# How to cut threads on a lathe



## vascon2196

I have searched and searched and cannot find a good place to see how threads are cut on the lathe. I attempted a 3/4"-10 this morning and kept cross threading it. I then tried a 1/2"-20 and that did not work either...the nut threads on but very loosley. I think I am setting up the levers correctly, starting the cross slide in the same location, feeding the compound .003 to .005 each time, and the set up is very rigid.

The compound was angled at about 30-degrees as suggested. I am running the lathe on its lowest speed.

I have watched a few videos on threading but none seemed to help. It always looks easier when someone else does it.:wall:


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## peterw51

Hi Chris, I will be watching your reply's with interest as this is something I am trying to pick up, I am sure you will get the information you need from the members as they seem nice folk!


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## albertorc19

Try mrpete222 videos in Youtube, he has a few videos on screw cutting including Acme threads. You can also try a small book by Martin Cleeve "Screwcutting in the lathe"


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## kuhncw

Hi Chris,

South Bend's book How to Run a Lathe has a pretty good part on thread cutting.  Martin Cleeve's book is good also, but the South Bend book would be my first stop.  Atlas published a great book on lathe work, but I don't recall the name of the book.

What lathe are you using and how are you picking up the thread for each pass, threading dial?

Chuck


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## Lockstocknbarrel

Hi Chris,
If I can give you one piece of advice "My Son",
*YOU TUBE IT.*
How to cut threads on a Metal Lathe.
Or any other How To questions you may ever have.

This has opened up a whole new world for me in drawing CAD, why reinvent the wheel when somebody has already learned how to do the job and is prepared to share.
Good Luck.
Kindest Regards
Beagles.


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## kvom

Assuming your leadscrew is 8 tpi, cutting 10 tpi means you need to feed the carriage back to the start using the leadscrew.  You can't use the thread dial unless the tpi is a multiple of 4.  For the 20 tpi, you can use the thread dial.  Since it's a multiple of 4 but not 8, use either all even numbers or all odd numbers.

The compound should be at 29.5 degrees, not "about 30 degrees".

1/2-20 threads have a minor diameter of .4392, so the depth to cut is (.5-.4392)/2 = .0304.  At 29.5 the infeed is .0349.  You can take a bigger DOC at the beginning of the thread and reduce it as you go in.  Rather than relying totally on the compound dial, I would use a nut to try the fit.

You need to have the cutting bit aligned perpendicular to the work or the threads will be malformed.  Use a 60 degree "center gauge" for this.  You can order one from Enco for $6.  The center gauge also is useful in grinding a HSS tool to verify the 60 degree angle at the tip.  To save the hassle you can get carbide inserts for threading that are precise.  The tip needs to be at the midpoint of the work.

The other way to get a good final thread is to single point it most of the way and use a die to finish.  The single point assures that the dies will thread on straight.  This is what I like to do on small diameters especially.


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## vascon2196

kuhncw said:


> Hi Chris,
> 
> South Bend's book How to Run a Lathe has a pretty good part on thread cutting. Martin Cleeve's book is good also, but the South Bend book would be my first stop. Atlas published a great book on lathe work, but I don't recall the name of the book.
> 
> What lathe are you using and how are you picking up the thread for each pass, threading dial?
> 
> Chuck


 
Hi Chuck,

I'm using a Supermax engine lathe. I am bringing the tool out, moving the carriage back to the beginning, increasing the compound by .002 to .005. I am also choosing the same number on the spinning dial. I have heard of the South Bend book before...I'll try to pick up a copy.

Thanks Chuck.


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## Mosey

There is a very fine video by the late Rudy Kouhoupt on cutting a thread on a South Bend 9" lathe that I found easy to learn from as a beginner. I assume it is still available.


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## Tin Falcon

Army TC 9-524 chapter 7 page 49

http://metalwebnews.com/machine-tools/ch7.pdf
Tin


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## MachineTom

I looked for info on Supermax lathes, and find lots of models, all of commerical size. So I hate to assume that model you have, and what leadscrew you have. 

