Newbie thread diameter question

[zeeprogrammer]

Finally able to get to my shop and turn some metal...but as usual...I get stumped when things aren't clear as crystal.

Making two identical hose couplings. Drawing shows to turn the diameter to 0.175 and then thread the end for 3/16-40.

Stump 1: Drawing doesn't show how far to thread. I looked at the parts where the couplings are attached. Looks like one shouldn't be more than 0.125. The other is not so critical and can be longer. I would cut a thread relief using a bit of hack saw blade that's about 0.025 thick. So the threaded bit is really 0.1.

Does that seem sufficient? 40 TPI would result in just 4 threads.

Stump 2: 3/16-40 means to me a 0.187 outer diameter which is the diameter of the brass I'm using. As a test...I threaded the rod anyway...it cut easily and the coupling fit (loosely) in a hole tapped for 3/16-40. 0.187 to 0.175 seems a lot.

Is there a rule for what you cut the diameter down to when making a thread?

Further, as a hose coupling, why would I turn it down from 3/16? Seems like a standard size to fit hose to.

So I'm thinking I won't turn the rod down. I'll simply cut the relief and thread and then polish the rod. Comments?

Sorry for what has to be a pretty fundamental question...but you all know me...very new to all this.

On a happy note...I have improved my ability to measure incorrectly. Here's a pic of a thread relief cut...

Thanks.

 

[kvom]

3/16-40 is not a standard unified thread nomenclature. The closest is 10-40 which has a major diameter range of .1891 - .1840. 3/16 is in the middle of the range.

 

[zeeprogrammer]

Thanks Robert.

3/16-40 is not a standard unified thread nomenclature. The closest is 10-40 which has a major diameter range of .1891 - .1840. 3/16 is in the middle of the range.

Thanks kvom. That would explain why it seems difficult to get much information on the 3/16-40. But it seems to be a 'common' thread in model engineering. (See also PM Research site.)

So for the 10-40...what would you have turned the rod down to before threading it? 0.189?

Thanks.

 

[Maryak]

Zee,

Perhaps it's an English Model Engineers thread. http://www.oldengine.org/members/diesel/Tables/me1.htm

Best Regards
Bob

 

[mklotz]

I've never heard of a 10-40 thread. Are you sure you don't mean 10-32?

As to the diameter of a numbered thread, I've published this formula many times before...

D = 0.060 + N * 0.013

D = major diameter (in)
N = thread number

so, for a #10, we have:

D = 0.060 + 10 * 0.013 = 0.190"

 

[Tin Falcon]

Z:
just out of observation I have measured various machine srews etc and it is not uncommon to see the actual measurement ten thousands less than nominal. Check the Machinery Handbook there is usually a tolerance window for all thread dimensions except pitch.
sometimes it is necessary to have minor deviations in nominal measurements. for example a roll pin.
the HOLE for a 3/8 roll pin is .375 the spring/roll pin is .385 and the pin punch is .365.
A sliding fit and a press fit are different animals for example.
Tin

 

[zeeprogrammer]

Thanks Bob, Marv, and Tin.

I'm still suffering some denseness.

Marv, in the past you've also given an equation used to determine the diameter drill bit prior to tapping. That equation has a variable (a percentage) that governs how tight/loose the threads will be. (I suspect the way I just stated that isn't entirely correct.)

Using the example you gave above...do you turn down the rod to 0.190 for a #10 prior to threading with a die? Or is there some variable or rule that governs tight/loose? So far, the two projects I've looked at are specifying the turned diameter to be smaller than the diameter of the thread. That just seems odd to me.

In quiet anticipation that this is a 'doh' slap on the forehead moment...

 

[Tin Falcon]

Z:
1) take a breath
2) Relax
3) make the part.
4) Amire the part
5) check for fit
I think you are worrying more than needed I think the smaller than nominal diameters are just to make die threading a little easier.
6)After machining have a drink if you want /need one.
7)admire the part again
8)Relax You aint building the space shuttle.
You are not machining unabtainium that costs a a hundred bucks an inch!!
remember you are doing this for fun and to learn.
If a part needs to be remade you probably learned something.
Tin

 

[kvom]

I've never heard of a 10-40 thread. Are you sure you don't mean 10-32?

I'd never heard of one either, but it's listed in Machinery's Handbook. I assumed zee knew the thread pitch, so went with that.

 

[Tin Falcon]

10-40 aka 3/16 -40 is a bit of an odd ball used for air steam fittings on some models IIRC the PM research engines call for these threads. It is good for attaching 3/16 tubing to an engine the 40 tpi alows for a little more strength in the tubing(pipe). On my PM engine I just used a 10-32 and made my own barb fittings.
Tin

 

[zeeprogrammer]

Tin, not to worry. Quite relaxed. (I'm on meds for a recent back injury :big

It's a learning question which stills stands unanswered. When tapping holes there seems to be some choice in what diameter to drill. I haven't seen a similar thing with respect to turning down a diameter for threading. Given a thread's outer diameter...is there a choice to be made? Or is there a simple rule? The two places I've seen this show to turn a diameter down to less than the diameter of the thread. Why?

kvom...definitely 3/16-40. Just as Tin mentioned and Bob gave a chart on. Apparently for model engineering. One other fellow a while back also seemed to think a #10 is the same as 3/16. You mentioned the machinist books shows a 10-40. If I ever get my hands on one (a 10-40...I also want to get the book)...I'll see how it fits a hole tapped for 3/16-40.

