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Thread: Half Nut Lever

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    Default Half Nut Lever

    Can anyone explain the function of the Half Nut Lever? This is the lever located on the upper right hand corner of the apron

    I have a Harbor Freight 9 x 20 lathe and am trying to understand how some of the controls work,an example the half nut lever.
    As far as I can gather it is used only when threading. When you are set up for cutting threads this lever is engaged by putting it in the downward position.

    Here is the problem,what does it control. Does it work in conjuction with the thread dial by allowing it to mesh in with the lead screw. Also,when you want to engage this half nut is the lathe on or off. With the machine off I used a slight pressure to push it down,it doesn't budge. Rather than applying excessive force I felt I should ask. What do the numbers on top of the thread dial represent.

    Any helpfull comments greatly appreciated
    Thanks
    John

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  2. #2

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    The half nut engagement lever controls half of a nut (picture a nut cut in half leaving 2 matching threaded pieces) that is spread open when disengaged when its engaged it closes to 2 halfs around the lead screw to advance the carriage. It can be engaged running or not doesnt really matter. If you engage it not running you may have to rotate the chuck some to get the threads on the leadscrew to mate up with the threads on the half nuts. It is used for threading and also used to feed the carriage automatically so you dont have to crank the handles all the time. That can help with the surface finish and making the cuts easier on the machine by not having a quick advance of the tool into the work, it will stay constant. The threading dial runs off of the leadscrew and turns "x" number of rounds per revolution of the leadscrew. To use it for threading you use the chart that came with your lathe to see what number or numbers you can use with the thread youre wanting to cut. If your wanting to cut a 16 thread per inch it might say you can use 1,3,5, meaning when the dial gets to any of those numbers you can engage the halfnuts. This keeps the thread starting at the same point in the rotation each time. If you go to www.mini-lathe.com or www.littlemachineshop.com they have some different information you could use to figure out some of the terms and different functions. Hope this helps.
    Rick



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    The Half Nut lever engages the half nut with the Leadscrew when screwcutting so the carriage is moved by the rotating screw. The threads on the nut and screw have to be aligned to engage it so if the drive between the spindle and the leadscrew is not engaged or if the machine is not running it will not easily engage. To check that it will engage with the screw stationary just close it gently and move the carriage until the threads align and then it should drop in.

    The half nut does not move the thread dial but is used inconjunction with the thread dial. On most machines the thread dial is mounted on a pivot with a nut clamping it so it can be moved out of or into engagement with there screw. If the dial rotates when the carriage is moved while the screw is not rotating it is engaged; alternatively if the dial rotates when the screw is rotating it is engaged.

    The numbers on the dial are used as a guide to ensure the halfnut is engaged at the correct point of rotation of the leadscrew. When screw cutting the screw is driven by a gear train of 'change gears' and gearboxes from the spindle to get the correct screw pitch based on the pitch of the lead screw. If you have an eight tpi lead screw running with direct drive then the screw runs at the same speed as the spindle for and you can cut an 8tpi thread. Fot 16 tpi the drive ratio has to be 2 to 1 through the drive train, 32 is 4 to 1, etc.

    When the drive ratio is an even number it does not matter at what point in the rotation of the lead screw the nut is engaged the threading toll will align with a previous cutting pass. However for odd numbered threads, say 15 tpi the drive ratio is fractional so it is important where the nut is engaged in the rotation of the screw. This is where the thread dial comes in, and you will have to read your manual because the dial numbering sometimes vary. However, if you always engage the nut on the same number the cuts will align correctly.

    An open mind is a virtue...so long as all the common sense has not leaked out.


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    Default Thanks

    Geof:
    Thank you for taking the time to explain,very informative

    John Romano
    Philadelphia Pa.
    jroma1@comcast.net



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    In addition to the good explanations given so far:

    When threading, the advance is fairly rapid, and is an exact number of threads per inch, i.e., 16, 20, 24, etc. Or in the Metric system, this is designated as a precise movement, or 'pitch' per revolution, i.e., .50mm, 1.0mm, 1.5mm, etc. In either system this cuts distinct grooves (threads) in the workpiece.

    When feeding, you don't want distinct grooves, so the advance is much slower, and does not have to be some exact number per inch. You might set the gears for a final feed of only .003" per revolution, equivalent to 333.33 threads per inch! With a slight radius on your tool bit, each pass will slightly overlap the previous one, producing a smooth, continuous surface.

    Typical home lathes and 3-in-1 machines only have a single leadscrew, used for both feeds and threads. More expensive lathes have both a leadscrew for threading, and a feed shaft for feeds. The feed shaft is not a threaded leadscrew, just a shaft with a drive slot in it. In this type of lathe, the half nuts are only used for threading to conserve them from wear, and a clamping arrangement drives the feeding operation.

    Dennis



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