Tap Size Question?

[MacLeod5]

Hi folks - total newbie - forgive me. With standard / imperial system, anyone know why small taps don't use fractional measurements (e.g. 10-32, 8-32, etc.), and then at 1/4-20, it switches to fractional, which is much more logical to me? I am setting up my shop and organizing taps, dies, drills, reamers, etc. and it's a bit of a mystery to me. Thanks, looking forward to engaging in this awesome hobby!

 

[NC Rick]

#10 or maybe #12 are the largest taps and dies you are likely to need to have on hand In common number sized fasteners. #4-40 is about the smallest non specialist size. I do keep 2-56 (#2 x 56tpi.) but don't remember the last time I used it. Having National fine and National Corse in #6,8 and 10 will prove useful in my opinion. Adding a #4-40 (#4 with 40 threads per inch) will do you well. I like to keep all those sizes in a separate little box. A tap and drill chart is good to have at hand. Things like #5 are often used in gunsmithing. Other special sizes are best purchased when and if a need arises.

while I have never had a need for this information, I just looked it up... 0.060 + (0.013 × n). Where "n"=the screw # designation will give you the inch diameter of the screw. Why this came about, I don't recall but it's screwy...

 

[JRaut]

Welcome aboard, @MacLeod5!



*** COMPLETE SPECULATION ALERT ***

I suspect the numbering system originally came about to roughly match the numbering system for nails (i.e. 4d = 4 penny, 8d = 8 penny, etc.)

 

[macardoso]

The numbered taps (e.g. #4-40 or #10-32) are "machine screw" sizes. Tried to find some history on where the divide occurred but wasn't able to come up with much. After #10 all the screw sizes become fractional nominal ODs. There are also 3/16" and 1/8" fractional taps as well as #12 and #14 screw taps, but those are uncommon. I'm currently working on a steam engine that has some #5 screw threads and 3/16-32 fractional threads. Pretty annoying to find taps for!

Here is some literature I found

Generally speaking, Unified National screws are available in Number sizes from 0 to 12. The Number screw size system is based on a major diameter of .0600 inches being size 0. Each size larger than #0 adds .0130 inches to the major diameter (i.e. a #10 screw has a major diameter = 10 * .0130 + .0600 = .1900 inches). Number sizes 7, 9, and 11 were removed from use in the 1920’s leaving sizes #0, #1, #2, #3, #4, #5, #6, #8, #10, and #12 in common service. The #14 size screw is not commonly available. #0 screws are only available in the UNF pitch. Sizes #1 through #10 screws are available in UNC and UNF pitches. Size #12 screws are available in UNC, UNF, and UNEF pitches. Each size smaller than #0 subtracts .0130 inches from the major diameter (i.e. a #00 screw has a major diameter = .0600 – 1 * .0130 = .0470; a #000 screw has a major diameter = .0600 – 2 * .0130 = .0340 inches; while a #0000 screw has a major diameter = .0600 – 3 * .0130 = .0210 inches). Number sizes smaller than #0 have mostly disappeared in favor of (metric) Unified Miniature Threads.

Fractional screw sizes are generally limited to ¼ through 5 /8 (by 1 /16 ths), ¾ through 1-¼ (by 1 /8 ths), and 1-½ through 2 (by ¼ths). They are commonly available in UNC and UNF pitches. UNEF pitches are available, but not commonly stocked. Standards for screws up to 6 inches major diameter exist, but are rarely readily available.

https://littlemachineshop.com/images/gallery/instructions/A_Brief_History_of_Screwthreads.pdf

BTW, Welcome to HM, we are glad to have you here!

 

[macardoso]

For a home shop, I like to have the following:

• 115 pc. Imperial drill index
• UNC and UNF taps from #4 thru 3/8"
• Metric taps, M3 thru M10, coarse and fine (only if you do metric work)
• Pipe taps 1/8" NPT thru 1/2" NPT
• Fractional reamers, 1/8" thru 1/2" by 1/16"
• Over/Under reamers, 1/8" thru 1/2" by 1/16"

Don't care much for dies. Single point on the lathe. Too easy to get them crooked.

 

[Asm109]

I have been doin mechanical engineering since 1981.
Starting with #10 screws I use coarse and fine threads on a regular basis. It depends on the application.
In sizes below that I have needed a fine thread in exceedingly rare circumstances. One being the use of ball plungers. Just recently I needed a #4 size ball plunger and they were only available in 4-48. I understand why, the fine thread leaves more room inside for spring and wall thickness at the root of the thread.

# 5 is a major diameter of 1/8 inch. Makes it nice when you need male threads to run a tap down a piece of stock dimension instead of trying to turn a spindly piece of stock to an odd diameter. I've seen that size on model engines and such but never encountered it in a professional setting.

 

[Larry$]

Macardoso, A perfect example of why we should be on the metric system! To find the tap drill size for a metric fastener, subtract the thread pitch from the nominal screw diameter. Quick, simple, works with fine or course threads. 6mm screw with a pitch of 1mm = 5 mm tap drill. Only one measuring system used no matter how small or large the screw. Watch out for some Japanese screws. Some use non-standard pitches.

 

[Tozguy]

Macleod, welcome aboard,
I have wondered the same thing for years, now I have the answer, thanks for asking the question.

 

[Asm109]

Larry, the same - pitch = tap drill works for Imperial screws too. It is a function of the 60 degree angle of the threads.
You just have to invert the TPI

 

[MrWhoopee]

Hi folks - total newbie - forgive me. With standard / imperial system, anyone know why small taps don't use fractional measurements (e.g. 10-32, 8-32, etc.), and then at 1/4-20, it switches to fractional, which is much more logical to me? I am setting up my shop and organizing taps, dies, drills, reamers, etc. and it's a bit of a mystery to me. Thanks, looking forward to engaging in this awesome hobby!

You are searching for logic in a completely illogical system. Stop it, you'll hurt your brain.