Question about piloted drill bits.
Close
Login to Your Account
Results 1 to 18 of 18
  1. #1
    Join Date
    Apr 2020
    Country
    SPAIN
    Posts
    4
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    1

    Default Question about piloted drill bits.

    Greetings to all.

    As you can see, this is my first post in the forum.

    Just to tell you that -as a good newbie- I hope to learn as much as I can from you!

    The thing is I am about to start a metal sheet project, however, some doubts come to mind after watching this video:



    Can anyone explain me why this gentleman uses a "piloted drill bit" for the second hole enlargement, instead of a regular twist drill -just like in the previous enlargement-? In other words, when should we use a piloted drill bit exactly?

    Thanking in advance.

    Regards.
    Last edited by FeMC; 04-10-2020 at 01:58 PM.

  2. #2
    Join Date
    Jul 2006
    Location
    Hillsboro, New Hampshire
    Posts
    8,892
    Post Thanks / Like
    Likes (Given)
    2137
    Likes (Received)
    6142

    Default

    Aerospace work calls for accurate, generally perpendicular holes to allow for best fitting and tightening of rivets and screws/bolts. But given that there's no clear reason why to use a piloted drill bit in the demonstration, I'd say it's just to show an option, not a "must do".

    Keep in mind that regular drill bits will frequently "grab" when enlarging pre-existing holes, so extra care should be used in those circumstances.

  3. Likes michiganbuck liked this post
  4. #3
    Join Date
    Nov 2008
    Location
    Madera county california usa
    Posts
    2,529
    Post Thanks / Like
    Likes (Given)
    21
    Likes (Received)
    639

    Default

    Like a step drill the pilot helps align new hole with existing.

    Sent from my SAMSUNG-SM-G930A using Tapatalk

  5. Likes greggv, Trueturning liked this post
  6. #4
    Join Date
    Feb 2006
    Country
    UNITED STATES
    State/Province
    Indiana
    Posts
    13,133
    Post Thanks / Like
    Likes (Given)
    720
    Likes (Received)
    4070

    Default

    Long experience has shown me that conventional twist drill bits are not suited to drilling holes in thin sheet metal. By thin, I mean the metal thickness is less than the drill diameter. Ordinary drills usually create a three-lobed hole in thin material, good enough to put a screw through, but not very nice if left visible.

    Many years ago, I was shown by an older guy how to hand grind the end of a twist drill in such a way as to let it drill a clean, round hole in thin metal. It is quick and easy to grind once you have some skill. The idea is to create something similar to the brad point drills that woodworkers use. The bit has a tiny sharp point that can locate in a punch mark to keep it centered, but the outer diameter next contacts the sheet metal, establishing a circular cut. The result is as good as a punched hole, but will leave a burr, also like a punched hole.

    Many years ago, Black and Decker came out with "Bullet Point" drills and they did very well in sheet metal. They have been discontinued. Black and Decker owns the DeWalt brand and the replacement for the Bullet point is the DeWalt "Pilot Point." They are very good for general drilling and also do well in sheet metal, but I would use just the bit that does the desired hole size, not try to enlarge an existing hole with a Pilot Point drill. Please note that the DeWalt "Pilot Point" is not the same as a piloted drill bit. Piloted drill bits are a specialty product meant for mass production use in special applications.

    My favorite drill bits for sheet metal are step drills. One brand name, perhaps the first on the market, is Unibit, but there are many copies these days. Some will start their own hole, but the larger sizes usually only increase the size of an existing hole. They all work very well. Not only do they make a clean round hole, but they allow low power battery hand drills to make large holes.

    https://www.irwin.com/tools/browse/d...it-step-drills

    Larry

  7. Likes Trueturning, digger doug, Kyle Smith liked this post
  8. #5
    Join Date
    Feb 2005
    Location
    Owl's Head, Maine
    Posts
    2,312
    Post Thanks / Like
    Likes (Given)
    391
    Likes (Received)
    629

    Default

    Larry - I don't have much experience with step drills for sheet, but do like another style. When you wrote,

    Quote Originally Posted by L Vanice View Post

    ...Many years ago, I was shown by an older guy how to hand grind the end of a twist drill in such a way as to let it drill a clean, round hole in thin metal...

