Regrinding a drill bit to an emergency countersink?
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  1. #1
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    Default Regrinding a drill bit to an emergency countersink?

    Hi,

    I have some broken Chinese machinery made of cheese that I have to get functional by tuesday morning. I have to countersink (M5 din 7991 bolt. About 1/4 allen head machine screw) two dozen holes that are inaccessible with anything less than 4 " shank to very mild and chewed up steel.

    I was going to take a 10 mm (5/8) HSS drill bit and single flute grind it and peck away. Is this viable? Or would I have better luck with keeping both flutes? I don't want to break a bit in there unless it's just a calculated risk.

    I wouldn't really know what to look for in the grind. Not really my area of expertise.

    I'm in Finland; nothing's open until it's too late, and finding a long shank countersink won't be easy anyway.

    Any pointers would be appreciated. Google didn't seem to help.

    With regards,
    Jon

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    I have made "emergency" countersinks a number of times and can say that while not ideal, it does work.

    I always used both flutes so I have no experience with using just one. Even with two cutting edges you will not get a perfect cone; there will be some waves in it. I suspect that a single flute tool, even a 10mm one, with a four inch extension will wobble like crazy. I would do two flutes.

    Grind it just like a normal drill bit sharpening: the same clearance is needed. Just change the tip angle to produce the 90 degree countersink angle. Start with the drill at a 45 degree angle to the wheel.

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    Like EPA said, just grind it like a drill with a 90 degree angle rather than the normal 118 degrees. Since the material is soft, I would take a hand stone and slightly blunt the cutting edge after grinding as this will greatly reduce the tendency of the drill to snag the entering edge of the existing hole as that's the most likely place to cause the drill to break.

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    It is quite easy to make a countersink out of any tool steel that can be heat treated. Take a 10mm bar, machine a 90 degree point and mill half the diameter - after heat treatment stone the flat and it will cut nicely. I make oll sort of countersinks and similar tools this way, sometimes using broken end-mills (but those must be ground to shape).
    A drill will make a very poor countersink.

    countersinks.jpg

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    If this is a 1/4 shank countersink, just weld a 1/4 rod to the end of the shank, any length you need.

    Jeff

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    I've made a number of "extensions" for countersinks over the years. If it's something that will be used for only a few times you can use a piece of mild steel or aluminum slightly smaller than the diameter of the countersink itself. Bore one end the diameter of the shaft, put in a couple set screws to hold it in place and be done. I have half a dozen different length and diameter ones I made sitting in a rack. They get used for getting into tight places where the chuck would rub on one side of the work piece or deep holes.

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    Thanks everyone.

    I don't have any suitable steel lying about for making one long enough. I'll try to whip up an extension, it's something I thought of while coming to work today. Problem is, I had my minions clean up and I've spent hours looking for the lathe chuck key...

    Might have to try the ground drill bit. Good idea, the dulling of the cutting egde.

    This is just a small maintenance and proto shop down the basement of our company, and most of the basic stuff is still unbought or in piles of unassorted junk from the auction I bought everything from.

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    Use one of these (preferably before drilling and tapping but will work after)...


    https://www.keocutters.com/product/24564/

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    I used to use extensions like this a lot in a shop I used to work in. The only thing retaining the counter sink is a .001" press fit.
    20190422_103001.jpg

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    Quote Originally Posted by kenton View Post
    I used to use extensions like this a lot in a shop I used to work in. The only thing retaining the counter sink is a .001" press fit.
    20190422_103001.jpg
    This is what I ended up doing, although the press fit was made fast with a heavy mallet...

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    Quote Originally Posted by Jonan View Post
    This is what I ended up doing, although the press fit was made fast with a heavy mallet...
    If that worked then job well one..

    I agree with the guys who say use both flutes in making out of a two flute drill.ad the hight rake attitude of a drill likely needs to be toned down some.


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