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  #1  
Old 01-24-2020, 9:11 PM
Usmc0844spare Usmc0844spare is offline
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Default Barrel threading - how?

I am addicted to YouTube gunsmith videos.

Watched a few where they pop a bbl on a lathe and thread it for the receiver.

How on earth does that work? I thought at first that maybe the cutter was angled just right to automatically drag itself up the tenon as it cut.

But just watched one where the guy cut the threads in like 4 passes. How does the machine cut threads in the exact same place every pass when the barrel is spinning at whatever rpm it’s spinning at?

Is there some sort of mechanical link between the chuck and the carriage that says “when point a on the barrel circumference gets to TDC, start the cutter moving up the tenon”?

If so, I would assume that if the barrel moves in the lathe at all before the threads are cut deep enough you are screwed.
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  #2  
Old 01-24-2020, 9:14 PM
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fast54vw fast54vw is offline
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Just google cutting threads in a leave. Depending on what gear ratios the lathe has set up And the speed the lead screw is turning will make the cutter move exactly the same every time
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  #3  
Old 01-24-2020, 9:22 PM
Usmc0844spare Usmc0844spare is offline
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Quote:
Originally Posted by fast54vw View Post
Just google cutting threads in a leave. Depending on what gear ratios the lathe has set up And the speed the lead screw is turning will make the cutter move exactly the same every time
Ok I’ll check it out.

If I asked my old Gunny a question like these he’d have said, “pretty simple really, it’s just FM.”

“FM?”

“Yeah, Fu****g Magic.”
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  #4  
Old 01-24-2020, 9:25 PM
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Single point thread cutting is a very common operation on a lathe.
It's easy once you get the hang of it, but you always need to pay attention.
And yes, there is a drive shaft that drives the carriage at the correct speed
to cut a given pitch thread.
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  #5  
Old 01-24-2020, 10:04 PM
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ar15barrels ar15barrels is offline
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Quote:
Originally Posted by Usmc0844spare View Post
Watched a few where they pop a bbl on a lathe and thread it for the receiver.

How on earth does that work? I thought at first that maybe the cutter was angled just right to automatically drag itself up the tenon as it cut.

But just watched one where the guy cut the threads in like 4 passes. How does the machine cut threads in the exact same place every pass when the barrel is spinning at whatever rpm it’s spinning at?

Is there some sort of mechanical link between the chuck and the carriage that says “when point a on the barrel circumference gets to TDC, start the cutter moving up the tenon”?

If so, I would assume that if the barrel moves in the lathe at all before the threads are cut deep enough you are screwed.
Most lathes have 2 gearboxes.
The first gearbox sits between the motor and the spindle and determines the spindle rpm.
The 2nd gearbox is fed FROM the spindle and turns a leadscrew known as the feedscrew.
The gearbox sets the pitch relation between spindle and carriage.
The carriage is what carries the cutter across the part.

In use, you have a threading dial that has a series of numbers like 1-2-3-4-5-6-7-8.
You pick a number and stay with the number through all the cutter passes and the cutter will follow the same path.

So if I am cutting 1"x16 threads for a tikka, I cut a 1" diameter thread tennon and then cut 16 pitch threads on it.
The thread cutter is placed in position near the end of the tennon and then you watch the threading dial until your chosen number is approaching and then you engage the feedscrew as the dial comes to your chosen number.
When the feed screw is engaged, the carriage (and therefore the cutter) advances across the part being machined until the feedscrew is disengaged.
After disengaging the feedscrew, the cutter is re-positioned to the starting side and run through the same process until the threads are complete.

You definitely do NOT remove the part from the lathe setup in the middle of threading as it's more difficult to "catch" an existing thread and follow it.
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  #6  
Old 01-25-2020, 12:34 AM
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Quote:
Originally Posted by ar15barrels View Post
Most lathes have 2 gearboxes.

You definitely do NOT remove the part from the lathe setup in the middle of threading as it's more difficult to "catch" an existing thread and follow it.
And you never take your machine "out of gear" to spin the chuck by hand.
Ask me how i know!
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  #7  
Old 01-25-2020, 7:19 AM
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Quote:
Originally Posted by ar15barrels View Post
In use, you have a threading dial that has a series of numbers like 1-2-3-4-5-6-7-8.
Lathes are different, mine is even threads on any line, odd threads on numbered only.
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  #8  
Old 01-25-2020, 7:47 AM
Dgr1dman Dgr1dman is offline
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Quote:
Originally Posted by 67Cuda View Post
Lathes are different, mine is even threads on any line, odd threads on numbered only.
... and 1/2 threads only on the same number ( ie: 11 1/2 tpi)!

Many older lathes will cut multiples of 12 anywhere the half-nut drops in. I can’t speak as to any machine made after 1960 or so, as I prefer old iron!