    Larry
    it sounded like what I do like for this purpose. In the Irwin line which you mention, they go by "TurboMax".

    -Marty-

  9. #6
    Join Date
    Apr 2010
    Location
    Justin, Texas
    Posts
    354
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    103

    Default

    Piloted bits get used more often when you start going through a material stack-up greater than .040" thick and need to ream the hole to final dimensions and are not using rigid tooling. You also tend to use them when drilling fracture critical assemblies and in locations where freehand drilling is difficult.

  10. #7
    Join Date
    Apr 2020
    Country
    SPAIN
    Posts
    4
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    1

    Default More questions about drilling.

    Quote Originally Posted by Milland View Post
    Aerospace work calls for accurate, generally perpendicular holes to allow for best fitting and tightening of rivets and screws/bolts. But given that there's no clear reason why to use a piloted drill bit in the demonstration, I'd say it's just to show an option, not a "must do".

    Keep in mind that regular drill bits will frequently "grab" when enlarging pre-existing holes, so extra care should be used in those circumstances.
    Thank you to all of you for the explanations. Saving up money is always very welcome. I thought it was a common practice when it comes to enlarging.

    In another order of things, I would like to ask you -if not abuse- about pilot holes.

    Researching about it I found machinists placing an emphasis on the relation between "spot angle" and "drill angle" because of vibrations, etcetera. A photo paints a thousand words:

    1a-400x350.jpg

    According to the pic the initial contact is quite critical, however, the case placed on the left is similar (not equal) to the one that we face every time we enlarge a pilot hole.

    Does anyone know if this pernicious effect is present while enlarging pilot holes? I am trying to find a proper method to drill precision located holes and I am not sure whether or not this is the way to go.

    Thank you!

  11. #8
    Join Date
    Nov 2003
    Country
    UNITED STATES
    State/Province
    Texas
    Posts
    5,347
    Post Thanks / Like
    Likes (Given)
    187
    Likes (Received)
    1733

    Default

    That drawing is good as far as it goes, as far as was intended by the author/illustrator. BUT the left side of the drawing is NOT a good representation of what happens when you enlarge a pilot hole. It shows an existing hole or dimple that is LARGER than the drill bit being used and that causes the outer corners of the drill bit to make first contact. A pilot hole will be SMALLER than that final size bit. What that drawing is showing is the use of a SPOTTING bit that makes a dimple that is larger than the final hole. Many machinists in commercial shops like to do it that way because the larger diameter dimple will also serve as a chamfer on the final hole: NO deburring step is needed after drilling to final size and that saves TIME. Time is money in a commercial shop. This can be considered an advanced skill. This is also why that side of the drawing shows a 90° angle: chamfers are often specified at 90°. And spotting drill bits are frequently found with 90° tip angles.

    So, in addition to the two cases shown in the drawing, there is ALSO the case of a small pilot hole followed by the full size bit. Using a small pilot hole will do two things for you.

    First, if you are at least somewhat careful/skillful in drilling it, it will be located with an accuracy that is smaller than the radius of that pilot drill. So, a 1/16" pilot drill will, in all probability, be centered on your mark within 1/32" of the intended location (punch mark) and likely much better than even that. The pilot hole's diameter should be about the size of the center web of the final, full size bit you will use. The pilot drill should also be a short, screw-machine length bit to best prevent wandering off course.

    The second thing that a small pilot hole will do is center the full sized drill well, REGARDLESS OF IT'S POINT ANGLE. 90°, 118°, 135°, or whatever angle, they will all center well on the pilot hole. You can do the drawing, but the edges of the small pilot hole will contact the edges of the full sized bit near it's center, not at the outer corners. And since this full size bit will follow the initial, pilot hole, your hole is still located withing that radius of the pilot bit that you used. So enlarging a pilot hole is more like the right side of the drawing where the initial contact/centering is at the center, than the left side where contact/centering is at the outer edges.

