I cut a thread and I have some questions

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hustlebird

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Hey All!

Been prepping to make a stud for my toolpost on my lathe, and I decided to practice single pointing a little bit today:
KlBw3E5.jpg


It came out well, I was trying to test how much stick-out I could manage and get it to cut down. The bottom thread I'm holding was actually my first try, and after doubting myself, I slipped a die on part of it, only for it to slip in the chuck and force me to start over. On the second try I did the threads on top.

Overall, I cut 1/2-13, because I had hardware I could test fit with, and my 'production' run will be m16-2 threads, which is comparable speed to the 13tpi.

So, on to my question. I saw diameter #'s out of the book as .500 and ~.405, or 95 thousandths to cut on the diameter. Half that being 48 thousandths to cut, and with my compound at 30*, doing a bit of math, I figured 55 thousandths on the compound would get me to where I needed to be. <-- Does this track? Did I do this right?

I cut down my stock to .499 diameter, and then cut the threads with the numbers above, but it was way too tight for the handful of nuts I tried (only thing I have to test with). After several spring passes without it fitting, I ended up cutting another 10 thousandths (65 total) on the compound (55 thousandths total depth on x-axis). It now threads smoothly with every 1/2-13 thing I could find in the garage, so it works.... but I feel like I got to the right answer the wrong way here. I don't have a way to measure minor diameter, any problems with that extra 15 thousandths cut?

Appreciate any advice as always, and if you see anything wrong with the threads in the picture I'd love your feedback! Go hard on me here, trying to get better.
 
Looks pretty good to me. I'm still learning myself, so I don't really have an answer to your question. The only thing I don't understand is your comment about the (55 thousandths total depth on x-axis). When I am threading, I start my cross slide at zero and when I'm done my cross slide is still at zero. I do all my cutting with the compound, but I can't tell you how much of a cut for a 1/2-13 thread.
 
Nicely described in a previous thread, but also data is available in pther tables and calculators. On smaller lathes, the compound is preferred because it cuts primarily on one side and can give a cleaner cut, bigger/heavier lathes typically just use the cross slide. I prefer the latter as it gives me an easier to follow DOC, that being said I almost always need to cut my thread deeper than the book values, you will also remove additional material with a spring pass, and then knock down the peaks. Once I have determined the maximum cutting depth for a material to match the corresponding thread, I can repeat parts with a high level of accuracy/fitment.
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Cutting Threads, Depth Of Cut

Yesterday, I had to thread some 5/16" stainless rod and did not have a chart of thread depths handy. This morning, I decided to make one based on the definition of unified National thread geometry. The external thread geometry consists of a 60o vee shape with the top of the vee truncated to a...
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Just for fun said:
Looks pretty good to me. I'm still learning myself, so I don't really have an answer to your question. The only thing I don't understand is your comment about the (55 thousandths total depth on x-axis). When I am threading, I start my cross slide at zero and when I'm done my cross slide is still at zero. I do all my cutting with the compound, but I can't tell you how much of a cut for a 1/2-13 thread.
Click to expand...
Yes, the cross slide stays at zero, the compound moved 65 thousandths @ 30 degrees, which translates to 55 thousandths into the part (along the x-axis). Think of a triangle, the compound is the hypotenuse, and movements along the compound move the cutter both in x and z directions (though that isn't captured on a DRO). So 10 thousandths on the compound moves the cutter to the same spot as moving the saddle 5, and the cross 8.5.

I saw this image in the intro to lathe thread on here when I was planning my stuff out. It doesn't show the tool movement cross-ways, but hopefully it helps clarify what I was saying.
fig7-48-gif.186540


mksj said:
I almost always need to cut my thread deeper than the book values, you will also remove additional material with a spring pass, and then knock down the peaks. Once I have determined the maximum cutting depth for a material to match the corresponding thread, I can repeat parts with a high level of accuracy/fitment.
Click to expand...
Well that's reassuring. I did knock down the threads with a file, but I didn't check major diameter when I was done. It wasn't loose on the bolt and it didn't occur to me to check.

I'll try making another one tomorrow with the same compound movements, see if I get the same results. Thanks for the link!
 
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On the thread calculator that I use the range for the minor diameter for 1/2 x 13 tpi ranges from .3877 to .4069. The major diameter ranges from .4877 to .4985

UN imperial screw thread calculator

theoreticalmachinist.com theoreticalmachinist.com

Your .055 depth of cut on the compound put you at the very top end of the range. Typical nuts are not made to fit that tight. They are made to fit towards the low end of the range. Your .065 depth of cut put you at the bottom of the range and the nut fit.

Try making a .060 depth of cut. The nut will must likely just barely fit.
 
That's a great link, thank you!

I think I'm going to give it one more practice run tomorrow, but I'm going to start a bit smaller too. I can't remember when I decided to file it down, but I feel like it may have been after I'd already gone too deep. May have been the major diameter I was struggling with to begin with.
 
hustlebird said:
Yes, the cross slide stays at zero, the compound moved 65 thousandths @ 30 degrees, which translates to 55 thousandths into the part (along the x-axis). Think of a triangle, the compound is the hypotenuse, and movements along the compound move the cutter both in x and z directions (though that isn't captured on a DRO). So 10 thousandths on the compound moves the cutter to the same spot as moving the saddle 5, and the cross 8.5.

I saw this image in the intro to lathe thread on here when I was planning my stuff out. It doesn't show the tool movement cross-ways, but hopefully it helps clarify what I was saying.
fig7-48-gif.186540



Well that's reassuring. I did knock down the threads with a file, but I didn't check major diameter when I was done. It wasn't loose on the bolt and it didn't occur to me to check.

I'll try making another one tomorrow with the same compound movements, see if I get the same results. Thanks for the link!
Click to expand...

Thanks, Got it. That make perfect since now.
 
+1 on getting some type of thread measuring tool. When you are getting close to your desired size, run a triangle file in the threads to knock down any burrs that are sticking up, then run a flat file on the top of the threads to knock down those on top. The burrs can lead you to believe that you're still not close enough to your final mark and result in an over cut, or a very loose thread. Look here for an online thread calculator for Imperial threads. They also have one for metric.
 
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