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Thread: Reaming bolt holes

  1. #1

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    Reaming bolt holes

    Can someone explain the process of drilling undersize and then reaming a bolt hole? Is this done on wood AND metal?.....what exactly is a reamer?

  2. #2
    Neil's Avatar
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    A reamer is a tool that is used to precisely size a hole. It is not designed or intended to be used to remove large amounts of material but as a follow up to a drilled hole. Unlike a drill, the reamer will not bend and can be used to true a drilled hole that has wandered off just a bit. Best used in machine tool applications (milling machines,
    lathes, drill presses) it is possible to use in a hand held drill and some are intended for use by hand when held in a tap handle. They are usually purchased in specific sizes but they do make adjustable reamers which are handy if you would like to add just a little extra size to a fit for whatever reason. There are all sorts of specialty reamers and job specific reamers but you don't really need any of those in aircraft building. (Generally speaking. There is always an exception.)

    Most builders will need some of the standard size reamers. 1/4", 5/16", 3/8" will cover most needs. You will need a reamer for the AN3 fasteners as well. An AN3 is NOT 3/16". An AN3 is a #10 screw. A #10 screw is .190 diameter. A 3/16 reamer will make a .1875" hole. An AN3 bolt will not fit. You will need a # sized reamer and they are sort of hard to find. A #10(.193") or #11(.191") reamer is used for an AN3 bolt.

    Reamers are best run at about 1/3 of the recommended speed of the same size drill bit for a given material. They are not intended for wood but I have been guilty of passing a reamer through a wood spar assembly. Most of my reamers are what is known as Straight fluted machine reamers. That is what I would recommend that you buy.

    It is much easier to get a good round hole with a reamer. Drills should be used to get you close then finish the hole with a reamer. Larger reamers (over 3/8") will take out 1/64" with ease. Smaller reamers should be limited to less than that. Reamers should always be used with a cutting fluid.

  3. #3
    Neil's Avatar
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    A reamer is a tool that is used to precisely size a hole. It is not designed or intended to be used to remove large amounts of material but as a follow up to a drilled hole. Unlike a drill, the reamer will not bend and can be used to true a drilled hole that has wandered off just a bit. Best used in machine tool applications (milling machines,
    lathes, drill presses) it is possible to use in a hand held drill and some are intended for use by hand when held in a tap handle. They are usually purchased in specific sizes but they do make adjustable reamers which are handy if you would like to add just a little extra size to a fit for whatever reason. There are all sorts of specialty reamers and job specific reamers but you don't really need any of those in aircraft building. (Generally speaking. There is always an exception.)

    Most builders will need some of the standard size reamers. 1/4", 5/16", 3/8" will cover most needs. You will need a reamer for the AN3 fasteners as well. An AN3 is NOT 3/16". An AN3 is a #10 screw. A #10 screw is .190 diameter. A 3/16 reamer will make a .1875" hole. An AN3 bolt will not fit. You will need a # sized reamer and they are sort of hard to find. A #10(.193") or #11(.191") reamer is used for an AN3 bolt.

    Reamers are best run at about 1/3 of the recommended speed of the same size drill bit for a given material. They are not intended for wood but I have been guilty of passing a reamer through a wood spar assembly. Most of my reamers are what is known as Straight fluted machine reamers. That is what I would recommend that you buy.

    It is much easier to get a good round hole with a reamer. Drills should be used to get you close then finish the hole with a reamer. Larger reamers (over 3/8") will take out 1/64" with ease. Smaller reamers should be limited to less than that. Reamers should always be used with a cutting fluid.

  4. #4
    Neil's Avatar
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    Don't know how the double post happened.

  5. #5
    bwilson4web's Avatar
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    Quote Originally Posted by Neil View Post
    A reamer is a tool that is used to precisely size a hole. . . .

