Scribe and center punch

  1. Memmeddu

    Memmeddu Member

    Messages:
    2,943
    Location:
    Italia Sardegna
    How to do it properly?
    Every single time I use a scribe to find out where to drill and then I use a center punch I never hit the perfect center .
    No matter if I put the point of the punch in the exact center of the cross ,the center mark will be always off .
    Why?
     
  2. hotponyshoes Member

    Messages:
    3,373
    Location:
    Somerset. Uk
    3 words.
    Optical center punch.
     
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  3. brightspark

    brightspark Member

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    31,670
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    yarm stockton on tees
    give it a gentle tap then look at it. if its slightly off move it over a touch and tap again if its correct give it a good bash :laughing:
     
  4. hotponyshoes Member

    Messages:
    3,373
    Location:
    Somerset. Uk
    Or, mark the hole. Center punch it.
    Workout the required drill bit size.
    Use the next drill up.
     
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  5. Hopefuldave Intergalactic pot-mender

    Messages:
    1,804
    Location:
    The Shed of Danger, surrey, England
    A prick punch is a good thing to have - finer point (30* or less included angle not 120*), with practice you can run it along one scribed line and it sticks where the other crosses it - press it to make a small dent - using only hand pressure, check position, then put the point of the normal centrepunch in the dent and tap once, check it's still centred, then whack it as normal :)

    On one US site the language checker changed it to "Jerk punch" - which seems worse to me!

    Dave H. (the other one)
     
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  6. Wallace

    Wallace Member

    Messages:
    7,511
    Location:
    Staines, Middlesex, England.
    This is something that bugged me for many years, only resolved for me by adding extra time to the process. I like accuracy and marking the lines accurately with a height gauge then center punching by eye goes against the grain. I blamed punches going blunt and all sorts eventually resorting to marking the centre with the scriber then checking with a loupe and centre punching. I bought myself an optical centre punch as a birthday present some years back, something I should have done years ago. Time consuming but satisfying.

    What it all comes down to is how accurate you need the hole centres to be. Unless you drill the hole by cenrtreing, centre drilling and going to size in increments then all the accuracy goes out the window. A drill bit into an accurately centre punched hole will drift.
     
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  7. Turbo Member

    Messages:
    3,846
    Location:
    Fermanagh, Northern Ireland
    I have a diamond tipped scribe that is great at scribing the lines. I have a steel masonry nail (obo) sharpened to a fine point that I use to feel the intersection of the scribed lines then I hit it a light tap with the hammer, leaving a small punch mark. If it all still lines up it then gets marked with a proper centre punch.
     
  8. mtt.tr

    mtt.tr Member

    Messages:
    4,012
    Essex England
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  9. Memmeddu

    Memmeddu Member

    Messages:
    2,943
    Location:
    Italia Sardegna
    I'll try again
    Anyway I'm using a Lidl centre punch
    And a 1kg hammer (smaller I have:D)
    I never strike hard because I know what can happen when two hardened surfaces hit on one each other.
    Then I use a split point 3mm drill bit to make the pilot hole
    I'm not making nothing extremely accurate (truck fenders brackets) but I don't like misaligned holes on my projects:)
     
  10. awemawson Forum Supporter

    With a scribed line you should be able to feel when the point of the centre punch drops into the minute ‘ditch’ as you Lightly draw it across The line
     
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  11. mtt.tr

    mtt.tr Member

    Messages:
    4,012
    Essex England
    A prick punch has a very sharp point, typically a center punch is duller
     
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  12. Hopefuldave Intergalactic pot-mender

    Messages:
    1,804
    Location:
    The Shed of Danger, surrey, England
    MUCH sharper, only needs hand pressure to make a mark - some have a mushroom shaped handle like a cobbler's awl, which spreads the load and makes 'em comfy to use

    Dave H. (the other one)
     
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  13. Memmeddu

    Memmeddu Member

    Messages:
    2,943
    Location:
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    Might be this the problem .
    I put pressure when I use my tungsten carbide scribe
     
  14. Munkul

    Munkul Jack of some trades, Master of none

    Messages:
    3,144
    Location:
    Cumbria
    I lean the punch over to see the lines before tilting upright and bashing. I use an auto punch for this, and then give it a heavier whack with a bigger punch and hammer a second time.

    It's not perfect, but the marks are always within 0.25mm I'd estimate, and you can always move the mark in the way Brightspark suggested.

    If you want more accurate than that, then optical centre punch would be the way forward :)
     
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  15. Rrobson

    Rrobson Member

    Messages:
    247
    Location:
    West Yorkshire
    Check the point on the end of your centre punch first. You may find it’s not particularly sharp and will have a tendency to end up anywhere. What size punch are you using? There are various sizes available and smaller punches will help in more accurate scenarios. And don’t worry about the hammer hitting the punch, the punch will be softer but safety specs should be used regardless, very hard punch shanks will chip when excessively bruised which is why they should be dressed now and again.
    Also, once you’ve lined up the punch give it ONE moderate tap and then look at the hole. If you give it a gentle tap first then hit (akin to a practice swing) this will make the punch jump. If it doesn’t hit dead centre then you can angle the punch in the direction you need to move the punched indent over and elongate the hole. Then give it a bigger whack and it should be near enough.

    opticals are very good also and if you need greater accuracy then it’s one of those or you’ll be limited to a milling machine. As said, you should be able to get within 10 thou. Failing that, you can always force the drill over slightly when making the hole.
     
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  16. Memmeddu

    Memmeddu Member

    Messages:
    2,943
    Location:
    Italia Sardegna
    I'm using a cordless drill.
    I don't have a milling machine or even a drill press.
    I would get more accuracy if in the future I'll make a fixture table using my mag drill .
    And also because I've asked about a job as apprentice in a machining shop
     
  17. lchris21

    lchris21 Member

    Messages:
    1,515
    Location:
    South Wales
    ....there ya go...grab a coffee..every day is a school day!

     
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  18. mtt.tr

    mtt.tr Member

    Messages:
    4,012
    Essex England
    Are you using a pilot drill first?
     
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  19. Memmeddu

    Memmeddu Member

    Messages:
    2,943
    Location:
    Italia Sardegna
    Just a normal 3mm split point HSS Co 8% drill
     
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