When to use pilot hole before drilling?
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    Default When to use pilot hole before drilling?

    I understand if you want to drill a 3/8" hole in steel you just drill it. And if you want to drill a 2" hole you should drill a pilot hole at least the diameter of the web of the 2" drill. But at what size do you start using a pilot hole? Thanks.

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    Nothing says you have to use a pilot. In many CNCs, you just use a spotter drill to hold center. We drilled over 2 1/2" in stainless without a pilot.
    JR

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    The pilot hole reduces the drilling forces.

    Whether you need a pilot hole depends upon your machine z axis thrust horsepower. A Fadal might need a pilot hole to drill a 1" hole in stainless without stalling, while a Mazak or Makino might not blink at 1.5" drill in the same material without a pilot.

    The same applies to a Bridgeport or manual lathe. It all depends on your machine power and rigidity.

    Also drill point geometry will make a big difference. For example a split point verses a chisel point. Too many variables to make an absolute rule in my opinion.

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    The thickness of what you're drilling also makes a difference.

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    As you may have gathered from the responses above, there is more than one reason for using a pilot hole.

    One big one is to reduce the amount of force needed for the full size drill. This is probably why you are thinking that they are needed for larger hole sizes and not for smaller ones.

    But another big, perhaps even bigger, reason for using a pilot hole is for better accuracy in the location of the final hole. If you punch the location of a hole and then use a large diameter drill, the chisel point on that drill may be bigger than the punch mark and that punch mark will not provide much or even any guidance for starting the drill. In fact, the punch mark, with it's raised edges may even encourage the drill to wander off the desired location while starting the hole. This can happen with manual drills and in drill press work if a firmly mounted vise is not used. Heck a drill can wander off center even in a rigid set-up.

    My thinking is that when drilling an initial hole I can count on it to be centered to within the radius of the drill I am using. And once a starter hole is established, a larger drill will follow it fairly faithfully. Thus a 1/2" drill will certainly start a hole withing 1/4" of the marked or punched location. But, for many purposes, that is just too much slop. Now, if I start that hole with a 1/8" drill, then it will be centered to within 1/16" and that is a lot better. And a 1/16" drill will guarantee even more accuracy once it is properly started.

    I often use a three step sequence: a very rigid spotting drill (or the very small diameter of a center drill tip) to get the initial location properly fixed. These drills will be well guided by a punch mark. Then a web sized drill to make a path for the final drill to follow. And finally the full diameter drill. Using this procedure for ANY sized hole will give me positional accuracy in the range of just 2 or 3 thousandths. So, I have used a smaller diameter drill with holes from, perhaps 3/32", and up. And I have used spotting drills or center drills for virtually every size hole from, perhaps 1/32", and up. Thus, if I need good accuracy in locating the holes, a sequence of 1/16", 1/8", and finally 1/2" may make a lot of sense. Or 1/16" followed by a final size of 5/32" would also make sense. In short, all sizes of holes.

    Of course, if the work does not demand it, then I often skip the smaller sized drills and just hog them out with the full diameter. Again, this applies to all sizes of hole. There is NO real size that separates one technique from another.

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    Piloting for location might be useful in something like a drill press. On a machine tool with any inherent positioning accuracy it really isn't necessary. Use a proper spotting drill with correct point geometry for your selected drill (and a correctly ground sharp drill) and your hole will be awfully damn close to where you wanted it. On very large drills with very small machines you are going to pretty much be stuck drilling a series of holes regardless, because the machine doesn't have the power to push a 2" drill. On a serious machine, you just punch the big drill through in one shot. Another problem with the piloting theory is that small drills may START the hole in the right place, but they are VERY prone to wandering if they encounter a hard spot or if the machine tool has any geometry errors.

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    As stated, it depends. Today I had to drill a handful of #29 for some canvas hardware. By hand, in a nicely painted 50+ year-old car. I was pretty sure the area had at least a skim coat of Bondo/whatever under the paint, and also pretty sure that there was a filled and painted original hole under there very near my target area. No centerpunching, due to said Bondo chip risk. Pilot hole was a 1/32" drill in a hand-held Dremel. Yup, Bondo skim coat. No chips. finished with the #29, also hand-held. Clean holes for next steps. Had it been my somewhat rusty Suburban, no pilot at all. Might have even used a powder-actuated Hilti on it...

