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  • Spotting Drill Lesson Needed

    In another recent thread there was talk about using a spotting drill to start drilled hole and obtain more accurate placement. There was also some discussion about the use of a spotting drill vs a center drill. I have always used a center drill, carefully, to prevent breakage of the smaller drill point, however I can see the advantages of the use of s spotting drill, plus, it's just the proper way to do things. So, I looked up spotting drills and learned that they come with different angles on the point. MSC has them in 50, 82, 90, 118, 120 and 142 degrees.
    So, why the different angles and when is the preferred use of each?

    Obviously the 60 degree could be used for centering and the 82 degree could be used for countersinking and if I understand correctly 90 degree for countersinking metric screws, but these are spotting drills, not countersinks. I suppose 118 degree spotting drills are used because they match the drill point angle of some drill bits although some bits are 135/138 degrees and none of the spotting drills mentioned have that angle.

    So, what's the deal and what is a good spotting drill to use before you drill?

  • #2
    And while a seminar is requested, how about the use of piloting vs spotting. With larger drills I often pilot with a drill slightly smaller than the web of the bigger drill. I have no idea if this is common.
    Location: Jersey City NJ USA

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    • #3
      http://www.guhring.com/Documents/Cat...SpotDrills.pdf solves the 142* part for me.
      I was under the impression you wanted the angle a bit flatter (bigger?) than the final drill angle but find it curious that some knowledgeable folks over on PM quoting various sources that you would hope would actually know, engineers and manufacturers and such (hate when the wrong thing is recommended by someone who is supposed to know, that is why I ask them...) some of whom take opposite views regarding the larger or smaller included angle of the spot drill relative to the finish drill.

      As an aside, IIRC several go to sources (oft quoted books) recommend that you not use a center drill for anything other than establishing those for turning on a lathe...does one assume it is because of the danger of breaking off the much smaller tip? If that is the reason, would not just taking some care be in order?
      Last edited by RussZHC; 01-24-2016, 12:08 AM.

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      • #4
        The idea, AIUI, is that the 90 degree included angle allows the 118 to 135 degree bits to ride on and cut with the well-formed cutting edges and not the chisel point at the center on the web. In the machine shop class I took last year, the instructor recommended using a center drill, probably because they were more readily available in the shop.
        http://pauleschoen.com/pix/PM08_P76_P54.png
        Paul , P S Technology, Inc. and MrTibbs
        USA Maryland 21030

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        • #5
          Oh boy! This is one of THOSE subjects. Who's making the popcorn? I like it buttered please.


          LET THE GAMES BEGIN!
          Paul A.
          SE Texas

          Make it fit.
          You can't win and there IS a penalty for trying!

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          • #6
            I don't know why there are so many different angles for spot drills.
            I guess there's an application for each one.
            I only use a 118° Spot drill, since all of the drills I use are standard 118° drills.
            And, I only use center drills for making centers in the lathe, or for spotting for small diameter drills (I hate snapping off the little tips)

            Also, in reply to gelfex, I always make a pilot hole, the size of the drill web, if I'm using any drill over 3/4".
            Pilot holes make drilling ALOT easier, and more accurate.
            Last edited by KiddZimaHater; 01-24-2016, 01:45 AM.

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            • #7
              Originally posted by Paul Alciatore View Post
              Oh boy! This is one of THOSE subjects. Who's making the popcorn? I like it buttered please.


              LET THE GAMES BEGIN!
              You forgot the beer

              Originally posted by KiddZimaHater
              Pilot holes make drilling ALOT easier, and more accurate.
              Um, no. The smaller bit can easily wander off as it flexes more and the bigger drill will follow that hole you made. There is something wrong with the drill sharpening if larger drills require a starting hole (not counting for the issue of having undersized machine here). Usual hand sharpening gives freely going action on a 40 mm drill and what I've used factory ground bits, the biggest is 76 mm and also can be hand fed to steel easily. With proper grinding the chisel is next to nothing and cuts almost perfectly.

              As for the different angles for spotting drills, the 82 and 90 degree ones are so called quick cheating tools, as you can just drill a starter hole and get a nice chamfering at the edge if you drill it deep it enough for the following drill. Used very often in CNC applications when doing production runs.

              The 120 degree is the one to use for the basic 118 degree sharpened drills, as it allos the drill to center itself in the divot, even if it is teeny tiny bit off from the center. The 142 degree one is meant for drills sharpened to 135 degree, though I've rarely seen them.

              If you make a center hole and start drilling, it's the drills lips that contact first, taking easily a bigger bite, and thus not guaranteeing a proper location as the drill can flex wherever it wants to.
              Amount of experience is in direct proportion to the value of broken equipment.

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              • #8
                Originally posted by Jaakko Fagerlund View Post
                ...Um, no. The smaller bit can easily wander off as it flexes more and the bigger drill will follow that hole you made. There is something wrong with the drill sharpening if larger drills require a starting hole (not counting for the issue of having undersized machine here). Usual hand sharpening gives freely going action on a 40 mm drill and what I've used factory ground bits, the biggest is 76 mm and also can be hand fed to steel easily. With proper grinding the chisel is next to nothing and cuts almost perfectly...
                Meanwhile, in the world where most of us operate, drilling a pilot hole for a larger drill is perfectly normal procedure--absolutely nothing wrong with it. A 40mm drill is about 1-9/16"--my biggest is about 1-3/4" and I can tell you that pushing it into a piece of solid steel without a pilot hole is not easy. Anyone who does this a lot should know that a drill is not an accurate cutting tool; it just makes a hole. For a quick and dirty repair requiring a spacer or bushing or sleeve a drilled hole is often all I need--if I need something more accurate I'll drill under-size and bore to finish.

