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Basic lathe drilling procedures

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  • Basic lathe drilling procedures

    As mentioned elsewhere, I'm a newbie lathe owner and am trying to learn as I go so be gentle if my questions are ignorant.

    Is there a standard process for establishing the progression of drill sizes to work up the finished hole?

    Example: Trying to advance my skills, last night I machined a tap-holder for the tailstock that slides on a length of surplus 3/8" hardened computer printer rod. A 1" 6061 alum. rod had to be drilled & tapped 1/2-20 on the chuck end and a 3/8" hole drilled/reamed 3 5/8" deep on the other end. I center drilled both ends and drilled the holes progressively using vegetable oil as a lube beginning with a 1/8" up to the final hole in 3 stages on the small hole, 4 on the larger. I tapped the chuck end by hand in the lathe using the live center to keep the tap holder centered. When I put it all together and spun it, the #$%@*& chuck wobbles so much I'm going to have to trash it and start over!

    In thinking back over the process, the bits seemed to wobble some as I was drilling and I don't think the holes are centered and straight. Is this just the work wobbling since the 1" rod wouldn't fit through the headstock hole and maybe 4" was sticking out of the chuck? It ran straight when center drilling but probably it and the bits wobbled some during the drilling. Is there a progression of bit sizes that would help or are my bits just wandering due to their quality? They are medium priced TiN coated bits bought from H.F. I tried to feed the bit into the work at a rate that kept a pair of nice spiral chips coming out plus cleared the chips pretty often.

    I realize turning down a stub and cutting the threads in the lathe for the chuck would have been better but I felt my skills weren't up for the threading process, hence threading the piece for a short stud to screw the chuck onto.

    Any suggestions?
    Milton

    "Accuracy is the sum total of your compensating mistakes."

    "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

  • #2
    If you are chucking the work you will NEVER get the holes to line up exactly. Unless you have a 4 jaw chuck, and a magic indicator, that you can get between the chuck body and the work, that will not be struck by the jaws as you turn and true the work from the headstock side once you have re-chucked the work. The only way you can get them to line up to anything near exact is with a collet. The work projecting 4" out of the chuck does not help yo either, though that appears to be out of your control. The other problem is the depth of your 3/8" hole. The 3/8" hole is about 9 time deeper than the diameter of the drill. The deeper a hole gets in relation to the drills diameter, the more likely it is to drift away from the center line. As for stepping up drill sizes, the web of the next drill you wish to use should be no larger than the diameter of the previously drilled hole. How much materail did you leave for reaming? The rule I was taught and still follow was: up to 1/2" leave no more than 1/64", over 1/2" 1/32". Reamers will only follow a drilled hole, intention being to be producing a better/smoother finished surface and producing an accurate diameter. They cannot bring a hole that has drifted back to the center line.

    [This message has been edited by ERBenoit (edited 04-29-2004).]

    [This message has been edited by ERBenoit (edited 04-29-2004).]
    Paying Attention Is Not That Expensive.

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    • #3
      First of all are using a center drill to start the holes? And are your drills sharpened evenly on both sides?

      Don't crowd the drill. Let the drill cut and withdraw it often to clear swarf.

      I know this seems basic , but you'd be suprised how many overlook them.

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      • #4
        Thanks ER & Rusty, exactly the kind of info I was needing.

        Yes, I did center drill both ends after checking for runout. I think I was guilty of pushing too hard and not letting the bit do it's work plus jumping up too far on my bit sizes.
        Milton

        "Accuracy is the sum total of your compensating mistakes."

        "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

        Comment


        • #5
          Chuck up the bits you used and slowly rotate and try to see if there is any wobble or use a dial indicator to check if the bit is warped or not.
          I once purchased a new set (imports) from a reputable dealer and found that the 1/2" bit had a runout of over .050" !! The set was not the cheapest set available either.
          Most of the other bits in the set was not much better.
          That was my lesson and now only buy U.S. made cutting tools. The cost of parts that I've ruined using cheap or dull tools have far exceeded the extra cost of quality bits.
          Also... the easiest way to drill off center is to have one side of the drill bit not sharpened even with the other. Just the slightest difference can cause the hole to be off.
          One of the first and most important pieces of equipment for any shop, (in my opinion), is an appropriate grinding station and jigs. Then take all the time that is needed to master the grinding techniques of all your various cutting tools.

          Tom M.

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          • #6
            I've started to use the stub method when both ends of a workpiece need machining. I will drill and ream, or bore, the longer or larger of the holes, then mount a shaft and turn it so the workpiece will fit tightly but smoothly over it. Often, I'll leave a slightly larger diameter on the stub near the chuck so when the workpiece is slid onto it, the last quarter inch or so will be tight. Now it will run true to the drilled, reamed, or bored hole, and the other end can be machined to be on axis with the first hole. Only at this time will I put the final turn on the diameter, so the whole thing is true and concentric.
            There's more to it than this, of course, and I would suggest starting be making sure your tailstock bore is axial with the spindle bore. All future operations using the tailstock will benefit from having done an alignment here first. Not only does the point of a dead or live center have to be concentric, it has to remain that way as the tailstock is cranked through it's range of travel, and as it is brought closer or farther from the spindle.

            [This message has been edited by darryl (edited 04-29-2004).]
            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

            Comment


            • #7
              When I need an accurate hole,I forget the reamer,drill to within a 1/32 and bore the front end of the hole with a boring bar for a distance of one bore size THEN ream.Drilled holes done in steps walk off center(especially deep holes) as each drill has some runout.
              I just need one more tool,just one!

