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  1. #1
    Join Date
    Jun 2011
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    gold coast
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    303

    Default Off centre drilling--lathe

    Hi Guys,
    I feel a bit helpless at the moment when I cant track down a problem myself, but in act Im stumped.

    I set up a simple turning job in aluminium on the weekend.
    Drill a 5 mm hole through a 65 mm long piece of 20mm diameter stock.
    Counter drill 50 mm deep to 13 mm ID.
    Of course I took it in stages (8mm, 10mm, 12mm, 13mm)
    When I took the part out of the chuck I noticed the 5mm hole was off centre at the 'other' end by almost 3 mm.

    I set up a second piece of stock and drilled right through to 5 mm diameter again. Same outcome.
    I used WD40 as a lubricant and drilled in small 5mm stages.

    I set up two 200mm lengths of 6mm ground stock, one in the three jaw and one in the drill chuck in the tailstick so they almost touched.
    I lay a straight edge along the top and there was no light gap so I assumed the centre lines are the same???
    Maybe a bad assumption?/
    So Im stumped.
    I'm open and grateful for any suggestions please
    warm regards
    Brian
    (PS Ive tried to upload a pic without success---sorry

  2. #2
    Join Date
    Jun 2011
    Location
    gold coast
    Posts
    303

    Default Offset drilling lathe problem pic

    Heres a pic I think
    regards
    Brian
    Attached Images Attached Images

  3. #3
    Join Date
    Apr 2012
    Location
    Healesville
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    2,129

    Default

    3mm... sheeezz... is the chuck mounting clean and tight?
    If so extend a length of stock say 100mm out of the chuck and put an indicator on it near the chuck and then at the end and see what you get

  4. #4
    BobL is offline Member: Blue and white apron brigade
    Join Date
    Feb 2006
    Location
    Perth
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    7,183

    Default

    What sort of tolerances are you talking about?
    I assume you want a 5mm hole 15 mm long at one end and a 13 mm diam hole 50 mm log at the other?
    First thing I would look at is the bit - is it and old 5mm bit? If so I'd try a new drill bit.

    It's not what I would call precise but if you are not after high precision the way I would do it is with a 6mm (minimum) drill to a depth of 50 mm and then use a 13 mm bit - ie no staging. Then drill a 5 mm hole from the other end.

    To drill long holes from one end with small diameter bits I usually start with a shortened bit so the first 6mm bit will be short - then I switch to a standard bit and then to a "long" bit.
    I keep broken bits and repoint them so over the years have most of a set of so called "shorts" - "longs" you have to buy.

  5. #5
    Join Date
    Jul 2016
    Location
    Melbourne
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    35
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    1,522

    Default

    I would definitely suspect the drills, I would probably start the hole with a very short stub length starter drill then I would drill the counterbore first all the way to 15mm then punch the smaller hole the rest of the way with the shortest 6mm I could use.

    I definitely wouldn't use that many drill to open the counter bore up.

  6. #6
    Join Date
    Oct 2011
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    Norwood-ish, Adelaide
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    59
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    6,542

    Default

    As others have said, you are stepping too much. A pilot drill only needs to be around 25% of the diameter of the finished drill - it's only purpose is to provide clearance for the web of the next size drill up. Now if your drills are not sharpened properly or have some damage on the tips, then one side will cut more than the other and that will tend to make the drill wander. Drills also follow an existing hole, so if one of your drills has wandered then the hole is doomed...

    I have a small carbide boring bar (Kennametal I think) that takes a CCMT06(?) insert. That will start in a 9mm hole, so ideal for boring out a small part like that. I must admit that I rarely drill holes bigger than around 9 or 10mm on the lathe these days as the boring bar makes it really simple (and I need the practice)

    Michael

  7. #7
    Join Date
    Oct 2008
    Location
    Cairns, Q
    Posts
    666

    Default

    Cheat. Use oversize stock, drill it through then turn the O/D to size between centres.

    Frank.

  8. #8
    Join Date
    Oct 2001
    Location
    ACT
    Posts
    667

    Default

    a well sharpened drill and clearing the drill of swarf frequently can make a big difference also.

  9. #9
    Join Date
    Apr 2012
    Location
    Healesville
    Posts
    2,129

    Default Adjusting your hole size

    Just a quick bit of info, to drill an oversized hole in the lathe.

    Just say for instance you need a 5.2 mm hole but you don't have an incremental drill that size, no reamer either, what do you do??

    You sharpen the drill with a slightly longer cutting edge, and naturally the other side is a bit shorter and the tip of the drill is slightly offset, centre drill for your hole first.

    Yep...you get a bigger hole and the hole that it drills is still pretty close to the centreline of the spindle.

    or you can blunt one of the the cutting edges for the same purpose, drill pilot hole first.

    So what I am getting at here is that even a bent drill should drill a straight hole in a lathe if it has a centre hole to start it first.

    Just sayin"..... 3 mm is a hell of a lot to be out over 65mm ?

    cheers, shed

  10. #10
    Join Date
    May 2011
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    Murray Bridge S Aust.
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    5,945

    Default

    Have you had this problem before? If so, the chuck may not be aligned sideways.
    Is your lathe a flat bed, a slight bit of play sideways may be enough to throw it out, I have that trouble with my wood lathe.
    Kryn
    To grow old is mandatory, growing up is optional.

  11. #11
    Join Date
    Dec 2010
    Location
    Syd
    Posts
    492

    Default

    I know lots of people get lazy, start with a centre drill?

    2 and 1mm steps seem excessive, short of lathe having no balls....or slipping tailstock.

  12. #12
    Join Date
    Nov 2007
    Location
    melbourne australia
    Posts
    3,228

    Default

    When drilling deep holes I drill from both ends where possible. Even if the holes don't line up in the middle, the subsequent drill bit tends to align the two holes. I always start with a centre drill as mentioned above.
    Chris

  13. #13
    Join Date
    Nov 2007
    Location
    melbourne australia
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    3,228

    Default

    Further,
    On my reading of the OP, it’s the initial 5mm hole that is wandering, so it’s not the stepping of the drills to get to 13mm that is causing the problem.
    Chris

  14. #14
    Join Date
    Sep 2012
    Location
    York, North Yorkshire UK
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    6,444

    Default

    Hi Guys,

    When drilling small deep holes, it is vital that both edges of the drill cut exactly the same, otherwise you will get the effect that you have experienced.

    Years ago I had drills specially made for me by Dormer so that I could drill a 5/32" diameter 6" inch long hole in full hard aluminium bar, and it had to exit within a few thou of spec ! A very difficult task indeed.
    Best Regards:
    Baron J.

  15. #15
    Join Date
    Jun 2011
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    Australia east coast
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    2,713

    Default

    Quote Originally Posted by BaronJ View Post
    Hi Guys,

    When drilling small deep holes, it is vital that both edges of the drill cut exactly the same, otherwise you will get the effect that you have experienced.

    Years ago I had drills specially made for me by Dormer so that I could drill a 5/32" diameter 6" inch long hole in full hard aluminium bar, and it had to exit within a few thou of spec ! A very difficult task indeed.
    This is why gun drills were invented.

    I've never ever had a hole wander off centre that badly and I've shoved some awful dodgy hand-sharpened bits into all sorts of material in a lathe chuck.

    Normally I centre drill, pilot drill and then use the biggest drill I can to finish the job. Step drilling is a PITA and wastes time.

    PDW

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