Anyone know anything about machining threads?

[John Milmont]

Hey guys, i figure someone here knows their way around a machine shop and could give me some advice.

I'm working on a project that's 2 5/16" in diameter. This is a thin walled piece that needs fine threads. The only standard thread that I can find (i'm not an engineer) is a 2 5/16 -16 UNS thread. The problem is that this is too coarse for my application, and i think the thread depth would be through the piece.

I have a machinist making the part for me, but hes doing this as a favor and asking me what threads to use. I don't know what to tell him. Can someone point me in the right direction with this?

Thanks,
John

 

[Tim]

The great thing about threads is you can make them any size you want if you are making both parts.

On a thin wall part I would probably use a UN form at 24TPI. Standard form carbide tips are available for both internal and external on this form. I am guessing you are making both the male and female thread. So in reality you can make it anything you want.

What you need to look at is the thickness of your part and then the depth of the threads to be cut to make sure you still have enough meat to do the job. If it were I doing the job I would probably put a 1mm pitch (as I have both tips for this pitch) on the Dia you have. I know that would be a bastard thread but does that really matter if you are making both parts.

You are unlikey to find a fine pitch tap at that size that will do the job, No thread that size listed in the Machinery's Handbook.

Hope this is helpfull

 

[John Milmont]

Yes, we will be making both parts and im sure its going to be done as a single point operation. The parts wall is only .09" thick (2.286mm).

I dont fully understand the relationship between the depth of the thread and the pitch. I would think that, for a given threading insert in a single point operation, that the shallower the thread is, the tighter the pitch would have to be.

Am i wrong in thinking that if a triangular threading insert is going to make a cut thats .05 units deep and .05 units wide, the pitch would also have to be .05 so there's no un-cut space between the threads? This is given that the threading insert is a equilateral triangle with all 3 legs being .05 units long....

 

[Gus Richmond]

Close but don't forget you don't want a sharp point at the top or bottom of the thread. They should have a nice radius.

 

[Tim]

as a rough guide I work on for metric threads is to take the pitch away from the OD to give a tapping size.

The thread angle is 60^o. The thread depth is 0.614 x pitch so for a 1mm pitch thread you are looking at a depth of thread of aproximatly 0.024"

 

[flatchat(Chris)]

1 - 1.5 mm pitch (.041" -.0625" / inch / rev ) --it could be multiple start , for a quicker transition ... all about freds :- www.google.com.au/search?q=thread+p...g&biw=1233&bih=718&sei=oFxPT_CmBevnmAWPzomyCg

 

[Grant S]

John what is the part or parts being used for ? does it need to make a seal ? is it under pressure? but if it is just a simple thread your looking for, you should be safe with UN 24 tpi nothing special very easy thread to do.
Cheers,

 

[John Milmont]

This is for a custom flash hider for my rifle I'm working on. I think I will go with the UN-24 thread because it seems to be used a lot in the industry. The standard 308 muzzle thread is a 5/8-24 which I'm sure is a UN pattern thread. Though, my part has 2 pieces which I need to thread together and that's what I'm trying to figure out.

Its been hard for me to grasp all of this threading information all at once. I've been doing a lot of research on the topic and have been somewhat overwhelmed. The information I'm finding is usually leading to more questions than answers.

The question I'm on now is this:
I have added some callouts for some standard UN threads in my drawling, but I have some confusion about what I should make the diameter of the parts. Do I need to take into account how deep the threads are on a male part? So, if the thread is .0361” deep, do I need to add .0361” (x2 because it’s a round piece) of material to the piece to allow for threading? The part diameter + the thread depth would be the Major diameter...



Or is it understood from the callout that the piece has to machined a little bit larger for the threading?

 

[Grant S]

Hello John, this is easy as well, look up "Tap drill charts" what ever size of threaded hole you want, it will give you the size drill you need to start with.
But if you are thinking about maybe using a suppressor you will want to use a two or maybe even a three start thread (wink).

 

[John Milmont]

Its not a sound suppressor, I guess the closest thing its like is a Noveske kx3. I cannot use a die and tap for this - thats why I'm going through all this trouble of figuring out how to thread this.

It will be threaded on a CNC lathe. All of the CNC stuff I have done in the past I had to designed either oversized or undersized to have the extra material to allow for the threads.

This project is a bit different than the things I've done in the past since its a tuning part which I have never done before, and I'm also not personally doing the machining. So, I've been trying to learn how to communicate in machinist language.

When I callout threads like this: 2.400-24 UN 2B, is the first number the diameter of the threads (the major dia?) or the diameter of the piece?

 

[John Milmont]

I think i just found my answer:

"ANSI Y14.6 defines standards for indicating threaded parts. Parts are indicated by their nominal diameter (the nominal major diameter of the screw threads), pitch (number of threads per inch), and the class of fit for the thread. For example, “.750-10UNC-2A” is male (A) with a nominal major diameter of 0.750 in, 10 threads per inch, and a class-2 fit; “.500-20UNF-1B” would be female (B) with a 0.500 in nominal major diameter, 20 threads per inch, and a class-1 fit."

So it looks like i do have to size the part a little bigger for the threads.

 

[Grant S]

Good to do a practice piece first if you can.
Cheers,

 

[John Fitzpatrick]

If you're having the part made by an outside shop, you may get the best result if you sit down with the lead machinist before you commit to your design. There are more factors to consider in machining than you'll ever learn from books. I was involved in designing prototypes of remote transponders back in the '70s, and I'd take my designs to our machinist and he'd look them over and smile. After going over what I wanted the part to do, he'd ask about materials alternatives, heat treating possibilites, corrosion requirements, strengths in tension, torsion and flex, thread galling, thermal expansion limits, and Idiot Proofing. Then he'd patiently explain about chip breakers, tool chatter, cutting tool geometry, surface finish, concentricity factors in workholding fixtures, and finally explain that he could make any part I designed, but not at a price I could afford to pay. Usually after an hour's give and take, we'd have a marked up drawing that did the job and saved me from considerable embarrassment. Just a thought.