              Comment


              • #8
                <font face="Verdana, Arial" size="2">Originally posted by DICKEYBIRD:
                ...... a length of surplus 3/8" hardened computer printer rod. A 1" 6061 alum. rod had to be drilled & tapped 1/2-20 on the chuck end and a 3/8" hole drilled/reamed 3 5/8" deep on the other end. I center drilled both ends and drilled the holes progressively using vegetable oil as a lube beginning with a 1/8" up to the final hole in 3 stages on the small hole, 4 on the larger. .....

                They are medium priced TiN coated bits bought from H.F. I tried to feed the bit into the work at a rate that kept a pair of nice spiral chips coming out plus cleared the chips pretty often.

                .......
                </font>
                I see several things here. First, I think it'a a bad idea to use 4 or more drill sizes to make a 1/4" or 3/8" hole. I would use a center drill to start, a drill that's approximately the size of the web of the filan drill second and then the final size. Why? Well, each drill will have some assemetry that will cause it to wander a little and by using several, you just build up a larger error. Besides, it's a lot of work.

                The drill quality is another thing. I tried to buy a set of import (China) drills and was so dissatisfied that I sent them all back. You wouldn't believe the actual shapes of some of them. And TIN coating over a poorly sharpened drill does not improve anything.

                If you have to have 4" sticking out of the chuck I would first use a 4 jaw and center it with a Dial Indicator and second use a steady rest to support the outboard end.

                As for "hardened" rod, I would forget that. It may not be hardened evenly throughout and soft spots could cause the drill to wander. And unless this is for a production environment where it will be used hundreds of times a day, hardened is just not necessary.

                I built a tap guide with hardware store 1/2" rod and didn't even finish off the outside. I drilled a 1/4" hole through it and added a 1/4" rod to the back end of a commercial tap holder. I had to hand straighten the the 1/4" rod for best performance but I had no trouble with the 1/2" bushing. It works far bettrer than the commercial one I bought. I posted a picture a few weeks ago so check the past posts.

                Paul A.
                Paul A.

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

                Comment


                • #9
                  A couple of reasons why your drilling gets off of center:

                  One of the nice things that happen when one is using the lathe for drilling is that physics can work in your favor. Since the work is turning the torque forces on the drill bit want to equal out thereby forcing the bit into the center of rotation (dead center). In other words, the bit wants to take the path of least resistance. However, to take full advantage of this property one must have a drill bit that has perfect symmetry. If one fluke is longer, sharper, or has greater rake angle, etc. then the forces on it want to equal out and the resultant neutral force may not be in the center of rotation as expected, but off to one side. The same thing applies when using step drills to enlarge a hole.

                  So if you, like most of us sharpen drill bits by hand, then chances are your bits won’t drill exactly dead center. Even some of the new drill bits are not sharpened perfect.

                  The best way I know of to insure a true hole on a lathe is to use a boring bar for final size as previously mentioned.



                  [This message has been edited by Mike Burdick (edited 04-30-2004).]

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                  • #10
                    Excellent, excellent information gentlemen. Thank you so much for taking the time to share your knowledge and experience!

                    Sounds like I need to keep the cheap imported drills over at my drill press for the looser tolerance work and save up for a quality set for the lathe. Does anyone have a recommendation for a brand and source for a decent set of bits? Same question re: bit sharpener?

                    A boring bar was mentioned several times....is it possible to bore a 3/8" hole? I know it couldn't go very deep but what does a boring bar that will bore a hole that small look like? I know, it looks just like a tiny little boring bar, right?
                    Milton

                    "Accuracy is the sum total of your compensating mistakes."

                    "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                    Comment


                    • #11
                      You could just go with a set of three or four flute drill bits instead of the standard two flute, and get a better drilled hole from the start. Sharpening these would be a challenge though. I don't even know if such a thing is available, but if so it would be an improvement in drilling more accurate holes. Probably cost about as much as a new drill press.
                      What Mike said about the uneven sharpening on a drill bit is essentially correct, but the bit will still try to follow to center. (when the workpiece is rotating, as in the lathe). It will drill a larger hole than it's diameter, that's the main problem with uneven length cutting edges. The important thing to get a drilled hole on center in the lathe is spotting the pilot drill. Even a well sharpened bit can't help but follow an off-center pilot hole. I find it helps sometimes to have a blunt metal piece in the toolpost that you can use to bring up against the side of the pilot drill, near the cutting lips. With this you can actually flex the point of the bit past center a tad, then as you crank the drill bit in, back out slowly on the crosslide. The bit will flex back as you do, and find it's center in the workpiece. Conversely, you can start the pilot hole, then as you see the point of the bit wobble, bring the crosslide in until the point of the bit is restrained from it's wobbling. At that point it will have found it's center in the workpiece. You have to be cranking in the pilot bit as you are nudging it in order for this to work. Once the hole is spotted right on, continue with the pilot hole.
                      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                      • #12
                        J&L has a Hertel set on sale right now at a very good price, you might want to check with them.

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                        • #13
                          Well, it wouldn't be polite not to finish the story.

                          My set of Chinese endmills came in yesterday so I decided to try to straighten up my drilling mess. I chucked up and indicated the work as close as I could then slowly ran a 1/2" endmill into the crooked 3/8" hole using the tailstock chuck. I took my time, cleared the chips often and it straightened the hole very well as far as the endmill would reach. The 1/2" drill bit in my cheap set had never been used and indicated in very close. I finished the hole with the new bit and it stayed straight. I then retapped the 1/2"-20 hole on the other end several times, installed and torqued the chuck and finally the thing runs close enough to true to suit me.
                          Milton

                          "Accuracy is the sum total of your compensating mistakes."

                          "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

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