Gear tooth repair

  1. Pete.

    Pete. Member

    Messages:
    8,260
    Location:
    Kent, UK
    I had this gear from a DSG lathe to repair. It had one single busted tooth on it, probably from being dropped as there was no other damage at all. I decided to fit a new piece in and cut the one single tooth with an involute cutter.

    Here's the gear. 90 teeth 12DP 14.5PA. An inch thick with a 1.25" bore. I milled a slot in it where the broken tooth was.

    millslot.jpg

    Then I cut a piece from a slab of grey iron I have kicking about for no purpose.

    iron.jpg

    Milled the piece I cut to fit in the slot nice and snug. There were four possible orientations so I picked the best and marked it with an arrow.

    fillpiece.jpg
    Bonded it in with a high-strength structural araldite. Not your ordinary stuff this is £25 for a small tube. I put it in my mill to keep the load on it for a couple of days of curing. I thought about brazing in the new piece but I wasn't sure if the red heat would affect the grain structure of the adjacent teeth and make them weaker or more brittle. I figured that a good fit and some decent adhesive should hold that piece in very well. I suppose a scotch key could be added for peace of mind, if the owner was worried.

    bondingpiece.jpg
    Once the part was cured I stuck in in my chuck to turn the OD. I put a piece of thin card under one jaw with the block dead opposite and this gave it a few thou runout so that I could just turn it down until it skimmed the adjacent teeth half a thou.

    cardpacking.jpg

    Did one half width then turned it around to do the other side.

    turnod1.jpg
    Gear now ready to have the new tooth cut.

    turnod2.jpg
     
    decca, slim_boy_fat, Seadog and 15 others like this.
  2. Pete.

    Pete. Member

    Messages:
    8,260
    Location:
    Kent, UK
    Next to cut the teeth. I had the loan of a suitable cutter but it needed to be sharpened first. I f you use a blunt involute cutter it rounds off the edges right back along the profile and renders it totally useless. You can see the corners already rounded off in the 2nd pic.

    First to set the cutter up in a dividing head.

    sharpencutter1.jpg

    I dressed the thin dished wheel and got ready to sharpen the cutter. You have to take particular care when doing these cutters as they must be turned radially to always sharpen them across the centre line. The Astra dividing head allows for this with a pair of jack screws you can just see on the very right edge of the pic above.

    Here's the cutter and wheel, note the rounded corners that have to be ground back.

    sharpencutter2.jpg

    After sharpening. No more rounded top and the cutter has very sharp edges.

    sharpencutter3.jpg
    I need to put the gear in my larger dividing head, an old IXL unit which though it looks rough still works very well. First I have to line the cutter up on the centre-line. I put the cutter & mandrel in the spindle and then put a pin in a 1/8" split taper into a MT3 taper sleeve and loaded that into the taper on the dividing head. This allowed me to visually line the cutter to the centre-line.

    aligncutter1.jpg
    Now the backplate onto the dividing head and load the gear onto that, adjusted to run true. The dividing head is a bit big for my mill but I manage it by hanging it right off the front and using extra clamps. Once it's all on and squared up I line up the cutter with one tooth gap.

    Note that the cutter is fully in the tooth but there's still a gap at the bottom of it.

    rootgap.jpg
    Now a simple indexing jig using two setup blocks and a 15 thou feeler gauge. Easy to use and foolproof, just loosen the blocks, turn one tooth and slide them back in, hold the tooth against the feeler gauge and tighten the clamp. Saves meddling about with index plates not worth it for this little repair.

    indexing.jpg

    Because of the gap in the root of the tooth I was wondering if the cutter was getting full depth, so I pulled it out of the gap and touched off on the OD of the gear. The cutter is marked as .1798" full depth and I measured .1755" which is only 3 thou different but there's 15-20 thou gap at the end of the cutter so I concluded that DSG must have added clearance at the root.

    infeed.jpg

    Cutting the tooth, this is the second side being done.

    cutting.jpg

    Each side cut now and you can see where the originals were cut deeper.

    toothcut.jpg

    I dressed the bottom of the root gap with a thin file. I could have done it with a slitting blade or end mill but I wanted to leave a rounded corner at the base which would make for a stronger tooth. It's just a clearance space anyway.

    fileroot.jpg
    And that's it. All done and ready for service. I've never used this method (or see it used except for Pressbrake's 3-tooth dovetailed repair piece but I think it'll hold up ok. Time will tell.
     
  3. jordhandson Member

    Messages:
    393
    Location:
    Dorset
    Nice job Pete
     
  4. Pete.

    Pete. Member

    Messages:
    8,260
    Location:
    Kent, UK
    Thanks!

    Just glad the other 89 didn't need doing as well :)
     
  5. Spark plug

    Spark plug Member

    Messages:
    3,242
    Location:
    Durham, England
    Excellent work Pete, and a brilliant how to :thumbup:
     
  6. slimjim Member

    Messages:
    932
    UK Cumbria
    Fantastic repair job, well done.
     
  7. 8ob

    8ob Forum Supporter

    Messages:
    4,292
    Location:
    moscow on thames
    Brilliant work as ever Pete, two dowels drilled through would give it a fair chance of being permanent, don't think I would trust glue.

    Bob

    Inkedgear_LI.jpg
     
  8. Luke

    Luke Forum Supporter

    Messages:
    1,931
    Location:
    Whitchurch, Shropshire
    Thanks, enjoyed reading that.
     
  9. Bill Edwards Member

    Messages:
    4,818
    Location:
    Scarborough, North Yorkshire
    I too enjoyed reading that.

    Have you had much to do with that expensive type of Araldite? I'm curious how it would perform compared with Loctite?
     
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