Metric/Imperial thread match

[northernsinger]

Do some standard metric thread sizes thread on some standard Imperial threads?

I happened to try a 2 1/2" by 6 TPI faceplate on my Weiler lathe spindle, which is metric 60 by 4 thread--I was just musing that I could rethread the faceplate to fit--and the darn thing screws on and fits pretty well. I was surprised.

I assume the threads are not close enough to use in this situation but thought I'd mention this. I'm sure others here know a great deal more about this than I do.

Northernsinger

 

[alwyn]

you got away with it because its only a short thread length,if it had been say 10"long,then you have seen the difference,6tpi is 4.23mm the error is per thread,multiply all the .23s over 10".

 

[northernsinger]

That's right, of course, Alwyn.

Does this semi-match happen with other metric/Imperial pitches we can cite?

Northernsinger

 

[franco]

northernsinger,

There are a couple of other close matches between metric and imperial threads. Should you ever come across 1/4 BSF or 1/4 BSB threads on old British equipment (1/4 x 26 TPI), the nuts will screw onto a 6 x 1 metric bolt (25.4 TPI) with a slighly loose fit.

I don't have one handy to check, but a
20 x 2.5 (10.16 TPI) metric nut should screw onto a 3/4 x 10 UNC bolt, once again with a slightly loose fit. Neither of these examples is a good enough fit to be used.

Another combination which can cause problems is the similarity between 5/16 x 18 UNC and 8 x 1.25 metric (16.9 TPI). In this case the metric nut will start on the 5/16 bolt and vice versa, but will lock after a few turns.

franco

 

[Rich Carlstedt]

Seems to me I read that there is a very dangerous combination out there.As I recall (?) it is a 19mm-2.5 and a 3/4-10.
It's dangerous because of some construction safety equipment that uses one or the other
Rich

 

[matt_isserstedt]

Yeah, Rich, I was thumbing thru a box of A-325 structural bolts, 3/4-10s x 2" and there was an oddball long one in there ~3-1/2".

The box was filled with deformed thread nuts, and the oddball long bolt would zip right past the deformity....the head had a 12.9 or some such marking on it typical of metric fastener but it was black oxide coated and looked identical to the A-325s.

*Very* dangerous in a structural situation....I promptly removed it.

-Matt

 

[Grandpa Jim]

The table lock screws for all 3 axis on my Bridgeport copy, Taiwan, (Jet) mill are an odd thread. They are a 12mm diameter and a 12 thread per inch. Weird, so close to a 1/2 - 13 but no go!!! I had to make new handles when putting on my digital readout and so cut them on the lathe to .472 inch diameter, and threaded them to fit at the 12 per inch. Weird, but worked real nice. Wish we'd gone METRIC in this country..... say 100 years ago!!
Would have been cheaper!!

 

[JST]

Wish we'd gone METRIC in this country..... say 100 years ago!!
Would have been cheaper!!

We DID, About 125 years ago!

Sometime like 1879, but nobody paid attention....... AS I understand it, that's when metric measurements became legal (for interstate commerce), as an official national system of measurement.

 

[alwyn]

Just one uk thread matches metric 0BA is 6X1mm,the BA system was for used for electrical things Oba the largest down to 16 ba about the same as 0 80 size.
The 1/2 12 on the Jet mill could be 1/2" whitworth,thats a match,
as already said its not a good idea to codge a thread if its structural,bad practice,
I wish we had gone metric 100 years ago,in fact it was around 30 ago,still has,NT sunk in to some people,mainly the older ones,kids only know metric

With a calculator eg 1 divide 6 = .166 x 25.4 = 4.23mm 1 divide 13 = .0769 x 25.4 = 1.953mm ect
most people may do the conversion like this