View Full Version : Odd thread size

09-28-2010, 01:31 PM
I have a mounting stud that says it's threaded as ".938 – 16 UN". Is this a 15/16-16? If not, where can I get this tap? I can't find it at any of my normal sources.

09-28-2010, 01:42 PM
34.00 here : http://www.victornet.com/subdepartments/Special-Pitch-Taps-17/32-to-1-1/8-inch/1270.html

09-28-2010, 02:00 PM
I'm sorry, I wasn't clear. I can find a 15/16-16 tap, but I don't see any taps designated as ".938 – 16 UN"

09-28-2010, 02:08 PM
15/16 = 0.9375 rounded to 3 digits = 0.938 ????

09-28-2010, 02:22 PM
.0005 will not make any difference at that size of Tap.....
1 half of one thousandth of an inch....

The Fixer
09-28-2010, 03:12 PM
Most (all maybe) SAE threads and bolt sizes come in "nominal" sizes which means .9375 requires a 15/16" ..... If it's a critical fastener then you may want to get into fit classifications which is likely a discussion for another thread topic...


Forrest Addy
09-28-2010, 03:13 PM
Decimal equivalent chart and a pair of specticals anyone?

Woodnerd, If you don't already have the decimal equivalents memorized to 16ths, now would be a good time to start. In the meantime a decimal equivalent and drill chart on the wall is a VERY good thing.

Ahem! Sermon: These days even cabinet makers are holding tolerances in the thousandths. "Slip fit;" "firm fit"? Too subjective. In these days of low cost measuring tools and well etablished need for quantifing manufactured part data, there's no hesitation in making parts to precision tolerances Preciion benefits the bottom line as the reject rate dwindles. This was widepread practice at the dawn of the 20th Century and is becoming SOP even for the home woodworker here at the start of the 21st.

I have dial indicators on magnetic bases to quantify how much I nudge my rip fence, a height gage to set the blade, and a digital protractor to set the angle. I cut tenons and tongues to .003 to 0.005 glue allowance from scratch in the time it takes another guy to make a series of trial cuts on scrap.

The Sage was right. Human enterprize un-guided by objective fact and quantified by precise numbers is doomed.

09-28-2010, 03:22 PM
Yes, I understand that 15/16 is almost .938. I own a calculator.

Let me clarify: Are "UN" threads different somehow? Do I need a special tap because they're "UN" threads?

The Fixer
09-28-2010, 03:26 PM
Another important aspect re UN (Unified National) threads is the 60 degree thread form. Whitworth etc are 55 degrees etc.

09-28-2010, 04:52 PM
The original question was:
I have a mounting stud that says it's threaded as ".938 – 16 UN". Is this a 15/16-16?

The answer is: Yes if the 15/16-16UN

Not revelant but just curious what uses a 15/16 stud.

09-28-2010, 05:05 PM
Yes if the 15/16-16UN
Sorry, I don't understand what you mean. Are you saying there's a tap out there that specifically says "15/16-16 UN" ?

Not revelant but just curious what uses a 15/16 stud.
Well, it's not really a stud, per se, but that's the best word I could come up with at the time. "Mounting collar" may be more like it, it's on the end of a Roton ballnut: http://www.roton.com/Mating_Components.aspx?family=7060076

09-28-2010, 06:54 PM
The UN stands for Unified National (That is a 60° V thread form). A commercial tap would normally be marked 15/16-16 UN

You don’t say where you are. It would help if you posted your location. Here in the US the UN thread form is common enough that it is sometimes not marked. Here are a couple of taps:
I am photographically challenged so the markings don’t show but the tap on the right is 1 1/16-16 H4

The tap on the left is a special oversized piloted tap that is also 1 1/16-16 +.010”

In both these cases the Unified National thread form is a given because of the application and therefore it is not so marked but normal commercial taps would contain that designation. Taps can be made in any size. The tap on the left could also be marked 1.073 – 16 UN

In your case the 15/16-16UN or the 0.938 –16 UN are the same thing.

Large taps are fairly expensive so when you need an internal thread that large most people single point them on the lathe.