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Chipping Drillbits?

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  • Chipping Drillbits?


    I am drilling deep holes, in some stock (stressproof) on my South Bend 13x40 (the POS I mentioned in previous posts) and I am having a problem with my 5/8 inch drill bit getting a chip taken out of one of the flutes. I start the hole by using a center drill, then drill a pilot hole with a 5/16 inch drill bit. (This drill bit is only about 6 inches long.) This all goes fine. Then, I use a 5/8 inch drill bit to both enlarge the hole and drill the hole out to 7 1/4 inches deep. The final step is to enlarge the hole, for the first 4 inches, to 3/4 inch.

    I am having a problem where the 5/8 inch drill bit is getting a chip taken out of one of the flutes when I am starting to enlarge the 5/16 inch hole. This 5/8 inch drill bit is about a foot long and does sag a little until the hole is started. I am thinking that this sag is what is contributing to the problem as the drill bit is fine once it starts enlarging the hole.

    I am thinking that maybe I need to use a drill bit between 5/16 and 5/8 (maybe 1/2 inch and only drill out to 1/2 depth) just to enlarge the hole enough that the 5/8 drill bit will start properly.

    Can anybody give me any advice as to what I need to do to start this hole without the drill bit chipping?

    Also, the tailstock is centered to within 0.001 and the height has been shimmed (with 0.016 shims) such that it is now only about 0.003 low.


    There are only 10 types of people in the world: Those who understand binary and those who don't.

  • #2
    Your pilot hole should only be as big as the web-thickness of your 5/8 drill.
    5/16 might be a little big.


    • #3
      I dont know if you need a pilot drill at all, 5/8" is not very big. spot drill it and ram that sucker in. 1140 machines like a dream.

      Too bad you dont have a good old turret lathe, it would be a gravy job then.

      Deep drilling with a tailstock just sucks, at least you are using good metal for the job.

      You might check the drillbits geometry, thin the web and dont give it too much clearence so it can hog and bust.


      • #4
        General rule of thumb, never stick a drill down a hole that is larger than 1/3 its diameter. As Kidd**? said your pilot only need to be as big as your web. The only real reason to even use a pilot is if you are HP limited and can't drive the chisel point or you are actually trying to get an accurate hole, and even then the pilot doesn't need to be that deep, just enough to get the bigger drill up on it flutes,and it will hold size surprisingly well.

        If I was you, I would spot, not center drill (center drills are for prepping stock for use with a center), pilot with something in the .220 range for an inch or so(if needed), drive in the 3/4 and then stuff in the 5/8(maybe pilot again). Depending on HP and tolerance requirements, that could change a bit.


        • #5
          Ease it in

          5/16", 1/2", 5/8.

          Reduce the front clearance on the drill - it may be getting "pulled in" and over-loading the weak edge.

          This is a common problem with longer drills.

          If you have a shorter 5/8" drill - try that first - then when the 5/8" hole is formed for say 1/4">1/2" deep, change to the longer drill.

          The problem is to do with the flexibility of the longer drill and the "chattering" that goes on until the (5/8") hole is formed and supprts the drill.

          It will also be "easier" on the tail-stock quill and the key or screw which restrains it fro spinning (with the drill) in the tail-stock.

          Slow the speed and "up" the feed. The "feel" at the tail-stock quill hand-wheel should "tell" you if you can "feel" it.


          • #6
            I agree with Tiffie that excessive relief on larger drills is often a problem. I reduced the relief on my S&D drills to about 10 degrees which helped considerably. Getting the cutting lips exactly the same - they weren't on the inexpensive Asian S&D set I have - was also helpful.

            My little 7x12 doesn't have the HP to drill with large drills easily if the pilot hole is the size of the web so I generally go up by 1/8" at a time. One thing I've found is that a chamfer the diameter of the target hole is a big help getting the next larger size drill to start nicely. A sharp edge on the hole you're enlarging catches one lip of the drill, throws it off center, the other lip catches, and it chatters badly until the outer edge of the drill engages; rapid variation in chip load is a good possibility as a source of chipping.

            You might try enlarging the 5/16" to 3/4" at the outer end before drilling the 5/8" section - this would automatically provide a chamfer matching the 5/8" drill that has the chipping problem, where the shorter 3/4" drill may be less prone to chatter. A chamfer made with a countersink helps, even if the angle doesn't match the drill point angle.

            Adding an SPA is also a help in getting large drills to cut smoothly and easily when stepping through drill sizes, see:

            Here's a picture of a drill with reduced relief and an SPA:



            • #7
              Thanks for the advice, everyone. I might get some time to try again tonight.


              There are only 10 types of people in the world: Those who understand binary and those who don't.


              • #8
                The long drill is unwinding when it starts to bite. That makes it longer instantly and is equivalent to ramming it into the work. If you have the option start the hole for that drill with a short one of the same size to develop the full cutting area for the bit so it won't grab as it unwinds. Otherwise try lowest rpm in back gear to start the hole. If you can't do that then cut a wide champfer on the hole with a boring bar.
                Free software for calculating bolt circles and similar: Click Here


                • #9
                  "Chipping Drillbits"?

                  Is that near Wales?


                  • #10

                    I sharpened my 5/8 inch drill bit and put another piece of round stock in the lathe. Then, I center drilled it and started drilling with a short 5/8 inch drill bit. Next, I put the long 5/8 inch drill bit in the chuck and tried to drill out the stock. It was working, but it was taking a lot of pressure. So, I then drilled a pilot hole and put the long 5/8 inch drill bit back in. Everything worked fine!

                    So, in summary, here is what worked:

                    1. Center drill
                    2. Drill about 1/2 inch with final size bit
                    3. Drill pilot hole
                    4. Finish drilling with final size bit

                    Thanks again,

                    There are only 10 types of people in the world: Those who understand binary and those who don't.