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:

 

[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!

 

[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"

 

[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

 

[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.

 

[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.

 

[vascon2196]

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.

 

[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.

 

[Tin Falcon]

Army TC 9-524 chapter 7 page 49

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

 

[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

 

[kuhncw]

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

 

[vascon2196]

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?

 

[vascon2196]

Army TC 9-524 chapter 7 page 49

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

Thank you Tin.

 

[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.

 

[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

 

[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!

 

[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

 

[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

 

[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

 

[kuhncw]

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