View Full Version : unusual thread?

12-07-2003, 10:43 PM
Help Please. I am building a replacement component for a very uold set of outdoor speaker horns. When i measure thr inside dimension of the existing thread it measures 1.335 I/D against the inside of the thread. ALike a nut if you know what im trying to say. The thread pitch is 18 TPI. It is a very unusual part to say the least. Any help would be great. Thanx Audrey

Al Messer
12-07-2003, 11:11 PM
Audrey, I'm a bit confused. Are you saying that you need to make a "nut" to fit a threaded piece of this dimension (1.335" x 18 tpi)? If so, just plan on boring to the minor diameter in a lathe and then threading it with a 60 degree internal threading tool.

12-08-2003, 02:07 AM
Hi Audrey,

Am I reading right:

You want to dupicate the existing female thread -- what you really want is the existing threads major dia. (internal)?

Easiest is to measure mating male thread -- guessing you don't have access to it.

Got to be an eiaier way, but if its critical, you could make a mating plug, then cut new part to fit.

You can get a rough idea by adding twice the thread depth:

18TPI = pitch of .0555
Trig. gives .048, twice is .096
plus 1.335 is 1.431

so you know 1.431 is too large as we have disregarded clearance, % of thread depth and any root radius.

Could guess thread depth at 75% and try 1.407 on your shiny new "practice part"! : )

Or wait two more posts 'till someone tells us how to actually measure it.


12-08-2003, 03:08 AM
Or could you measure inside to outside with a dial caliper (like wall thickness on tubing),
then slip in a 5/16 18 threaded rod, measure from outside to rod on wall.

Subtract the difference in the two measurments from 5/16ths for single thread depth.

Outside of horn is probably tapered. : (


PS, could it be 1-3/8" 18, that's pretty close to our range?

[This message has been edited by uute (edited 12-08-2003).]

12-08-2003, 03:18 AM
Aaw, hell. Where's Evan? He can measure a ticks Johnson, on a whale's back, a hundred meters under the icepack,, IN the dark!

He'll show us the way! : )


I better go to bed now. : |

[This message has been edited by uute (edited 12-08-2003).]

John Stevenson
12-08-2003, 04:54 AM
I think I remember seeing a post of yours where you did some screwcutting using the taper attachment?
If so try turning a tapered pice up from about 1.370 up to 1.450 and screwcut this 18 tpi.
Try this into the part concerned then measure the diameter where it stops, that's the diametr you need.

John S.

12-08-2003, 11:49 PM
John Stevenson - Thats too darn simple! Could you please try again? http://bbs.homeshopmachinist.net//smile.gif

Forrest Addy
12-09-2003, 01:52 AM
Maybe not such an oddball thread.

A standard N-07 bearing nut has 1 3/8 - 18 UNS threads. The handbook calls out minor diameter 1.315/1.321 for an N-07 bearing nut. 1.335 isn't that far off. Look at the crest truncation (flat). If it seems a bit excessive maybe an N-07 bearing nut thread is on the button.

A $3.48 bearing nut would give you an more convenient ring gage to fit to a trial thread.

12-09-2003, 10:03 AM
ID of nut is 1.335 Thread pitch is 18. The OD shaft thread is a 1 and 7/16 inch 18 UNEF thread according to Machinerys Handbook.

Thread "PITCH" x 2 1/18 = .0555 x 2 = .11111 +1.335 = 1.44611 (but less due to 75% or more thread depth bore on nut.

1.4375 is 1 7/16 but the OD speced for this thread in machinerys handbook is 1.436 / 1.427. Make this OD, thread 18 tpi, use nut as your gauge.

PLEASE make sure this is 18 TPI, could also be a 1.5 metric thread, as this is VERY close to the 18 TPI reading.

The 1.335 ID makes this strange, because the bore should actually be 1.377 according to my calculations, thus my big question - is this metric. Double check this....

Forrest Addy
12-09-2003, 12:24 PM
Maybe not such an oddball thread.

