[OC_]

Hi guys,

I'm working on a round piece that has thin walls and needs to be threaded. The piece is 2 5/16" in diameter (I.D.) and .09" thick. I really don't know what kind of threads to put on the inside of it. I'm just a hobbyist, and even though I have a machinist making the part for me, hes doing this as a favor so I don't think he will give me too much input.

Obviously, the threads need to be very fine and not very deep. He's asking me what the diameter and pitch should be. I don't know what to tell him. Anyone have any advice?

He will probably do this as single point threading.

Thanks

[txcncman]

How much strength do you need to have left in the material after the thread is completed? Here is a chart for you out of Machinery's Handbook. On a 10 TPI thread you would have about 0.004" material left per side.

[OC_]

The 20TPI thread on that table seems about right for my application. So the "depth of internal thread and UN external thread" of .02706 is actually how "deep" the threads will be cut into the wall of the part? Most of this information seems to be expressing the profile of the thread, thats what all that truncation stuff is?

Sorry to be such a newbie at this..

[txcncman]

Yes, truncation describes the limits of the thread profile. What you are needing is basic thread call out data. With the 2.3125" OD and 0.090" wall, If you are trying to achieve maximum thread engagement with a 20 TPI thread and maximum remaining material, put you at an ID of 2.1325" plus 2 times the V thread height (0.0433 X 2 = 0.0866) which gives a nominal thread diameter of 2.2191".

[OC_]

Is the OD of 2.3125" you describe above the OD of the part or the outside of the thread? The ID of my part is 2.3125" so I'm a little confused. I have attached a picture of what I'm working on. The banned area is the threaded portion.

[txcncman]

Sorry. I misread and mis-remembered your original post. If your ID is 2.3125", just add double the thread height to get the nominal thread size needed. Normally tubing sizes are given as OD X wall thickness.

[OC_]

ok. i think i get it then. So, basically, the thread diameter is the the diameter of the part (the ID in this case) Plus how deep the threads are. So, it would be 2.3991".

Then, what is the proper way to call this out?
would it be: 2.3991x20 UN-3B

Thank you so much for your help in making heads and tales out of this.

[txcncman]

Nominal thread size is always called out for the OD thread size. xA or xB determines external or internal thread. 1A/B, 2A/B, 3A/B determines class of fit. Class 3 fit is tight and difficult to machine and should only be called out when absolutely needed for precision. Class 2 fit is standard and applies to almost all applications. Class 1 fit is for threads such as anchor bolts for concrete foundations. So, for your application, I would call out:

2.400 - 20 UN 2B

[OC_]

So now for the male piece thats threading into this. I understand that this piece has to be a little larger than the base dimension to engage the threads.
Could I use a UNR thread for the male piece? Or does it really matter since that piece is heavier and thicker for this application.
It says the UNR for that dimension is .02977". I'm guessing I just add that to my base diameter (x2) to get the final thread diameter?

So my minor diameter would be 2.3125" and my major would be 2.372"?

[txcncman]

Oh wow. I am not sure I can explain any better than I did before.

Thread call out for external thread:

2.400 - 20 UN 2A

Thread call out for mating internal thread:

2.400 - 20 UN 2B

The call out is the same except for A and B.

The nominal dimension (2.400) is taken at the largest part of the threads (crests of the thread for external and root of the threads for internal). Calculations are done from there to determine major diameter, minor diameter, and what is called pitch diameter. What you can easily measure on an external thread is the major diameter of the thread. What you can easily measure on an internal is the minor diameter of the thread. A seasoned, knowledgeable machinist should know how to do these calculations. If I were to machine these parts, I would machine the external thread first and measure it with thread wires to make sure the pitch diameter was correct then use it as a gage while threading the internal thread until I got a good fit.

[OC_]

Sorry about being so dense. I'm coming from a blank sheet with this stuff and I find that whatever new thing i learn usually leads to more questions.

[txcncman]

Ok. i was just trying to think of a different way to explain it to help you understand.

[OC_]

Just to be clear, its understood from the callout for the threading that extra material has to be left on the piece for the threads. So I should design the male piece to be the nominal diameter of 2.3125".

I would think that if it don't add the .0433" (the thread depth) to it, it will be machined too small! I'm sure all of this is going to be done on a CNC lathe if that makes any difference..

But again, i dont have to add the .0433?

Thanks again for spending the time to explain all this.

[txcncman]

According to your original post, your machinist wanted to know diameter and pitch.

After a closer look and further consideration, I think you would be better off with a standard 2-3/8 - 20 UN 2B thread.

Now, I can break it down for you. But your machinist did not ask for any other information.

Internal thread specifications for 2-3/8 - 20 UN 2B:

Minimum Minor Diameter = 2.321"
Maximum Minor Diameter = 2.332"
Minimum Pitch Diameter = 2.3425"
Maximum Pitch Diameter = 2.3491"
Pitch = 0.050"
Thread per inch = 20

External thread specifications for 2-3/8 - 20 UN 2A:

Maximum Major Diameter = 2.3735"
Minimum Major Diameter = 2.3654"
Maximum Pitch Diameter = 2.341"
Minimum Pitch Diameter = 2.3359"
Pitch = 0.050"
Thread per inch = 20

[OC_]

Bah! I think I need to have a talk with my machinist to see exactly what he wants. I'm guessing more than anything at this point it seems.