So the old saying, the better the question, the better the answer.

The supermax series seem to have both metric and imperial threading capabilities, You need to know if you have an imperial leadscrew or a metric lead screw. so grab a caliper and measure 3 threads on the leadscrew, is it .375" or .480. If .375 its an 8tpi screw if .480 its metric 4mm pitch screw. The threading dial, are the numbers 1-4, with tic between, or a circle of numbers 1.125, 2. .7 etc. that will also tell 
if metric or imperial. And last check all the levers and knobs that the threading is set for imperial turns not metric. It may also be possible that a gear set in the quadrant has to be changed to effect metric/imperial threading. For that a nmanual would help.

Lets hear how you do. 

If you have a metric leadscrew, then threading needs to have the half nuts engaged all the time to cut imperial threads


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## kuhncw

vascon2196 said:


> Hi Chuck,
> 
> I'm using a Supermax engine lathe. I am bringing the tool out, moving the carriage back to the beginning, increasing the compound by .002 to .005. I am also choosing the same number on the spinning dial. I have heard of the South Bend book before...I'll try to pick up a copy.
> 
> Thanks Chuck.


 
Hi Chris,

That Supermax should do a fine job.  My 9X42 mill is a Supermax.  

To me it sounds like you have the right approach.  You are cutting even number threads and my SB Lathe book says to throw the half nuts in at any line on the dial for even number threads.  This is for an 8 tpi lead screw, but I'd think your dial is matched to your lead screw if it happens to be a different pitch.

Please let us know what you find that is causing the problem.

Regards,

Chuck


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## vascon2196

kvom said:


> Assuming your leadscrew is 8 tpi, cutting 10 tpi means you need to feed the carriage back to the start using the leadscrew.  You can't use the thread dial unless the tpi is a multiple of 4.  For the 20 tpi, you can use the thread dial.  Since it's a multiple of 4 but not 8, use either all even numbers or all odd numbers.
> 
> The compound should be at 29.5 degrees, not "about 30 degrees".
> 
> 1/2-20 threads have a minor diameter of .4392, so the depth to cut is (.5-.4392)/2 = .0304.  At 29.5 the infeed is .0349.  You can take a bigger DOC at the beginning of the thread and reduce it as you go in.  Rather than relying totally on the compound dial, I would use a nut to try the fit.
> 
> You need to have the cutting bit aligned perpendicular to the work or the threads will be malformed.  Use a 60 degree "center gauge" for this.  You can order one from Enco for $6.  The center gauge also is useful in grinding a HSS tool to verify the 60 degree angle at the tip.  To save the hassle you can get carbide inserts for threading that are precise.  The tip needs to be at the midpoint of the work.
> 
> The other way to get a good final thread is to single point it most of the way and use a die to finish.  The single point assures that the dies will thread on straight.  This is what I like to do on small diameters especially.


 
Thank you...this makes a little more sense now. The lathe is 8tpi. So I have to stop the lathe, reverse the lead screw direction, and have the tool go backwards through the same threads I just cut?


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## vascon2196

Tin Falcon said:


> Army TC 9-524 chapter 7 page 49
> 
> http://metalwebnews.com/machine-tools/ch7.pdf
> Tin


 
Thank you Tin.


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## kvom

Retract the tool with the crossfeed while reversing.  The crossfeed dial should be set to 0 before starting the thread.  Then after reversing so can put it back at the correct position via the dial.

The technique I use for the crossfeed dial is as follow:

1) At the start, position the crossfeed with the tool fairly close to the stock and the handle at approx. 11 o'clock.  Zero and lock the dial.

2) Using the compound, advance the tool so that it just touched the stock.  Zero the compound dial.

3) While cutting the thread, I keep my left hand on the crossfeed handle and right hand on the feed lever (assuming I'm using the thread dial).  At the end of the thread a quick downward movement of my left hand disengages the tool from the stock while I also disengage the feed.  Once you get used to this you can run the lathe fairly fast.

