Noob SP threading problem - compound not graduated for it?
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  1. #1
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    Default Noob SP threading problem - compound not graduated for it?

    Hey all - I'm trying to get a lathe that's been sitting for a long time going. I have two lathes in my shop, and I have a good number of hours under my belt doing repair type jobs on my 15x48 Colchester that came to me with no half nut, just using taps and dies when threading is required.

    So, I'm not a lathe noob, but am to single point threading. I'm trying to learn (because I ran into a job for which I don't have a die, haha) to single point on my smaller Portuguese Adira lathe. However, I'm either misunderstanding something or it's not set up for it??

    I made a screw (pic below) and as you can see, the thread is asymmetrical. (The insert is a standard 60 degree thread cutting insert which I squared to the chuck using a proper fishtail, I don't think any of that is suspect.) Obviously (I think) I had the compound at 30 instead of 60 or vice versa depending on how you read it. The compound graduation reads zero when the compound is moving parallel to the ways, perpendicular to the axis of spindle rotation. I cut the pictured thread with that at 30, which I'm pretty sure is wrong, as it should be 30 (29.5) from the spindle rotation axis, not 30 degrees from the saddle movement, correct? So the results I got aren't surprising given the mistake.

    However, I made the mistake because of something that is surprising: the compound angle graduations stop at 45! I think I need to set to 60.5 degrees if zero is compound movement parallel to the ways, right? Well it's not graduated that far. I have to be overlooking something simple, because what lathe is made not to be able to thread, and then shipped with change gears and a chart to do exactly that? This isn't a chinese tray top. It's no beast, but it's a real lathe. I also thought maybe there was a zero mark on the handwheel side of the compound, 90 degrees from the mark in the pics, so that maybe you'd line that one up on the graduation marks at 30. But there isn't, and that would be really hard to see even if there was.

    I think I can get around the problem with an angle gauge against the chuck, but doesn't this seem strange, or am I totally missing something?

    img_20190125_112621.jpg img_20190125_112624.jpg img_20190125_112631.jpg

    img_20190125_111840.jpg

  2. #2
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    (The right side of the thread pic was the chuck side.)

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    Yep, what you said is correct. You should be at 60.5 on your compound.
    You could set it with a sine bar or use an angle block off the face of the chuck.
    Make a few test threads and when it looks good, mark the carriage.

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    It is somewhat unusual for the compound graduation to be set up that way, but it certainly does not imply that the lathe is made "not to be able to thread". Technically, you can use any angle between 0*-30* for your infeed, it need not be exactly 29-1/2*. Some materials actually thread better at 0* infeed. If you are cutting a coarse thread and tool pressure is a concern, you can, with the compound set perpendicular to the cross slide, work one side of the thread at a time.
    Of course, if you want to have the compound at 29-1/2*, you could always dial it in with an angle gage as you mentioned and than make a mark at that position for future use.
    As a side note, you're not doing that machine any favors running it in the condition in which it appears to be in your pictures. It looks as though it has not been cleaned or lubed since it left Portugal!

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    Yeah, your compound was just jacked.

    But if you are using halfway decent threading tool/inserts you can just feed in straight though.

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    We just covered this in another thread. Take a look at it and see if you still have any questions. single point threading help

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    If you turn the compound 90 degrees so that it is perpendicular to the ways, is there perhaps another 0 marking?

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    Quote Originally Posted by awake View Post
    If you turn the compound 90 degrees so that it is perpendicular to the ways, is there perhaps another 0 marking?
    I had hoped so, but no.

    Since it didn't need to be accurate to 0.1 degree, I came up with a faster approach. I turned to 45.5 degrees and marked the compound where it lined up with 30 degrees. I turned that mark to zero to verify that the factory zero mark lined up at 15.5 to make sure the graduation was linear, and it worked. So now, I just need to put that new mark on the factory 45 graduation and I'm at 60.5. Worked great, I think.

    Thanks, all!

