AL320G - Threading Dial

Discussion in 'Tips and Tricks' started by joco-nz, Apr 9, 2017.

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  1. Apr 9, 2017 #1

    joco-nz

    joco-nz

    joco-nz

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    I've been looking at the threading dial on my AL320G and thinking it must be usable even though the "manual" that comes with the lathe doesn't tell you anything other than how to set up the change gears.

    So first off some research into threading on the lathe by reading Workshop Practice Series #3: Screwcutting on the Lathe by Martin Cleeve.
    Now this is pretty jolly complete and after reading things several times I felt I had enough of a handle on things to make the next steps.

    So looking at the threading dial the gear is 20 teeth, the lead screw is a 3mm pitch and the dial has 4 "lines" or sectors on it.

    So what does all this tell us ... well after some manual measurements the following ...
    We get 60m travel of the saddle from a single rotation of the threading dial and the lead screw turns 20 times during that travel.
    i.e. 20 teeth x 3mm pitch = 60mm travel.

    So back to the book, specifically from pages 71 with specific attention to pages 81 and 82. So from reading this section it appears that the key to being able to use the threading dial is that the pitch of the thread needs to fit some rules with regard to:
    [a] the thread pitch being a whole multiple of the leadscrew pitch
    the thread pitch being able to divide evenly into the amount of saddle travel represented by sectors on the dial

    So armed with this hopefully correct info I built a chart to see what pitched threads I could use the dial for on the lathe and which I could not. And that is the kicker with metric threads, not all of them will work with the threading dial without some changing in the threading dials gearing.

    The calculated table of threads. Blue lines show thread pitches not possible with the stock threading dial gearing.
    AL320G-Dialchaser-Metric.png

    Now to do a proof. I picked a 1.5mm thread, setup the lathe gears, found some scrap and got it ready to do some light cuts.
    threadingdial-metric - 0.jpg
    threadingdial-metric - 1.jpg

    So, set up the and ready to go I had the lathe running nice and slow (60rpm) and leadscrew turning and ready to engage the half-nut on a any whole sector line on the dial. Let it run a light scratch, disengaged the half-nut, pulled back the cross slide, brought the saddle back to start position, shutdown and checked with a gauge. Photo's not the best but you can see that the scratch is bang on the 1.5mm pitch gauge. So far so good. Now for the moment of truth, can we get repeated pickup?
    threadingdial-metric - 2.jpg


    Well repeated the same process and engaged the half-nut on one of the whole sector lines. Not the same as the last one, just to make sure there was some more variability in the mix. Re checked the slightly deeper cut, BINGO, bank on the same lines as last time.
    threadingdial-metric - 3.jpg

    Okay according to my understanding from the book learning I should be able to engage the half-nut at ANY point on the dial gauge as the 1.5mm is a whole multiple of the leadscrew pitch. So thats what I did, engaged the half-nut at a point between the sector lines on the threading dial.
    threadingdial-metric - 4.jpg

    And the result was bang on!

    I ran out of time tonight to do some more variations on threads, also will need to dig out some more scrap round to trial on. But so far the math seems sounds and practical testing is lining up with the book learning.
    threadingdial-metric - 5.jpg

    Conclusion at this point is that while not all threads can be done using this method a good chunk of them can be so the threading dial is no longer looking like the useless appendage it seemed to be for metric threading.

    I'll do some trials over the course of the week plus I want to look into what gearing would be needed on the thread dial to do the other threads and if that is even worth the hassle given the non-disengaged half-nut method will work just fine on those.

    Cheers,
    James.
     
