Internal threads, how do I know how deep?

[Investigator]

I'm needing to cut threads inside a 1" tube. Actual ID is .875"
My plan is to take a small skim cut on the ID, and then cut either 36 or 28 TPI. Since this is not a standard piece, I have nothing to use as a gauge to thread in and see how it fits.
So, How do I know how deep to cut the threads, when its deep enough and not too deep?

To answer the obvious question, I will make a threaded 'cap' to fit the tube. The cap will have external threads of course and I will have the tube to use as a gauge for the cap.

 

[ChazzC]

Option 1: Make the cap first and use it as a gauge.

Option 2: Cut the internal threads first, stopping when the flat on the crest just disappears (don’t forget to make spring passes - cuts without going deeper - the last few cuts. Then make the cap to suit: it’s easier to make adjustments on an Od than on an ID,

 

[machinistnoob]

Option 1: Make the cap first and use it as a gauge.

Option 2: Cut the internal threads first, stopping when the flat on the crest just disappears (don’t forget to make spring passes - cuts without going deeper - the last few cuts. Then make the cap to suit: it’s easier to make adjustments on an Od than on an ID,

I vote for option 1. Best possible result this way.

 

[mmcmdl]

Option 1: Make the cap first and use it as a gauge.

Absolutely . Thread plug gage .

 

[benmychree]

Yes, I would make the "plug gage" first.

 

[MrWhoopee]

It's unanimous, make the cap first.

 

[machinistnoob]

...I love this f×*$&$% place. Y'all are the best! Singlehandedly carring my hopes for humanity.

 

[Investigator]

Thank you all, looks like the cap gets made first.

But I'm having trouble wrapping my mind around this. Tell me if this sounds right:

1. take skim cut and measure ID of threaded area
2. look up thread depth in the Machinist Handbook for threads chosen
3. use that major depth diameter as the OD on the plug to be threaded for cap
4. cut external threads on cap until sharp crest
5. thread ID of tube and use cap as plug gauge

sound good? Did I miss anything?

Edited to correct major diameter from depth

 

[Investigator]

I really have no update to make, Just wanted to express my frustration with thread cutting (attempted).
I have a long list of woes and errors but the short version is that my compound was set at 30* on the scale. However it was not 'the correct' 30*'s. Took a while to figure that out. Now compound is on 60* which is 30*, if you know you know.
AND THEN, threads still look horrid. Looks like I'm still dragging the back side against the thread, lots of tearing and really rough finish.
And in the mix of all that I swapped from carbide threading insert tool, back to HSS, then back to carbide.......
I was tired and finally realized it so I stopped. Deep breath.....

Tomorrow is a new day and we'll start again. Just wanted to vent.

 

[GeneT45]

I think about half of us have probably done the 30 / 60 thing. Why so many lathes are graduated like that is baffling. On the bright side, you'll never get it wrong again.

What kind of material are you cutting? Some steels are never going to cut nicely.

Take relatively deep cuts to start, then go shallower as the thread progresses. Take spring passes (repeat passes, with no additional feed) as you're getting close.

Breathe. Stopping when you're frustrated is a good idea. Tomorrow will be a better day.

GsT