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    You could always leave the compound slide at zero, ie as it would be set for normal machining operations, then feed in 0.003" on the cross slide ad 0.001" on the compound when screwcutting

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    For better or worse, there is no standard for the angular markings on lathes. Big Brother has not visited there yet. So, when the compound is set perpendicular to the long axis of the lathe, some scales read 0° while others read 90°. I have not seen it, but I would not be surprised if there were some with a third choice, whatever that may be. This seems to cause no end to the confusion that beginners have when told to simply "set the compound to 29.5°". That simple instruction is just that, a SIMPLE instruction. And, if you are going by the numbers on the scale, it is WRONG for a good percentage of the lathes in the world.

    The real instruction should be "Set the compound at 29.5° FROM THE POSITION WHERE IT TRAVELS PARALLEL TO THE CROSS FEED. ". In other words, set the compound square to the lathe's main axis and then move it 29.5° CCW as viewed from above. That's the angle you want, regardless of the reading on the compound's scale.

    And that 29.5° does not need to be all that accurate. The idea is to have only one edge of the threading tool remove most of the material from one flank of the thread while the other edge only skims the other flank. In theory, any angle that is less than or equal to the 30° angle of the thread's flank (more accurately the angle between the flank and a line that is perpendicular to the axis of the thread) would work. At any angle that is greater than 30° the right hand flank will become distorted as seen in the OP's photo and at 30° you may start to see some steps beginning to form on that flank. So the general advice is to make that angle just a bit less than 30° so that edge of the threading tool skims the following flank on each cut. Then that flank stays nice and straight and at the 30° angle set by the cutting tool's geometry.

    A sine bar would certainly be gross overkill for this. A simple protractor or protractor head on the blade from your square will be accurate enough and would take a lot less time.

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    Quote Originally Posted by EPAIII View Post
    For better or worse, there is no standard for the angular markings on lathes. Big Brother has not visited there yet. So, when the compound is set perpendicular to the long axis of the lathe, some scales read 0° while others read 90°. I have not seen it, but I would not be surprised if there were some with a third choice, whatever that may be. This seems to cause no end to the confusion that beginners have when told to simply "set the compound to 29.5°". That simple instruction is just that, a SIMPLE instruction. And, if you are going by the numbers on the scale, it is WRONG for a good percentage of the lathes in the world.

    The real instruction should be "Set the compound at 29.5° FROM THE POSITION WHERE IT TRAVELS PARALLEL TO THE CROSS FEED. ". In other words, set the compound square to the lathe's main axis and then move it 29.5° CCW as viewed from above. That's the angle you want, regardless of the reading on the compound's scale.

    And that 29.5° does not need to be all that accurate. The idea is to have only one edge of the threading tool remove most of the material from one flank of the thread while the other edge only skims the other flank. In theory, any angle that is less than or equal to the 30° angle of the thread's flank (more accurately the angle between the flank and a line that is perpendicular to the axis of the thread) would work. At any angle that is greater than 30° the right hand flank will become distorted as seen in the OP's photo and at 30° you may start to see some steps beginning to form on that flank. So the general advice is to make that angle just a bit less than 30° so that edge of the threading tool skims the following flank on each cut. Then that flank stays nice and straight and at the 30° angle set by the cutting tool's geometry.

    A sine bar would certainly be gross overkill for this. A simple protractor or protractor head on the blade from your square will be accurate enough and would take a lot less time.
    I agree with using a professional tractor...I find the amateur ones to be a little obtuse...

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    The compound on my Whacheon has zero marked parallel to the spindle and the graduations stop at 45. There is however a mark at ~60. That’s what I use for threading, I don’t know if it’s 60.5 or 60, either way my threads look nice.

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    You don't even need a protractor at all. Any angle between 60 and 65°, as your lathe is marked, will do so you could simply eyeball by splitting the angle on your thread gage and err towards the perpendicular.

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    one picture is worth far more than a thousand words! (from How to Run a Lathe- South Bend)

    img_0818.jpg

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    hey Don, maybe that pic should be a sticky? title maybe "before posting a question about manual threading..."

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    Quote Originally Posted by plastikdreams View Post
    I agree with using a professional tractor...I find the amateur ones to be a little obtuse...
    they are acute as often as obtuse

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    Quote Originally Posted by cyanidekid View Post
    they are acute as often as obtuse
    Only the female ones...


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