# 3 tries and 3 fails to make a 1/4 20 thread



## twooldvolvos (Feb 28, 2021)

Well, I've given up for the day on cutting my first threads on my South Bend 10K.  My objective was a 1/4 20 thread in a piece of brass stock.  I took my OD down to 1/4 inch and set my lead screw speed.





I put my drive into back gears and set the RPM to about 150.




My compound was at 29 1/2 degrees.  My tool is ground to 60 degrees.  My first pass looked OK and measured 20 threads per inch as expected.  But after several passes of about 5 thousandths, I kept coming up with a mess that looked like this.




The threads were rough and read at 40 TPI, not 20.  I tried to engage my half nut on any of the 8 lines on my thread gauge since it was an even number of threads.



I did notice that my gauge does not allow me to engage exactly when the lines are on the mark.  I can go a little before or a little after.  Maybe that is where where I am making my mistake.  The next time I can try always going a little before the mark.  Does anyone know if I can adjust the threading gauge to engage closer to the mark?

Anyway, any advice is appreciated.


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## T Bredehoft (Feb 28, 2021)

twooldvolvos said:


> Does anyone know if I can adjust the threading gauge to engage closer to the mark?


I would expect some way of adjusting the disc relative to the mark. can you turn it by hand,  twist the face?


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## Brento (Feb 28, 2021)

A good practice you can try that i think alot of us do is try to stick to the same line when you engage. Are you going back to your zero everytime?


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## epanzella (Feb 28, 2021)

An easy way to at least figure out if it's you or the machine is to lock the carriage as close as you can to any whole number then mark that spot with a sharpie. Go back to that same mark every time. If you find you can't lock the half nut at that exact spot, something is slipping.  An easy way to get the same spot is to put some pressure on the half nut lever ahead of the spot so when you get there it'll drop in. If the thread  comes out OK doing it this way then you were doing something wrong before. Use your lowest speed.


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## jcp (Feb 28, 2021)

I had to adjust the dial of my 9A when I first got it. Pretty sure there's a set screw in the gear to shaft connection of the thread dial.
 On a Southbend thread dial for any even numbered thread can you can close the half nuts on any line. For odd numbered threads close on any numbered line. This is not true on all lathes.


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## mattthemuppet2 (Feb 28, 2021)

if it's 40tpi you're splitting the thread. I only ever use the same mark on the threading dial. It's slower but simpler.


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## RJSakowski (Feb 28, 2021)

It is important that  your marks line up with the index mark.  If they don't, you will experience what you have seen; that you can engage slightly before or slightly after.  This will give you two different tool paths.

You should be able to rotate the dial relative to the gear so that when the half nuts are engaged one of the marks lines up perfectly.  

Another cause of your problem with an older lathe is wear of the lead screw, half nuts, or thread dial dear.  Any of these can cause enough backlash that you could jump a tooth when you engage.  To minimize any issues make sure that you start your pass several inches before your thread and that you engage at the same position on the dial each time.  It might be just before the dial mark or just after.  You should be able to use any of the allowed marks for your thread pitch.

Rather than attempting to cut a thread while you are figuring this out, just repeat your initial pass without advancing the cross feed or compound.  You should be tracking the same path.  Once you are sure of what you are doing, go on to actually cutting a thread.


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## markba633csi (Feb 28, 2021)

RJ makes a good point- practice engaging and disengaging till you get consistent tracking
-Mark


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## tjb (Feb 28, 2021)

This may be a bad idea, but when I was first practicing cutting threads, it worked for me.  1/4-20 is a fairly fine thread to learn on.  I started by trying much coarser threads like 12 or 14 TPI.  I didn't even care what round stock I started from - I just wanted to see if I could cut the threads successfully.  For me, it was lot easier to visually see what was happening with the coarse threads.  And it was a lot easier to see if my subsequent tweaking was behaving as I anticipated.

Veterans, comment on this please.  If it's a bad idea, I'd like to know that myself.

Regards


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## AGCB97 (Feb 28, 2021)

Like someone mentioned the sharpie mark on the dial works for me but if the TDI bolts to the right side of the carriage you may be able to shim it with a washer or 2.
All the above posts are good but mainly the practice till you got it.
Aaron


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## benmychree (Feb 28, 2021)

You need to reset the thread dial so that the lines line up with the half nuts is engaged, and then try to engage the half nuts when threading just slightly before the lines line up with the witness mark.  No need to use the same mark for even threads, numbered or plain lines are all fair game, for odd numbered threads use any numbered line, opposite numbers for half threads, the same number for quarter threads; this works on every lathe that I have ever run.