                If you've got a big machine with lots of power pushing a big drill without a pilot is relatively easy--with a smaller machine not so much. Your example of a 76mm drill tells me you've got a pretty good sized lathe--I'll be that not 1 in 1000 people on this site have a drill that size...
                Keith
                __________________________
                Just one project too many--that's what finally got him...

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                • #9
                  "So, what's the deal and what is a good spotting drill to use before you drill? "

                  Heh. As I said in the other thread, there is argument over these things. Always something to learn, which is why I brought it up. There is no definitive answer...many opinions come from shops that earn their keep making holes the way they do because that's what works for them.
                  My own background in hole making involved getting the most accurate placement & size with the least amount of steps. For dead accurate repetitive work nothing beats a jig plate w/bushings.
                  (You CNC guys lighten up- all manual pre DRO here)

                  Let's rule out using combo/centre drills, yes of course they work.
                  The arguments over angles concern how well the following drill intersects the previous divot. A broad angle divot with a narrower angle follow will intersect the centre first. Vice versa, the outer lips. Carbide drills don't like that.
                  The "ideal" would be matching angles. Weren't many to choose from in days gone by.

                  Regardless your view on that, best practice (for me) is short stout spotter, split point screw machine drill, then a jobber if depth is needed.

                  Like Keith says, a drill is a crude but efficient tool. True centre sharpening is best for accuracy expectations.

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                  • #10
                    Why not a small endmill to start a hole?
                    Andy

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                    • #11
                      Originally posted by vpt View Post
                      Why not a small endmill to start a hole?
                      How are you going to hit a punch mark or layout lines with tooling that has a nearly flat bottom? For the OP, he'd be depending on his DRO which seems to have questionable dependability.

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                      • #12
                        Originally posted by LKeithR View Post
                        A 40mm drill is about 1-9/16"--my biggest is about 1-3/4" and I can tell you that pushing it into a piece of solid steel without a pilot hole is not easy.
                        If the tool is properly sharpened (i.e. not having that chisel end on it), it will cut very easily, with less heat produced and thus for longer. When you drill holes day in and day out on various machines you really learn what sort of sharpening works and what doesn't work. The best and easiest by far is a 4 facet sharpening (provides a self centering end) and the chisel point edges ground to cut with the edge of a grinding wheel.

                        My example of a 76 mm drll cutting easily is just an example that a proper sharpening on a tool makes it a lot easier on you, the tool and the machine, no matter what size they are. Heck, if you jump to U-drills (carbide inserts), they cut with even less power.
                        Amount of experience is in direct proportion to the value of broken equipment.

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                        • #13
                          Hi,

                          A small end mill will tend to wander when plunged and produce an oval hole.

                          As far as spot drills vs. center drills. To me it doesn't matter, I use what ever is handy at the moment.

                          Spotting drills did not become popular until CNC became common. They became popular because they are much stronger than center drills and allow increases in feed rates, (think on the order of 10ipm vs 2ipm rates). Which in turn, decreases cycle times because seconds matter on long part runs. Oddly enough, that also makes them widely available in tool catalogs. So HSM'ers can get easy access to them.

                          Back in the day, center drills where what was commonly available to everyone. And they worked fine as a dual purpose tool. They not only made center spots for lathe centers, but could also do spotting work for drills and do combo drill and countersinks on thin materials. A very versatile tool all together. Center drills still excel at those things today.

                          Which angle to get? Depends on what you might need to do. For general spotting drills, I like 90deg. Not only does that give a good general use spot, I also tend to spot drill deep enough to leave a chamfer of the size I want after drilling the hole. This works well whether I'm programming turning centers or machining centers, it saves a lot of cycle time. A center drill would be a more fragile tool if I did that. Other angles can provide other advantages, like counter sinking for screws. It all depends on what you think you want the tool to do.

                          I think that for the average HSM, it really doesn't matter all that much. Use what you've already got or buy a spot drill if it will make you happy. A HSM'er often has more time than money, so a more versatile tool like the center drill might be a cheaper choice. But if using a spot drill makes you happy, go for it. 3 billion Chinese won't care.

                          Dalee
                          If you think you understand what is going on, you haven't been paying attention.

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                          • #14
                            A word of caution here about terminology. In drill sharpening circles a four facet tip usually refers to adding additional clearance to a two facet tip. This would still have the standard wedge at the web. I think what you are talking about is a split point grind which provides cutting action to the center and a center point that helps to stop wandering.

                            Pass the popcorn, please.



                            Originally posted by Jaakko Fagerlund View Post
                            If the tool is properly sharpened (i.e. not having that chisel end on it), it will cut very easily, with less heat produced and thus for longer. When you drill holes day in and day out on various machines you really learn what sort of sharpening works and what doesn't work. The best and easiest by far is a 4 facet sharpening (provides a self centering end) and the chisel point edges ground to cut with the edge of a grinding wheel.

                            My example of a 76 mm drll cutting easily is just an example that a proper sharpening on a tool makes it a lot easier on you, the tool and the machine, no matter what size they are. Heck, if you jump to U-drills (carbide inserts), they cut with even less power.
                            Paul A.
                            SE Texas

                            Make it fit.
                            You can't win and there IS a penalty for trying!

                            Comment


                            • #15
                              Spotting drills did not become popular until CNC became common.

                              Not so. They were used on the screw machine induatry many decades before CNC cme along.


                              If you're going to be drilling deep holes with a narrow drill the point angle of the spotting drill should be the same angle as the drill. It is easier for the drill to find center that way.

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