  2. Apr 9, 2017 #2

    Journeyman

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    Heres a bit of an article I wrote ages ago when having trouble cutting a 1.75mm (M12) pitch thread on my WM250, very similar to yours. It may or may not help!
    The screw-cutting thread indicator dial meshes with the leadscrew via a 30 tooth gear. To get the indicator dial to rotate once the carriage has to move 90mm (number of teeth x leadscrew pitch). Only those metric pitches that divide exactly into 90 will be able to use the dial when screw-cutting. Thus pitches of:-
    0.5, 0.6, 1.0, 1.25, 1.5, 2.0, 2.5 and 3 will work, pitches of 0.7, 0.8 and 1.75 won′t work.
    Replacing the 30 tooth gear with a 28 tooth gear would work for the missing pitches but not for all. With a 28 tooth gear the carriage has to move 84mm for a full turn of the indicator and 84 is divisible by 0.7, 0.8, and 1.75. So either change the gear, make an indicator that has both gears or just leave the leadscrew engaged and reverse the lathe.
    Cheers
    John
     
  3. Apr 9, 2017 #3

    joco-nz

    joco-nz

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    Thanks John.

    I think on my leadscrew a 21 tooth gear on the dial indicator will get me the missing pitches. 21 * 3 = 63mm per full turn

    For 0.45 pitch: 63 / 0.45 = 140.
    For 0.7 pitch: 63 / 0.7 = 90.
    For 1.75 pitch: 63 / 1.75 = 36.

    Therefore a 21 tooth gear will give these pitches on a full turn and half turn. It will give the 1.75 and 0.45 on each quarter segment.

    So I guess I need to source a 21 tooth gear. :thumbup:
     
  4. Apr 9, 2017 #4

    Journeyman

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    James, I thought of putting both gears ie a 20 and 21 tooth in your case on the same shaft and find a method of sliding the indicator shaft up and down to engage the right gear. Never got around to it though as I don't do much in the way of screw-cutting on the lathe. Could make for an interesting lttle project though.

    John
     
  5. Apr 9, 2017 #5

    joco-nz

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    Oh dear - another project for the list. :hDe: Dang good idea though.
     
  6. Apr 9, 2017 #6

    Blogwitch

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    My drop in dial uses 3 separate gears to obtain the full range of metric threads on my machine (Chester Crusader), and it was a real PITA to swap them over when you went from one pitch to another.

    http://www.homemodelenginemachinist.com/showthread.php?t=24110

    Even when you have done this mod, by changing the gears (even single gears) it is a PITA to get the drop in lines to line up with the zero mark, so this is how I solved the problem and now only takes seconds to realign everything.

    http://www.homemodelenginemachinist.com/showthread.php?t=24111

    Hope this helps

    John
     
  7. Apr 9, 2017 #7

    joco-nz

    joco-nz

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    John - very nice and great pointer on how to approach that problem.
     
  8. Apr 10, 2017 #8

    bazmak

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    very interesting topic
     
  9. Apr 10, 2017 #9

    joco-nz

    joco-nz

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    Managed to do a test this evening using a 2mm pitch and following what I have worked out got the thread restarts using the threading dial bang on each time.

    The key of this test was that the restarts had to be done on a half turn. Which I did and it worked perfectly.

    I have updated the chart a bit on the basis I need a 21 tooth gear to get the extra pitches covered.

    View attachment Hafco-AL320G-ChasserDial.pdf

    Cheers,
    J.
     
  10. Apr 10, 2017 #10

    purpleknif

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    Uhhh , why not just start at the same no. each time ?
     
  11. Apr 11, 2017 #11

    joco-nz

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    Yes you could do that which would be a full turn restart which will work. But you then have to wait that much longer for the dial to come into alignment before engaging the half-nut.

    Cheers,
    J.
     
  12. Apr 11, 2017 #12

    Blogwitch

    Blogwitch

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    That is why it is sometimes a lot better to use a permanently engaged leadscrew, no waiting about, just put the cut on and start the machine. It only works if you have fwd/reverse on your machine.

    John
     
  13. Apr 11, 2017 #13

    Journeyman

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    A flip up tool holder is also a useful add-on for this. Saves having to wind the tool out and then back in again. I know John (Blogwitch) has a version of this but I can't find the post at the moment. (Might be on another forum)
    John
     
  14. Apr 11, 2017 #14

    goldstar31

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    You are thinking of the swing tool holder originally provisionally patented by Kenneth C Hart and writing as Martin Cleeve.

    I think that my crude copy came out of Popular Mechanics but Cleeve certainly wrote modifications both in Model Engineer and also Engineering in Miniature.

    Hemingway kits has the newest one which does boring but also screwcutting retraction.

