Advice for a small but deep hole
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  1. #1
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    I need to bore an ~.220 diameter hole 4 inches deep. It needs to be pretty accurate and straight. I don't have exact tolerances yet, just think of minimal angular displacement between the part axis and the bore axis. How would you guys approach this using a standard mill or lathe as your tool?

  2. #2
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    Start with a short drill and work your way to the 4 inch lengh.

  3. #3
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    Shop where I once worked had to drill many deep, small diameter holes. Their solution was to disassemble a lathe tailstock and make a hardened drill bushing to fit snugly in the tailstock. A long twist drill, guided by the tailstock bushing and powered by a 3/8 hand drill was used to make the holes as the workpiece revolved in the headstock, supported by a steady rest.

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    I don't know how close that number has to be but you might have a hard time finding a drill bit to cut that hole. Here is a small chunk of my drill size chart.

    Drill Size
    7/32 .2187
    #2 .2210
    #1 .2280

    5.5mm = 0.2165348
    6mm = 0.2362198

    My first reaction was to drill to 7/32 and then ream to size but I couldn't find a .220 reamer longer than 1.5 inches. Do you know anyone with an EDM?

    I did find a 5.6mm which works out to .2205 which is pretty damn close, however, its not 4" long, its solid carbide, and it costs about the same as gold by weight.

    http://www.jlindustrial.com/catalog/...?id=GUR-02205A

    If you can work into using a #2 (.2210) drill then there are a lot of options available to you.

    Good Luck

    jkilroy

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    gundrill it.

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    What exactly is a gundrill and how does it work? I've heard the term before, but have never seen one.

    Also, the .220 is not hard and fast so the .221 should work just fine. I'll be trying this out in the near future.

    Right now it looks like I will be pilot drilling, then drilling in steps with some smaller drill, then drilling to "close" and finally ream to finish. I like the idea of the tailstock thing as well and will have to investigate. I'll keep checking to see what else shows up here as an option. I won't be doing the work until early next week. Thanks!

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    a gundrill is a solid carbide coolant fed drill that was made for the situation you describe. I use one often on my machining centers. You need a coolant inducer and a special high pressure spraymist unit. I have had awesome success with half round gundrills from drillmasters. I have used them in all variety of steel and aluminum alloys. I have put 1/4" holes in 7" blocks that you can take a hardened and ground ejector pin and drop right in, no interference. You have to be able to create an accurate pilot hole on size and location, the rest is up to the machine. For 7/32 approx. you are probably looking about 2800 rpm and about 0.8 IPM feed. www.drillmasters.com for more info.

  8. #8
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    Yah, just what Willie said! We used to buy Eldorado, but that's Drillmaster now.

    JR

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    Thanks guys, the gundrill seems to be the way to go and the hole is important enough to consider the expense of setting up to do this. I will be contacting them tomorrow. Again, thanks!
    Joe

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    KBC (or equivalent) 1-070-013 13/64 X 6" Aircraft Extension Drill @ $1.76 ea.
    KBC (or equivalent) 5-016-010 #2 X 6" OAL Chucking Reamer @ $5.10 ea.

    A gundrill would be ideal...but could be a wee bit pricey unless the part run is large enough to justify the cost. You're going to have to pre-drill a short pilot, too. They pop up cheaply enough on Ebay, but of course, it's a pig in a poke when it come's to finding the right size. If you choose a conventional twist drill, I'd recommend an extension drill, rather than an extra length drill, because it's mostly solid shank & has more rigidity...of course this is offset by the downside of the increased number of chip clearance peck's you'll have to make.& reduction of coolant flow. Good luck! - John


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