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  1. #1
    Pat G is offline Hot Rolled
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    Default Best tool for flat bottom hole drilling CNC?

    I'm boring 7/8" cold roll steel to get a flat bottom at 2.250" depth- .563 in diameter.

    What I've been starting with is peck drilling an 11mm hole with a carbide bit I have an abundance of, then following with a 1/2" x 4" long 4 flute carbide center cutting end mill to get a flat bottom, then using a boring bar to finish.

    The carbide end mill works well until the edge chips and breaks, then it takes out my .250 I.C. boring insert.

    Is there a better 1/2" bit to use for this? Roughing end mill? Roughing finishing mill? Flat ended dill bit? HSS center cutting end mill? Coated? Or?....

    The cycle time is taking longer to get the end result than I would like. Sure would like to find a better way.

  2. #2
    jcoehlo is offline Aluminum
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    Depending upon your diameter tolerance & finish requirements, I would drill 2.250" deep with a 9/16" drill (carbide or HSS), and then go back in with a long 9/16" HSS, 2flute endmill to a flat bottom. If you need a truly flat bottom, grind the endmill flat first. You could hand relief grind the endmill flutes, leaving about 1/8" t0 1/4" of full size flutes at the end. This would reduce the chance of the endmill chattering on its sides as it goes to the bottom of the hole.

  3. #3
    Pat G is offline Hot Rolled
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    Thanks for the fast reply ...

    It has to be .563 in diameter within .0005 and flat, but it can taper deeper towards the side edges like a mill. Finish needs to be reasonable smooth. Doesn't have to be a mirror.

  4. #4
    Pat G is offline Hot Rolled
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    I just realized I do need to finish with the bar because of a fairly big chamfer with corner edge breaks at the opening.

    I also remembered I have a 14mm coated HSS 4 flute long end mill I could try. It would have to hog out .120 on the next pass after my 11mm drill.

  5. #5
    Metalcutter's Avatar
    Metalcutter is offline Titanium
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    I don't know if this applys here, but I really like to finish ream with a half round drill ground with a flat bottom to the center. The hole doesn't seem to get any bigger than the half round drill and the bottom is flat because there is no flute gullets to mess up the flat grind.

    Mine have been in brass though. Really beautiful finish.

    Regards,

    Stanley Dornfeld

  6. #6
    Limy Sami is online now Diamond
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    Quote Originally Posted by Metalcutter View Post
    I don't know if this applys here, but I really like to finish ream with a half round drill ground with a flat bottom to the center. The hole doesn't seem to get any bigger than the half round drill and the bottom is flat because there is no flute gullets to mess up the flat grind.

    Mine have been in brass though. Really beautiful finish.

    Regards,

    Stanley Dornfeld
    Over here we call em Dbits,............ easy to make out of drill rod or HSS drill blanks,.... and as Metalcutter says they don't chew up the sides of the holes.

    Work equally well in steel, just don't push things too fast, give em time to work as it were.

  7. #7
    Pat G is offline Hot Rolled
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    You have a picture of one?

    Anybody ever make a flat bottom drill out of a regular jobber twist drill?

    I'm gonna try the 14mm end mill this morning and see how that does.

  8. #8
    Metalcutter's Avatar
    Metalcutter is offline Titanium
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    Quote Originally Posted by Limy Sami View Post
    Over here we call em Dbits,............ easy to make out of drill rod or HSS drill blanks,.... and as Metalcutter says they don't chew up the sides of the holes.

    Work equally well in steel, just don't push things too fast, give em time to work as it were.
    Thank you Mr. Sami...

    I'm still looking for fish and chips, and some Ale.

    The bit about steel you left in here is welcome information. T'You

    Best regards,

    Stan-

  9. #9
    Limy Sami is online now Diamond
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    Quote Originally Posted by Pat G View Post
    You have a picture of one?

    Anybody ever make a flat bottom drill out of a regular jobber twist drill?

    I'm gonna try the 14mm end mill this morning and see how that does.
    In that case I'd drill 1 X D deep with an ordinary pointed drill first to use as a guide for the flat bottomed bit,........ and only grind 1 flute to do the cutting - back the other off so it's clear of the work, grind / stone a small flat across the outside corner of the cutting edge 15 thou wide is enough, makes the edge last much longer.

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