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## RJSakowski (Feb 28, 2021)

AGCB97 said:


> Like someone mentioned the sharpie mark on the dial works for me but if the TDI bolts to the right side of the carriage you may be able to shim it with a washer or 2.
> All the above posts are good but mainly the practice till you got it.
> Aaron


A good point!  Registration is determined by the distance from the half nut engagement to the thread dial gear engagement with the lead screw.  If there is no other adjustment for a misaligned dial on the South Bend, a shim on the pivot pin will work.  The maximum thichness required won't be more than the lead screw pitch.


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## martik777 (Feb 28, 2021)

Are you backing off enough with the crossfeed before each pass and returning to zero correctly?  I'm assuming you are only advancing the compound.


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## Mini Cooper S (Feb 28, 2021)

I always use the same lime on the threading dial to engage the half nuts. You waste a little time waiting for your line to come around but it never fails, even number of threads or odd number, doesn't matter.  The other thing that I do if there is room, I position the tool maybe a half of an inch or so to the right of my work piece, and start my cut there.  This gives me some time to disengage the half nuts before the tool enters the part in case I missed my mark or the half nuts don't engage well.

Richard


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## twooldvolvos (Feb 28, 2021)

T Bredehoft said:


> I would expect some way of adjusting the disc relative to the mark. can you turn it by hand,  twist the face?


There is a set screw near the face that is accessible from an oil hole on the side of the threading dial.  I loosened it and tried to turn the face on the shaft but did not get it to budge.  I may not have been using enough force.


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## twooldvolvos (Feb 28, 2021)

Brento said:


> A good practice you can try that i think alot of us do is try to stick to the same line when you engage. Are you going back to your zero everytime?


I was not using the same line every time.  But yes, I did take my cross slide back to zero on each pass.  All my advancement was done with the compound.


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## twooldvolvos (Feb 28, 2021)

tjb said:


> This may be a bad idea, but when I was first practicing cutting threads, it worked for me.  1/4-20 is a fairly fine thread to learn on.  I started by trying much coarser threads like 12 or 14 TPI.  I didn't even care what round stock I started from - I just wanted to see if I could cut the threads successfully.  For me, it was lot easier to visually see what was happening with the coarse threads.  And it was a lot easier to see if my subsequent tweaking was behaving as I anticipated.
> 
> Veterans, comment on this please.  If it's a bad idea, I'd like to know that myself.
> 
> Regards


Good point.  I used 1/4 20 because I use that size quite often and I had easy access to nuts for testing my threads.


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## twooldvolvos (Feb 28, 2021)

AGCB97 said:


> Like someone mentioned the sharpie mark on the dial works for me but if the TDI bolts to the right side of the carriage you may be able to shim it with a washer or 2.
> All the above posts are good but mainly the practice till you got it.
> Aaron


Good idea.  Shimming on the mount would be easy to do with washers.


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## twooldvolvos (Feb 28, 2021)

martik777 said:


> Are you backing off enough with the crossfeed before each pass and returning to zero correctly?  I'm assuming you are only advancing the compound.


Yes.  I am pretty sure I am doing this correctly.  Disengage the split nut.  Back off the cross slide.  Wind the carriage back beyond the starting point.  Advance the compound.  Engage the split nut.


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## cathead (Feb 28, 2021)

Sometimes it helps to apply a slight down pressure just before the mark.  That way you can time when the
half nuts engage with the exact position of the dial.(You can feel the half nuts engage just as the mark comes up) Also just
a dry run with the cutter close to the work but not touching it will allow you to verify that the cutter is tracking properly against the
previous threads.  The slowest speed helps too till you get the hang of it.  It won't be long and you will think it is easy.  Good luck.


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## twooldvolvos (Feb 28, 2021)

mattthemuppet2 said:


> if it's 40tpi you're splitting the thread. I only ever use the same mark on the threading dial. It's slower but simpler.


My scratch pass was right on 20.  I found it too much of a coincidence that I measured 40 (2x what I wanted) when I stopped the operation.  Would I get 1/2 a thread advance if I engaged the half nut one position late?  I well could have engaged the nut just before the mark one time and just after the mark the next since my thread counter would not let me engage exactly on the mark.


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## twooldvolvos (Feb 28, 2021)

benmychree said:


> You need to reset the thread dial so that the lines line up with the half nuts is engaged, and then try to engage the half nuts when threading just slightly before the lines line up with the witness mark.  No need to use the same mark for even threads, numbered or plain lines are all fair game, for odd numbered threads use any numbered line, opposite numbers for half threads, the same number for quarter threads; this works on every lathe that I have ever run.


By reset the thread dial, I have read 2 possible ways to do this.  How do you recommend?

1.) Shim the threading dial mount with washers.
2.) Somehow turn the face of the thread dial to make the marks line up better with the half nut.


