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Thread: Tap hole in bandsaw cast iron table

  1. #1

    Tap hole in bandsaw cast iron table

    Have a grizzly 14Ē bandsaw and need to drill to tap 2 holes into the cast iron table. Everything Iíve read says any bit that works with metal will work and itís relatively easy.

    I bought the heftiest 13/64 bit at the store and tried lots of my other HSS bits I have too. But so far I can only get a small dimple. Should this take really really long or are these machine tables different kind of cast iron that canít be drilled?

    Anyone else do this successfully?

  2. #2
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    Many times.
    I don't know why you're having trouble, 13/64th's is a relatively small bit. I know folks think that you don't need cutting fluid for cast iron, but I always found it helps.
    Are these quality drill bit's? What's the speed of the drill?
    Odd.
    "The first thing you need to know, will likely be the last thing you learn." (Unknown)

  3. #3
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    I've drilled and tapped several cast iron tables. I've always used a little cutting fluid (or light oil) and a slow speed on the drill motor. Trying to drill fast with cast iron doesn't work.

    What size thread are you trying to cut? I assume the 13/64-inch drill is the pilot hole and you have the correct drill for the tap you will be using.

  4. #4
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    Probably going too fast; you need slow for metal. A little oil, or even water, helps. If all else fails, try a masonry drill.

  5. #5
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    A little oil and a speed of between 75 and 100rpm should work for CI.
    Of all the laws Brandolini's may be the most universally true.

    Deep thought for the day:

    Your bandsaw weighs more when you leave the spring compressed instead of relieving the tension.

  6. #6
    Iím trying to tap for a 5mm bolt if I remember correctly. There isnít an exact fractional not but whatever I came up with was close enough on the charts.

    Could it be that I need to put a lot of force into the drill? I might try to do it on my drill press too.

    Some of the machinist sites talk about a layer of extra hard cast iron on some things. Iím going into the corner of the table that is about an inch or so thick. Not sure if that matters.

    Iíve tried slow and fast and even hammer drill..

  7. #7
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    Something is going on here. I’ve drilled three eights inch holes in cast-iron using a battery powered hand drill at about 200 RPM. Very easy.
    Buy a man a plane ticket and heíll fly for a day.
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    and heíll fly for the rest of his life.

  8. #8
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    Quote Originally Posted by glenn bradley View Post
    Something is going on here. I’ve drilled three eights inch holes in cast-iron using a battery powered hand drill at about 200 RPM. Very easy.
    I agree a hole less than 1/4" shouldn't be hard at all. My first thought was bit but he said he tried multiples. I have drilled much bigger holes in CI tops with a hand drill and crappy Asian bits and normally don't lubricate, CI generally self-lubricates. I guess it could be unusually hard (but Grizzly?) worst case get a quality cobalt bit, probably with a 135* point.
    Of all the laws Brandolini's may be the most universally true.

    Deep thought for the day:

    Your bandsaw weighs more when you leave the spring compressed instead of relieving the tension.

  9. #9
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    Do not hammer drill! you can crack the cast iron. I would switch to a 1/8 bit for a pilot drill. Saw table should not be hardened. Poor casting methods may have made chilled iron near the edges.
    Bill D.

  10. #10
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    You may be applying insufficient pressure, and/ or too fast rpm.

    Steel and Cast Iron needs WAY more feed pressure than drilling any wood.

    Watch a few Youtube videos to see what type of cutting action you want to see.

    If the above was your problem, and pretty sure it is, you may have dulled the bit some by burnishing it.

    Marc
    Last edited by Marc Jeske; 07-23-2018 at 1:06 AM.
    I'm pretty new here, not as as experienced as most. Please don't hesitate to correct me

  11. #11
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    Quote Originally Posted by Dave Rosner View Post
    Iím trying to tap for a 5mm bolt if I remember correctly. There isnít an exact fractional not but whatever I came up with was close enough on the charts.
    The drill size for the M5 tap is 4.2mm, which is about 0.1654 inches. The 13/64-inch drill is 0.2031 inches, so your M5 bolt will have a looser fit and won't take much torque before it strips out the threads in the table. If you can't find a 4.2mm drill, the #19 is the closest match at 0.1660 inches.

    For what it's worth, I buy all of my SAE and metric drills and taps from McMaster-Carr. If you can't find the correct drill locally, McMaster-Carr has fast delivery.

  12. #12
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    Just checking that you are running the drill in forward position and not backwards? Sometimes i forget to reset switch position after backing out of a hole.

  13. #13
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    Quote Originally Posted by Dave Rosner View Post
    Some of the machinist sites talk about a layer of extra hard cast iron on some things. Iím going into the corner of the table that is about an inch or so thick. Not sure if that matters.
    Some people anneal the cast iron before machining. I've never tried that - would need a good sized oven or a way to cool very slowly.

    If the carbides are just on the surface, maybe you can use a diamond bit on a dremel or something to get through. I've drilled and tapped cast iron but haven't run into the need for anything other than standard drill bits. I know you can buy diamond drill bits but don't know if you can get one that small, and even if you did tapping might be a problem. I might start thinking about another way to do the job.

    JKJ

  14. I am retired now but worked in the machining trades for 40 years so know a bit about machining most kinds of metal. First, if you can't drill it you would never be able to tap it. A tap will be much more brittle than a drill so if a good quality drill won't go through it just forget it. I also agree that the drill is probably toast by now and you will need a new one. A few drops of motor oil should be a big help and absolutley use some on the tap. Sometimes cast iron does get harder near the edges of a casting so if you can move the holes further away from the edge that might also help.
    IIRC a 13/64 dia. is the correct size for a 1/4-20 thread, whatever size threads you choose use the correct size drill. If the hole is too big for the thread size the threads will pull out too easy. If the hole is too small it will be harder to tap and the last thing you want to do is have a broken tap in the table. Good luck, Jim.

  15. #15
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    Beware of carded taps and tap drill sets at the hardware store. I have seen them with the wrong tap drill where they just throw in the closest fractional size. If it is too hard to drill consider a through hole with nut on the bottom. The nut could be epoxied if needed.

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