Metric is a whole different game on standard lathes!

Cheers,
Digger1
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  #9  
Old 01-25-2020, 11:29 AM
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Quote:
Originally Posted by 67Cuda View Post
Lathes are different, mine is even threads on any line, odd threads on numbered only.
Mine is like that too, but it's still a "best practice" to stick to the same start number vs using different numbers because using the same number guarantees you catch the same start on the leadscrew.
Also no need to confuse someone with minutiae that was asking the most basic question.
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AR work: www.ar15barrels.com
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Barrel, sight and trigger work on most pistols and shotguns.
Most work performed while-you-wait, evening and saturday appointments available.
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  #10  
Old 01-25-2020, 11:31 AM
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Quote:
Originally Posted by Dgr1dman View Post
Metric is a whole different game on standard lathes!
oh yeah baby.
Engage the nut once and never disengage until the threads are done!
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AR work: www.ar15barrels.com
Bolt actions: www.700barrels.com
Foreign Semi Autos: www.akbarrels.com
Barrel, sight and trigger work on most pistols and shotguns.
Most work performed while-you-wait, evening and saturday appointments available.
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  #11  
Old 01-25-2020, 1:45 PM
baih777 baih777 is offline
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Easiest.
Take your barrel to Randall to have it threaded.
And watch.

Setting the barrel up to the axis of the barrel bore . Is key to getting it straight.
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  #12  
Old 01-27-2020, 11:02 AM
Usmc0844spare Usmc0844spare is offline
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Thanks all... found a basic youtube vid on threading and it all makes a lot more sense now. In laymans terms, there is a physical link between the chuck and the carriage that basically enables the machine to mechanically do the same motions each time.
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Old 01-27-2020, 11:11 AM
kcstott kcstott is offline
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I remember my trade school days and the lathe single point lecture and demo day. I just took what the instructors said as gospel as i didn't know enough to ask a question let alone argue.
follow the direction in the book, that's all we could do. 30 years later it's like looking back on when you learned to walk
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  #14  
Old 01-29-2020, 7:28 PM
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All lathes will cut the thread pitch of the lead screw, and MULTIPLES of that pitch, when closing the half nuts any place on the screw.
Rule of thumb for closing half nuts:
Even No of threads/any line
Odd No of threads/any number
Multiples/any place


Not so sure about division numbers Example 8tpi lead screw/4tpi thread wont work on my lathe. Yours may be different.

8, 16, 32, 64,128, 256 works OK on mine. Haven't had the occasion to cut 128's or finer yet!
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  #15  
Old 01-30-2020, 6:11 AM
kcstott kcstott is offline
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Quote:
Originally Posted by kendog4570 View Post
All lathes will cut the thread pitch of the lead screw, and MULTIPLES of that pitch, when closing the half nuts any place on the screw.
Rule of thumb for closing half nuts:
Even No of threads/any line
Odd No of threads/any number
Multiples/any place


Not so sure about division numbers Example 8tpi lead screw/4tpi thread wont work on my lathe. Yours may be different.

8, 16, 32, 64,128, 256 works OK on mine. Haven't had the occasion to cut 128's or finer yet!

My machine will do from 8 to 56 with no gear changes. with gear swaps I can go to as little as 2, 2.25, 2.5, 2.75, 3, 3.5, 4, 4.5, 5. 5.5 tpi including the odd ball 19 tpi
I can also swap gears to do metric from .11 to 2.56 mm pitch.

6 TPI lead screw on my machine.
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  #16  
Old 01-30-2020, 7:31 AM
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Quote:
Originally Posted by kendog4570 View Post
.... snip.....
Haven't had the occasion to cut 128's or finer yet!
I had a request from an engineering grad student once to cut an adjustment assembly (+/- 1” dia.) for his project at 156 tpi.

I explained to him that would be a feed rate, not a thread! His response: “No, it’ll be fine, I drew it out on CAD and it’ll be fine!”

After showing him a trial cut, he agreed that 40 tpi would be better!

Cheers,
Digger1
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  #17  
Old 01-30-2020, 7:38 AM
kcstott kcstott is offline
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Quote:
Originally Posted by Dgr1dman View Post
I had a request from an engineering grad student once to cut an adjustment assembly (+/- 1” dia.) for his project at 156 tpi.

I explained to him that would be a feed rate, not a thread! His response: “No, it’ll be fine, I drew it out on CAD and it’ll be fine!”

After showing him a trial cut, he agreed that 40 tpi would be better!

Cheers,
Digger1
everything works on paper
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  #18  
Old 01-30-2020, 9:01 AM
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I lied. My Clausing will only go to 128 on the lead screw pitch gear. It has the capability to cut 224. Probably work for cutting Edison cylinders!


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