thread depth chart

[SB1997]

hello, im new to this forum and machining for that matter so forgive me if his isnt the right place for this post.

lets get down to it. im just starting to learn threading on my old south bend 13" lathe and i was trying to do some male threads (12 tpi) on a piece of aluminum tube for practice and trying to make it thread into a small pipe fitting we had laying around the shop. i got the inside diameter of the pipe (1.517) and the first try i just turned the OD of my aluminum down to that diameter and didnt realize what i did until i tried to thread it.

so does anyone have advice on how to calculate the thread depth of cuts like this or is there a chart somewhere? any advice would be extremely helpful. Thank you

 

[Rickyb]

hello, im new to this forum and machining for that matter so forgive me if his isnt the right place for this post.

lets get down to it. im just starting to learn threading on my old south bend 13" lathe and i was trying to do some male threads (12 tpi) on a piece of aluminum tube for practice and trying to make it thread into a small pipe fitting we had laying around the shop. i got the inside diameter of the pipe (1.517) and the first try i just turned the OD of my aluminum down to that diameter and didnt realize what i did until i tried to thread it.

so does anyone have advice on how to calculate the thread depth of cuts like this or is there a chart somewhere? any advice would be extremely helpful. Thank you

Plenty of advice in the thread on "what causes this gaulling?"
Plenty of YouTube videos also. These may be the most helpful as they are easier to follow and no visualization required.

 

[Phil in Montana]

You need a starrett fish tail gauge for setting your tool, it has full depth chart on it...Phil

 

[Gordon B. Clarke]

so does anyone have advice on how to calculate the thread depth of cuts like this or is there a chart somewhere? any advice would be extremely helpful. Thank you

http://flexiblemeasuring.com/images/support/manual/1basic/1.09.pdf

 

[4GSR]

SB1997,

You have to realize that pipe threads are totally different from regular straight vee threads. First of all, they are not 12 threads per inch on 1", 1-1/4", 1-1/2", and 2" pipe sizes. They are 11-1/2 threads per inch. You may not be able to cut this odd thread pitch on your lathe. Second, most all pipe threads are tapered, meaning they are not straight or parallel but have a taper to them. They are also cut to a near sharp vee root and crest, (tops and bottom of the threads).
if you don't already have a copy of the South Bend LAthe book on "How To Run A Lathe" you need to get one. It does show you some details on threads as well as a detail of a pipe thread. Also, get a copy of the Machinerys Handbook, a lot of good information in that too, that may be of help.

Ken

 

[TDegenhart]

The depth of the thread isn't as important as the pitch diameter. To measure that you can use either the three wire method or a thread micrometer. To check a tapered pipe thread you will need a pipe thread ring gage.

Tom

 

[theperfessor]

I'll second the idea of getting hold of a copy of Machinery's Handbook.

 

[rons]

The threads in a pipe fitting are tapered and yet you are trying for some straight threads on a mating aluminium tube. It may fit for little bit of travel but
this is not correct. Another thing is that pipe dimensions are not the same as for tube. You can look into that.

For straight threads:
----------------------------
The pitch is P = 1 / 12.
The 30 degree travel distance for making a UNF thread is D = 0.625 x P.
The height of the thread would be H = D x cos 30 = 0.541 x P.
I worked this out from reading the Machinery Handbook.

So for a common 1/4-20 thread the D would be 0.03125. The handbook would tell you it's 0.031.

The numbers for H and D are adjusted down a few thou for the fit tolerance, which in your case would not matter much. You just want to get it to screw in.
If your female fitting is tapered then you need a taper attachment on your lathe to get a tapered male thread.

