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Reaming - stock allowance, speed, and feed - machine vs hand

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  • Reaming - stock allowance, speed, and feed - machine vs hand

    There were three questions on reaming in my shop class mid-term exam, and I got all of them wrong, so I need to learn more about the correct values for this operation.

    The first question was, "The feed rate for reaming a hole in a drill press should be ___ that of cutting. I didn't read it carefully and thought it was speed, so I chose 1/2. The other choices were 2 times, 4 times, and 1/8. I think the correct answer is 2 times.

    The second question was, "The stock allowance for reaming a 0.375" diameter hole is:" I thought the answer was 1/64", which is the next smaller fractional drill size, but the choices were: 0.003", 0.060", 0.012", and 0.032". Since the decimal equivalent of 1/64" is 0.016, none of the answers were correct, so I chose the next larger fractional equivalent of 1/32" or 0.032", which is correct for a 1/2" hole. The instructor put an "X" next to 0.060", which I know is wrong, and when I questioned it, he said it should be 0.012", which is a 0.365" drill. The closest drill is a "U", which is 0.368".

    The third question was, "The stock allowance for a hand reamer with a 0.500" diameter is:" The choices were 0.015", 0.009", 0.004", and 0.032". I chose 0.032" which is correct for machine reaming, but the correct answer was marked as 0.009", although the nearest drill size is 31/64" or 0.4844" with a 0.016" allowance. I don't recall hand reaming discussed in class, although it may have been on the day I was late.

    I'm not protesting these, as I think some of them come directly from the NIMS certification test, and the instructor has said that the test is not well designed and some answers are questionable, but the idea is to be able to pass the test.

    I found the following information on reaming, although mostly on machine reaming:

    This mentions hand reaming and recommends 0.002 to 0.004 allowance:

    I have a set of reamers in commonly used sizes from 1/8" to 1/2" but they are labeled RCST or maybe RC5T which is probably for an RC5 running/sliding fit. I used the 1/4" reamer for a part I made, and a 0.250" shaft fit smoothly, but I wanted a press fit. So I used a chisel to make a rough "knurled" section where I wanted it to grip, and that worked well.

    In another case, I found that I could get a decent press fit by first drilling 1/64" undersize and then following with the exact drill size to shave off a few thousandths, and that seemed to work quite well. This was using drills from my $69 HF 115 piece HSS TiN coated set, which seems to be quite good, but I got them long ago and quality may vary.
    Paul , P S Technology, Inc. and MrTibbs
    USA Maryland 21030

  • #2
    Paul - In the real world, being different from theory, you would drill 1/64 smaller than the finished machine reamer size. Over one inch you would go to 1/32.
    Speed would be 1/2 of drilling speed for the material.

    It is to bad that our young people still have to fight with the crazy imperial system of fractions, numbers and letters.
    In metric you would drill -.1 to -.2 for up to 5mm dia. -.2 to -.3 for up to 30mm dia and -.4 to -.5 for over 30mm dia.

    The RED is "low carbon to high carbon" steel'
    The YELLOW is "cast iron"
    The BLUE is "copper, brass, aluminum.
    The GREEN is "plastics hard and soft",

    This is what you would use in a shop environment. Like I said, theory - that is what school is - is a somewhat different matter.

    When you enter into theoretical absolutely correct numbers than many more things need to be considered. These include among others the type and material of reamers, exact material specifications, lubricants, machine stability, machine capability, rigidity of set up etc. etc..
    Keep in mind that all the schools can do is give you the tools for you to use later on in life when it comes to making the right decisions, be it on the shop floor or as an engineer.
    Last edited by Juergenwt; 03-27-2015, 07:32 PM.


    • #3
      Get a copy of Machinery's Hand Book (mine is 27 Edition) - MHB27 - and you will find all of the answers you require in there- but they are (for) "guidance" and not "absolute/s" - so a bit of experience and common sense with MHB will see you there - where you want to be/go.

      Remember that a drilled hole will probably be a bit over-sire and maybe off centre (ie "wandered off") as well but it will otherwise do the job if ground/sharpened correctly. The best solution all-round (mostly) is a pre-drilled hole bored to pre-reaming size (and position).


      • #4
        I hate tests you have to answer wrong to get right.... that explains the planners and engineers at work....

        Oldtiffie is "spot on" as our English speaking cousins say.

        Machinery's Handbook is generations of knowledge... always a little behind times but, always gold.