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Cutting very fine threads

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  • Cutting very fine threads

    Hi to the group. I have need to cut extremely fine threads for camera filter adapters and telescope accessories. These threads are both very fine and large in diameter, up to 3 inches across the filter holder. Looking at a typical camera filter the thread is vanishingly small. I can't begin to cut that fine with anything I have in the shop. Is there a guide somewhere to cutting such small threads over such large diameters? Can you get or make a die that could be used for this purpose?

    Thanks

    DonM

  • #2
    Do you have a lathe, what is the thread pitch, a single point tool should work.

    Phil

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    • #3
      Half millimetre pitch IIRC. Very low forces, so no real accuracy needed for the thread profile, and since you'll probably be doing it in ally, or brass at a pinch, the cutting forces will be low. This all means that a small HSS tool that you grind freehand will be more than adequate.

      What is more difficult is setting the work up concentric again if you need a second operation. So it's important to plan the job for as few remounts as possible.
      Richard - SW London, UK, EU.

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      • #4
        You WILL need a lathe for that. Not a chance to find a tap in those ranges, if you could afford one. If you do not have a lathe or the knowledge it will be way cheaper to just buy what you need. Bob.

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        • #5
          I've done a lot of these recently. 0.5mm pitch is common and usually has to be done without half-nuts (unless you have a metric leadscrew).

          Hand grind the tool and use a jeweler's loupe to check it against a fishtail. Stone as necessary to get it touched up just so. Then it's off to the lathe where the loupe comes in handy again. Use it and the fishtail to get squared up on the workpiece or to pickup the existing thread if necessary. Hopefully the lathe has an electric brake to take away a little of the pucker factor if you're going towards a shoulder!

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          • #6
            These threads are fairly easy to cut in aluminum. As mentioned above, a freshly ground and honed HSS tool works best. It's also easier to turn the lathe spindle by hand. You can make a handle that fits on the outboard side of the spindle, even better is a large wheel (If you accidentally turn the lathe on, the wheel will be a little safer). I use a ratcheting wrench as a handle as it is easy to remove. Once the lead screw is engaged, I think it is better to leave it engaged when reversing.

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            • #7
              I like the idea of hand turning the lathe. Haven't ground a tool that small before, but I guess there has to be a first time. One head scratcher is how to set the threading gears for something that fine but with a diameter that large. Have to slow down the lateral movement in relation to the circumference of the work otherwise i won't get accurate pitch across that large a diameter. A commercial telescope part I own is about a foot across with camera threads on the outside. No idea how you would do that. Can't buy the parts I need since I am experimenting with adapters for my own telescope design.

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              • #8
                The diameter doesn't make any difference. The thread count is per revolution. If you post which lathe and gears you have, someone can tell you how to set up the gear train.

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                • #9
                  Traditional methods would make use of a chasing apparatus.

                  It's still a very good way.

                  Those short thread length, fine pitch rings are a pain in the butt any other way. (numerical controls excepted)

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                  • #10
                    Give thought to turning the tool up-side down and running (or hand cranking) the spindle in reverse. That way you don't have to worry so much about running into the shoulder.
                    Errol Groff

                    New England Model Engineering Society
                    http://neme-s.org/

                    YouTube channel: http://www.youtube.com/user/GroffErrol?feature=mhee

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                    • #11
                      I don't believe you will ever be pleased with the surface finish of fine threads produced by "hand cranking".

                      I could be wrong, but typical filter rings are made from 6061 (or worse), Anodize color uniformity being of high importance.

                      To produce the fine finish expected by the optics community, cutting speeds (sfpm) needs to be adhered to.

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                      • #12
                        Yes, cut them in a lathe.

                        You need to properly support the ring or tube you are cutting them in properly or it will distort. A six jaw chuck would be helpful. Or a heavier ring that clamps over it to keep it round.

                        A DEAD SHARP tool. HSS is probably best as carbide can be hard to fine tune. After sharpening it to a sharp vee, use a 10X magnifier to check and to guide you in putting a small radius on the tip. I just eyeball these radii. Be sure to continue that radius down the relief angle. I use a side to side motion across a fine stone, with the tool at the relief angle and rotating from one side of the vee to the other on each stroke. Or, at least more than half that angle on each half stroke. This only takes a few strokes on tools for fine threads, perhaps only one.

                        I assume you are working with aluminum. Use a good cutting fluid, like WD-40 and keep it wet.

                        Check the thread depth often and use a fine feed for each cut.

                        Get a fine, brass wire brush to clean the burrs off the threads after each cut and before each check. Emphasis on FINE and BRASS.

                        Oh, and I also like the idea of a hand crank on the lathe spindle.

                        Paul A.
                        SE Texas

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

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                        • #13
                          I've only ever cut one thread for a filter, it's a bit of a pig, 1st go imploded, it was a peice of tube, second go out of solid bar, cut the thread then gently bored it out with a tiny homemade file tang boring tool, it fitted but not too good so it was adjusted with silicon grease!
                          I'm sticking to big course nuts and bolts, btw a microscope is handy, these days a usb one and tablet would fit the bill, I need glasses to find my glasses these days.
                          Mark

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                          • #14
                            I've made a good many camera lens fittings of one kind or another in the '70's and 80's. Most hag very short threads with maybe a turn or three full threads and a relief so they shoulder freely. I found the simplest way is to cut the thread without power; turning the chuck by hand and reversing back. The cutting force is very low. Usually it only takes three or four passes with maybe a spring or low indeed pass. I used WD40 on the aluminum, brass dry, and the one stainless 303 steel adapter I made I cut with bacon grease.

                            One thing: aluminum camera adapters absolutely MUST be anodized or they are sure to gall at the worst possible time.

                            One more thing: practice the thread to prove set-up and tools and make gages as you do.
                            Last edited by Forrest Addy; 04-16-2015, 05:06 AM.

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                            • #15
                              Thanks everybody for all the good advice. Got a handle planned for the lathe to try some hand turning. I will waste a lot of aluminum and see if I can get the thread size I need. The consensus is that the usual thread is .5mm? I have a King lathe, 10X22, with the usual change gear assembly for threading. Given that it does not have a metric lead screw, how would I go about setting this lathe up for a 0.5mm thread? Anybody have a similar lathe?

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