 

[Gordon B. Clarke]

SB1997,

You have to realize that pipe threads are totally different from regular straight vee threads. First of all, they are not 12 threads per inch on 1", 1-1/4", 1-1/2", and 2" pipe sizes. They are 11-1/2 threads per inch. You may not be able to cut this odd thread pitch on your lathe. Second, most all pipe threads are tapered, meaning they are not straight or parallel but have a taper to them. They are also cut to a near sharp vee root and crest, (tops and bottom of the threads).
if you don't already have a copy of the South Bend LAthe book on "How To Run A Lathe" you need to get one. It does show you some details on threads as well as a detail of a pipe thread. Also, get a copy of the Machinerys Handbook, a lot of good information in that too, that may be of help.

Ken

I'll start with a

Re normal pipe threads the only thing the USA and the rest of the world can agree on is 14 TPI. The US 60º flank angle has 5 pitches and the Whitworth profile pipe thread has 4.

Again, this gives good details.

http://flexiblemeasuring.com/images/support/manual/1basic/1.09.pdf

It's true that a Machinery Handbook gives a lot of information but not always enough on threads as is sometimes necessary.

 

[Gordon B. Clarke]

The depth of the thread isn't as important as the pitch diameter. To measure that you can use either the three wire method or a thread micrometer. To check a tapered pipe thread you will need a pipe thread ring gage.

Tom

There are other alternatives.

http://flexible-measuring.com/images/thread_inserts/03_3.06.19.pdf

Wires and thread micrometers are for external threads so with internal and tapered threads not much use.

 

[crossthread]

Welcome. I would not start out learning with a tapered thread. Start simple. You need to learn how to use the half nuts, coordinate your hand movements to stop the lathe and withdraw the tool bit and get it back to the proper starting point etc. If you want to practice threading then get yourself some stock of a standard bolt size, 1/2" , 3/4" etc. Get a nut of the same standard size (1/2", 3/4" etc.) Set the lathe up to cut the proper number of threads per inch. Cut the threads until they are just sharp. Try the nut and see what you would have to do to make the fit better. I know I have left out a lot but perhaps you get the idea. Keep it simple.

 

[EPAIII]

Yes, those fish tail thread gauges are almost indispensable when you are cutting threads. Starrett is not the only brand, they are made by many manufacturers.

Also Machinery's Handbook is like the bible of machining. DO get one, even if it is a 25 or 50 year old edition. I have three of them and the really old one is the one that I keep in the shop because it is the smallest. Most of the information in it is still valid unless you are working to NASA specs - and probably good there too. Most of the more modern standards were developed to be compatible with the older ones so hardware stores did not have to have two stocks of screws and nuts.

You say you are trying to make a thread to fit a "pipe fitting" that was laying around the shop. But you did not say if this fitting was already threaded or if you are going to also cut the internal thread in it. If it is already threaded, it is probably a tapered thread because that is how 99% of pipe threads are made. So your measure is probably only for one point on that taper and you will have to cut a tapered, male thread to fit. This does not sound like a good first thread to cut. I would recommend finding a standard nut, perhaps a large one and make a male thread to fit that for your first effort.

Others have said that the best way to measure a thread is to use the pitch diameter. That is the theoretical half way point between the top and bottom of a sharp Vee thread. But no one makes sharp Vee threads: real threads will have a flat on the tops and some fill in the valleys. Measurements can be taken from the flats on the crests of the threads, BUT many threads will have various percentages for the size of this flat. So such measurements can be misleading. This is why methods have been developed to make finding/measuring that pitch diameter possible. But this need not be a concern with your first practice thread. You can start with a good guess of the OD and just use a nut as a gauge. Continue taking small cuts until the nut fits properly. I have made many threads this way and the others have probably also used it. You can save the super accurate measurements for later jobs that dictate their use. So don't overthink this on your first single point thread.

You need a starrett fish tail gauge for setting your tool, it has full depth chart on it...Phil

 

[Gordon B. Clarke]

Others have said that the best way to measure a thread is to use the pitch diameter.

Probably the main reason for using pitch diameter as the best/preferred diameter is that, as far as UN and metric go, pitch diameter is the one with the least tolerance. The tolerance on the OD diameter is about 30% more than for the pitch diameter and the minor diameter (bottom of thread) is about twice that of the pitch diameter tolerance.