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  • Accuracy

    I did some creative thinking last night (had a nap in my chair). Got up and went downstairs to the shop. I figured out how to turn round brass balls up to 1.5 inches without a ball turning attachment. I made one .875 round ball with stem for a project that needs two of these. This evening I did the same to make the second. Hadn't changed the setup completely but did change the work and position of the cross slide and tool. I spent a lot of time last night finding exactly where the center of rotation of the compound is in relation to the centerline of the spindle, then scribed a mark on the carriage to reference the cross slide so I can put it back to that point. The thing I realized when drifting off to sleep was a new (to me) geometry for the tool holder that would allow me to draw the tool well behind the center of rotation of the compound.

    Turning a ball this way is done by swiveling the compound on the cross slide. On an SB9 you loosen the allen screws that hold the compound. I use a lantern tool post and have a custom made tool holder that can tuck up very close to the post. I don't think this could be done with a quick change tool holder.

    The point is that I turned another ball tonight and it is the same diameter to within .0002 / .0001

    I know my lathe is good with little wear but is it common to hold this tolerance? I can't take all the credit.

    [This message has been edited by Evan (edited 12-05-2003).]
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  • #2
    i was thinking about making a gizmo for making rollers for english wheels,but maybe i don't have to.i saw a radius turning attachment on ebay and thought to myself "self,if you ever finish all those other projects laying around you could make something like that out of scrap metal,if you had any scrap metal laying around." think it would work on steel as well as brass?


    • #3
      There are many ways of producing a ball or radius on the lathe. Slipping the compound is one. They all depend upon location of the center of rotation for accuracy. The more accurately it is located, the more accurate and round the ball will be.
      For most purposes such as ball handles, close enough is good enough, and tapered posts and lathe centers suffice. For extreme accuracy, a lot of fiddling to get everybody on center is needed.
      Use of a scribed line on the carriage is a good idea for future reference, and probably is repeatable to +- 0.001"-0.002", usual eyeball accuracy.
      A question arises when mention is made of turning a ball to 0.0001"-0.0002" in that measurement to that accuracy is pretty difficult. Some pretty sophisticated equipment is needed to measure in this range, particularly of a spherical item. How did you determine what the exact dimension is?
      Jim H.


      • #4
        Good idea Evan, but don't you ever think of any thing but your BALLS?

        but you know, i think i will go up to the shop and try it.


        • #5
          JC makes a good point.

          A couple of weeks ago, while browsing, I found a textbook titled "Fundamentals of Metrology". For $4.95 it was a no-brainer so I ordered it, along with a couple of other titles. It's an outstanding guide to using all manner of machine shop measuring devices, including verniers, mics, gage blocks, CMMs, and optical flats.

          I had been happily reading my mic to the tenth until they started explaining the difference between resolution, precision, accuracy and repeatability. Factor in technique, and it's clear to me that I shouldn't be using anything more precise than a yardstick.

          So I've decided to continue measuring to tenths - I just won't expect my measurements to have any relationship with reality!

          To get to the point:
          According to the book, it takes a good deal of skill (experience/technique), as well as accurate equipment, to consistently and accurately read a mic to a tenth.

          Note: I'm not slamming Evan or his ability to measure. I've seen the pictures of some of his work and it's first rate.



          • #6
            Point well taken here as well. However, the two balls are the same dimension to within a tenth or so. That is easy to determine with a mic. It's not the absolute dimension but the fact that the two are so similar that I was suprised by. Measuring that is much easier than determining the "real" absolute dimensions. Just like a go/no go guage.

            On the scribe line; I can place it much closer than .001 since once the cross slide is at the line I then refer to the dial which I can interpolate to maybe .0002. The dial on the SB9C is fixed in relation to the lead screw so it will be repeatable.

            I work to tenths all the time. When I make a holder for a ball bearing I make it a light finger press fit, maybe 5 pounds pressure at room temp. That requires .0001 accuracy at least.

            [This message has been edited by Evan (edited 12-05-2003).]
            Free software for calculating bolt circles and similar: Click Here


            • #7

              It should work fine on free cutting steel as well. I'll give it a try.

