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  • Extension to leadscrew

    I want to drive the leadscrew of my lathe from the opposite end with a dc gearmotor. The problem I am having is figuring out how to attach an extension to the end of the leadscrew for a pulley or gear. In the picture you can see the end I’m talking about. There are thrust bearings on each side of the end block and the nut is a locking type that holds the adjustment. I would prefer to have something that is reversible, but realize that may not be possible. One thought is to bore the end and press fit an extension on. Another would be to grab the nut with a socket and build an extension from that, but I can see the motor loosening or tightening the lock nut and losing the tension on the thrust washers. If I were to make a longer nut with set screws with lead or brass pads to protect the threads, would the set screws have enough holding power to keep nut from coming out of adjustment? Any thoughts on what would work?


    Stu
    Last edited by Stu; 03-11-2015, 03:23 PM.

  • #2
    File a flat on the threads to accept a set screw lock
    and make the bore of your coupler threaded.


    -D
    DZER

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    • #3
      Well, I have a couple ideas, but I need to know about the thrust bearings. Are they ball/needle or bearing material like bronze? What is the keeper thread - 1/2/13, 3/4-10 etc. (60deg.?)

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      • #4
        Attach a pulley to the leadscrew, on the shaft side of the block? Might lose an inch or two of travel on your carriage but..
        Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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        • #5
          Drill the end of the leadscrew and silver solder in an extension slightly smaller than the thread's minor diameter? Choose your extension from hex, keywayed shafting etc.
          Rules are for the obedience of fools, and the guidance of wise men.

          Holbrook Model C Number 13 lathe, Testa 2U universal mill, bikes and tools

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          • #6
            Originally posted by Carm View Post
            Well, I have a couple ideas, but I need to know about the thrust bearings. Are they ball/needle or bearing material like bronze? What is the keeper thread - 1/2/13, 3/4-10 etc. (60deg.?)
            The thrust bearings are roller type and the thread is 1/2-20

            Stu

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            • #7
              Originally posted by Black_Moons View Post
              Attach a pulley to the leadscrew, on the shaft side of the block? Might lose an inch or two of travel on your carriage but..
              That's very interesting, there is a 3/4" space from the leadscrew to the lathe bed and the LS has a keyslot. I really won't miss a couple of inches of carriage travel. I like that, I'll have to checkout the needed pulley sizes.



              Stu
              Last edited by Stu; 03-11-2015, 05:04 PM.

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              • #8
                Originally posted by Hopefuldave View Post
                Drill the end of the leadscrew and silver solder in an extension slightly smaller than the thread's minor diameter? Choose your extension from hex, keywayed shafting etc.
                I like that way too, but would I weaken the shaft by leaving such a small amount of material under the threads. The shaft end is 1/2"-20, minor dia is .435, wouldn't a smaller bore maybe .250 be better?

                Stu

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                • #9
                  Turn a round hunk o'stuff that has clearance from the mounting bracket (the two allen heads). place a jam nut in lieu of the full hex nut in place. Either blind bore and thread/tap 1/2-20 or get out to the end of the threads and fasten a cap that has your desired pulley that features a pilot hole that acts as a drill bushing into the outer periphery of the lead screw end (or 1/2-20 0 minor diameter). Draw the ass'm up snug, drill the hole with a hand drill, ream for a dowel or roll pin. You won't have to take anything apart ('cept that full nut) and won't have burrs that could make you cuss when you want to remove...just back off the jam nut.
                  You could c'bore the leadscrew end over the jam nut and setscrew on a flat if you think that is necessary.

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                  • #10
                    Originally posted by Stu View Post
                    I like that way too, but would I weaken the shaft by leaving such a small amount of material under the threads. The shaft end is 1/2"-20, minor dia is .435, wouldn't a smaller bore maybe .250 be better?

                    Stu
                    That's pretty much what I meant, the actual extension would want to be smaller than the thread's minor diameter so you could get the nut and bracket off, the spigot going into the leadscrew could be a fair bit smaller - there's probably a formula in Machinery's or similar for equal torsional shear strength in the hollow leadscrew and the spigot if you're pernickety!
                    The deeper you drilled the screw (and the longer the spigot), the more strength the join would have, I'd guess the limiting factor would be the spigot's torsional load, chamfering the leadscrew bore and matching it with a "cone" where the extension fits into it might help reduce stress raisers and make it a little stronger (especially if the two conical faces got some silver solder too) - I guessed that last bit!
                    Rules are for the obedience of fools, and the guidance of wise men.

                    Holbrook Model C Number 13 lathe, Testa 2U universal mill, bikes and tools

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                    • #11
                      This is not, or SHOULD NOT BE, a high strength issue.... if you reef on the leadscrew that hard, you will likely pretzel the lathe as well...

                      So, back to reality....

                      One way which should carry all that you need to carry load-wise, is to drill the end for a stub shaft, turn it and put it in, then cross-drill for a pin to hold them together. If the pin ends up under that nut, it will be held in and be secure.
                      1601

                      Keep eye on ball.
                      Hashim Khan

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                      • #12
                        Try McMaster Carr Part # 3084K31. Tap the unmachined end to match the thread. Looks like the torque rating would be just fine.

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                        • #13
                          Drill 2 or three holes in both lead screw end and separate pulley shaft. Attach with roll pins. (Will, of course, need some kind of bearing for pulley shaft.)

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                          • #14
                            You might want to consider getting rid of the home made heavy duty lead screw support and going back to the original Zymac support.
                            They made them flimsy on purpose, it fails rather then sending the lead screw through the gear train.

                            Grind that nut down to half the thickness and you will have a little more shaft to work with.

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                            • #15
                              A few thoughts- in use the lead screw is driving the carriage towards the head stock for the most part, so the thrust bearing would be on the left side of the bushing support. You would probably want a permanent shoulder on the lead screw on that side so that nothing shifts unexpectedly in use. That leaves the threaded portion to the right of the bushing for the nut to go on, and that's where the play would be taken out. At the same time it makes it impractical to drive the lead screw from the nut itself. Otherwise you could just put a roll pin through the nut and screw, then turn the nut to turn the screw.

                              Perhaps this still would be the best way to do it- drive the nut, which you pin to the lead screw shaft. Make the play adjustment before cross drilling for the roll pin.

                              Perhaps the more difficult thing to do is to align the drive with the lead screw so a positive engagement can be made without play, and without binding.
                              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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