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## WobblyHand (Feb 28, 2021)

Whatever works.  For my mini-lathe, there's no dial adjustment, so all I can use are shims.  You actually don't even need to shim the threading dial. All you need to do is to consistently hit your mark.  If making the dial line up helps you hit the mark, everytime, by all means do it.

I made a screw like yours.  It happens.  Started out well, and went south from there.  What irritated me was I had threaded the first end perfectly. But on the second thread I was not consistent hitting the same place.  Pretty much looked like yours.  I looked at it as me practicing making scrap...


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## RJSakowski (Feb 28, 2021)

Either way will work.  Shimming may be a bit of trial and error.  It shouldn't take too much.  If you have an 8 tpi lead screw, the most would be .125".  You can get an idea as to the thickness required by engaging the half nuts close to a mark and driving in the direction that you will thread.  Loosen the set screw on the pivot pin and carefully back the pin out while maintaining the gear mesh.   When the mark is aligned with the index, lock the pon and measure the gap with a feeler gauge to get the thickness required.

If you can adjust the thread dial, you will want to drive the carriage with the half nuts engaged in the direction that you thread and and adjust the dial to match the index mark. I'm not familiar with your lathe but it doesn't appear that you have a screw holding the dial  Someone above had mentioned a set screw in the gear which is a little more difficult.


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## savarin (Feb 28, 2021)

You could just keep the half nuts engaged, stop machine at end of cut, retract, reverse back to past the beginning, advance cut, start cutting again.
This should show up if its the whole threading system slipping somewhere of if its just the threading dial.
All my threads are cut this way as they are mainly metric on an imperial lead screw.


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## martik777 (Feb 28, 2021)

I just ran 2 passes, one on the line and the next was one position late. This resulted in 40tpi so I think this is your problem.  Loosen and adjust the gear on the bottom of the thread dial so the marks line up to make it easier to read.


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## twooldvolvos (Mar 1, 2021)

martik777 said:


> I just ran 2 passes, one on the line and the next was one position late. This resulted in 40tpi so I think this is your problem.  Loosen and adjust the gear on the bottom of the thread dial so the marks line up to make it easier to read.


Thanks for the test.  That's good info.  It also explains why a couple of passes were screechy.  It was probably the tool trying to split the threads.


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## WobblyHand (Mar 1, 2021)

I found the embarrassing threads I made (described earlier) and a second attempt.  There was some improvement between the attempts.  It won't make the machinist's hall of fame, (still pretty rough) but the second one was good enough for what I needed.  Long threads at 1/4-28 and short threads at 1/4-20.  I did the 1/4-28 thread first, followed by the 1/4-20.  2nd attempt on top, 1st attempt on the bottom.  Clearly was off in hitting the mark on the 1/4-20 side.


What threw me off was changing the gearing and the timing/cadence was slightly different between the two different threads.  Once you get the hang of it, it will get much easier.  For what it is worth, the second version was turned at a higher speed than the first.  Got a lot of tearing at the lower (too low!) speeds.  First threads for beginners are nerve wracking.  After some successful threads, it is much less so.

Before you know it, your threads will be great!  You might want to practice on 12L14 or 6061 rather than brass, since those materials are cheaper, just to get the timing down.  Even 1/2 PVC conduit is fine for getting your timing right.


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## Winegrower (Mar 1, 2021)

Close the half nut and make a new mark with a sharpie.


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## WobblyHand (Mar 1, 2021)

@Winegrower, anything that works is great.  A new sharpie mark, a shim, remembering to hit just prior to the mark, leaving the half nut engaged, they are all equally valid.  You just have to consistently be on the same "part of the lead screw".  

I'm using "part of the lead screw" because I don't know what the correct terminology is.  What is the correct term for "part of the lead screw" in this context?

Inspired by @RJSakowski's comments, I'm going to try to shim mine.  It annoys me that the engagement doesn't line up with the markings on my mini-lathe.


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## RJSakowski (Mar 1, 2021)

If you really want a challenge, my Grizzly G0602 Had a thread dial where the marks weren't concentric with the shaft so while some marks lined up exactly, others were off by a  half a thread.  A new dial cured the problem.  








						Threading On A Grizzly G0602
					

On March 19th, I replied to a thread begun by Cody Killgore entitled "Threading, Something Went Wrong Here" (post #19) where I show evidence of an problem with the threading dial on my Grizzly G0602.  I had determined that the dial markings were not concentric with the gear shaft, creating an...




					www.hobby-machinist.com


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## WobblyHand (Mar 1, 2021)

@RJSakowski  Wow, not a challenge I'd like to experience!  How did you instrument that error?  Don't want to thread jack, but the methodology of the measurement intrigues me.


