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  • Drill Rod and Lobing

    This is a follow on question based on a response to my earlier bearing question. (Man, I have a lot to learn!) It's great to have a place to come for an education.

    In the responses to my earlier post someone mentioned that drill rod may not be truly round and sometimes is lobed during the grinding process. My guess is that it isn't ground on centerless equipment, but I may be way off on that.

    My question is with regard to testing roundness of drill rod. Can it be placed between centers or in a lathe chuck and indicated as a test or must it be put in V Blocks and rotated on a surface plate to gauge it's roundness, and, if so, why?

    Were I to make a wild guess at the reason for V blocks it would be that during the centering process in a 4 jaw chuck the lobing might be compensated for giving a false impression that the drill rod is round. Am I even close?

  • #2
    What I equate drill rod with is music wire, also known as silver steel somewhere else in the universe-

    When I look at my music wire, it appears to have a bit of a twist to it, and yes it's not round. It also seems to be about a thou large in diameter. Normally I'll spin a piece in my fingers while holding it against my sanding drum. It polishes up nicely, and as I begin sanding I can sometimes see the 'lobes' in a spiral pattern.

    One method I use to true it when the need arises is to carefully grind a point on one end, doing my best to keep the point centered. I use a cordless drill to spin it, and a piece of wood with a hole in it as a handle to control the end at the grindstone. Then I chuck it in the lathe and support the pointed end in a little jig on the tailstock, kind of the opposite of using a center in the tailstock. Then I use the tool post grinder to take off the 'high' spots and hopefully not reduce the diameter below nominal. Easier said than done, since there's rarely more than a thou to work with.

    A second method I've used, for short pieces, is to grind a point on both ends, then hold the piece between dimples in a jig that rides the bed. I can TP grind it like that, turning it with a bow. The bow is a piece of wood with a bow in it- - The string makes one turn around the music wire and is tensioned to some degree. You basically 'saw' the bow back and forth and the wire turns several rotations each way. The TP grinder is worked back and forth during this process to cover as much length of the wire as possible before re-orienting the bowstring.

    Of course, alignment is critical if you want to keep the piece the same diameter from end to end.

    I have toyed with the idea of making a jig that would ride the wire (or rod I guess you'd call it if it was a significant diameter). The jig would hold the wire in a groove, and you'd snug it up to eliminate play. There would be a cutting tool on the jig, which you'd adjust to skim the wire, and you would play the jig back and forth while the cutter took off the high spots. I don't know- this would probably be problematic-

    But in your case, you're probably looking to use drill rod as a shaft, and of significant diameter and not too long. I'd probably spin it in a drill while using a file to 'touch up' one end. Then chuck that end and center it as best as possible and see if you can true a section on the other end with a sharp cutter and a shallow depth of cut. If that will work with the end unsupported, then continue truing towards the chuck. You'll want to keep an eye on the diameters as you go, since the outboard end will probably be fatter due to spring in the workpiece.
    Last edited by darryl; 01-19-2011, 12:49 AM.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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    • #3
      If you need to know if the rod is round, bound a laser beam from it while it is held in a clamping device (a chuck or collet). The light beam needs to hit the rod close to the chuck/collet. You need an index mark on the rod and clamp so you can index the rod. You need to isolate the lobs on the rod from the errors in the chuck. The laser beam will bounce off the rod and hit a wall. Where it hits the wall is determined by the angle of incidence, the roundness of the rod, and the error in the clamp. By indexing the rod and clamp you can separate out these errors and arrive and a roundness value. Clamping error will be a repeating sine wave per revolution that is directly related to the index on the clamp and lob error will be a smaller signal on the sine wave and at a varying angular relationship as it is indexed in the clamp.

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      • #4
        on centreless ground stock, if you have trilobular form then your only looking at 10/1000's", not a lot really!, obveously the lobe form gets bigger as diameter increases, it seems a common ginding fault, i can remember checking a pile of ceramic fuse bodies, the big ones like 2" diameter, micked them up, to drawing fine, tried to stick an end cap on, no go, checked the hole in the end cap, ok now i'm confused.
        put it on a vee block on a comparator set up with slips to maximum material condition, looked great, little throw but not that much, this is odd
        Drag out bench centres and DTI, wobble wobble wobble, oh bloody great, now i get to check them all, trouble is theres 1000 of them.
        QA plan no 1, take sample of 100, which wasover what the table said, all wobbly, end up checking them all.
        All useless, my first encounter with trilobular grinding.
        A hard defect to pick up on small diameters
        regards
        mark

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        • #5
          Lobing is common on centreless ground material. Drill rod probably would be centreless ground - it is the obvious choice. 3 or more lobes are possible.
          The causes and avoidance of lobing has been an active area of research for at least 40 years - probably a lot more, and good setup based on this work can minimise the effect.
          Centreless grinding is used a lot for automotive components because it is so fast and stable when properly set up. If you can easily detect the lobing on a component such as a gudgeon (piston) pin with bench centres and DTI then this is fairly extreme. Typically you need something like a Tallyrond to really see what is happening, which puts such measurement outside the normal capabilities of a home shop.
          Bill

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          • #6
            I may be quite wrong,but my old memory tells me that I have read that centerless ground drill rod can have 5 lobes.

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            • #7
              firbikrhd1, if your only wanting to measure the high/low spots and it's a short rod you can use 2 V blocks on a surface plate with a dial indicator that reads ten thousands of an inch.

              My question is do you need it to zero runout or are you just curious?
              It's only ink and paper

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              • #8
                In "Foundations of Mechanical Accuracy" by Moore they cover the issues involved in determining whether something is truly round. ( "How Round is your Circle" by Bryant is a more theoretical book with some interesting info and mechanisms.)

