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Thread: Cutting a nut

  1. #1
    Join Date
    Oct 2011
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    Norwood-ish, Adelaide
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    Default Cutting a nut

    Coincidentally, I've been cutting nuts for machines while Kenny_10 has been thinking about options for repairing his vice.
    Steve has suggested a technique used for sometimes repairing leadscrew nuts, but the nuts I've been cutting have been real, dinky die ACME style nuts so I thought a short write up might be helpful to others who have not made them before.

    Before cutting a nut, one thing I would strongly recommend is a matching male thread, so if you haven't got 'the other bit', you will need to make a short length. Industrially, there are thread gauges that can be used but lots of people will use a bolt. A sample thread piece can be checked with wires, so any nut checked with a sample will be better than guessing it is about right.
    I was cutting nuts to suit parts that had been given to me. This leadscrew was so long that I had to feed it in through the headstock to be able try it in the thread.
    P1040588.JPG

    To cut a nut you will have to use the interior version of plunge cutting, as you are unlikely to have the room inside a nut of any decent length to set the compound over. Most nuts you will single point cut (rather than tap) will be a non-V form anyway, and that technique is primarily for V forms anyway.

    To cut interior threads (where I have the space) I use a HSS boring bar holder with (in my case) a small piece of 3/16 HSS ground to the correct shape. For smaller threads, an all in one boring bar can be used, either the type that Pete F demonstrated how to make or one of those forged types with the bent end. One nut was MS but the others were Phos. Bronze. A diamond hone to keep the tool sharp is a must, especially with Phos. Bronze.
    P1040585.JPG

    Once you have set your lathe to the appropriate pitch and drilled out your clearance hole (drilling is important - see later), with the lathe not running, zero the cutting tool on the side of the hole. Then wind the tool back to touch the other side and note the reading. 1/2 of this will be your safety distance - that is, the distance you will wind to when at the end of the thread to move the tool back to the start.
    P1040583.JPG

    The other thing that helps is a mark on the threading tool that shows the limit of the thread - either for a LH thread how deep to start the tool or for a RH thread, when the tool has cleared the thread and can be brought back for the next pass. Usually I'll use a paint pen or a piece of masking tape with a pen mark on it.
    P1040584.JPG

    Why is drilling important? I find that because of the shape of the tool, a slight burr can be thrown up on the side of the cut. I will periodically clean the cutting tool away and then run the drill down the middle of the thread to restore the centre clearance. That way, when I try my sample thread I know that if it is not working, it is because the cut is not deep enough rather than a burr that is preventing the thread from engaging. You can of course bore the hole but if you do, there is nothing to clean the minor diameter of the thread you are cutting. (I suppose you could use a round or half round file but that is more effort than I want to put in).
    P1040586.JPG

    Depth of cut is one of those things that handbooks will give you, but it depends a lot on having a correctly ground tool. Normally I find for ACME type threads that I have not got the tip distance correct and so need to go deeper (or move along when at depth). Steamwhisperer came up with a jig to get the tip size correct, but I haven't gotten around to making one yet, so if it is important I'll measure it with a microscope.


    Michael

  2. #2
    Join Date
    Oct 2011
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    Norwood-ish, Adelaide
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    Default

    Interesting to note - 'This Old Tony' has just done the same thing https://www.youtube.com/watch?v=11pcIJN1Gd8

    Michael

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