              Further on the measuring thing; The absolute dimension of the balls in this project is unimportant. Measuring comparative size is as simple as setting the mic so that one ball just lightly slips between the jaws and then trying the other ball without changing the mic. If the force required to push both balls past the jaws is similar then they are at least good to a tenth or even better. I base this on my experience making press fits.
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              • #8
                Use of the micrometer as a snap gage can be misleading with this type of measurement for several reasons.
                Heat of the hand is enough to throw it off, the longer the mic is held, the larger it gets, same thing happens with part being inspected.
                With a sphere, if the mic is moved only very slightly from the perfect vertical or centerline, the dimension will change.
                Snap gages are normally provided as a two part set, go and not go, thus the size of the part lies between the two size ranges. This eliminates the subjectivity of feel when reading a dimension.
                According to the Johansson System of tolerances, a push fit in the range of 1"-2" hole diameter is +-0.00024"-0.00031", so anything within 0.0005" is okay.
                Jim H.


                • #9
                  A difference of .0005 will make one heck of a big difference in a press fit. That would be the difference between a finger press fit and using a press.

                  I have quite a stash of small ball bearings, 5/8 OD, that I use on various projects. To make it easier to machine accurate housings for them I made a plug guage. It is .6251 at one end and tapers over one inch to .6241. That makes it really easy to measure to tenths as each .0001 is .1 of the length of the guage. Gotta watch the temps though.

                  [This message has been edited by Evan (edited 12-05-2003).]
                  Free software for calculating bolt circles and similar: Click Here


                  • #10
                    That is the Johansson system, stated as push fit.
                    Easy driving fit in the same range is +0.00031-+0.00087. Again a 0.0005" plus range.
                    Press fit is +0.001"-+0.0025"/inch of diameter, with +0.0015" being a fair average.
                    The actual push fit range is -0.00024-+0.00031".
                    Information is from Machinery's Handbook, generally regarded as accurate.
                    Jim H.


                    • #11
                      Small things get tricky quick in reguards to press fits,usually the biggest b problem isn't the tolerence but the alignment anyway.

                      On my multi-spindle head I just built the diffference on the .375" od shafts and the gear bores was no more than .0005" and even that was tough as a press fit goes,I made a few extra shafts just incase,the very first was .0001" oversized,it siezed in the hole,after that I checked all dems.and grouped the parts according to size,I found that the shafts averaged +/- .00015 and the gears were about the same so it was a matter of matching under and over sized together,I only had to re-ream one gear.

                      As for small ball bearings,I learned my lesson on ultra-light aircraft spools,the bearings would not at all function with any sort of press fit for very long in the end they had to be retained with loctite and grasping grooves.
                      I just need one more tool,just one!


                      • #12
                        Have had a similar problem as WS when press fitting small bearings. They get tight and rough.
                        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                        • #13
                          The small bearings work fine as long as the fit is very light. As I said, a finger press fit. That's where working to .0001 is needed.
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                          • #14

                            You're dead right about the alignment. In fact, it seems the lighter the press fit the more critical the alignment becomes since it is easier for the part to cock in the hole. I fit the bearing to a shaft in the drill press and use the drill press quill to push it straight in to the hole. Again, I emphasize that these press fits are only a few pounds of force. It doesn't take much to deform the bearing outer shell when the clearances are measured in micro-inches. The last few projects I built use around about 26 different bearings, including tapered rollers, ball and needle bearings. All must fit the housings with no slack and be able to operate properly down to -20C (NOT my shop temp). I use a small set screw with a lead plug between the screw and the bearing shell to place a small amount of axial force upon the bearing OD. It ever so slightly deforms the shell (microinches) and with very carefull adjustment can take up the normal built in clearance in a ball bearing to just about zero without hampering operation. I build with full allowance for temp variation of +20C to -20C in my astronomical instruments without any "slop" or unwanted clearance appearing in critical dimensions. It means very carefull selection of material combinations and designing for for dimension changes. Calculation of CLE is very important.
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                            • #15
                              I find this very interesting, remember all this stuff is extremely new to me, so I have some probably stupid questions, but thats how I learn by asking.

                              How do you make your bearing housings, do you use your lath to bore out the dim. but how do you get the front of the hole to be narrower then the inside if your pressing a round ball bearing in, how exactly is this done.
                              If I had to do this I wouldn't really know how to start, but what is the best way this is done to get the desired results..