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Thread: threading on jet 9x20 lathe

  1. #1
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    Nov 2013
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    plentywood,montana,usa
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    Default threading on jet 9x20 lathe

    i am just getting up the courage to try threading on my jet 9x20 lathe. my lowest spindle speed is 130. seems a little fast for my liking. can it be reduced further with a pulley change? also, can a person thread just by turning the chuck manually or does the cutter rely on some speed to do the cut? i am assuming it would.

  2. #2
    Join Date
    Feb 2015
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    If you just want to get the feel of single point threading, start with maybe a 1/2" aluminum rod, maybe get a 12" piece, and maybe only thread 6" or less so you have time to react. At the end where you are going to stop, cut away maybe a 1"spot less than the minor diameter. 130 should be fine, maybe thread a finer thread like 1/2-20, it will give you more time to react. Good luck

  3. #3
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    A popular mod/accessory for small lathes is a hand crank that locks into the outboard end of the mainshaft for just this sort of thing. It allows for easily threading up to a shoulder or to a very fine stop point.

    And no, you don't need a minimum speed. Otherwise we'd have to work a LOT faster with a hand driven tap or die....

    Really though 130rpm for finer threads is quite manageable. For example a common thread size of 20 tpi running at 130 rpm is going to take 20/130 minutes or 0.15 minutes or just over 9 seconds to travel 1 inch. A more coarse 16 tpi still takes 7.4 seconds to do 1inch of threads. So unless you're working in towards a pretty tight fit against a shoulder there's more time than you might think.

    I would not suggest doing a thread in tight to a shoulder for your first thread even so. Lots going on and it would not be that tough to have an "oopsie". I'm sure we've all had a few when learning to thread.

    There's also some tricks you can do that allow you to thread out away from the chuck by running the lathe in reverse and mounting the threading cutter upside down or by putting a right side up cutter on the rear of the part. Look up on You Tube for something along the lines of "threading in reverse" "Threading away from the chuck" . You'll get a bunch of them to watch which I think you'll find very helpful.

    I also found a video that shows an OUTBOARD CRANK HANDLE TO CUT SINGLE POINT THREADS. Note that due to lost motion in the gear train it is imperative that you back out the cutter for the return or it'll make a mess of the threads you just cut. He mentions this in passing as he backs out of the cut to reverse the carriage at around the 3rd pass. But it's VERY important or it'll basically ruin your thread. For the same reason once you start a pass you can pause but do NOT back up. It's not a tap or a die. You need to do keep the gearing all loaded for the duration of each pass.

  4. #4
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    My first attempt at thread chasing on my HF 9x20 lathe was cutting 3/4"-8 LH square threads in 4140 steel for chuck screws. This video shows what happened at 130 RPM, and how I switched to using a hand operated spindle crank, for a good arm workout:



    So before I could do that, I had to make and install a tumbler gear to reverse direction of travel, as well as making the crank.

    The whole project, almost a half-hour video:

    https://www.youtube.com/watch?v=fO_WpIo5gzs&t=2s

    I ground my own HSS cutter(s) for this. The 130 RPM was just a bit too fast for my comfort. It was perfectly fine when I chased 3/8"-16 UNC threads in mild steel for my machine shop mini-vise project.

    Here are some images of the threading job, but on the school shop's Clausing lathe:





    Note that I had the cutting tool upside down, and the spindle running in reverse. The carriage moves to the right where there is plenty of space and time to stop cutting. This also allows for keeping the threading half-nuts engaged so you don't need to use the threading dial.

  5. #5
    Join Date
    May 2015
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    I have a small multiple purpose machine named Astoba. For threading, it has a hand crank keyed to the spindle. With well sharpened and positioned cutting tools, it does a marvelous job of threading. I intend to make a hand crank for the spindle of my Harrison 10AA. Hand crank threading takes the anxiety out of threading.

  6. #6
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    Paul, I noticed in the video that you dubbed over with the fact that this was your first ever single point in the lathe? Talk about diving in the deep end of the pool with concrete overboots ! ! ! ! 3/4-8 AND an Acme or square thread AND left hand to boot! That was a bucket load of difficulty! ! !

    I was going to ask about your bench wobbling around then you dubbed in a note about needing to fix the legs or set them up with a wedge... .

  7. #7
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    Necessity (or at least desire) is the mother of ambition. I didn't absolutely need new chuck screws, but I wanted them, and I enjoy a challenge. I did practice and make mistakes first, on a piece of Delrin.







    First attempt on steel:

  8. #8
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    Nice... And from the pictures of the plastic it looks like some practice and learning proved useful.

    I have to admit to skipping ahead to the end and condensed your half hour video down to about 5 minutes stopping where the preview still seemed like something interesting. So I got the very last still of the fit in the jaws without the earlier one showing it WAS a jaw. So I thought it was a half nut. Now that I see it's for a four jaw that's even doubly more interesting.

  9. #9

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    Watch the attached video from Joe Pieczynski on threading in the lathe: https://www.youtube.com/watch?v=Z-dqOi_z5bk
    He shows how to thread away from the tailstock as well as many tips. His second threading video does away with most of the threading math involved as well.
    I can vouch for this as I have done a number of threads this way before I got my Hardinge and still do most this way today. Toughest I did was a left hand inside Whitworth form, special for a pre-war Rolls-Royce rear axle yoke puller. Had to tighten release bolt over 100ft-lbs so it had to be right!

  10. #10
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    Ronnie, it's not as hard as it seems. Most single point threaded parts include a "runout" area where you can safely disengage the half-nuts without cutting extra threads. The picture below has a VERY exaggerated runnout area. In practice the runout area should be a few threads long and as deep as the bottom of the thread. While you are learning you can make that area wider as you build up confidence.

    In use, you watch the thread and as soon as your cutter enters the runout t area you disengage the halfnuts. This will stop the carriage immediately while the spindle its still spinning. There were several JET 9x20 models built throughout the years. One model has a safety clutch that comes into play when it's running at it's lowest speed. When you crash the tool it will stop the spindle and carriage immediately and make a terrible ratcheting noise. This is alarming but causes no problems. The one at http://www.micro-machine-shop.com/9x20_lathe.htm has the safety clutch from the EMCO Compact 8 that it was copied from.

    At the end of the project, there is a profound difference between spare parts and extra parts.

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