitch gears by brazing after filing in dovetails to aid retention ( probably not necessary , but made me feel better ) ....heated the gear shy of color change , brazed & shoved it into sand to cool....turned the gear to size & used index head & fly cutter/formed cutter to finish ......they are all still in use years later...
    best wishes
    docn8as

  8. #8
    JoelS is offline Cast Iron
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    This subject seems to come up often. A quick search shows that the 'pin and braze' solution is certainly a popular one:

    http://www.practicalmachinist.com/ub...30.html#000003

    http://bbs.homeshopmachinist.net/sho...r+tooth+repair

    I have a worm gear with a large braze repair that must be 30 years old - and still looks to be holding up great.

  9. #9
    kendall is offline Cast Iron
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    my old Hendey had 4 gears on the bull gear that were apparently cut from a similar gear, then soldered into a pocket milled in the bull, never noticed it till I tore it down degreased and found very faint silvery lines under them. Don't know if they pinned them, but the gears fit so well in the pocket it would have been a waste of time.

    Ken.

  10. #10
    Asquith is offline Diamond
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    Here’s a tooth I saw in Venice, repaired by screwing in studs, welding, and filing. Unlike Don's, it wasn't a non-critical application ..... it was on a crane!

  11. #11
    Bruce Griffing is offline Stainless
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    I do think brazing (or silver brazing as SMT suggests) is the way to go on a cast iron. But if you do a conventional braze, be sure to use high tensile rod.

  12. #12
    willbird is offline Banned
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    What about having it professionally welded with high nickle rod ?? I have done bores that way and the weld cut like butter, I do not recall if I had to machine into the nickle/CI interface or not, but I was amazed at how nicly the weld machined.

    Bill

  13. #13
    northernsinger is offline Titanium
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    In about 1987 or so I had an old machinist (now long dead a great guy) attempt to repair a bad gear and do it by the method described by Thomas above. He milled out a section of the and fit closely a piece of CI, I can't recall if he milled the teeth--about three or four if I recall--or left me the blank.

    However the silver solder--or whatever he used--was not well done and the piece fell out. He perhaps could have pinned it in but didn't and he was getting old. And I was so young and sensitive--not now--that I was too shy to tell him that his work failed, didn't want to hurt his feelings.

    I had a brother in law welder fill the void with brass, then turned the glob into the proper conformity with the rest of the gear and then hand filed the teeth for this slow turning gear.

  14. #14
    Forrest Addy is online now Diamond
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    If at all possible find a stock gear of like count, pits, width, and pressure angle to the broken one. Mount the new gear in a pot and bor it out about six addendums short of the root diameter. This giver you a ring with brand-new teeth on it. Machine the old gear to accept the new replacment teeth ring leaving either silver braze/Locktite clearance or a small shrinkage (0.0005" per inch) secured with drilled and tapped setscrews straddling the fit diameter and radially aligned with the tooth.

  15. #15
    Mike C. is online now Diamond
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    The ring replacement suggested by bob and Forrest are the best repair... if you can find the proper gear. That was where I hit a brick wall in my repair. Nobody made a 5DP 37 or 35 tooth bevel gear I could cut to a ring. If you can get the gear and make a ring, by all means, yes.

    As for the pins leading to cracking, the gear I repaired was very large and solid. Cracking may be an issue on a smaller gear, but it wasn't in this case. I tried to fit the pins close to the width of the top of the tooth at the smallest part, which ended up being 1/8" diam. The idea of the pin is the same as rebar in concrete, they make the brass a lot stronger. Now, if your pin can't be 1/8" or so, it probably isn't going to make much difference whether there is a pin there or not. No sense drilling for sewing needles. I also heavily fluxed the area and tinned the broken areas as well as the pins to start, THEN I built up the teeth.

    The only thing I would definitely avoid is the nickel rod. Nickel rod on cast iron is a quickie arc weld fix that usually doesn't work. I have tapped a series of holes through a crack in my lathe apron that was "fixed" with nickel rod, and prompty broke again (probably as they watched it cool). The iron down to the crack drilled like soapstone. When I hit the crack, it was like drilling granite.. burned up three drill bits getting though it. To make it worse, I had to grind out all the nickel to be able to make my braze repair because brass will not stick to nickel. My new repair has held.

    I would imagine a set of gear teeth of nickel rod would just snap right off at the weld zone.

  16. #16
    mobile_bob is offline Stainless
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    another option is to get an accurate drawing of the gear and take it to your local steel company with a laser.

    here in tacoma we have a steel supplier with two such lasers, one of which is the largest west of the mississippi. they can cut a steel gear that is spot on requiring no machining.
    cost is about 50 bucks so it ain't cheap but they can hold very close tolerances,
    a replacement ring gear could then be heated and shrink fitted to your turned down hub.
    and then do as Forrest mentioned.

    bob g

  17. #17
    mobile_bob is offline Stainless
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    btw, they have made gears for obsolete machinery on numerous occasions

    bob g

  18. #18
    awake's Avatar
    awake is offline Stainless
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    The only thing I would definitely avoid is the nickel rod. Nickel rod on cast iron is a quickie arc weld fix that usually doesn't work. I have tapped a series of holes through a crack in my lathe apron that was "fixed" with nickel rod, and prompty broke again (probably as they watched it cool). The iron down to the crack drilled like soapstone. When I hit the crack, it was like drilling granite.. burned up three drill bits getting though it. To make it worse, I had to grind out all the nickel to be able to make my braze repair because brass will not stick to nickel. My new repair has held.
    There are two (at least two) types of nickel rod for use on CI; one is non-machinable, and the other is machinable. I'm guessing your repair was made with the non-machinable! FWIW, I'd guess that the problem on your lathe wasn't the use of nickel rod per se, but rather the technique. I'm no expert, but I've done a lot of reading on the welding forums (okay, you can all start wincing and raising your eyebrows now!) - from what I have read, you either need to pre-heat, or supposedly there is a way to do very short beads with immediate peaning.

  19. #19
    Mike C. is online now Diamond
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    Oh, I know exactly how they screwed it it up, But very few folks take the time and use proper procedure. An acr is still going to be more likely to cause a brittle zone, though. It's the extreme of heat caused by that 5000F arc.

    I have a friend who is a welding engineer. His words, "braze cast iron."

  20. #20
    eKretz is online now Titanium
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    I've welded very large rabbit (fit between the rod's cap and the rod) fits on big connecting rods for punch presses (300 + ton range) with Ni-rod many a time, and it has machined almost identically to the parent metal every time. (Except once I had another junior fellow weld one up...he didn't pre-heat hot enough...or post-heat...or slow cool! Grr. Glass or granite, as posted above, comes to mind). It will work just fine if you pre-heat, post-heat, and slow cool properly. The whole thing needs to be HOT and STAY hot (600-800 degrees) the whole time you're welding. Give a quick thorough peen with the pointy end of a slag hammer, then a good post heat (same temp as pre-heat or maybe even a little warmer) and either wrap it in an asbestos-type insulating blanket or plunge it into a BIG barrel of preheated sand. I usually preheat the sand by heating a similar size piece of scrap steel to the same temp as the iron I'm working on and popping that in the sand 15 minutes or so before I'm ready to put the cast piece in. I've never tried the short weld "peening" method myself, so haven't got any comment on that other than to say I've heard of it. For such a small item or thin sections, however, heat will be difficult to control...so you're better off with the ring repair or brazing, like has already been suggested.

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