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Thread: Drilling precise holes

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  1. #1
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    Default Drilling precise holes

    To date I have not been able to bore a precise hole. Tonight was no exception. I had a piece of 12L14 about 2 inches in diameter and 4 inches long chucked up in the lathe and wanted to drill a 3/8 hole lengthwise through it. The 3/8 drill bit created a hole that is bigger than 3/8". What do I need to bore a precise hole? Do I drill undersize and ream it to spec? I have been using a boring bar for other jobs but coming up with a precise 1/4" hole would be an issue.

  2. #2
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    Depends how precise, but you drill a smaller dia hole to start and increase in steps, finish with a reamer if the accuracy is needed.
    Oh yes,the drills need to be sharp, and coolant helps with chip clearance.
    Regards Ian.

  3. #3
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    Default Poor man's reamer.

    I've read the OP several times. I'd guess that where he says "bore" he means "drill". Further, size seems more important than "straight". The level of accuracy is not stated either and reaming seems to be either a second option or an after-thought.

    Assuming that is the case, I would centre-drill and start small-"ish" and "work up" toward the required size. Stop short at about 1/64" to 1/32" under-size.

    Grind the drill of the required size (3/8"??) accurately.

    Hone the cutting edges.

    Hand-hone a small (say 1/64") radius on the outer "corner" (where the cutting edges meet the outside diameter) of both sides/edges.

    Slow speed, high/medium feed, lots of cutting oil - and "go for it" - don't stop if you can help it until you get to correct depth.

    I know, I know, I know, the "Purists" will shriek with dismay, have fits, palpitations, apoplexy and coronaries etc. (and blame Tiffie for giving bad advice and "tear-arsing" and "get 'er right" etc. etc. - again).

    Maybe so.

    But give it a go.

    You might be quite surprised.

    For another surprise - try pushing the drill down the hole by hand and you might find that the round shank of the drill is a pretty good fit.

    "Poor man's reamer".

    It was very old - I was told - when I was taught it - 55+ years ago!!

    There are others too that I will leave for later.

    I am quite confident that this will meet the OP's requirements as I understood them.

  4. #4
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    Good advice Mick.
    I also recall a post, on this board I think, re grinding masonry bits for small boring bars.
    Must add to this, my usual comment. Turn the boring bar upside-down and cut at the back
    May be worth a try.
    Agree also, the difference between drilling and boring must be very clearly in mind!
    Just got my head together
    now my body's falling apart

  5. #5
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    Tiffie, lots of good advise there but don't centre drill, spot drill! better results and you won't bust the tip off your centre drills

    Richard, you have to define precise. The smaller a number precise becomes the more challenging, and the more specialize equipment you need to get there.

    drills drill an inaccurately sized hole because of a bad grind - the lips are different lengths - as a drills axis is around its point (not the chuck's axis) you get an over sized hole. this also tends to make them wander of centre. with a T&C grinder i can get drills to drill within a couple of thou becuase they are very close to dead on, but for all practical purposes, a drill is NOT used to make a precision hole. Reaming is the next step in the hierarchy , drill undersized and search here about reaming, there was just a thread on techniques. The other trick is to creep up on the final size, drilling in several stages - this helps with straightness but its not going to be better than reaming for dia.

    you can bore a 1/4 hole, you just use a smaller boring bar - albeit not for 4" Holes with a high length/dia ratio that must be precise are very challenging for everyone, although more for keeping it straight than dia - dia to a reasonable accuracy is quite well handled by reaming.
    Last edited by Mcgyver; 08-22-2008 at 07:11 AM.

  6. #6
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    Default

    Quote Originally Posted by oldtiffie
    ...

    For another surprise - try pushing the drill down the hole by hand and you might find that the round shank of the drill is a pretty good fit.

    "Poor man's reamer".
    ... .
    You will find that most modern drills have the shank a couple thousandths smaller than the cutting diameter. for example, my quarter-inch drills measure .248 at the shank.

  7. #7
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    Richard, you'll never "drill" and accurate hole. You can "bore" and accurate hole and sometimes you can ream an accurate hole.

    As stated, the cutting edge of each flute have to be exactly the same length and the angle of each flute must be exact to get the best performance out of a drill bit. To do that you would have to sharpen the drill bit on a grinding machine that would be comparable accuracy to grinding an endmill. With most drill sharpeners and doing it by hand the tips will be close but not perfect.

    If there is one flute dull or short or the wrong angle the drill will most likely cut an oversized hole no matter what you do.

    You can drill the hole in steps but if the last drill you use is not PERFECTLY ground the hole will oversize.

    If you want a hole to a certain size drill undersize and ream. Even then the speed, feed and type lube or no lube will affect the final size of the hole. Many times it's a crap shoot and the reamed holes may vary by .0005" or more depending on the size of the hole.
    It's only ink and paper

  8. #8
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    Default "D"-bit

    Get some 3/8" HSS rod or "drill-rod" ("Silver steel" in UK and OZ) and make yourself a "D"-bit. Drill rod will need to be hardened - and depending upon the material to be bored - "tempered"/"drawn".

    Drill to a bit less than 3/8", bore accurately to 3/8" (0.3750") for say 1/4" to 1/2" deep (use the HSS or drill rod as a guage) then follow through with the "D" bit.

    It will be as close to 3/8" as .............................

  9. #9
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    I was always told that you could drill a hole that the drill would not go back into easily. You had to sneak up on the final size in stages, never succsseded myself but I am usually in a hurry to get to the next step. I also not sure as to the concentricity that can be obtained with an ordinary drill that why gun drills were made. Most gun barrels are made today by swaging down onto a former much easier and accurate.

    Peter
    I have tools I don't know how to use!!

  10. #10
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    Yeah Peter, they're swaged down onto a carbide form which also does the rifling.
    It aint gonna appear in the home workshop real soon

    Try googling Glock 17 frinstance.
    And gun drills are made for long skinny holes, not necessarily accurate, axially.
    A gun (rifle, pistol, shotgun, even artillery) can be straightened after machining.
    An engine cylinder, for example, cannot.
    Just got my head together
    now my body's falling apart

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