4) After moving the carriage back to the start, then it's easy to move the crossfeed back up to its 0 position.  Advance the compound, and go again.

if cutting 10 tpi or metric or another similar thread you can't disengage the leadscrew, but the crossfeed technique is still good.  Just stop the spindle afterwards and reverse.

Threading steel I always use back gear for torque at low speed.  For brass and aluminum I don't bother.  On the first pass I make a "scratch cut" that I can measure with a thread gauge.  This is to ensure that the lathe's gears are set for the proper TPI.  If not, then that can be fixed without messing up the stock.


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## Philjoe5

Chris,
I haven't seen it mentioned and it may be obvious but I was getting inconsistent threading because of one thing I had failed to do.  When I was at the end of the thread, I'd stop the horizontal feed, back out the toolbit and wind the carriage to the starting point.  But I failed to account for the backlash in the drive.  When you wind the carriage back to start the next cut be sure to account for any backlash in the drive by going past the starting point, then advancing the carriage to the start of the cut.

Phil


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## vascon2196

Well thanks to all I finally cut my first successful 1/2"-13 on the lathe! Why did it work this time you ask? I have no idea.

I chose the "1" on the dial and stuck with that because of the 8tpi lead screw right? Maybe that was it?

I also turned the diameter down between the max and min of the major diameter...I did not do that for the 1/2"-20 or 3/4"-10 threads.

Now I just need to practice...thanks again for all of your valuble input!


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## Jtrain

Just my thought, I have very little experience, but could you not use _*any*_ *position* on the thread dial?  So as long as what ever position on the thread dial you *start* your first cut on, you *must stay* with that position for every cut start.
John


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## Tin Falcon

even number of threads engage on any graduation  on the dial
Odd number  of threads engage on any main division 
fractional thread the same division each time

Multiples of the lead screw any time the split nut engages.

you are always safe using the same one each time on a main division. 

Tin


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## Tin Falcon

A decent pdf of the SB HTRAL 

http://www.scribd.com/doc/62662262/South-Bend-How-to-Run-a-Metal-Lathe

Here is a 60 guide to lathe threading it was written for Mach 3 cnc but a lot of the considerations are common with manual threading.


http://www.machsupport.com/docs/Mach3_Threading.pdf

Tin


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## kuhncw

kvom said:


> if cutting 10 tpi or metric or another similar thread you can't disengage the leadscrew, but the crossfeed technique is still good.  Just stop the spindle afterwards and reverse.
> 
> .


 
Hi KVOM,

I don't understand the comment that the leadscrew cannot be disengaged when cutting 10 tpi threads.  I am assuming you are saying not to open the half nuts, when you say "disengage the leadscrew"  

My 13 inch Sheldon with an 8TPI leadscrew will pick up a 10 TPI thread just fine when I open the half nuts, crank the apron back, and close the half nuts on any of the marks on my thread dial.  

Maybe I've misunderstood what you said.

Regards,

Chuck


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## kvom

> My 13 inch Sheldon with an 8TPI leadscrew will pick up a 10 TPI thread just fine when I open the half nuts, crank the apron back, and close the half nuts on any of the marks on my thread dial.



You are correct.  Brain fart when posting above.  :hDe:


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## kuhncw

No problem, KVOM.  I was just puzzled by the comment.  Those brain farts happen to me as well.

Regards,

Chuck


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## vascon2196

Tin Falcon said:


> A decent pdf of the SB HTRAL
> 
> http://www.scribd.com/doc/62662262/South-Bend-How-to-Run-a-Metal-Lathe
> 
> Here is a 60 guide to lathe threading it was written for Mach 3 cnc but a lot of the considerations are common with manual threading.
> 
> 
> http://www.machsupport.com/docs/Mach3_Threading.pdf
> 
> Tin


 
Thank you Tin...I love that SouthBend book!!!


