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Thread: Trying to enlarge a 3/8" hole to 5/8" hole in a chrome alloy part with RC60 hardness

  1. #1
    Derek 122

    Trying to enlarge a 3/8" hole to 5/8" hole in a chrome alloy part with RC60 hardness

    Anyone got any ideas how to drill a bigger hole in a chrome alloy part?

    I have tried everything I can find including masonry bits (apart from carbide drill bits, but I hate to spend $200 just for a few holes). The part is not flat, so I can't punch the holes. Is there such a thing as a reaming bit to make the hole bigger? The bits I have tried vibrate something horrible.



  2. #2
    Technical Fellow
    Join Date
    Feb 2011
    Hi Derek,

    Well, first off, using a 2-flute drill is always going to vibrate badly (or well, actually ) To prevent that you could get a 3-flute drill but at that hardness you will need it to be Carbide and not cheap. Also going from 3/8" to 5/8" would have to be done in steps as the 3-flute drills do not have a lot of lead-in. That means maybe three drills.

    You could try a step-drill something like the following link, slow speed and lots of cutting fluid, but I don't like the chances of success...

    You could also try a 3-flute carbide insert end-mill, but it would need a rigid machine set up otherwise it will just break up the inserts?

    Have you asked at a local machine shop? They may have the tooling and equipment to do it for a lot less than 200-bucks.

    Generally, I will not give you the answer to your question, but I **will** guide you into discovering how to solve this yourself.

  3. #3

    from 3/8" to 5/8"

    What is the thickness , and the shape.?
    Could you upload a image.

  4. #4
    Lead Engineer RWOLFEJR's Avatar
    Join Date
    Mar 2011
    Rochester Pennsylvania
    Depending on depth of holes... number of holes... size of part... accuracy required to name a few... You might want to take to a local shop with EDM? Could be the same money as you'll have tied up in a cutter? Would also assure accurate hole size and location… and direction.

    If on the other hand... This isn't critical. And let's say the entire part was heat treated, not because the area at these holes necessarily needed to be hard, but because it was more economical to heat treat the entire part than to scan a portion... then you could soften the areas around the holes to make them easier to drill.

    Would need to see the print and know how this is being used…

    Good luck!

  5. #5
    Project Engineer
    Join Date
    Feb 2011
    Spokane, WA
    I am probably missing something, as usual, but this seems straight forward to me.
    The last 5/8 carbide end mill I bought was about $41, last week.

    Put it in the mill, clamp down the part, set your speed/feed,
    and let it go.


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