(inserting a more complete discussion)

The handy rule for calculating minor diameter for a tap drill is nominal size minus one thread pitch. This rule produces results acceptable to Handbook H-28 and its NIST successor and its ISO counterpart.

In this case we have the minor diameter so we need to add one thread pitch to get the probable nominal diameter.

In our example, one thread pitch equals 1/18 or 0.0556". 1.335" + 0.0556" = 1.3906". The nearest English nominal size to 1.3906 is 1 3/8" with only 0.0056 error - well within probability.

Working the problem in Metric units: Converting into metric 18 TPI = 1.411 mm. 1.4 is a preferred metric pitch. 1.411 minus 1.4 equals 0.011 mm error per pitch. 0.011 mm = 0.011 mm = 00043".

This is an almost imperceptable error over three or four pitches on a thread gage but detectable over eight or more pitches when the error would produce a significant "rock" in an 18 TPI thread pitch gage.

1.335" measured minor diameter = 33.91 mm. 33.91 + 1.4 = 35.31 mm which rounds to 35 mm. 35 mm is a preferred metric thread diameter which = 1.378".

A 1 3/8 -18 UNS screw thread is not quite interchangeable with a 35 x 1.4 M.

- 1 /38 - 18 UNS male thread will fit in a 35 x 1.4 M thread with a looser than preferred fit - 0.005 clearance when at basic pitch diameter. The loosness will be irritating to the picky but have little effect on thread strength or function.

- A 35 x 1/4 M male thread will balk in 1 /38 - 18 UNS internal thread unless both male and female threads are at the lower limit of their respective minimum material condition. (Balk in this case means partly engage but not start.)

35.31 - 35 mm = 0.31 mm = 0.012" error. Working backwards: 35 mm - 1.4 mm = 33.6 mm. 33.6 mm = 1.322". On the low side of the minor diameter allowances according to the thread formulae.

A standard N-07 bearing nut has 1 3/8 - 18 UNS threads. A $3.48 bearing nut would give you a convenient ring gage to fit to a trial thread suited for both English and Metric threads provided the engagement is short.

[This message has been edited by Forrest Addy (edited 12-09-2003).]

John Stevenson
12-09-2003, 12:54 PM
<font face="Verdana, Arial" size="2">Originally posted by Forrest Addy:
Maybe not such an oddball thread.

(Working the problem in Metric units: Converting into metric 18 TPI = 1.411 mm. 1.4 is a preferred metric pitch. </font>

1.4 Preferred pitch?
Writing from the land of the endless metric change I have yet to stub my toe against one of these animals.
Plenty of large fine threads here but they still stick to set sizes. 1.0, 1.25, 1.5, 1.75, 2.0 etc , etc.
Below 1.0 anything goes like 0.8, 0.85, 0.9 etc but after 1.0 they do stick rigidly to the 'metric quarters'.

John S.

12-09-2003, 12:58 PM
As the standing joke in the computer business goes, "That's the nice thing about Standards, there are so many of them."

Forrest Addy
12-09-2003, 04:52 PM
1.4 mm pitch isn't ...

D'oh! (slap!)

OK! OK! It's definitely 1 3/8 18 UNS. Not Metric, no how.

12-10-2003, 02:41 AM
I have to agree with Forrest - D'Oh!

Sounds like Altec Lansing up to their old horn driver tricks...

12-13-2003, 02:11 PM
Thanx again everyone for your time and replies. This is a very old gizmo and has sentimental value to my friend. Thanx Audrey

12-13-2003, 04:30 PM
Many horn drivers use a thread to assemble to the horn. I have some old WE units, but they are not 18 tpi. Most Altec drivers I've seen use a 2 or 3 bolt flange. University sound on a lot of PA type gear did use a screw thread, but again, I don't have any samples to check. I'll try to do that when I next see one.

12-13-2003, 08:15 PM
"Beach Master" by chance?

12-16-2003, 12:34 AM
Hey Audrey, careful with that "old Gizmo" crap! I resemble that remark...