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## vascon2196

I just spent about 4 hours slowly turning the first part for my Quadricycle and attempted to turn a 5/8"-11 thread in the process.

Everything was going great until the 5/8"-11 nut that I have did not fit. I grabbed a thread guage and sure enough it was off....it does not even match anything??????

Here is a picture of the sticker on the lathe....for an 11 it looks as if lever "B" and "D" need to be selected and "1" "V" for the other two knobs. I then set up my dials and chose the #1 on the spinning dial. I then proceeded to use the same number, #1, for the entire process.

Can anyone see what I am doing wrong?

The first couple of threads seem okay but then it goes to hell.

50" long piece of stock is now scrap.


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## sssfox

Do you have the 60 and 66 tooth gears installed at positions A and B on the banjo?

You know you don't need to turn the entire thread before you check the pitch?  If you just put a "scratch" on the stock, you can make sure it's correct before you ruin something.


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## vascon2196

sssfox said:


> Do you have the 60 and 66 tooth gears installed at positions A and B on the banjo?
> 
> You know you don't need to turn the entire thread before you check the pitch?  If you just put a "scratch" on the stock, you can make sure it's correct before you ruin something.


 
Unfortunately I am unfamiliar with the "banjo" and I do not know anything about installing gears. I thought the levers took care of that.

I made a 1/2"-13 thread the other day without a problem and now having difficulty with the 5/8"-11?

Do I have to take the gearbox cover off to change gears?


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## sssfox

If you look at the lever diagram you attached, in the upper left-hand corner, it shows three circles that represent gears: A, B and 120t.  If you look at the chart, it shows that you need A= 60 and B= 78 for 13tpi.  Look at the top of the 11 and it shows A= 60 and B= 66.  Those gears are on an adjustable bracket called a banjo.  With your current settings, you should still be getting 13tpi.

The gears should be behind the cover where the chart is.  You should have some extra gears that came with the lathe.


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## sssfox

Chris,

Your shaft isn't necessarily trash.  You can make a nut that fits the threads that you put on there.  I guess 5/8-13?


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## vascon2196

Ahhhhhh!!!

Thank you for pointing this out!

I took the platic guard to the left of the headstock nad staring me in the face were the change gears! I then looked in the bottom drawer of our toolbox and there was a pile of spur gears that have been kicking around for years. Now I know what to do with them!

Oh boy....you learn something new everyday.

Thanks again to everyone for all of your help.


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## vascon2196

:wall::wall::wall::wall::wall::wall::wall:

Now I cannot locate the 66 Tooth spur gear to make the 5/8"-11 thread!

Looking through the Supermax manual the gear is MOD1.25x66T

I cannot find anyone who stocks metric gears and the supplier is no longer in business. So without this gear I cannot make a 5/8"-11 thread on the lathe!:rant::rant::rant:


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## sssfox

Chris, sorry to hear that.  It always seems to work out that way, doesn't it?

Sounds like a good time to learn to make your own.  It really isn't that difficult.  There are a lot of places on the web that describe the process.  All you need are a mill with a flycutter and some type of dividing head/rotary table.  Make the single point cutter to fit the 60t gear and it will work fine.

It doesn't need to look perfect, it just needs to fit.  I've made several for my old Atlas lathe from PVC plate.


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## sssfox

Chris

Here is a PDF of what I believe the gear should look like.  According to Gearotic Motion, it should be 85mm outside diameter (3.35").  I didn't know what the shaft diameter was, so I used 5mm.  If you print this file full size, it should be exactly that and the tooth profile should match.

If nothing else, you know what you are working with.