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    Thanks for the help. I understand better now.

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    Quote Originally Posted by rickseeman View Post
    I understand if you want to drill a 3/8" hole in steel you just drill it. And if you want to drill a 2" hole you should drill a pilot hole at least the diameter of the web of the 2" drill. But at what size do you start using a pilot hole? Thanks.
    .
    2" dia steel hole pi r squared so 3.14x1x1= 3.14 cubic inches per 1" depth
    .
    roughly 1hp used to turn 1 cubic inch into chips per minute
    .
    2" dia drill going a slow 2ipm feed thats over 6 hp required
    carbide drill going 20 ipm thats 60 hp required
    many a apprentice has had problems cause they dont realize hp levels required
    .
    spot drill often used before a long length drill that might have 0.1" or more runout wobble at drill tip. i have seen 20" long .75 dia spade drill get bent like a pretzel in less than time to say what the fck ?

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    When I used to do aircraft structure assembly, we piloted all our fastener holes that were through fracture critical parts. Way easier to pull a #50 hole for edge distance than a #21. As most of the holes I drilled were close tolerance, they were reamed to final dimensions.

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    Don't drill a pilot hole if you're using an Allied type spade drill.

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    Don't drill a pilot hole if you're using ANY spade drill. If you do, come tell us why not afterwards...

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    With manual drilling I use center drill with a 5x lens to get the point exactly located.
    Then remove center drill and install larger drill bit.
    Hole alignment always comes out perfect.

    A carbide center drill bit usually will chip apart with manual methods so I use HSS/Cobalt.

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    Quote Originally Posted by dkmc View Post
    Don't drill a pilot hole if you're using an Allied type spade drill.
    .
    i every day usually use pilot hole with spade drills. if i need a 3.5" dia bore drilled horizontally through 30" of cast iron normally i use a
    .
    1.125 spade drill as pilot from each end that meets in the center
    then hole is opened up farther might use a
    2.0 spade drill as a 2nd pilot drill
    2.5" spade drill as a 3rd pilot drill
    3.5" spade drill as a final drill
    .
    and i usually use a big spot drill to help start the 1.125 dia by 18" long spade drill so it does wander to side on hole start and get bent like a pretzel.
    .
    reason for pilot hole is machine hp limits AND ability to hold part without it moving. hearing a thunk of part moving 0.010" in fixture cause it could not hold part against 1000's of lbs of drilling thrust is normal reason to use a pilot drill. i probably have drilling many hundreds if not thousands of pilot holes with spade drills and finished drilling with bigger spade drill
    .
    i often have used pilot spade drill and bigger spade drills with steel parts too. most times its cause of hp limits

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    First of all, that's not pilot drilling. Pilot drilling is usually defined by being barely larger than the web of the following drill. Next, please post a video. Also, if your parts are able to move at all short of something bending or breaking, you don't have proper fixturing.

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    Ive found most SST doesnt like a pilot.
    Esp. Inconel.

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    Quote Originally Posted by DMF_TomB View Post
    .
    i every day usually use pilot hole with spade drills.
    Of course you do.
    Was school an uphill walk both ways as well?

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    I use a pilot hole for just about every hole over 3/16.

    Mostly to pick up the pin prick when using the DP. On the mill, or lathe, the spot drill comes to play. By the numbers mind you. If on the mill and using a prick to locate, I still like a flexible pilot drill, to start the hole at least.

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    only time i ever had problems with a pilot hole is a pilot hole .002 smaller dia than a .531 drill thats 4" deep (short carbide drill drills 10x faster) for a 2nd drill thats 18" long to make hole deeper
    .
    long length to dia ratio drill bits make bad reamers. if pilot hole is bigger even .001 no problems. if pilot hole is .001 smaller it often develops a vibration to breaking point that no feed or speed change will stop

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    i am always amazed when i hear

    1) spot drilling is bad

    2) pilot drilling is bad or impossible even with spade drills

    3) peck drilling is bad or impossible

    now i am not saying they are 100% foolproof but with care every day thousands if not millions are drilled with no problems


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