    I have the original tool but the new kit has yet to be started.

    I hope that this helps

    Norm
     
  15. Apr 11, 2017 #15

    Journeyman

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    Found the post I was thinking of by John, it's on Model Engine Maker. Useful bit of kit. Have a look at the videos to see it working.
    Who needs a thread cutting dial;)
    Sorry James, you have another project!
    John
     
    Last edited: Apr 11, 2017
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  16. Apr 11, 2017 #16

    Blogwitch

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    About 2/3rds the way down that post I did a rough sketch on how to make one that just fits into a normal tool holder in the size you require.

    I should have mentioned that little tool when I said you need a reversible machine to carry out that sort of permanently engaged lead screw.

    John

    BTW Norman,

    "You are thinking of the swing tool holder originally provisionally patented by Kenneth C Hart and writing as Martin Cleeve."

    He designed the tool to be retracted manually by cam action when doing the reverse operation, mine works in a different and more simple and economical way.

    http://www.hemingwaykits.com/acatalog/Swing_Tool_Post.html

    Mine uses a much more lightweight head swinging on a lowered pivot point that allows the cutter to lift itself automatically when in reverse, so no cutting action can take place, and in fact, the next cut can be put on before it reaches the end as the tip is well away from the surface of the job, and when it is sent in again it will start to cut straight away, fully automatically again without having to touch anything.
     
    Last edited: Apr 11, 2017
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  17. Apr 11, 2017 #17

    John S

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    Don't worry John.

    If is wasn't written by Hart, Cleeve, Thomas, Westbury then it's not worth considering. Or should be according to a few diehards who never had a unique thought in their tiny brains and can only regurgitate the same old again and again.
     
  18. Apr 12, 2017 #18

    joco-nz

    joco-nz

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    Thanks gents.

    I'm aware of the swing/lifting tool holder for threading and even have a CAD model completed for one.

    I also have done the permanently engaged half-nut with reverse threading model on my lathe and its very successful if a little on the slow side.

    The challenge I have been looking to solve is when threading up to shoulder, either external or internal how to solve the challenges that brings when your lathe does not have a brake on it. Even going slowly its all a bit nerve racking. Heck thinking about it even a brake would still rattle my nerves.

    I then found some very interesting discussions/videos on threading in reverse. That is running the chuck in reverse with the tool suitably positioned to cope with the change in rotational direction. The benefit of this being you could run at a higher rpm and the tool threads away from chuck. Nice and safe, relaxed and no stress. To do that easily having a working thread dial seemed to be very handy. You could of course do a similar thing with a continually engaged leadscrew using the lathes forward/reverse. It becomes even easier if your lathe has fully variable speed not gear based speed. i.e. fast out threading and nice and slow and controlled back to start position.

    Anyway the game plan was to get to a point that I could
    (a) use the threading dial with confidence and know what threads I could use it for and what changes I might need to make to get the main threads I want covered.
    (b) with the above under my belt I could then setup and do the "reverse" threading model and enjoy the lower stress, higher threading speed and extra safety this would deliver.

    A reasonable video on the technique I am referring to is here:
    https://youtu.be/Z-dqOi_z5bk

    Anyway that was part of my thinking in beginning this research. The other part was quite simply "why can't I use this blasted threading dial!?" :cool:
     
  19. Apr 12, 2017 #19

    John S

    John S

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    There are plenty of variations on the swing threading tool.
    John has shown his and I have seen some where they use index-able tips to achieve the same result.

    This is mine which has two tool holders. A normal single point one, made from a parting off tool, the red one, and a holder made to take one spare insert out of a Coventry die head.

    [​IMG]

    Simple enough device as as Bogs says you have to have the pivot point below the tip of the tool, how much doesn't seem critical.

    I also have machined a tenon inside the tool holder which mates with a key on the bottom plate to stop any sideways movement as you have to have some clearance in the pivot bush or it doesn't work.

    [​IMG]
     
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  20. Apr 12, 2017 #20

    joco-nz

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    John - nice and chunky and solid looking. They are a really smart tool for threading, no doubt about it. :thumbup:
     

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