    This does not translate to sheet metal work, but for most holes it is the most accurate way for drilling holes by hand.

    If you want an accurate hole in sheet metal I would again use a small sized pilot bit and follow it with a step bit. You will get an accurately located hole and a nice, round one; no lobes. A small diameter pilot bit (1/16") will probably have it's full diameter inside the hole before it breaks through the rear side so it acts more like it would in a thicker piece of stock: it is right at home in sheet metal. And step drill bits excel at keeping the hole round. If you are enlarging an existing hole in sheet metal, the step bit will still do an excellent job, with one small caution. If the original hole is not the same size as one of the steps on a step drill, then there is a possibility that the center of the enlarged hole may wander by an amount up to half the step size. If you need the best accuracy, a taper reamer can be used to enlarge the hole to the size of one of the steps on the step drill and then use the step drill.

    If you are going to use a spotting drill bit which makes a large dimple as per the second side of the drawing, then yes, it is best to have a spotting bit with a 135° angle or larger.




    Quote Originally Posted by FeMC View Post
    Thank you to all of you for the explanations. Saving up money is always very welcome. I thought it was a common practice when it comes to enlarging.

    In another order of things, I would like to ask you -if not abuse- about pilot holes.

    Researching about it I found machinists placing an emphasis on the relation between "spot angle" and "drill angle" because of vibrations, etcetera. A photo paints a thousand words:

    1a-400x350.jpg

    According to the pic the initial contact is quite critical, however, the case placed on the left is similar (not equal) to the one that we face every time we enlarge a pilot hole.

    Does anyone know if this pernicious effect is present while enlarging pilot holes? I am trying to find a proper method to drill precision located holes and I am not sure whether or not this is the way to go.

    Thank you!

  12. Likes dian liked this post
  13. #9
    Join Date
    Jul 2003
    Location
    savannah, jaw-ja
    Posts
    1,802
    Post Thanks / Like
    Likes (Given)
    1397
    Likes (Received)
    443

    Default

    Quote Originally Posted by cvairwerks View Post
    Piloted bits get used more often when you start going through a material stack-up greater than .040" thick and need to ream the hole to final dimensions and are not using rigid tooling. You also tend to use them when drilling fracture critical assemblies and in locations where freehand drilling is difficult.
    We used to use piloted spotface tools when hand drilling large-ish holes in thin aluminum when I was at the plant.

  14. #10
    Join Date
    Apr 2020
    Country
    SPAIN
    Posts
    4
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    1

    Default

    Quote Originally Posted by EPAIII View Post
    That drawing is good as far as it goes, as far as was intended by the author/illustrator. BUT the left side of the drawing is NOT a good representation of what happens when you enlarge a pilot hole. It shows an existing hole or dimple that is LARGER than the drill bit being used and that causes the outer corners of the drill bit to make first contact. A pilot hole will be SMALLER than that final size bit. What that drawing is showing is the use of a SPOTTING bit that makes a dimple that is larger than the final hole. Many machinists in commercial shops like to do it that way because the larger diameter dimple will also serve as a chamfer on the final hole: NO deburring step is needed after drilling to final size and that saves TIME. Time is money in a commercial shop. This can be considered an advanced skill. This is also why that side of the drawing shows a 90° angle: chamfers are often specified at 90°. And spotting drill bits are frequently found with 90° tip angles.

    So, in addition to the two cases shown in the drawing, there is ALSO the case of a small pilot hole followed by the full size bit. Using a small pilot hole will do two things for you.

    First, if you are at least somewhat careful/skillful in drilling it, it will be located with an accuracy that is smaller than the radius of that pilot drill. So, a 1/16" pilot drill will, in all probability, be centered on your mark within 1/32" of the intended location (punch mark) and likely much better than even that. The pilot hole's diameter should be about the size of the center web of the final, full size bit you will use. The pilot drill should also be a short, screw-machine length bit to best prevent wandering off course.

