Beginner help: Single point threading doesn't follow previous track
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  1. #1
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    Default Beginner help: Single point threading doesn't follow previous track

    Hi all,

    I could use some help diagnosing what I'm doing wrong cutting a M12x1.75 thread on a lathe, on some mild steel rod. The threads passes aren't being cut on top of each other, and seem to be offset by 1/2 pitch.

    My process is;
    1. Turn the outside diameter down -.030mm (to guarantee concentricity without aligning part every time), zero DRO in X/Y axis
    2. Put a gutter in, 1mm deep x 3mm wide, starting 10mm from end of rod
    3. Move cutter out approx 20mm from end of rod, change to carbide threading tool, set speed to 300rpm, configure gearing for threading
    4. Wait for '1' to come round on thread dial, engage half nut, check a pen mark lines up on dial while it's covering the 20mm of air
    5. Wait for it to enter gutter, disengage half nut, retract cutter +.5mm from diameter, move cutter back out to roughly 18-20mm from start of rod. Sometimes I'll double check the pitch (it's correct), generally I won't because I can see it looks reasonable and it's just practice.
    6. Move cutter in -0.180mm, wait for 1 to come around on thread dial, engage half nut, check pen mark, repeat with depths -.360, -.460, -.560, -.640, -.720, -.8, .880, .950, -.9, -1 (or until it's obvious I

    I'm using a carbide insert, have the tool holder and work piece choked up.

    I have a metric leadscrew (roughly 26mm x exactly 3mm pitch), and the numbers on the dial match up with instructions (I can use #1 or #5, but always use #1 for simplicity). I've tried advancing top slide at 29.5*, vs the cross slide, and had the same affect, and just use the cross slide now for simplicity. The lathe is brand new, so minimal chance of a previous owner messing with it.


    Normally, the second real pass I put down seems to be offset from the first one, I think it's by 1/2 a pitch, but I haven't measured.

    I've been able to produce an acceptable thread keeping the half nut locked, and reversing the chuck.

    I also had better luck when I returned the tool to *exactly* 20.000mm according to the DRO, but I understand I shouldn't need to do this with a dial, and even then, on pass #5 it wrecked the thread, same as above.

    I'm begininig to wonder if the dial has a half count of teeth on it, I'm really stumped what I could be doing wrong. Any pointers or things I'm obviously doing wrong?

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    Metric leadscrews often need the gear inside the threading dial changed to screwcut different pitch metric threads. Almost counter intuitive - if you have a good manual for it it may tell you what you need for this.

    I have an imperial lathe, but screwcut 99% metric threads, just keeping the halfnuts engaged and reversing the leadscrew.
    Can you try the same on yours?

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    Quote Originally Posted by Peter Neill View Post
    Metric leadscrews often need the gear inside the threading dial changed to screwcut different pitch metric threads. Almost counter intuitive - if you have a good manual for it it may tell you what you need for this.

    I have an imperial lathe, but screwcut 99% metric threads, just keeping the halfnuts engaged and reversing the leadscrew.
    Can you try the same on yours?
    I bet that's it
    I saw this on one machine:

    Note: The thread dial is not used when cutting metric threads. Leave the half-nut engaged until the threads are complete.

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    Quote Originally Posted by ChrisBruno View Post

    Hi all

    The lathe is brand new, so minimal chance of a previous owner messing with it.
    Hi, you said brand new then minimal chance? Well is it NO chance or minimal, if it’s brand new then no chance right?

    Peter neill suggests leaving your half nuts locked and I agree.

    From my understanding, on a imperial lathe with metric change gears you CANT disengage the half nuts even with a threading dual, the timing will NEVER line back up due to the imperial to metric conversion going on. The thread dial in this case is a liar.
    Last edited by Homebrewblob; 09-17-2020 at 03:45 PM. Reason: Spelling due to Ohio public education

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    My Emco S11 has an Imperial leadscrew, as mentioned I havta leave the half nut engaged to cut metric threads.

    Don't know how this works the other way around as in your case. Are you positive you have a metric leadscrew?

    What happens if you cut an imperial thread? Can you use the dial then?

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    A pix of the op's set up would explain allot too.

