SB 9: repairing back gear
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  1. #1
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    Default SB 9: repairing back gear

    The back gear on my SB 9 had 3 teeth missing on the small gear. I decided to attempt a repair.

    I cleaned the part thoroughly, then heated it up and filled the area of the missing teeth with braze. Let it cool (slowly) and evened out the top of the blobby braze a bit.

    I made a fixture to mount it in my milling machine, and set up a 16DP 14.5PA #5 gear cutter. I used an indexer to carefully rotate the gear 15 degrees at a time, to carve slots in the braze to make new teeth.

    The technique seemed to work, but something's not right. When I mount the gear back in the lathe and turn it on, I get a noticable "clacking" noise when the newly cut gear teeth go by the gear driving the chuck (bull gear?). I can feel vibration through the back gear handle, and if I don't hold the handle, it will kick itself out.

    Needless to say, I'm not going to put any kind of load on this thing until I get it sorted out!

    It acts like the cuts between the teeth are not deep enough or wide enough, such that the teeth on the bull gear are not meshing clean, and trying to kick the back gear out. But I've mic'ed the width and height of the teeth, and the depth of the cuts, and they seem fine. If anything, there should be a little *more* clearance than the stock teeth.

    When I look at the cutter and the original teeth under magnification, they're close, but not a perfect match. The cutter doesn't get quite to the bottom of the cuts between the original teeth, and the cutter appears to have just a touch tighter curvature than the teeth. That last might be wear.

    So what else should I look at? Anybody got experience repairing these things? I could just go buy a used gear, but I feel like a machinist, even an amateur one, should be able to fix this thing

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    First question would be is the gear 24 tooth? (the 15 degrees)

    Second question is how did you determine it was 16 DP?

    Third question is the cutter domestic or otherwise? The numbering is backwards on many from Asia - a #5 being like a USA #4

    Van Keuren says a 24 T 16 DP 14.5 gear will measure 1.6515 over a pair wires .108" is a way to check your work

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    Yes, it's 24 tooth (that's why 15 degrees was easy)

    I didn't save the arithmetic which convinced me it was 16DP, but basically what I did was google for measurements for calculating DP, then measure the thing several different ways to convince myself I had the right diametrical pitch measurement. I'm pretty sure the diameter measured from the tops of the teeth was something like your 1.65. I'll go back out to the shop and collect some fresh measurements this evening.

    I'm not sure where the cutter came from. I didn't know the numbering was backwards on some of them, how confusing! If an asian number 4 is like a US number 5, that would mean it was for a larger diameter gear (more teeth) and thus the tops of the teeth I cut would be too close together. That might be the problem.

    The cutter says TTC, but it was probably made in China, nearly everything is these days.

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    Yup a 24T 16 DP will be 26/16 OD outside dia.

    26/16 is the same as 1.625"

    But this does not tell you much about what the cutter did - that is the reason for the wire measurement

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    Ok, learning time: Can you point to a description of the wire measurement?

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    Quote Originally Posted by jrd-sb View Post
    Ok, learning time: Can you point to a description of the wire measurement?

    Pretty good little sketch here from the Van Keuren 1952 catalog - "M" being the measurement over "wires". This would be Even - and Odd is a second bunch of tabulated information

    wires-gear.jpg

    I would imagine this subject is covered at length in such as Machinery's Handbook

    Van Keuren - now gone I suppose - made a business of providing such things as the "wires"

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    my memory may be off but i thought i remembered the SB9 had 14DP backgears.

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    I measured the gear. I get 1.623 diameter from the tops of the teeth. That sounds like it confirms 16DP.

    I couldn't find any .080 wire to do the wire check. I'll dig around some more tomorrow.

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    Quote Originally Posted by jrd-sb View Post
    I measured the gear. I get 1.623 diameter from the tops of the teeth. That sounds like it confirms 16DP.

    I couldn't find any .080 wire to do the wire check. I'll dig around some more tomorrow.

    .108 gear wires are hardened and precision ground - be nice if some wire gage drill shanks were that size

    A source

    Gear Measuring Wires | Willrich Precision Instruments

    good luck

    (for what its worth department....both 4 penny and 5 penny NAILS are listed as having .109 shanks)

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    rougly 7/64"

    since youre dealing with the repair of an OLD gear with some wear a couple drill bits should tell you whats going on.....even a one wire measure as a comparative against one of the unrepaired teeth...(may even be the way to go)

    you could also TURN a .108" diameter yourself with a nice sharp tool.

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    Yeah

    I don't know why I thought it was .080. I did carefully turn something down to .080 and it was way too small. Experimenting with drill bits produced exactly what you said: 7/64 or .109 made it look close to johnoder's pic, above.