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## RJSakowski (Mar 1, 2021)

WobblyHand said:


> @RJSakowski  Wow, not a challenge I'd like to experience!  How did you instrument that error?  Don't want to thread jack, but the methodology of the measurement intrigues me.



It was actually fairly simple.  I just recorded the DRO reading for the z axis position at the points where the dial markings lined up with the index mark and plotted the difference in the readings between adjacent marks.  A dial indicator could be used as well or a digital caliper with the depth rod connected to the carriage and the body clamped to the bed would work.


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## ErichKeane (Mar 1, 2021)

If you have an 8TPI lead screw, every other engagement is usable for 20 TPI.  I'm guessing you are off by 1 on at least 1 of your cuts.  The result would be a two-start thread, each of 20 TPI 

So the problem it seems is you just need to practice engaging the same way each time.


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## pacifica (Mar 1, 2021)

twooldvolvos said:


> Well, I've given up for the day on cutting my first threads on my South Bend 10K.  My objective was a 1/4 20 thread in a piece of brass stock.  I took my OD down to 1/4 inch and set my lead screw speed.
> 
> 
> View attachment 357509
> ...


When I was first learning single point threading I had rpms at less than 50, as low as 30 until I gained knowledge of the lathe and confidence. Now 200 rpm is no problem. Also I thread straight in (90 deg.) to remove a variable. Until you get more experience can you leave lead screw  engaged and reverse to the start?


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## twooldvolvos (Mar 1, 2021)

pacifica said:


> When I was first learning single point threading I had rpms at less than 50, as low as 30 until I gained knowledge of the lathe and confidence. Now 200 rpm is no problem. Also I thread straight in (90 deg.) to remove a variable. Until you get more experience can you leave lead screw  engaged and reverse to the start?


Thanks for the tips.  I could reverse to the start but from what I am reading, disengaging the half nut and cranking the carriage to the start is a preferred method, so I thought I would try to get into a good habit.


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## BladesIIB (Mar 1, 2021)

Based on everything I read here and just checking the thread chasing dial on my lathe, here is what I recommend you test quickly. My thread dial had 8 lines, 4 numbered and 4 not. However, it will engage in 16 total places around the dial so it will engage between each Mark. Your earlier post was that you were not consistent in engaging just before or just after the Mark. Mine is also off slightly and I always engage just after the Mark on the dial. If you did one just before you would have split the thread being in one of the 8 non marked spots. Quickly check your engagement and test for this. You don’t even have to run the lathe. Just move the carriage slowly by hand and engage the nuts in the next position and I bet you will see there is an extra spot between marks. Good luck. And let us know when you get it sorted.

Edit:  as others have said a little pressure before helps it drop in. I am fine with mine off a bit I start to push on the line and it drops in just past. Easy to keep up with.


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## derf (Mar 2, 2021)

One thing nobody mentioned...make sure that the thread dial gear is fully engaged with the leadscrew. If there is excessive clearance, the dial will not be spot on, and the backlash will be evident. Matter of fact, while your at it, remove the whole assembly and clean it. The gears have a habit of accumulating chips and debris preventing full engagement.


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## BGHansen (Mar 3, 2021)

Hopefully you are close, but make sure you don't touch the feed reverse lever.  That synchs your spindle to the gear box.  If you disengage the feed reverse lever and rotate the spindle, you'll very likely be off your mark.

You may also try hitting a mark on the thread dial and sticking with that number (mentioned many times above).  At the end of your pass, shut off the lathe, back off the cross feed and move the carriage back past the starting point.  Keep going until your mark lines up again.  Engage the half-nuts with the lathe shut off and chuck the carriage back and forth to confirm you are right on your mark.  Advance the cross feed and compound and start the lathe for the next pass with the half-nuts already engaged.

Bruce


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## Forty Niner (Mar 3, 2021)

Couldn't see your compound, so I ask the question about the 29 1/2 degree angle.  
29 1/2 degrees from the X axis or the Z axis?
Some compounds are marked with zero one way and some another way..  On some compounds, setting the angle at 29 1/2 degrees to thread will indeed make a mess.


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## twooldvolvos (Mar 3, 2021)

Forty Niner said:


> Couldn't see your compound, so I ask the question about the 29 1/2 degree angle.
> 29 1/2 degrees from the X axis or the Z axis?
> Some compounds are marked with zero one way and some another way..  On some compounds, setting the angle at 29 1/2 degrees to thread will indeed make a mess.


Thanks Forty Niner.  That was in deed my problem!  I was 90 degrees off.


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## BladesIIB (Mar 3, 2021)

Congrats!! What this forum is all about, helping people solve problems.  Glad you got it sorted out, well done Forty Niner.


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