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Thread: how to drill out threads in hardened steel drive flange (Rotax B gearbox)

  1. #1
    Senior Member 100 Club
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    how to drill out threads in hardened steel drive flange (Rotax B gearbox)

    Hi
    Im about to drill out the threaded M8 holes in the Rotax B gearbox drive flange.
    I will fit longer AN5 bolts with nyloc nuts on the gearbox side of the flange.

    I'd appreciate some wise advice:
    1. Any suggestions for drilling the hardened steel drive flange?
    2. Should I use a 5/16 drill (to match my AN5 bolts) or an 8mm drill?
    3. Do I need to support the flange while drilling (to avoid any strain on the gearbox castings)?

    Thanks in advance for your help.

    Cheers,
    Bruce (New Zealand)

    Background info:
    I own a CFM Shadow B-D microlight with a Rotax 582 engine, B gearbox and 3-blade Precision Prop.

    In the early 2000's, several Shadows lost their propellers when the M8 prop bolts sheared off in flight (they were threaded into the M8 holes in the prop drive flange)

    In June 2002, a mandatory service Bulletin was issued (CFM SB 16) to replace the six M8 bolts every 25 hours. It notes that prop bolts which pass thru plain holes in the drive flange are OK - the CFM factory drilled out the threaded M8 holes and fitted longer bolts with locknuts.

  2. #2
    Hi Bruce (really? :-)

    My advice isn't wise.
    Go talk to a local toolmaker. I don't know much but I know twist drill bits don't produce round holes. I know that if you don't mange to get these non-threaded holes exactly in the right place you'll alway have trouble balancing the prop.

    I think you would want to be doing this job in a jig with a mill.

    More than that I do not know.
    Good luck.
    Laurie (2)

  3. #3
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    Bruce,

    How hard is 'hard' in reference to the drive flange? A decent quality, properly ground HSS drill should cut material up to HRC50 at the correct speed.
    I doubt (but don't know) that the prop flange is harder than HRC50.
    In any case, the flange should be removed from the gearbox and the holes plotted out on a mill. Drills can do odd things when trying to follow a threaded hole.
    Pete T.

    "A closed mouth gathers no feet".

  4. #4
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    Fresh cobalt bit, lubricant, mill. If you know what a new hub costs then you know why.

  5. #5
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    Hi
    Job done. I took the gearbox to a precision engineer with a milling machine.
    She used a tungsten carbide cutter thingee to drill out the M8 threads in the prop drive flange. Drill speed = 100rpm.

    PS: I had a 'spare' drive flange that we could practice on (from a damaged Rotax B gearbox).
    I tried an tungsten carbide-tipped 'glass/ceramic' drill bit. It easly removed the threads, but the hole was not pretty.
    A cobalt drill was OK, but it needed sharpening after each hole.
    The solid carbide tool was controlled and lasted for all the holes.

    Out of interest, she tested the hardness of the spare drive flange.
    It was 51 Rockwell HRC (= 492 Brinell), so Pete Twissell's guess was right.

    Thanks, guys, for the advice on this forum (and to Mick Broom who emailed me with info).
    Last edited by Bruce Conway; 29th November 2017 at 02:30.
    Cheers,
    Bruce


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  6. #6
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    Anyone know if the holes can be reamed out rather than drilled? As it has been said on here that twist drills don't leave a round hole when drilling threads I wondered if it would be better to use a reamer instead?
    XL's forever! Well, one of them anyway. It's all I can afford, not to mention the Raven and the Mini-Max. Oh, and I almost forgot the Spectrum as well :-)

  7. #7
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    A reamer is designed to remove a small quantity of material only.
    Whatever the tool used, most important is that the tool is presented to the job correctly and it's position and orientation are kept under control.
    A milling machine provides a rigid location for the tool, preventing it from deviating from the intended position. It also holds the tool square to the job and allows for a controlled feed rate to prevent the tool from 'snatching' into the job. This is particularly important when using solid carbide tools, which are brittle and would likely shatter if one tried to use them in a hand held Black & Decker.
    Pete T.

    "A closed mouth gathers no feet".

  8. #8
    Beautifully explained Pete, many thanks

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