                Depending on the (unknown) number of lobes, measuring with a V block isn't always accurate. As I recall, you need to verify using a second set of V blocks which have a different included angle. Moore is a fussy bunch so this may not apply for typical home shop measurements but it may be worth tracking the book down just to see how the really crochety guys do it

                If your goal is to improve your rod's roundness a shop made lap should do it fairly quickly and easily, although you may lose several tenths in the process.

                John

                Edit: Shop made OD laps are quick and easy. I have some 1.25" aluminum round so that's used for my laps; copper or brass work too. The lap holder is a scrap of steel bored to take the aluminum and knurled on the OD. A setscrew is used to close the lap to size and adjust as it wears.


                Uploaded with ImageShack.us
                Last edited by GadgetBuilder; 01-19-2011, 01:46 PM.

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                • #9
                  Drill Rod

                  All the drill rod I've seen over the years is perfectly fine as is for most projects. Any minor out of round condition is likely to be inconsequential to an HSM project. If it needs to be better, it is usually ground in a spin jig after hardening.

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                  • #10
                    Originally posted by gwilson
                    I may be quite wrong,but my old memory tells me that I have read that centerless ground drill rod can have 5 lobes.
                    The lobes are caused by the runout on the bearings supporting the stock. So if it's a 3-mount centerless grinder (the one's that I've seen), you'll have 3 lobes.
                    It wouldn't surprise me if high-end centerless grinders have more lobes.

                    But I think the OP is overly concerned about roundness. Assuming it's quality drill rod, it should be rounder than you can turn on a hobby lathe anyway.

                    By the way, why the drill rod? Unless you're going to harden it, drill rod isn't going to be substantially better than mild steel that's been TGP.
                    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                    • #11
                      Thank you to all who have responded. My curiosity was peaked by a response to a previous question I asked about using a needle roller bearing and drill rod as a shaft. From what I can glean from the comments you all have posted, the out of roundness (error) of drill rod is on the order in the low ten thousandths of and inch error range, which, for me as a HSM, is pretty much Space Shuttle tolerances. I can shoot for within .0005" and achieve it most of the time when necessary and if I am very cautious but most of what I do other than running and press fits is OK within .001". If auto manufacturers can use center-less grinding for wrist pins and expect service life of perhaps billions of cycles then I guess drill rod is probably accurate enough for a shaft running in a needle roller bearing at very low rpm and expected to live only a million cycles or so. Toolguy's comments pretty much sum up what I now suspect. I may be wrong, but I believe I could use a piece of drill rod that is too large, turn it down to the proper diameter, finish with emery cloth and oil and have it be round enough to do the job I was considering. If I ever come up with a tool post grinder I may be able to better the outcome after hardening if necessary. The shop made lap is another great idea and might be the best solution given that I seldom have the need for a tool post grinder.

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                      • #12
                        You will not machine a piece of drill rod to more of an accuracy than it already is. Why do you think you can machine the ground drill rod to a higher accuracy?

                        Try measuring the drill rod as I suggested in the V blocks and come back and tell us what you found, if you could find anything at all.

                        What I am trying to tell you is your trying to do NASA work with home shop machines and it won't happen.

                        Your making a mountain out of a mole hill. Use the drill rod as is.
                        It's only ink and paper

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                        • #13
                          Originally posted by Carld
                          You will not machine a piece of drill rod to more of an accuracy than it already is. Why do you think you can machine the ground drill rod to a higher accuracy?

                          Try measuring the drill rod as I suggested in the V blocks and come back and tell us what you found, if you could find anything at all.

                          What I am trying to tell you is your trying to do NASA work with home shop machines and it won't happen.

                          Your making a mountain out of a mole hill. Use the drill rod as is.
                          I think you're right Carl. Thanks for the input.
                          Steve

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                          • #14
                            Lazlo-

                            I'm not sure that I understand what you are saying here:

                            "The lobes are caused by the runout on the bearings supporting the stock. So if it's a 3-mount centerless grinder (the one's that I've seen), you'll have 3 lobes.
                            It wouldn't surprise me if high-end centerless grinders have more lobes."

                            The stock isn't supported by conventional bearings so run-out in this sense is not really meaningful. The work is supported by two wheels and a blade. It has been a while since I took an interest in this process but as I recall, the tendency to lobing is due to dynamic instabillty arising from the geometry (setting) of the grinding wheel, the regulating wheel and the workrest blade, particularly for through grinding. The number of lobes that arise are the result of this geometry and its stability or otherwise. Some settings will cause lobing to grow and others will cause lobing to damp and and diminish - it is a dynamic property of the system.

                            As I said earlier, for good quality centreless ground work you will need a Tallyrond or similar to see the true shape of the work and its lobing. This means there is no process that you can undertake in a home shop that will improve such work in a way that you can actually measure and control. By comparison, any work with emery cloth and oil would be like taking a hammer and chisel to the roundness of the ground rod.

                            Of course you may just have some really badly ground drill rod!
                            Bill

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                            • #15
                              Originally posted by willmac
                              Lazlo-

                              I'm not sure that I understand what you are saying here:

                              "The lobes are caused by the runout on the bearings supporting the stock. So if it's a 3-mount centerless grinder (the one's that I've seen), you'll have 3 lobes.
                              It wouldn't surprise me if high-end centerless grinders have more lobes."

                              The stock isn't supported by conventional bearings so run-out in this sense is not really meaningful. The work is supported by two wheels and a blade.
                              The 3 lobes are the runout of the spindle bearings + runout of the support bearing + runout of the workpicece:



                              "In this setup, any high spot will create a diametrically opposite low spot. Over time, the workpiece will look like the exaggerated shape shown at the bottom."
                              "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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