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Thread: Slot Drill Size

  1. #1
    Join Date
    Apr 2009
    Location
    Kingswood
    Posts
    930

    Default Slot Drill Size

    I just did a job using a new 2-flute solid carbide slot drill in 6061 aluminium on my CNC mill.
    A quick check with a caliper said about 3.85 mm, so I "assumed" it would cut 4 mm.
    Not so, it actually cut like a 3.85mm - duh !

    So, measured carefully the flutes, also 3.85 mm.
    5/32" is 3.97 mm, 9/64" is 3.57 mm, so it does not appear to be imperial.

    Is it normal for cutters like this, probably purchased cheap somewhere, to be somewhat smaller than the expectation ?

    I will start checking actual cutter dimensions for future jobs.

    Keep well,
    John

  2. #2
    Join Date
    Aug 2008
    Location
    Melbourne
    Age
    34
    Posts
    1,075

    Default

    Just had a look through the Guhring catalogues and their tolerance on flute diameter is generally h10 or e8, which would make a 4mm endmill 3.952 - 4.000mm or 3.962 - 3.980 respectively.

    Some tolerance is to be expected, but 0.15 under is waaay not acceptable.

  3. #3
    Join Date
    Sep 2009
    Location
    Newcastle
    Posts
    341

    Default

    I've found cheap Chinese carbide is sometimes under size by a far amount. I assume they are starting with a nominal size blank and then grinding a bit off.

    Not a big issue for me using CNC so long as I remember to check actual cutter size...

  4. #4
    Join Date
    Mar 2013
    Location
    Mt Pleasant SA
    Posts
    126

    Default

    Back in the olden days HSS slot drills always had a (-) tolerance whereas endmills always had a (+) tolerance but agreed -0.15 on 4mm is a bit gross. Hardly the end of days tho'.

  5. #5
    Join Date
    Sep 2012
    Location
    York, North Yorkshire UK
    Posts
    6,439

    Default

    Hi Guys,

    I think I would prefer undersized slot drills to oversize ones, at least you can always take a finishing cut !
    Best Regards:
    Baron J.

  6. #6
    Join Date
    Jun 2016
    Location
    Sydney
    Posts
    283

    Default

    Dear me. Where did that come from???
    But yes, if it is undersize you can handle that. In fact, I often prefer to do it that way, after micing the cutter carefully.

    Cheers
    Roger

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