Thread Triangles for External Thread Measuring - Question

[bcall2043]

I have been gathering materials and tools for an upcoming project - making a 2 1/4 OD x 6 thread/inch plug gauge so that I can make a chuck adapter plate for an old 14 1/2 inch South Bend lathe. I read that the triangles are easier to use but the wires are more accurate. I found a set of thread triangles on Craigslist at a fair price so I got them. The seller had a set of wires also so I got them too.

The triangles came with a chart for the common threads from #0 through 1 1/2 inch nominal sizes along with the formula for use to find the measurements for other or special threads. The chart or container does not have a manufacturers name on them so I suspect they are import. The included chart says in the heading “Simmons Equivalents as measured over triangles". A search on the name "Simmons" found the following patent: http://www.google.com/patents/US2466490 this is interesting but not likely the manufacturer.

I have made a PDF file of the written information included with the tread triangles and can provide that if anyone is interested. However in my searches I read that included charts for thread triangle vary with manufacturer. Not sure if this is true or not.

For my use the thread triangles being import does not bother me but what does raise a question is in the formula instructions a class 4 fit (highlighted in picture below) is mentioned. I am not sure what that means. The included charts only mention classes 2, 2a, 3, and 3a.
View attachment 60574

Does anyone know what the “class 4 fit” means?

Benny
The Orphanage Never Closes

 

[DMS]

I've never used the triangles, but I have a set of wires. They are "fiddly" but other than that, not hard to use. I recommend a small piece of styrofoam to hold them (just push them in like pins). You may also want to make sure your chip pan is clean, as they are a bugger to find in a pan filled with swarf.

As far as a class 4 thread, unified national threads only specify 3 classes (1,2,3) for internal and external threads (A,B). The class 4 may refer to a metric thread standard, though the Machinists Handbook refers to this as "tolerance grade". Tolerance grades range from 3 to 9 it seems.

Not sure how that helps you. There is a description of the metric thread tolerances on p1797 of the 28th ed, but it was not immediately clear to me after a brief glance.

 

[Tony Wells]

Thread classes are immaterial for your project, but the general idea is to control the clearance between mating threaded parts. Backlash, if you will. And the fit classes run up to 5, if my memory doesn't fail me. It has nothing to do with Metric threads, which have there own nomenclature for fit. The most common, by far, is class 2. Class 3 fits with less clearance (tighter), 4 tighter still, and 5, which is actually an interference fit. It is not a free-running thread. You will see these in places where double ended studs are used and the intent is for the stud to stay in one component while the nut is removed from the other end, typically a class 2 thread. One example would be on automotive engines, the exhaust manifolds are sometimes held on with studs, and the end of the stud in the head will be class 5. When you remove the nut, the stud remains with the head. Think of them as self locking.

As far as triangles go, they have their uses, but IMO, as a comparator only. The root radius of the pattern thread must not interfere with the contact of the triangle on the thread flanks. That is where wires are probably a better choice. They are designed to measure the pitch diameter alone. Other features of the thread are inspected with other instruments.

 

[bcall2043]

REPOST of LOST REPLY: Thread Triangles for External Thread Measuring - Question

If the SB is on hand, just use your triangles (or wires) as comparators, making your gauge as accurately as you can dead nuts to the spindle. Limits and fits shouldn't be necessary.

Jono,
I plan to do as you suggest since the SB is handy.

Thread classes are immaterial for your project, but the general idea is to control the clearance between mating threaded parts. ………….. The most common, by far, is class 2. Class 3 fits with less clearance (tighter), 4 tighter still, and 5, which is actually an interference fit. ……….

Tony,
Thanks for your information about the fits. That makes sense to me. I am still learning this hobby and understand the question is immaterial to my project. I just saw the thread class note on the supplied chart and wanted to learn what it meant.

Additional information discovered since my last post went missing:
Tony’s comment got me to thinking that since someone in the industry is having to test his memory and that the chart was also quoting the original patent holders name that maybe the thread information chart was “old school” and not common usage any longer so I went digging. I looked in an old (about 1930) copy of Kent’s Mechanical Engineer’s Handbook where I learned that the 1918 Congress appointed a commission to establish standards for screw threads to be used by plants under the control of the army, navy, and for adoption by the public. The Kent’s text included typical application data for each of the thread classes in understandable language. Below I have included a photo of a comment about the thread classes and a photo of the data table about the “close-fit or Class 4” thread.



Noticing that the standards established also applied to gages I went back to my copy of Machinery’s Handbook (17[SUP]th[/SUP] Edition) and looked under “Thread Gages” and there was the classes mentioned. However it did not provide the simple explanation of their applications like the old Kent’s book did.

Thanks for the reply's. Sometimes asking and getting reply's helps one to dig in another direction to get answers.

Benny
The Orphanage Never Closes

 

[OldMachinist]

I have made a PDF file of the written information included with the tread triangles and can provide that if anyone is interested. However in my searches I read that included charts for thread triangle vary with manufacturer. Not sure if this is true or not.


Benny
The Orphanage Never Closes

Benny,
I'd be interested in a copy of the chart you have. A mouse got in my toolbox and chewed up the chart that goes to my thread triangles.
Don

 

[bcall2043]

Benny,
I'd be interested in a copy of the chart you have. A mouse got in my toolbox and chewed up the chart that goes to my thread triangles.
Don
View attachment 69150

Not a problem. I have attached the requested file plus a copy of the US Patent for a bonus.

Benny
The Orphanage Never Closes

 

[OldMachinist]

Thank You Benny

The numbers on your chart match exactly with what's left of my chewed up one.
Don

 

[JohnJ50]

I needed a copy too. Thanks for making it available.