Here we go.
As you can see from my set-up the barrel is bigger than the bore of the lathe, so a steady rest is in order. To set-up the steady I dial the outside of the barrel to 0 then bring up the rollers of the steady as I roll the chuck in neutral, that way they come in contact with the barrel in motion which is how they will work when threading.
The key to a good set-up is rigidity which means putting the support as close to the end to be machined as possible. In my set-up I placed the steady as close to the welded on port as I felt comfortable to keep deflection and vibration to the absolute minimum.
Alright, now to start off you have to make the bore the right size to have proper depth to the root of your thread and also the proper height of the crest. This is along the same lines as drilling a hole to the right size in order to tap a certain size thread into it. The only difference is that the tap drill size when drilling is based around the factor of a 75% thread depth where single point threading is 100% thread depth right down to the .001 for different fits of the male and female pieces. To do this you use a formula which is
.6495 x 1/the pitch of the thread is you are going to cut. This formula is for a class 2 fit which is a standard General Purpose thread.
The pitch being how many full threads there is over the span of 1. The answer will be the approximate SINGLE sided thread depth. To find what the overall depth will be just multiply what you get by 2. Now this is just an APPROXIMATE to put you in the general ballpark of the final depth. The end result on the piece may be over or below by tenths or even thousandths depending on how precise you bored your hole. I like to go .003-.005 bigger on my bore when making my hole so I dont have to do any sanding on the threads in case the hole is too small. Just as when turning a piece to thread externally I go .003-.005 SMALLER that the nominal OD to achieve the same result which is no sanding.
K heres what I was threading at work, it is a Buttress thread which is a self sealing/self holding thread with a completely different thread for than a standard V thread, but the same principles still apply. I had to make this gland with the mating outside thread
Fit into the end of this barrel by making the proper mating inside threads.
I had to make a 3 Ā¾-12TPI thread inside the hydraulic cylinder barrels. So I punched .6495 x 1/12 (the TPI of the thread) into my calculator which gave me an answer of .054. So that means that the SINGLE sided thread depth is .054. Multiply that by 2 and I get .108 which means the overall thread depth Im working with is the .108. So now that I have that answer I can figure out what size I need to make the hole by subtracting the .108 from the 3 3/4 nominal diameter I need to hit. My answer is 3.642 (plus the extra .005 I like to go over to prevent having to sand, so it is actually 3.647) so now I know what size to make my hole in order to achieve the proper thread depth.
Ok now that all the brain work is done its time to make the hands do what they need to do. First off we need to get our boring bar set-up to start making hole.
Then I start boring to the proper depth and length of what we need for fitment.
I tapered off the end of the thread so there wont be a sharp edge that would rip seals on the piston when I assemble the cylinder.
Once i have my depth you will need to make an undercut for the thread to run out properly and not hit the shoulder ive made so I take a radius insert and make a groove at the end of the bore approx .010-.015 deeper than the depth of the thread so it will have a clean run out. I use a radius because its a stronger shape than a square which has sharp corners that could be a breaking point under stress.
Now comes the fun part, setting up and cutting the thread. Like I said the thread I was cutting is a Buttress thread which has a specific form with a 45 degree leading edge and a 7 degree backside edge. So the insert looks like this
The mating threads have a cross section that looks like this
Once I have my tool squared to the barrel I touch off the insert to the bore of the barrel and set my ZERO. I then set the gears on the lathe to cut the right TPI I need in this case its 12 using the chart on the lathe for the different levers.
Once that is all set I fire up the lathe and start to focus on the thread chasing dial usually down and to the right of the carriage which will usually look like this
. With different pitches of thread, the lever should be engaged at certain numbers being the same number everytime you engage it to keep the pitch constant. If the TPI of the thread youre cutting is a multiple of the TPI of the leadscrew on the lathe youre using you can engage it anywhere on the dial over and over, a different spot everytime. My lathes leadscrew has TPI of 4 and the thread Im cutting is 12 so I can engage it anywhere, anytime because 4 is a multiple of 12.
For my first pass I just want to scratch the surface so I can check that I have the right TPI set. I do this with a thread pitch guage, I set it to 12 TPI and compare it to my scratch.
Once I check it and its good, I start taking my cuts. Once i get about .015 away from my .108 approx depth I start taking small cuts of .001 and check my mating guage in the case the threaded gland Im using, to come to the proper depth slowly so I dont go too deep and have a really loose thread.
The thread is gonna look ragged during the threading process so dont worry, you will have to condition it at the end to make everything smooth.
Getting closer. Now I have to start taking passes at my last depth to take the deflection out of the threading tool because it gets pushed out gradually as the insert moves through the piece. Doing the spring passes the tool pressure decreases which straightens out the thread making it even from front to back.
To the final product which is the gland threading all the way to the shoulder of the barrel.
So remember when I said that formula gives you an approximate depth and youd have to tweek it to suit? Well I had to go .003 deeper then my formula told me to in order to make a proper fit to its mate.
Some things I use to condition a thread at the end of the process are:
A pointed file run with the point in the root of the thread while the lathe is running because the thread will move the file along as it turns helping to clean out and burrs or sharp edges that would affect the pieces from mating together.
A deburring blade to fix the lead of the thread from sharp edges or burrs.
And lastly, a 120 grit emery cloth to clean or sand down the crest of the thread.
SO thats how you set up and cut an internal thread. To cut an outside thread you use the same formula and principles mostly but different tooling. Ill try and get an external thread how to up sometime soon. But for now I gotta jam cause Ive got 5 more of these bad boys to thread before lunch.
Im not a real good explainer, Im a better shower by doing so if this confuses anyone feel free to ask questions.