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I am a gear noob.. some help please...

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  • I am a gear noob.. some help please...

    I am going to be making a thread dial, but dont know they proper gear to fit my lead screw. The lead screw is 1-1/2 4tpi....

    Thanks...

  • #2
    You need a 16 tooth gear that matches up with your leadscrew. Because it's a 1/4" pitch it not a standard DP gear but what is know as CP [ circular pitch ]

    Your blank on the pitch line will be 16 x 1/4" or 4" which is 1.273" diameter but more important is the OD which is 16 + 2 x 1/4" / Pi = 1.433"

    Thread dial gears do not need to be that accurate and many people just turn a blank and gash 16 teeth into it using a flycutter that's been ground to fit the leadscrew.

    Technically the teeth are layed off in a helical to match the slope of the thread on the leadscrew but as it neary point contact where it works you can cut straight teeth.
    .

    Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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    • #3
      Sir John,

      Perhaps I should be more clear... Its going to take all my machining knowledge and HSM skills to make the general box... There isnt a snowball chance in Hell that I am going to attempt a gear at this point in my education.

      So... Where can i buy one??

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      • #4
        Closest off the shelf is going to be a 16 tooth, 12 DP 14.5 PA gear.

        Tried looking at rush gears, SPI, and Martin but crap websites won't give a price.

        McMastercarr list one at $27.

        http://www.mcmaster.com/#standard-gears/=mu5i8n
        .

        Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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        • #5
          John offers good advise for a stock off the shelf gear. 12 Dp is 0.261" circulr pitch which will mesh in a 4 pitch Acme but the action would be lumpy if you were using it for power transmission. You will be using it strictly for indication. There will be no load on the gear except for driving the shaft with the dial. You may see the thread dial lunge slightly from division to division but this will cause no problem in actual use.

          Another point is the lead screw has a helix angle and the spur gear teeth go straight across. This will not allow much of a mesh. The solution to that is cut the width of the teeth down to 1/4" wide or so. This will allow the gear to mesh deeper into the lead screw thread. Some older lathes have a keyway cut into the full length of the thread to drive the power feed gearing in the apron. This may pose a problem if the 1/4 wide thread dial gear can hang up in it.

          Another consideration: An Acme thread has a 14 1/2 degree pressure angle. If the gear's circular pitch is a bit large for the screw, the gear tooth crest is sure to interfer slightly as it comes around to mesh in the screw thread space. Maybe a 20 degree pressure angle gear could be used. The PA is all wrong but there is no load to speak of. The shallower PA will prevent crest interferance on approach.

          Maybe you could incline the thread dial shaft angle to match the hellix angle of the lead screw. Helix angle for a 1 1/2 - 4 Acme is 3 degrees 20 min or so.

          A narrowed 16 tooth 12 Dp 20 degree PA gear will work for your thread dial until your developing skills allow you to make a new circular pitch gear for it.

          BTW, you aren't making a dual Metric/Imperial thread dial thingy are you?

          Also: I once saw a home made thread dial where the thread dial gear was laid out on a brass blank and hand cut with dividers, hacksaw, and file, helix angle and all. Worked good for - what? - 40 years? If you elect to try hand-cutting your own gear, look up "Grant's Odontograph" in Machinery's Handbook. That's assuming you have the curiousity and time for it. I'd guess it as a one or two evening job if you start with a blank.

          Adding. I went looking for a 16 tooth 12 Dp stock worm gear that mates to a 1 3/8 PD worm with no luck. I looked in Boston Gear, Martin, Dodge, and a couple surplus outfits with no luck. Murphy's Law dicates if I found one it would be on the wrong hand.

          Regardless, good luck.
          Last edited by Forrest Addy; 05-21-2013, 02:36 AM.

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          • #6
            Should be quick and easy, if your leed screw doesnt have a keyway, then a
            thicker gear is needed. One of my lathes, same size leadscrew as yours, has
            no keyway (its a twin shaft machine) I made mine from a big brass washer
            cut the teeth with die grinder and used an exhaust valve for the dial. works
            perfect.

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            • #7
              now that's creativity at its finest. . . . my kind of guy.



              Originally posted by big job View Post
              Should be quick and easy, if your leed screw doesnt have a keyway, then a
              thicker gear is needed. One of my lathes, same size leadscrew as yours, has
              no keyway (its a twin shaft machine) I made mine from a big brass washer
              cut the teeth with die grinder and used an exhaust valve for the dial. works
              perfect.

              Comment


              • #8
                Originally posted by cuemaker View Post
                I am going to be making a thread dial
                Why do you want a thread dial anyway? Thinking outside the box might save you a huge lot of unneeded effort. Either that or people are going to make fun of me. Or both I suppose. This is not a strategy to get an "A" in shop class but it'll git er done.

                Don't laugh too hard but I saw a beater lathe where the "indicator" was a loop of duct tape and the "flap" of the tape meant zero. Could have used paint or nail polish just as well.

                The only purpose is to know when to engage the half nut, right? You don't need much angular accuracy for that. When da paint (or duct tape, or paint/ink on duct tape, or whatever) points up, slam the half nut lever, right?

                Another alternative, degrease a small spot on the leadscrew and smear some dykem or whatever it is for layout. Then engage the half nut every time the dykem points up or whatever. Should be easy enough to clean up, probably easier than the duct tape.

                If you're willing to mess up the leadscrew, there's all kinds of crazy ideas like make a little octagon with a lead screw major diameter hole in it and stamp each side of the octagon (presumably with different numbers...) and then cut it in half and bolt/screw/whatever both halves onto the leadscrew. Or if you're hot to learn how to internally thread ACME threads, well, nows the time, that and some locktite and you're all good. Try to put your "indicator" somewhere its physically impossible for the carriage to reach.

                If you're really willing to ruin the leadscrew, pick the easiest end, think really hard about it first, and file a nice shiny flat spot on the end of the leadscrew where it doesn't matter much. Thats your new zero for engaging the half nut. This will absolutely give some people heart attacks just thinking about it.

                Obviously if you're doing restoration (or repair!) work then this is all out the window and you gotta do it the hard way. But if all you want to do is thread some stuff, there's easier ways to skin this cat. Try the duct tape thing and see if it works well enough. You can buy a lot of duct tape or dykem or whatever for the cost of a $40 brass gear that may not mesh anyway.

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                • #9
                  I think I missed something....

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