    Most builders will need some of the standard size reamers. 1/4", 5/16", 3/8" will cover most needs. You will need a reamer for the AN3 fasteners as well. An AN3 is NOT 3/16". An AN3 is a #10 screw. A #10 screw is .190 diameter. A 3/16 reamer will make a .1875" hole. An AN3 bolt will not fit. You will need a # sized reamer and they are sort of hard to find. A #10(.193") or #11(.191") reamer is used for an AN3 bolt.
    . . .
    I need to drill two, AN3 holes for tube-in-tube, aileron bell crank mounting. I've assumed the #12 is .189". Since it is important to eliminate or minimize slop in the control system, would a #12 reamer be the right choice to finish the hole for an interference fit?

    I have assumed an interference fit requires a clamp with a block and a hole to press the bolt home. However, if the threads clear the hole and the AN3 shaft needs a little bit to be pulled through, could a bolt and washers be used to pull it through and make a better, interference fit?

    We have a calibrated torque wrench but would the bolt still be properly torqued if there is an interference fit?

    This is replacing a defective part so the original bolts were removed. The outer, aluminum tube has the original hole. I have new bolts to replace the originals so there is always the question of whether or not the outer, aluminum tube hole will still be tight . . . we used to call this an 'engineering tolerance' problem.

    Thanks,
    Bob Wilson
    Last edited by bwilson4web; 03-21-2012 at 08:33 PM.

  6. #6
    Neil's Avatar
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    If I had a #11 and a #10 I would use the #11 reamer as the first attempt and see how that works out as the bolts tend to wander a bit on size and all you will do with the #12 is scrape all the cad plating off the bolt if you get it in at all. The cad plating is a functional part of the bolt and care should be taken to retain the plating. For the most part the #10 reamer is the best over all choice for the typical builder.

    I don't know if you are following the wording in a manual when you are trying to produce the interference fit, but in most cases it is best if the bolt slides in easily with no detectable looseness. Since the bell crank is aluminum I'm sort of guessing this is not a 400mph turbine powered whiz banger so the clamping force of the AN3 bolts on the tubes with only a couple thousandths clearance should be sufficient.

  7. #7

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    Quote Originally Posted by bwilson4web View Post
    I need to drill two, AN3 holes for tube-in-tube, aileron bell crank mounting. I've assumed the #12 is .189". Since it is important to eliminate or minimize slop in the control system, would a #12 reamer be the right choice to finish the hole for an interference fit?
    An AN-3 bolt has a shank dia of .186" - .189", (.1875 +/- .0015) the threads are 10-32 x .406" A #12 reamer will provide a Class II fit or "free fit" which is exactly what I would shoot for.

  8. #8
    bwilson4web's Avatar
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    Thanks!

    It makes sense to have all three sizes and start with the #12 followed by the others as needed to make the hole fit the bolt.

    As for the structure, it is steel tube inside an aluminum tube of the aileron. The bell crank is a welded, steel plate to a steel shaft bolted to the steel tube. This end is not the part I'm worried about. Rather the aluminum tube that is part of the aileron that is the area I'm . . . interested.

    Thanks,
    Bob Wilson
    Last edited by bwilson4web; 03-22-2012 at 02:00 AM.

  9. #9

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    One thing that will help you make the best of it, is to run the reamer in under power and stop the drill motor and withdraw it while turning backwords by hand. You can also do just as well, if access allows, to simply unchuck the reamer after it's thru the hole and pull the shank on thru. Unless you have lots of practice with using reamers in your hand drill, or are using rigid tooling, it's very easy to allow the reamer to cut additional materal on the withdrawal if you continue to run the drill motor. Use some solid type lube on the reamer before you insert it and turn at the appropriate speed for the toughest materal in the stack. Also remember that the flutes are very sharp and can slice your hand easily.

  10. #10
    Neil's Avatar
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    It is generally considered bad machine tool practice to turn a reamer backwards but unchucking and drawing the reamer out the back of the work is a good suggestion.

    All the "book" numbers say that a #12 reamer would be ideal, but in actual practice I don't find it to work out. If you have all three give it a shot.

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