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Thread: Cutting thread

  1. #1
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    Cutting thread

    Hi. I recently acquired a Grizzly G8688 7" x 12".

    Still learning. Looking to make an hand adjustable air screw (motorcycle carburetor). Pictured below. OEM screw (requires screwdriver to adjust is bottom), what I plan on making (hand adjustable is top). Thread pitch is 6mm x 0.75 (red arrow). Brass is the material. I have a knurling tool.

    1. It was suggested I would be better off using a die rather than trying to cut the thread using the lathe.

    Tacoma Screw near me had a 6x0.75mm hex die for $4. Very similar to "http://www.amazon.com/6mm-Carbon-Steel-Hex-Rethreading/dp/B005KLW0D4"]this but they told me it was only for chasing threads (which is what Amazon thread title says). I told them application was brass but they shook their heads. I'm confused as I have a Craftsman metric die set that is basically the same and I've cut threads with them before. I called them back and they expanded the info saying that I need to get an adjustable (split) die to cut thread even in brass. I guess something similar to "http://www.amazon.com/6mm-High-Speed-Steel-Round/dp/B0007CNIR8"]this.

    2. Any advice on how to accurately measure the angle of the current tip (blue arrow) and to reproduce it using the mini lathe would be appreciated. Tailstock setover between live centers? It's pretty critical it match.

    Last edited by crazybrit; 05-17-2012 at 06:49 PM.


  2. #2
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    1. make the thread on the lathe. Your machine will cut pitch down to .3.

    2. Since the angle scale on your cross side is going to blow for want you want it to do (not accurate enough), forget trying to figure out the angle with an angle measurement tool first and then adjusting the compound rest to it, and just figure out the needed angle on the machine instead during set up.

    On your dial indicator, the tip should be removable, so do so, and install a fine needle tip instead of the standard hardened larger end ball tip.

    Now set the indicator up in the lathe tool holder so the tip of the dial indicator is Dead center of the existing Jet you have installed in the chuck, then while moving the compound rest back and forth across the tip of the jet, adjust the angle of the compound rest so the dial indicator stays constant across the entire angle tip of the jet.

    You now have the angle needed for the tip cut, set on the machine in the compound rest, and can use it for the needed angle to make the tip cut.


    As for making the pieces, here where the fun starts. The tip cut is a walk in the park since you only need a small section of the brass stock sticking out the chuck, but as you extend the piece of stock out of the chuck to start making the longer straight cuts back to the threaded section, it going to get a bit dicey. The problem is that the piece of brass is going to be turned so thin, the thin brass section is going to want to flex as it being cut (even with light cuts). Since there is not enough meat inside the tip of the jet to use a fine point live center tip, your going to have to get a live center with changeable noses, and create a live center nose that will receive the tip of the piece instead (read follower rest is going to suck, since you would have to readjust it every pass, and since you are just working with brass, the live center nose can be made out about any steel.

    Once you have the live center nose made and in play, then the rest of the jet is a cake walk to create, including threading the jet on the lathe as well (read for the thread section back, change out to a rotating tail stock chuck for tail end support).
    LittleMachineShop.com - Rotating Tailstock Chuck
    Note, I you are just making one of these, then make another live center nose that will accept the long front straight section before the threaded section instead.

    If you are going to be creating a few of these, then work the cuts in batches, hence do all the tips first when you have the compound slide set from pre-cut pieces of stock, then do the long narrow cuts for the batch, then threading (forget the die, you have a lathe that can cut pitch down to .3), the relief section behind the threads, then knurling, and final cut off.

    Note on threading, may just want to turn the chuck by hand to make the thread cuts instead of doing them under machine power.

    P.S, If you have figured it out yet, the lathe was the cheap part of the expense. It's the tools for the lathe/ all the project that you do that require such (what tools you don't or can't make yourself) that will really rack up the overall expense fast instead. Truth be told, your going to spend more time making specialty tools in the beginning that actual completion of projects instead. But on the positive note, at one point, you become real creative on converting the tools that you have already made so the become mulit-purpose tools in the end (so you don't have to make as may special tooling for each new project).


  3. #3
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    Very good post, I learned a few things, thanks!


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