Odd Thread Sizes

[Smrtman5]

While perusing the Machinery's Handbook thread section i noticed more than a few of what seemed to be odd ball thread sizes. Out of sheer curiosity i was wondering if anyone knew any interesting facts about them. Particularly the .390-27 UNS thread and the 1/4-27 UNS. I cannot think of any reason for their existence other than to make life more difficult for someone who stumbles upon one.

[handlewanker]

Hi, you can cut any thread you can dream up with a lathe and a bit of turning knowledge, for whatever purpose the"oddball" thread is required.
Machinery Handbook will give you a constant that you multiply against the no. of threads per inch or whatever, so that you can get the depth of thread for when you turn it.
For instance you might want to make a non standard thread screw of 2" diam X 40 TPI, with a thread form of BSW 55Deg, or UNf 60 Deg etc.
Measuring the finished thread is the problem, not the cutting.
To measure non standard external threads, there is a formular that is used with three wire gauges of a certain diam and a standard micrometer, for the thread being measured.
To measure non standard internal threads requires a plug gauge that is known to be of the thread form and fit that you require.
You can quite easily turn a plug thread gauge first, measure it and then bore and thread the internal thread using the plug gauge to test the fit.
Easier said than done, but I've done it many times when the occasion arose.
I would go so far as to say that any fairly competent turner could turn ANY thread devised using the applied formulae for thread fits, just takes a bit of practice, not magic or being gifted or being highly skilled.
If you really want to identify a screw, measure the diam and check the pitch of the threads with a thread pitch gauge.
Comparing this information with a screw thread chart will tell you what screw it is.
You will need both Imperial and Metric screw pitch gauges, as an odd size thread often turns out to be Metric or imperial if you're on the other side of the pond.
Once you have a collection of screws that are identified for what they are, put them in a plastic bag with a tag showing the ID, then you can identify a nut by using them as a gauge.
It goes without saying that there are fits and fits for various screw types and tolerences, but if you have a nut that is known and has been cleaned out with a tap then it will form a usefull gauge for checking purpose.
When you are making screw threaded components to a customer's order, then depending on the specifications on the drawing, some sophisticated screw gauges will be required, either calibrated thread plug gauges, ring gauges or screw caliper gauges, nuts and bolts will not do, unless the customer authorises the use thereoff BEFORE manufacture.
Ian.

[big_mak]

I've done work for some high end digital cameras' from a few different manufacturers, and they just LOVE to spec odd ball threads.

0.139-100tpi uns

1-3/32 -32tpi.

fun fun fun!!!!

[skmetal7]

"0.139-100tpi uns"

wow

[big_mak]

YAH,

try programming that and running it in a metric machine!!! took me a bit to get them to come out with threads, and not looking like a bored hole!

[Geof]

Originally Posted by Smrtman5 ...... I cannot think of any reason for their existence other than to make life more difficult for someone who stumbles upon one.

Correct.

If you are a manufacturer of camera equipment you want to make it awkward for a competitor to supply accessories for your product so you use a custom or uncommon thread.

Although sometimes the reason is not quite so cynically greedy. Sometimes the parts being threaded have such a small wall thickness it is necessary to use really oddball seeming threads.

[big_mak]

These 100TPI threads were through 3/8" of material.

There were left hand on one side and right hand on the other. I think they were used to position the aperature over the CCD.

You understand? lol

[handlewanker]

Bl#$y oath mate, it's as clear as mud.
No wonder it costs an arm and a leg to just look at one of those digital camera thingies.
I bought a digi cam on Ebay a while back for tuppence, to see how the wheels go round.
The wheels went round allright, all round the package it came in along with a hundred other parts that were dissasembled.
Still now I know what it looks like inside, but just like my long lamented grandfather, it'll never work again.
One of the hardest things is trying to discover what the thread type is when you just want to find a screw in your scrap box to replace a lost one in your video recorder or computer when it comes back from the service guy.
They always leave a few screws out of the casing, and you can never find one off the shelf.
So I just retap to the next size and live in peace.
Ian.