    The second thing that a small pilot hole will do is center the full sized drill well, REGARDLESS OF IT'S POINT ANGLE. 90°, 118°, 135°, or whatever angle, they will all center well on the pilot hole. You can do the drawing, but the edges of the small pilot hole will contact the edges of the full sized bit near it's center, not at the outer corners. And since this full size bit will follow the initial, pilot hole, your hole is still located withing that radius of the pilot bit that you used. So enlarging a pilot hole is more like the right side of the drawing where the initial contact/centering is at the center, than the left side where contact/centering is at the outer edges.

    This does not translate to sheet metal work, but for most holes it is the most accurate way for drilling holes by hand.

    If you want an accurate hole in sheet metal I would again use a small sized pilot bit and follow it with a step bit. You will get an accurately located hole and a nice, round one; no lobes. A small diameter pilot bit (1/16") will probably have it's full diameter inside the hole before it breaks through the rear side so it acts more like it would in a thicker piece of stock: it is right at home in sheet metal. And step drill bits excel at keeping the hole round. If you are enlarging an existing hole in sheet metal, the step bit will still do an excellent job, with one small caution. If the original hole is not the same size as one of the steps on a step drill, then there is a possibility that the center of the enlarged hole may wander by an amount up to half the step size. If you need the best accuracy, a taper reamer can be used to enlarge the hole to the size of one of the steps on the step drill and then use the step drill.

    If you are going to use a spotting drill bit which makes a large dimple as per the second side of the drawing, then yes, it is best to have a spotting bit with a 135° angle or larger.
    EPAIII, thank you for your answer. Now everything is more clear.

    Building on that train of thoughts, let me do not consider this thread closed without discussing about center drill bits.

    An old friend told me that using CDBs -apart from spot & stub drill bits- to locate precission holes is a widespread technique, however, questions come to mind -once more-, as follows.

    1st scenario: imagine we drill until the aluminum surface meets the "red area". Can we use that indentation as if it were a "partial/micro pilot hole"?

    cdb.jpg

    2nd scenario: I have some split point drill bits in handy (not stub). Taking into account that one of the main reasons to use this kind of bit is almost the absence of wander in "virgin metal" -probably due to its low web thickness-, do you think that using them to enlarge holes is a waste of money? I mean, the chisel point wouldn´t make contact with any portion of metal, Would it have more sense using a regular twist bit to finish the job and save resources?

    The secuence would be:

    1) Center punch.
    2) Center drill bit for location.
    3) Split point or twist bit? for pilot hole.
    4) Metric drill bit to final size.
    5) Reaming in inches (3/16).

    Thank you!

  15. #11
    Join Date
    Nov 2003
    Country
    UNITED STATES
    State/Province
    Texas
    Posts
    5,347
    Post Thanks / Like
    Likes (Given)
    187
    Likes (Received)
    1733

    Default

    As far as I am concerned, center drill bits are just that. They are drill bits for making center holes on stock that is going to be turned between centers in a lathe. They have a bad habit of having that small, short center drill at the tip break off. I do not need that problem when I am drilling holes in a part.

    On top of that, the cone on a center drill is going to have a 60 degree included angle. If you let that part of the bit come into the part you are drilling a hole in, that will almost guarantee that you will have the problems illustrated in the left side of the drawing in post #7.

    On what to use to enlarge existing holes, I never worry too much about the difference in the cost of standard bits vs. split point ones. That difference is small.

    A great advantage of center drills is they are very short. They are even shorter than spotting bits and therefore they will wander even less. But that may be why they often break. But yes, used properly they can accurately locate a hole. I just do not like using them for that.

    As for your sequence, I think you are over thinking this hole business. But, knock yourself out if you want.