    Following your suggestion, I tried walking my drill bit around, mic'ing across to the flat between the two teeth on the opposite side. I found two of my cuts where I got about .003 higher than the rest, or than the dimension of the stock cuts. I speculate that that might be enough to show the uneven mesh. So I think my next task is to revisit my procedure and my fixture, and see if I can find a way to take a little more out of those two, without messing anything else up.

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    I'd say your are looking at some light file work now, instead of continued milling.

    allan

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    Quote Originally Posted by kitno455 View Post
    I'd say your are looking at some light file work now, instead of continued milling.
    Exactly... You can even do this in-situ, so you don't have to disassemble anything again. You might try putting some Dykem on your build up to see if you can indicate high spots.

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    Yep

    I did clean everything, put dykem on the bull gear, and rotate the works by hand. I got what I think are some high spots. Carefully filed them off, and repeated. The result sounds a little quieter.

    My plan at this point is to just repeat that procedure a few more times, and see where I end up.

    I'm sure y'all are getting sick of my blow-by-blow learning how to do this sort of repair, but I'm trying to proceed cautiously and not break anything worse than it already was. Besides, it's fun to learn new things, especially when it's a hobby. I can take it slow without worrying about anybody's schedule.

    Thanks for the great input so far!

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    JRD, your post will provide a valuable reference to someone with the same or similar problem in the future.Jim

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    Quote Originally Posted by jrd-sb View Post
    I'm sure y'all are getting sick of my blow-by-blow learning how to do this sort of repair, but I'm trying to proceed cautiously and not break anything worse than it already was. Besides, it's fun to learn new things, especially when it's a hobby. I can take it slow without worrying about anybody's schedule.
    Aw heck no, I think its very entertaining -- you're showing us how to do it, and do it right! So yeah, years from now there's gonna be some other guy just like you, with the exact same problem, and he's gonna find this thread. By all means, please keep going!

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    Thanks for the encouraging words.

    I decided that I've made enough mistakes on the first part, that I'll start over with another one. Busted parts are available cheaply I'll likely go back and take another swing at the original part later. This new one has only one missing tooth.

    This time I'm documenting with pix and better descriptions.

    I started by cleaning the part of paint and grease, and wire-brushing the area to be brazed.

    img_0368-med.jpg
    I made a little oven out of firebrick. That'll come in handy a few steps further on.
    img_0371-med.jpg
    Heated the part more or less evenly, adding flux to try to get good adhesion of the braze.
    img_0369-med.jpgimg_0370-med.jpg
    Filled the area with a big blob of braze.
    img_0372-med.jpg

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    [Continuing due to attachment limit]


    Closed up my firebrick enclosure to help it cool slowly and evenly. Let it sit overnight.
    img_0373-med.jpgimg_0374-med.jpg

    This morning I took it out, and chucked in my (still non-back-geared) lathe. Knocked down the high spots cleaned away most of the excess braze. I couldn't get it all off the areas to the sides of the gears, as the shaft has a couple thousands run-out. So I left that stuff there, it won't hurt anything. I got down to about .005 of the tops of the existing teeth, but didn't want to get any closer for fear of dinging them. I'll file down the tops of the new teeth later.
    img_0381-med.jpg

    I had made up a fixture to mount the gear in my milling machine. Set that up again. Re-indexed it carefully, and took a cut. Looked pretty good, so I took a second cut.
    img_0385-med.jpgimg_0387-med.jpg

    I'm stopping here to work on my day job. Tonight if I get time I'll measure the gear (without removing it from the fixture), and decide whether to do another cut in the miller, or take it out and go the rest of the way by hand.

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    Success!

    I found a drill blank in 7/64 and used that as a measuring pin. Mic'ed the newly-cut gear teeth, using the pic posted by johnoder as a guide. Compared the dimension to what I got off the teeth from another part of the gear. Determined that I needed another .008. I wasn't sure how to calculate what depth of cut would result in .008 difference in the location of the pin, so I took .002 at a time (I'm not sure my milling machine is quite that accurate, but that's what I dialed in) until I got the dimension down to where I wanted it.

    img_0388-med.jpgimg_0389-med.jpgimg_0390-med.jpg

    I took the gear out of the fixture, cleaned off the extra flux that had gotten into the gear teeth, and mounted it in the lathe. It took a little bit of hand turning to crunch a few last bits of flux out of there, but then it turned smoothly. Took a breath, powered it up, and here's what I got.
    http://www.jrd.org/nate/05-12-20/MVI_0392.AVI

    It's making a small amount of noise, but there's no substantial vibration. I tried turning a couple of small bits of scrap, to put a load on it. It makes a little more noise under load, but not too bad.

    I'm counting this as a success. My repaired back gear (take two) seems to work pretty well. I'll work with it a little more, and then maybe go back to the original part and see if I can get it working as well.

    My main takeaways between the first part and the second: Have your technique for measuring ready to go before you start. Be super careful about getting the thing indexed. Proceed cautiously and check everything as you go.

    I hope this thread is useful to anybody else who wants to try a similar repair.

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