new to threading and have questions

Pete, it is common for threading on the lathe to cut a run out groove at the finish of the threaded area. So if you turn the major OD for your threading blank, and then cut the run out groove to the minor od as you cut the thread progressively deeper when you are at the correct depth you should just see the tool tip scratching the surface of the run out area. Provided you have the correct angle on your tool you would also see the thread come to a sharp vee point at this time too. Basic trig for a right angled triangle will tell you how many divisions on your compound (which is the hypotenuse) equals the distance "in" that you need to cut.
This is also where single point threading using the crosslide and the compound set at 90* is beneficial in that you can see directly on your dials how deep to cut according to your class of fit.
For threading on the lathe it is also common to run a die nut over the finished thread. This will give a much smoother finish to the thread and also check your diameters are correct.

Cheers Phil
 
awesome info guys !!!!!!!!!!!!! this really really helps me out. i'd rather ask know and learn the proper methods and be able to understand the whole process like you would do in a classroom. im pretty happy i joined this forum!! great bunch of guys willing to help a new guy out, it really means alot. im thinking i will do some trial test runs on pvc pipe i have sitting in the corner. seems like a good idea. any thoughts?
 
Just for fun, I'll illustrate some of the difference in hobby work and industrial here. Phil is spot on with his how to, but that won't work in many commercial settings with industrial specifications/requirements. By far, the majority of threads do not allow a thread relief groove. CNC is capable of very consistent tool pull-out, so it isn't necessary. However, like so many things, there are exceptions.

One exception. In highly stressed thread, a rotary shouldered connection used in oilfield tubular components for example, a generous radius is required in the corner of the shoulder and major diameter of the thread. Sometimes the required radius is so large that an undercut is needed to accommodate it. So in effect, there is a relief groove, but not so much for that purpose. There is a problem inherent here though. The surface finish comes into play with stresses, so that these reliefs and radii must be very smooth. And since threads are simply spiral, relatively sharp bottomed scratches, the reliefs must be smaller than the minor diameter so they can be without any scratch, including a thread runout. The same principle is used, but not quite as commonly seen in internal threaded tool joints (RSC).

Additionally, there are specific dimensional requirements that must be met in commercial threading. There are tolerances for every feature of the thread. In the end, depending on the application, various gaging and inspection procedures and instruments are used to qualify the thread as acceptable to drawing requirements. It is difficult to compete with CNC equipment when running most threads on a manual machine. The CNC simply has the speed and repeatability that no human can match. I've had some fun trying, but can't keep up for long, even on a simple thread.

All that aside, to make the best threads possible is a good goal for any machinist, since perfectionism is often a part of the mindset of a machinist. So the more you know about threads the better off you will be, and the sooner you will be cutting threads you are proud of.
 
A thread pitch gage just tell the tpi. If your measuring thread depth you need thread wires or thread mic. A little math, or if it's common the machinery handbook will have the math done.

Or cheat and test it with a nut till the nut fits

Though if your just learning your only cheating your self if you dont at least learn how to do the math. I like to practice math with a problem thats already been answered so I know if it's right
 
great stuff tonywells & chuckorlando. very informative. can never know too much! there must be a chart somewhere that tells the exact diameter for the start of the shaft size and end of the shaft meaning depth size no???????? lets say i start off with 3/8 rod but need a 1/4- 20 thread on the end what would i turn the shaft down to and what diameter to turn the shoulder relief down to so the thread blends in? is there a chart somewhere to do simple calculations???????? im really lousy at math!!!!!!! but mama tried!
 
pete1972 said:
awesome info guys !!!!!!!!!!!!! this really really helps me out. i'd rather ask know and learn the proper methods and be able to understand the whole process like you would do in a classroom. im pretty happy i joined this forum!! great bunch of guys willing to help a new guy out, it really means alot. im thinking i will do some trial test runs on pvc pipe i have sitting in the corner. seems like a good idea. any thoughts?
Click to expand...

Just don't breathe the fumes. PVC is nasty stuff.
 
No he aint joking. Your best bet is dont breathe any fumes or smoke from anything if you can help it. Alot of old timers have died way to early and alot more still to come from fumes that they thought would never hurt them.

And some things only take a small amount to do instant damage.
 
i have to say im totally screwed then. have smoked for 30 plus years and a welder for over 25. maybe the pvc will be the straw that broke the camels back. its all coming to a stop someday for all of us so im not to worried about a little practice threading on PVC. thanks for the warning guys
 
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