Hope this helps,
Steve Fox 

View attachment 66tMod1.25.pdf


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## Bastelmike

vascon2196 said:


> I just spent about 4 hours slowly turning the first part for my Quadricycle and attempted to turn a 5/8&quot;-11 thread in the process.
> 
> Everything was going great until the 5/8&quot;-11 nut that I have did not fit. I grabbed a thread guage and sure enough it was off....it does not even match anything??????
> 
> Here is a picture of the sticker on the lathe....for an 11 it looks as if lever &quot;B&quot; and &quot;D&quot; need to be selected and &quot;1&quot; &quot;V&quot; for the other two knobs. I then set up my dials and chose the #1 on the spinning dial. I then proceeded to use the same number, #1, for the entire process.
> 
> Can anyone see what I am doing wrong?
> 
> The first couple of threads seem okay but then it goes to hell.
> 
> 50&quot; long piece of stock is now scrap.


 
Hi Chris,just had a quick look at Your gearbox setup. If I'm not mistaken, Your 1-2-3-4 Lever should be in the 1 position. In the pic it seems to be 2 !Another point: You need a transition 60:66, if You would write the teeth number of all gears You have, maybe there's another gear combination that will do this.Mike


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## vascon2196

sssfox said:


> Chris
> 
> Here is a PDF of what I believe the gear should look like.  According to Gearotic Motion, it should be 85mm outside diameter (3.35").  I didn't know what the shaft diameter was, so I used 5mm.  If you print this file full size, it should be exactly that and the tooth profile should match.
> 
> If nothing else, you know what you are working with.
> 
> Hope this helps,
> Steve Fox


 
Steve..that does help. When I get back to work on Monday I will check the tooth form against what I currently have. I finally got in contact with a supplier in California regarding spare parts...I am waiting to hear back from them on stock and lead-time.

Thank you for your advice Steve.


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## vascon2196

Bastelmike said:


> Hi Chris,just had a quick look at Your gearbox setup. If I'm not mistaken, Your 1-2-3-4 Lever should be in the 1 position. In the pic it seems to be 2 !Another point: You need a transition 60:66, if You would write the teeth number of all gears You have, maybe there's another gear combination that will do this.Mike


 
Thank you Mike. A huge part of me goofing all of this up is that I never had basic machine shop training. If I was not so damn busy I would love to start right at the beginning....layout work, lathe work, mill work, fixturing, tooling, and everything related to machining. This issue has taught me a lot though. I now know more about threading in this past week than ever before!

I will sort through the gears I found to see if other combinations will work.

Thanks again.


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## peterw51

Hi all, I have been following this thread, and with some help from DR jo, I have just turned my first 1mm pitch thread & it looked good against the thread gauge. Thanks vascon for starting the thread & all the members who took the time to help others.


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## vascon2196

You can learn SO MUCH from everyone here....I use the forumn almost weekly in my Manufacturing Processes class when teaching. Whenever I get stumped, this is the first place I go to when I need the answer.


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## Jtrain

I found this, so I c&p it here, it seems like a good expanation in most parts to me, hope it helps others.
John


The lead screw is the part of the lathe that determines the pitch or number of threads per inch that are being cut. Machines that have inch dials and gear boxes will have a lead screw with an even number of threads per inch and will range from two threads per inch to perhaps 16 threads per inch with small bench lathes having 8 TPI or 16 TPI.
If you cut 8 threads per inch with an 8 thread lead screw then the spindle and the lead screw are rotating at the same speed so the half nuts can be closed in any position on the threading dial. You will pick up the thread in the work you are cutting when you close the half nuts. If you wish to cut 16 TPI then the lead screw will be turning at half the speed of the spindle and 12 TPI will be three quarters of the spindle speed or 3 revolutions for every 4 revolutions of the spindle.
If a machine does not have a device to tell you when the lead screw and spindle have come into synchronization then you would have to close the half nuts when starting to cut your thread and not open them again until the thread is complete. This can only be done by stopping the spindle retracting the tool and reversing the rotation until the carriage returns to a point where you can take another cut. This operation is repeated until the thread is finished. Needless to say this can be a very tedious and nerve wracking exercise.
Fortunately thread cutting dials are available that allow the half nuts to be opened and the carriage moved back to the start without losing control over the relationship of the screw and spindle. The dial reads directly from the lead screw using a gear that fits the screw. The gear on the dial will usually have a gear with 4 times the number of teeth as the pitch of the screw. For example an eight pitch screw will mesh with a thirty two tooth gear giving you four positions on the dial where the screw is synchronized with the dial.
The dial will be divided into usually 8 divisions that are numbered one to eight. Each lathe will have a chart that relates to the dial, listing the different pitches to be cut and the positions on the dial where the half nuts can be closed. For example there may be a group of pitches listed and the chart will say you can close the nuts at any number on the dial. You will see another group of pitches listed and the chart will say you can close the nuts only on one, three, five and seven.
You will probably find other pitches where the chart will say that you can only close the nuts at the same number each time. Thread cutting with the dial will become second nature to you with a little practice


