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  1. #1
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    Default Thread Cutting Problem

    Finally got the selector gears all back together in my Modiale Cletic lathe and today had a project envolving gear cutting. I could not get a cut to save my life. The gear dial has 1-4 numerals, and even when I select the 1 every time, the cuts would get off and shred the work.

    At first I thought it was wear in the half nuts. This is not the case. I then checked wear in the lead screw...no significant wear.

    I started just turning consecutive screw cuts until I figured out what is going on. I am turning 1.5mm pitch, and my pitch gage shows it is cutting right on...of course that is based on a 1/2" section, so there could be a slight discrepancy that I cannot measure. I discovered that so long as I start the cut on a number 1,2,3, or 4...it makes no difference which number I use to engage the following cuts. Either all 4 line up or none do. They all have the same alignment. But the weird part I cannot figure out is that the screw pattern advances each pass. In other words, I can make a cut on any number. If I immediately run the next cut, the tool is advanced ahead of the first cut. If I wait about 1 minute at 800 RPM, the alignment comes back into phase and I can run another cut right on the first. To run a third cut inline, I again must wait about 1 minute.

    I considered the thread indicator, but it is too simple to have any issues. Even if it was screwed up, I should be able to get the same cut alignment if I select the same number each pass.

    The problem must lie in the leadscrew gearset, but I am not sure how to go about figuring it out. Or, am I doing something completely wrong? I have been out of the game for several years, so I could be screwing something up.

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    I had a lathe with an inch lead screw and, just one time, set up the metric transposing gears to cut a metric thread. The thread dial will not work in that configuration. The entire thread must be cut without disengaging the half nut. I did get the job done OK.

    Larry

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    Wow. That hadn't even occurred to me. I was thinking a tooth or two off in the set-up, but when I tried altering teeth, the pitch went off. And, I don't think I've ever tried metric threads, now that I think of it. It makes sense that the pitch of the lead screw would determine the alignment of spindle and lead screw, though. I'll do some studying and get back.

    Thanks, Larry, you definitely set my thinking straight!

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    Toying with the lathe you might get a large screw. one metric and one inch...Chuck-up and then just follow the tread perhaps looking through a loop and being .010 away from the part..that would save a lot of wasted parts and time.

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    I single pointed a lot of metric threads on my old South Bend. I would back off the tool off with the cross feed, disengage the feed and then reverse the spindle in quick succession. I would crank the carriage back to the starting point and note the #1 on the thread dial, when the #1 was lined up again I would switch to forward and do the next pass. While I was waiting for the #1 to line up I would re-zero the cross feed and advance the compound. The advantage is saving time on the re-zeroing and setting the cross feed. On real long screws that took multiple turns on the thread dial this might be more problematic. When you are doing 20 or 30 of the same thing you will figure out some short cuts. Try it on a piece of scrap till you get the hang of it, it is easy.
    Last edited by FredC; 01-07-2020 at 09:58 AM.

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    With an imperial leadscrew you can't disengage the half nuts for metric, module or DP threads.

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    I'm just going to throw this out there not being familiar with the threading dial/setup on a Modiale Cletic but I know there are rules when using the threading dial on a South Bend lathe.

    From "How to Run a Lathe":

    Rules for Operating Thread Dial on South Bend Lathe

    For all even numbered threads, close the half nuts at any line on the dial.
    For all odd numbered threads, close the half nuts at any numbered line on the dial.
    For all threads involving one-half of a thread in each inch, such as 11 1/2, close the half nuts at any odd numbered line.
    Could your lathe have the same or similar rules?

    -Ron

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    Thanks for the responses. It turns out Larry and Illinoy are right on. If the lead screw is Imperial, then it will not line up when doing metric threads. I assume the corollary would be the same if you had a lathe with a metric lead screw trying to cut imperial threads.

    So the solution is that the thread dial is meaningless when cutting metric threads, and you must keep the half locks engaged throughout the cut. That’s what I get for choosing metric for my first cut in 20 years!

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    Quote Originally Posted by MetalCarnage View Post
    ...Could your lathe have the same or similar rules?

    -Ron
    It probably does, if cutting inch pitch threads on a lathe with an inch pitch lead screw. As has been said, the thread dial is useless for cutting metric threads on a lathe with an inch pitch lead screw. The half nut need to stay engaged.

    It is simple enough to just keep the half nuts engaged until the thread is finished. On my lathe that taught me that necessity, it was especially simple because I have a Swiss Multifix QCTP with a retracting threading cutter. Just lever the cutter back out of the work and run the spindle in reverse to get the carriage back to the starting point, lever the cutter back in position, feed the cutter in a little bit and take another pass. Super easy once you learn the basic method.

    Larry

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    Quote Originally Posted by CJD View Post
    Thanks for the responses. It turns out Larry and Illinoy are right on. If the lead screw is Imperial, then it will not line up when doing metric threads. I assume the corollary would be the same if you had a lathe with a metric lead screw trying to cut imperial threads.

    So the solution is that the thread dial is meaningless when cutting metric threads, and you must keep the half locks engaged throughout the cut. That’s what I get for choosing metric for my first cut in 20 years!
    Not necessarily. Your lead screw is indeed imperial at 4 tpi but there is a method suggested by FredC and also described by our friend Conrad Hoffman on the forum here on his web site:

    Metric Threading

    about half way down under the heading "You Have to Keep the Half Nuts Engaged for Metric Threading" - Not!", that you can use with threading dials and half nuts that lets you disengage the half nuts. I won't reiterate it, but apparently it works. I have an old Hardinge lathe that does threading by keeping the half nuts always engaged so I have no recent experience in using threading dials.

    Irby

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    Quote Originally Posted by IrbyJones View Post
    Not necessarily. Your lead screw is indeed imperial at 4 tpi but there is a method suggested by FredC and also described by our friend Conrad Hoffman on the forum here on his web site:

    Metric Threading

    about half way down under the heading "You Have to Keep the Half Nuts Engaged for Metric Threading" - Not!", that you can use with threading dials and half nuts that lets you disengage the half nuts. I won't reiterate it, but apparently it works. I have an old Hardinge lathe that does threading by keeping the half nuts always engaged so I have no recent experience in using threading dials.

    Irby
    Yeah, a lot of things that were given to us as hard and fast rules are really not impossible. Single point threading between centers is something the books say that can not be done. Same metric screws were done between centers, there are ways to over come the problems that normally make it a bad idea.
    Our Hardinges that keep the half nut engaged are also doing something that should be impossible. You disconnect the gear train to reverse the carriage, then somehow it is all good again when you throw the thing in forward. With the metric transposing gears, how does it do that?
    Conrad figured it out on his own as did I, makes you wonder how many others have done the same thing in the past.

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    I found that work around method while I was figuring this out. You can disconnect the half nuts...but the trick is you have to reengage them to the same exact position on the lead screw after you reverse the lathe. It’s a technique when threading towards the spindle to reduce the chance of clobbering your tool into the spindle. I have been threading away from the spindle, so leaving the half nuts engaged is the easiest way. No chance of hitting the spindle.

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    Just a closeout on this thread. Yesterday I ran the 1.5mm threads keeping the half nuts engaged through the entire cut. All went perfectly. Thanks again for the help...I don't think that incompatibility between screws would have occurred to me, like, ever!

    Now I wanna' go swap the gears back to God's given threads...BS Imperial.

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