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    by the way, it has never neen a problem to pick up a thread by eye.

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    I believe Peter Neil is correct.

    I have a Emco Maximat V-13, which I bought used. It has an imperial leadscrew- and a thread dial for that- but also came with a thread dial which has 3 "gears" which can mesh with the leadscrew. One, marked 16, I use with the imperial leadscrew. I also have ones marked 14 and 15. There is no reference to them in my manual.

    Some time ago I obtained a copy of the instruction for the thread dial indicator for the Emco Maximat super 11. It indicates #16 is used with the 1/8" imperial leadscrew. For the metric 3mm pitch leadscrew you were to use #14, markings 1,2 for pitches of .175, .35, .7, 1.75, and 3.5. #15 was for .45 mark 1, .9 mark 1, 1.25 mark 1 or 3, 2.25 mark 1, 2.5 mark 1 or 3, 4.5 mark 1, 5.0 mark 1 and 3. #16 in metric used marks 1,2,or 4 when cutting .4, .8, 2, and 4.

    1/2 of your 1.75 pitch is .875. If you were cutting .9 pitch treads it would look like 1/2 of what you expected. I would double check the pitch setting in the gearing, and look to see if you have other teeth options for the thread dial you add as needed, or raise or lower the thread dial to engage different ones.

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    Looked a little on the net and found a prior PM discussion on metric threading

    Thread chasing Dial

    Lots to read, not all of it helpful- but some is...

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    Ah, you guys are legends - got it sorted with advice from here - Peter Neill had it, I didn't realise I had multiple gears for the thread dial, and had the wrong one mounted. Put the correct one on and bam perfect thread.

    One of the good points of messing up so much is I now have a perfect technique, I'm really happy with how the thread is coming out.

    On the way home I even got hit on by a woman at the petrol pump, so I guess an added bonus of being able to thread properly is being more attractive to them - I suppose they can detect a good mate.

    For anyone following along, or finding this post in future, these are the steps I used to diagnose my problems:
    1) I confirmed I have a metric leadscrew (22 x 3, based in New Zealand using metric system), so using a dial for a metric thread should be possible
    2) I tried making a thread without disengaging the half nut, which came out well, so it was unlikely my process, and probably something to do with how I was engaging/diesngaging the nut
    3) I found out I had multiple gears in my parts box, without the keyway the main gear box had, with different tooth counts
    4) Swapping that over, I found the thread dial now worked as expected - re-engaging the half nut on the right number would now engage the thread in the correct place.
    Last edited by ChrisBruno; 09-19-2020 at 02:38 AM.

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    Quote Originally Posted by ChrisBruno View Post
    On the way home I even got hit on by a woman at the petrol pump, so I guess an added bonus of being able to thread properly is being more attractive to them - I suppose they can detect a good mate.
    You can't have a good screw without good threads.

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    Good to hear you got it worked out.

    Which threading gears came with the lathe? Which one were you using, and which one was the correct one?

    Someone in the future is going to have a similar issue, find this thread, and want to know more. If you could go back in time, what would you tell yourself before you started the threading. Ok, AFTER your petrol pump plan of action is decided- what will you tell yourself about threading?

    Ahhh, " I could make a good 1.75 pitch without disengaging the half nut (which made me believe it was something to do with how I was engaging the nut after disengaging it) " If you have the correct metric threading dial gear in use, you should be able to disengage the half nut and then reengage for the next cut. If you are not going to disengage, you will never need a thread dial... Did you mean to say what you said?

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    Does your lathe have a little hiding spot on the back with a extra set of imperial gears?

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    I think it came with 14, 15 and 17 tooth dial gears. The 17 was fitted from factory, and I didn't find any others until I knew to look, I needed the 14 (numbers are approximate, workshop is otherside of city from home).

    What would I tell myself... I'm not sure I would tell myself anything, up to this point. As a result of messing up so much, I can turn a good finish (without threading), I've got my parting tool set up properly, and I have a relatively efficient threading process down. Plus, it's reinforced there's nothing magical about machining, it's more or less following a good technique and knowing things.

    Good catch on the wording, I'll edit it a bit.


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