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## jerryc

If you have an old machinist's handbook, such as Machinery's or Audel's from the 30's/40's from when  lathes and mills had gears that you manually changed, you will be able to use the direct gear chain computations. I only know this stuff because I went through an apprenticeship with a couple of old world Germans who showed me how to do it. ...and got cussed at in English and German if I did it wrong!


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## Tin Falcon

the last couple of issues of home shop machinist  have articles by Randolph on lathe threading. my NOv/december issue came today with part 2. 
as far as older machinery handbooks I have links here for out of copyright ones.
IIRC top of the link section as well as the getting started in model enginebuilding thread.  
Tin


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## bb218

One thing you can do in the future, if you are cutting a 13 TPI divide 1 by 13 this will give you the amount the carriage should move with 1 rev of the spindle " .0769 "  Engage the thread half nuts and rotate the spindle by hand untill the carriage starts moving. pencil a mark on the head stock and spindle, Zero your read outs or travel dial indicator then rotate the spindle 1 rev.  If the Carriage did not move really close to the .0796 amount something is wrong with your settings.  This works for any thread just divide 1 by the TPI, With metric threads the carriage should move the " Pitch " in 1 rev.   
   Hope this helps.   Mike


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## vascon2196

Thanks again...it does help!


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## LSAGuy

A few things, One, I hate grinding threading tools so I bought insert tools about 20 years ago and never looked back. I was making lots of stainless steel pump shafts for the local Chevron refinery and they paid for themselves in no time. Put it on the top of your Christmas list. :-} 
Two, make your last passes at .001 infeed on the compound. Make two passes with no infeed to take out all the slack and rebound.
Third, Deburr before trying your check nut. A couple of light passes over the top with a fine flat file to get the main burr, then use a small triangle file rocked slightly to get the burr off the side of the crest. Rock the file both ways to get both sides of the crest. Clean the threads thoroughly with a fine brass wire brush to remove filing debris.
Four, you may find that the problem is the very first thread. The tool is pushed away, then settles in. My Birmingham 1440G lathe does this especially in tough materials like stainless. I've found a cheat for this. Dial in .001 undersize, cut the first two threads and dial out with the crossfeed before disengaging the threading nut.
Five, don't wait until you're at depth to start checking, start several thousandths out. Make your clean up passes, deburr, then try the check nut. If you're big, take another .001 and try again. At first it might take several tries but eventually you'll know how your  lathe cuts and you'll do this less.

Rick Girard


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## Tin Falcon

I was trained as a machinist by the USAF . The first cut you take  should be zero depth just a scratch. Prep the part with dykem and check your pitch. if it is wrong reset the lathe reapply dykem and try again until it is right. Less parts scrapped this way. 
Tin


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## bb218

I like to check the thread with a nut but NEVER us that for a final check unless the part is going to stay with that nut.  Most nuts are oversize, machine to the Pitch Diameter to control size.  Mike


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## vascon2196

Thank you again folks. I have been practicing with different types of threads and so far so good. I am checking the thread right away with my thread guage by just scratching the surface.

Works great.

When I get better this is something I would love to show my students...


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