Thanks all.

 

[Hilmar]

I also think the model thread 3/16 - 40 is a 55° and not a 60° thread.
Hilmar

 

[Hilmar]

Threads, 10-24 UNC ,10-28 UNS, 10-32 UNF, 10-36 UNS, 10-40 UNS, 10-48 UNS, 10-56 UNS
Look for the different UNS, UNF, UNC's
Machinery's Handbook 22nd Edition, Page 1231.

Hilmar

 

[shred]

I posted a chart I made up from Marv's formula a while back.. I'll see if I can find it. It's very handy when trying to thread with a die-- instead of turning down to the 'major' diameter, turn down to the thread-major diameter and the die will go much easier. Put a little chamfer on it if possible to get started.

ah, here it is: http://www.homemodelenginemachinist.com/index.php?topic=3554.msg35615#msg35615 Marv's discussion and my table in the next post.

 

[zeeprogrammer]

Thanks Shred (and Marv)...that's what I was looking for.

I thought what I was doing was a bit too easy. Probably because it's brass, a new die, and the diameter is small. I figured I would get into trouble with harder metal and larger diameters.

So here again I've asked a question that has been answered who knows how many times. Sorry for that. (To be expected though...why wouldn't every newbie who joins ask the same question?)

The search engine has been very helpful but seems difficult to use. At least you get a lot of stuff that isn't what you're looking for. I know some members have talked about putting together a library or such. Also difficult - who would do it?

What about creating a list of keywords that can be inserted into posts that could be searched for? Like '$Thread Cutting$'. Keep the list in the library. Nah...probably just as bad. People will end up inserting quotes and carrying that along with it...then we're back to the problem.

Too bad there's not some field in the post we could use to 'tag' it.

Ah well...I haven't finished my first cup of coffee.

 

[kvom]

Unless you need a very tight fit, it's OK to drill a hole a couple of thou larger for threading, or turn the major diameter a bit smaller. An extruded brass rod may not be perfectly round in any case, so a skim cut is a good idea. You can also thread a bit deeper than spec, which accomplishes the same thing.

 

[zeeprogrammer]

Thanks kvom.

Well it is a split die after all. I should do some experimenting.

 

[mklotz]

The Model Engineer threads with which I'm familiar are all 32 and 40 tpi with 26 tpi for the larger sizes. From my personal library I have the following chart.

               Model Engineer - ME

Size      T.P.I.     Tapping Drill Size.
                mm   Number/Letter/fraction
1/8       40       2.55      38
5/32      40       3.25      30
3/16      40       4.00      21
7/32      40       4.8       11
1/4       26       5.3       4
1/4       32       5.5       7/32
1/4       40       5.5       7/32
5/16      32       7.00      J
5/16      40       7.00      9/32
3/8       32       8.7       11/32
3/8       40       8.7       11/32
7/16      40       10.3      13/32
1/2       40       11.9      15/32
1/2       32       11.9      15/32
1/2       26       11.8
9/16      26       13.5      17.32
5/8       26       15.0      19/32
3/4       26       17.8      45/64

So 3/16-40 is a recognized ME thread.

As several have indicated, for our small, non-critical work one need not get too paranoid about sizing for cutting male threads. Lots of folks, myself included, regularly cut 10-32 threads on as-bought 3/16" stock with good results.

That said, it's still worth your effort to keep a copy of my thread diagram/equations (as referenced in my post that Shred pointed to) in your shop notebook. I can't tell you how many threading questions I've answered with that page.

As for making all this information easily findable by future novices, I think that's a lost cause. If there were an easy way of doing it, it would have been discovered long ago and every forum would already have it embedded in the software.

The best you can do is hone your perception of what is truly important and, when you encounter such material, capture it in your notebook in an organizational scheme tailored to the way your mind works.

 

[zeeprogrammer]

As for making all this information easily findable by future novices, I think that's a lost cause. If there were an easy way of doing it, it would have been discovered long ago and every forum would already have it embedded in the software.

Thanks Marv. Although I'm a programmer, I prefer paper. I like book references. So I'm going to get some plastic sheet covers and put together a shop notebook.

As for the 'lost cause'...you never know...I'm always surprised by the 'youngsters' at work. What was true yesterday isn't necessarily true today. Simple example - smileys. Forums have been around a while...someone probably wanted to embed a smiley...and was told...'if it could be done...they would've done it by now'.

Sometimes you get someone thinking about it just by asking the question.

 

[Foozer]

As for the 'lost cause'...you never know...

Forum software, simple search engine. Doesn't ask "Do you mean CENTER" when someone types in "centre" as the search word.

Kinda stuck with its limitations, that of course, becomes more profound as the board matures.

Robert