  16. #12
    Join Date
    Oct 2005
    Country
    UNITED STATES
    State/Province
    Pennsylvania
    Posts
    17,268
    Post Thanks / Like

    Default

    Might be time for the OP to add a little more info an the exact job at hand.

  17. #13
    Join Date
    Apr 2020
    Country
    SPAIN
    Posts
    4
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    1

    Default

    EPAIII, thank you for your clarification. I will test all the options you put on the table.

    Quote Originally Posted by digger doug View Post
    Might be time for the OP to add a little more info an the exact job at hand.
    Digger Doug, you are right. I did not just wanted to keep you busy.

    bp.jpg

    It is the mast of a gyroplane. Locations are critical because the chances of scouring the inner tube are relatively high. This is why I am a bit concerned about spotting.

    Wall thickness: 0,125 & 0,25 inches.

    Any comment would be very welcome.

    Thank you.

  18. Likes digger doug liked this post
  19. #14
    Join Date
    Feb 2004
    Location
    peekskill, NY
    Posts
    25,084
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    4998

    Default

    Quote Originally Posted by L Vanice View Post
    Many years ago, I was shown by an older guy how to hand grind the end of a twist drill in such a way as to let it drill a clean, round hole in thin metal.
    Larry
    They look sorta like this:



    [

  20. #15
    Join Date
    Nov 2007
    Location
    marysville ohio
    Posts
    9,792
    Post Thanks / Like
    Likes (Given)
    2807
    Likes (Received)
    6491

    Default

    One reply likes step drills, I do also, but keep in mind if you use them on steel and especially stainless steel you really need to be careful not to run them too fast or you will burn the cutting edge. If you don't melt it too bad you can grind it but most times they are junk, expensive junk.

  21. #16
    Join Date
    Feb 2004
    Location
    Staten Island NewYork USA
    Posts
    3,836
    Post Thanks / Like
    Likes (Given)
    1143
    Likes (Received)
    1880

    Default

    If you do not have a Bridgeport like mill to accurately clock off...

    ...and do not want to bring to a machine shop to have it done.


    and do not have a height gauge or granite table.


    Most likely your short on layout fluid too.




    But OK, he's what we do.

    Layout your holes with a scribe...if hard to see...go over with a Black marker and re-scribe.

    With a loupe locate a sharp punch at the center of crosshairs and give ONE good tap with hammer. Make sure you punch mark is where it needs to be...you can move punch mark a little if off by hitting sideways, but I recommend getting it right initially.

    Now you grab a nice small drill 3/32, 7/64 or so, then in a drill press easily bring the tip of the drill to that punch mark. The tip will/should find the center of the punch mark and not dance around it...if it dances your too far off center. Once its in the mark drill your hole and then go up a little bigger to a size that is aprox the size of the web of your finished drill size...or a size your Unibit will follow.

    Thin stuff is a pain as drill breaks through the bottom before the wall of hole can act like a drill bushing keeping it true...instead the flutes grab and you can loose position. It may pay to clamp your sheet to a piece of alum plate or similar to offer some thickness.

    If you do it right, you can expect to be pretty close.

  22. #17
    Join Date
    Mar 2020
    Country
    UNITED STATES
    State/Province
    New Jersey
    Posts
    19
    Post Thanks / Like
    Likes (Given)
    20
    Likes (Received)
    5

    Default

    I have the loupe and two punches ,60° and 82°. I rarely use it. Instead after layout fluid, accurate scribe , I install a " point" in the chuck. The point is a shop made tool made from a broken tap and ground to a sharp long angle. I move the X Y axis til the point is centered on the intersecting scribe lines. I install a spotting drill, then finish with what ever size drills needed.
    I found this as or more accurate than using the loupe.

  23. #18
    Join Date
    Feb 2010
    Location
    ch
    Posts
    2,439
    Post Thanks / Like
    Likes (Given)
    212
    Likes (Received)
    314

    Default

    maybe not the loupe but the punch for sure. i have such a tool in a collet and use it all the time. it really saves time.


Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •