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  1. #1
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    Default Help - drilling deep holes in steel

    I need to drill two holes 125mm long for 1/2 inch bolts.
    I can only allow about 1mm off centre at the far end.
    It's a drill press job.
    I have a 1/2" 2MT drill bit that's long enough.
    Any suggestions on how to go about it?
    I'm wondering if it would be more accurate to use the large bit instead of smaller ones first, as it would be stiffer?

  2. #2
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by nadroj View Post
    I need to drill two holes 125mm long for 1/2 inch bolts.
    I can only allow about 1mm off centre at the far end.
    It's a drill press job.
    I have a 1/2" 2MT drill bit that's long enough.
    Any suggestions on how to go about it?
    I'm wondering if it would be more accurate to use the large bit instead of smaller ones first, as it would be stiffer?
    What ID tolerance is required.
    Can you mark out and drill from both ends?

  3. #3
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    Doing that in a regular drill press is pretty hard, youll need to stiffen the table up underneath with a bottle jack to prevent it deflecting. youll need the drill resharpened halfway through too. Thats a job thats really beyond the tools you have. Im not sure i could do that even with the stuff ive got. I would probably use a carbide stub drill to start the hole, maybe 1" deep then swap over. Carbide drills cannot tolerate any runout once they are in a hole though so i would want one helluva drill press.

  4. #4
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    I'm making a puller for a stubborn 4 groove B section pulley.
    The part needing the holes is a split plate, 25mm thick with a big hole in it to clear the pulley hub.
    It's similar to the type of split plate used to pull bearings etc, and will also have tapped holes on the faces of the two plate halves, for long studs.
    But this one will be optimised for this particular job. I think I need to have everything going for me.
    Because it's to be split, I could indeed halve the hole depths if I split it first. But I thought I'd try for a small clearance bolt fit, as it could help keep the assembled plate stiff.
    That could be achieved if drilled in one go for each bolt, the full 125mm.
    I might not attempt the long holes though, as I've already spent hours on it and don't want it ruined now. The trade off is I'd have to accept some misalignment so needing more bolt shank clearance.
    I said drill press, but actually I have a small radial arm drill, so rigidity is assured.

  5. #5
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    Oh I think you'll be fine then, definitely drill a deep start hole with a stubby strong drill if you can. It'll really help with drift

    Sent from my Nokia 8 Sirocco using Tapatalk

  6. #6
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by nadroj View Post
    I said drill press, but actually I have a small radial arm drill, so rigidity is assured.
    Hummmmm . . . . small radial arm drills are notorious for not drill hard stuff straight. Maybe have a practice run is something say 50 mm thick first?

  7. #7
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by nadroj View Post
    I'm making a puller for a stubborn 4 groove B section pulley.
    The part needing the holes is a split plate, 25mm thick with a big hole in it to clear the pulley hub.
    It's similar to the type of split plate used to pull bearings etc, and will also have tapped holes on the faces of the two plate halves, for long studs.
    But this one will be optimised for this particular job. I think I need to have everything going for me.
    Because it's to be split, I could indeed halve the hole depths if I split it first. But I thought I'd try for a small clearance bolt fit, as it could help keep the assembled plate stiff.
    That could be achieved if drilled in one go for each bolt, the full 125mm.
    I might not attempt the long holes though, as I've already spent hours on it and don't want it ruined now. The trade off is I'd have to accept some misalignment so needing more bolt shank clearance.
    I said drill press, but actually I have a small radial arm drill, so rigidity is assured.
    Some pics would be good.

  8. #8
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    I'm probably thinking too creatively, but:

    I think you only need the accurate holes in the center where the split is.

    How about this:
    Split the part, accurately mark/drill holes from the split face, to just under final diam you want for the bolts.
    Open up the holes (from the non-split end) part depth to clearance , leaving some of the original hole (30mm depth remaining in each part??).
    Clamp or tack weld the 2 halves back together, then finish size the holes. You're only effectively cutting a 60mm long section, with minimal material removal so it should stay pretty straight.
    Hope that makes a smidgen of sense.

    Steve

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    In the past for something like that I've drilled half way from both sides with a smaller drill (with the theory that any drift will only be half as much) and then using that as a pilot, drilled through to get the full size hole that has (carefully) negotiated the potential mis-match in the middle with a hole that approximates a true centre line. You have to make sure the hole halves are lined up so you can meet in the middle but all things being equal, it makes sure that the ends are where you want them and that any drift is only half what it would be if you went all the way from one direction.

    Michael

  10. #10
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    Are you drilling through hollow tube such as square hollow section or is a solid section ?. If hollow then I would suggest marking and drilling from both sides. If its a solid material then I would suggest getting a long series drill, but be careful if the ie steel isn't very good quality and you hit a hard spot the drill will wander. I realise that the tool you are making is to remove a stubborn pulley. So assuming that the pulley is secured by a grub screw that you have removed, I would suggest that you have another look and make sure that there isn't a second grub screw remaining (some manufacturers use a second grub to lock the first grub screw in place.
    All The Best steran50 Stewart

    The shortest way to do many things is to do only one thing at once.

  11. #11
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    I've been in the workshop, and came back to see several more responses to my query.
    Thank you all, and Stewart - no hidden extra grub screw. I had checked because I was caught by this one before.

    Anyway, I took the bull by the horns, threw caution to the wind and have drilled the two holes without splitting the plate first.
    The result is acceptable, runout not too bad.
    Method:
    I mounted the plate in a vice on the small Rong Fu radial arm drill.
    I shortened a spare 1/2" drill bit and turned it into a nice rigid stub drill.
    I centre popped 2 spots, used a centre drill to start, then the stub drill to make 40mm deep pilot holes.
    Swapping to the 1/2" 2MT drill bit, the holes were completed, 5mm deep pecks plus cutting fluid. It went smoothly, didn't need to resharpen the bit.

    The main thing I learnt was to use a stub drill of the same size as the finished hole, for its rigidity. The follow up drill is then guided by this hole, so the flexibility due to its length does not cause much problems.
    The 1/2" capscrews have minimal clearance - perfect.
    As this is going to be a "once only use" tool most probably, it's not pretty. I didn't want to spend much time on it, but if I had gone to the trouble of milling true squares, flats and parallels it's likely the accuracy of the holes would be even better. I'm chuffed with the outcome, so thanks again fellas.

    Here's the result, prior to splitting it on the bandsaw.

    IMG_20190606_191020.jpgIMG_20190606_191012.jpgIMG_20190606_191238.jpg

  12. #12
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    Well done there Jordan, I just have a couple of comments, and not from personal experience either, just stuff that I have heard (or think I have heard )some time ago.
    The first tip was to start the hole where you wanted it to finish, ie. the most critical point.
    Secondly, and I'm not sure if you did this, is to drill in from both sides, that way the start and end points are correct, and hopefully the hole joins adequately in the middle.
    I have also heard that spade bits tend to drill accurate deep holes, apparently much better than normal twist drills, but I have never use on in metal. Interestingly I saw a YouTube vid from ClickSpring just a day or two ago, where he made one by carburising some mild steel, and then grinding it into a spade bit. His was only small, but it is worth a look, for future reference. Anyway I am pleased that you have had a satisfactory result for your efforts, Cheers,
    Rob.

  13. #13
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    Thanks Rob,
    To drill from both sides would have required cleaning up for parallel, more work which I avoided for this job.
    It was "fingers crossed" but had a happy ending.

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