Learning how to "back drill"?

Discussion in 'Sheet Metal' started by E28POWERM20, Jan 9, 2019.

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  1. Sep 17, 2019 #21

    pfarber

    pfarber

    pfarber

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    You'd really want about half that... about 2000RPM for small holes in Al. Most rivets are in the 1/8th range and don't need to be super fast. When I went through A&P classes we used the same 800/1200RPM pneumatic drill for Al and steel. A drill press is adjustable, most hand held pneumatic are not (although some do have hi/lo settings).

    I am currently using a HF 1/2 pneumatic drill to take out rivets and at 800RPM not having any issues.
     
  2. Sep 17, 2019 #22

    lakeracer69

    lakeracer69

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    You don't know what you're talking about. Perhaps you need to invest in a Machinists Handbook or actually work work in a machine shop and see how it's really done. Speeds and feeds are everything for "proper" material removal and tool life. Sure you can wallow along or burn up tools and maybe get the job done with just about any rpm.

    Doing the calculation a 1/8" drill in aluminum would be 3200rpm. 3/32" @ 4266rpm. Hence, to answer the original question posed, buy a drill in the 3000-4000rpm range. Don't know where you came up with adjustable? LOL
     
  3. Sep 17, 2019 #23

    N804RV

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    Try laying out the rivet holes on virgin skin with a Sharpie, using the already drilled part as a template. Center punch the Sharpie dots with a spring loaded punch, then drill an undersized hole (use a #31 for a 1/8 rivet, or a #41 for a 3/32, etc.), then match drill the parts and cleco as you go, using a chuckable reamer. This will minimize the amount of bugger to the original holes you're trying to match.

    As far as the drill motor debate, I have a mid-priced pneumatic drill motor that is rated for 2,600 rpm. It has a full-tease trigger and is controllable down to very slow speed. But, for most aircraft work, I'll tease it a little to make sure I got a good bite, then let'er rip. I think I used the same #40 drill bit for almost a year before I replaced it.
     
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  4. Sep 17, 2019 #24

    narfi

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    If you are just replicating a skin, do it on the table. Your table should have a sacrificial layer on the top you can drill and cleco into.
    beat your old skin into a flatish shape that can lay down over your new sheet of aluminum, drill through one hole in the old skin into the new skin, cleco it down to the table, press it out nice and flat and do the same in the oposite corner, when all your corners are done, then do holes half way between the corners, then just keep splitting the difference and clecoing it down to the table. Drilling symmetrically will keep you from walking the old skin around and magnifying errors.

    To do it even better, drill the holes undersized, #40 for #30 holes, etc.... then when done, that lets you cleco it securely to the (jigged if needed) structure where you can back drill to enlarge the holes without error.


    *of course this all depends on the damage done to the original skin. Rock holes and really mangled areas can be cut out instead of beat flat for a smoother template.
     
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  5. Sep 17, 2019 #25

    Angusnofangus

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    Those feed numbers you quote, I would guess, are for billet aluminum. But for drilling light aluminum skins, a 2600 RPM drill is pretty much the aviation industry standard. I worked beside a lot of guys in the aircraft sheet metal field for many years, and no one used a 4000 RPM drill. As for adjustable, that is a bit of a mystery to me also. Adjustability comes with a drill with a sensitive trigger and an 'adjustable' trigger finger.

    As for drilling off new skins from old, the usual method is lay the old one over the new, maybe put a few weights on it to keep it flat and in position, then start at one end copying the holes. Best to do this on a sheet of plywood or something you don't mind drilling. Use lots of clecoes as you work from one end to the other. Drill far enough into the plywood that the clecoes will hold in the wood. Continue until all holes are drilled, mark the edges with a Sharpie, remove clecoes, trim and deburr the new skin and Bob's your uncle.
     
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  6. Sep 21, 2019 #26

    Winginitt

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    When doing most machining, as was previously mentioned,"speeds and feeds are everything". A general rule of thumb is that the larger the drill bit, the slower the rpm you need. When working with small drill bits and drilling steel, its not just rpm. You also have to control pressure, or the bit just rubs as much as cuts and gets dull. It also workhardens the steel around the edge of the area causing replacement drill bits to dull quickly.
    When drilling aluminum with a small bit, for the most part basically any reasonably fast rpm works fine. Softer aluminums may ball and tear while harder aluminums may machine easily. The drill bit is always harder than the aluminum, but eventually it will dull no matter what you are drilling. With a sharp bit drilling aluminum, it should last a long time no matter what reasonably fast rpm is used.

    The biggest reason for quickly dulling drill bits is excessive rpms and lack of pressure, both of which seldom are a problem when drilling aluminum. As long as the rpm anyone is using is producing an acceptable result, thats really all thats important. When you start drilling larger holes, it becomes important to at least approximate a correct speed. With hand drills "approximate" is all you can do. When operating a mill or lathe you can actually control cutting speed (rpm) and feed (pressure of cut)......with a hand drill its just guesswork.
     
    Last edited: Sep 21, 2019
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  7. Sep 21, 2019 #27

    ScaleBirdsScott

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    For sure, with a drill it's all about feel and watching the chips that come out. If it sounds good, is making decent swarf, and is getting the hole cut in a reasonable amount of time, you're doing good as can be expected.

    Using drill press rated speeds/feeds thinking doesn't mean much with hand tools except as a general rule of thumb. More important to just always try and give good feed pressure without loosing control of the tool; and use good bits for the job.
     
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  8. Sep 21, 2019 #28

    Turd Ferguson

    Turd Ferguson

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    For the most part common hand power tools are not optimized for aluminum. So you use what's available. A&P's get stuck drilling aluminum with tools that are designed for ferrous metal. The twist drill geometry is not optimum for aluminum and neither are the drill motor speeds so you use what ya got. However, the optimal speed for drilling a 1/8" hole in aluminum is closer to 6400 rpm. (Four times the cutting speed divided by the diameter)
     
  9. Sep 21, 2019 #29

    BBerson

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    I use a cheap Black and Decker electric drill for everything sheetmetal. Why run a compressor?
    I don't care if it takes two seconds to drill a hole instead of one second.
    Starting really slow helps keep it from dancing around.
     

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