Measuring Screws for Replacement

[kgg]

2 minutes ago, Constabulary said:

Posting an AliExpress link may help

Temu:

 https://www.temu.com/ca/1pc-nut--thread-checker-inch-metric-26-male-female-gauges-stainless-steel-14-inch-12-metric-thread-checker-g-601099524615655.html?_oak_mp_inf=EOfbjp2m1ogBGiA5ZWNhMmYwZDdjNzU0NDAzOTViNDEwNjIyZTM1MWViZCDflrjLwTM%3D&top_gallery_url=https%3A%2F%2Fimg.kwcdn.com%2Fproduct%2FFancyalgo%2FVirtualModelMatting%2F1bd1d6e014704d792beadf80a72c5b49.jpg&spec_gallery_id=2025093704&refer_page_sn=10032&refer_source=0&freesia_scene=2&_oak_freesia_scene=2&_oak_rec_ext_1=MTY4OQ&_oak_gallery_order=2093112306%2C160354140%2C2045537314%2C271545300%2C1416541352&search_key=nut and bolt thread checker&refer_page_el_sn=200049&refer_page_name=goods&refer_page_id=10032_1769953176822_anhrd4lta2&_x_sessn_id=3raljzhp3j

Aliexpress:

https://www.aliexpress.com/item/1005009944813944.html?algo_pvid=598db5f1-c508-430c-ba85-99afda6fd421&algo_exp_id=598db5f1-c508-430c-ba85-99afda6fd421-6&pdp_ext_f={"order"%3A"7"%2C"eval"%3A"1"%2C"fromPage"%3A"search"}&pdp_npi=6%40dis!CAD!28.44!22.09!!!142.79!110.89!%402101c44f17699531248835014ee51d!12000050647125799!sea!CA!0!ABX!1!0!n_tag%3A-29910%3Bd%3A18b3922%3Bm03_new_user%3A-29895%3BpisId%3A5000000197831932&curPageLogUid=ItEaF9KDGd9H&utparam-url=scene%3Asearch|query_from%3A|x_object_id%3A1005009944813944|_p_origin_prod%3A

kgg

 

[Constabulary]

thanks, did not know what to look for 😉

[kgg]

1 hour ago, Constabulary said:

thanks, did not know what to look for

No problem. I've had one for about 5 years.

kgg

[friquant]

14 hours ago, 1hp said:

the "Machinery's Handbook" has enough info on threads to make your head explode.

Excellent reference, thank you. I suspected machinists would know things about this. Here is a link to a free copy of the book: 

https://www.tfgusa.com/wp-content/uploads/2023/11/Machinerys-Handbook.pdf

Starting on page 1802 of the text, "Threads and Threading" introduces different thread systems. (There have been a few.)

Page 1816 has a table of typical combinations in the american standard unified system. I think some slightly amended version of this system is still in common use today. It uses numbered sizes for anything smaller than 1/4". These numbered sizes mostly do NOT line up with 1/64" increments.

Let's cross-referencing page 1816 with page 3 for details

1/16 is a little bigger than size 0
5/64 is in between sizes 1 and 2
3/32 is between size 2 and 3
7/65 is almost as big as size 4
1/8 matches size 5 exactly!!
9/64 is barely bigger than size 6
5/32 is in between sizes 6 and 8
11/64 is between sizes 8 and 10
3/16 is almost exactly size 10
13/64 is between sizes 10 and 12
7/32 is almost exactly size 12
15/64 is between size 12 and 1/4"

Major Diameter of External Threads

My main curiosity is how to interpret the measured major diameter and choose a nominal diameter from which to place an order. Let's look at an example from the book.

Page 1818

The 1/4-28 UNF external (meaning screw thread, not nut thread) has a nominal diameter of 0.25"
The least precision version (1A) has allowable major diameter 0.249" - 0.2392"
This means the major diameter for the 1A series must be between 95.5% and 99.5% of the nominal diameter.

From this I deduce that when I measure a major diameter to be X, I can look for a matching nominal diameter that is 1-4% bigger than X.

I don't think I will always know which thread system the screws I'm searching for are from, either by my own ignorance or unavailability of data. But in general it makes sense that for a non-interference fit the screw major diameter has to be smaller than the accompanying nut major diameter. So I'll roll with the bolded deduction above until I find a more informed pathway.