Determining screw/thread size

[NoodleKnight]

Is it possible to do without a thread gauge?
Like, could I use calipers to figure out a certain dimension then plug it into a formula to get the answer i'm looking for (like for determining gear pitch).

[Tristan Lall]

American (unified national) threads are given as [Bolt Size]-[Thread Pitch] [Thread Series], so for a given screw (e.g. #10-32 UNC), you can look up the dimensions and measure with calipers. Pitch is in threads per inch.

Metric threads are given as M[Bolt Size] for coarse (standard) bolts, or M[Bolt Size]×[Thread Pitch] for fine or irregular bolts. Dimensions are here. Pitch is in mm per thread.

[sanddrag]

There actually is a formula for determining pitch diameter of macine screws, although that doesn't do a person a whole lot of good (and I don't remember what it is). But yeah, just look up OD on a chart and measure with calipers.

[NoodleKnight]

Tristan,

When you say bolt size, would that be the bolt's diameter?

Thanks for your help, guys.

[sanddrag]

It would be the bolt's number or major diameter. Bolts below 1/4" are measured by numbers (up to the number 12). Each number corresponds to a diameter (by reference chart, probably created by ANSI). Bolts 1/4" and over are usually specified by fractional (like 7/16"), whole (1"), or mixed (1 1/2") number. It should be noted that there is such thing as a #0 screw, a #00 screw, and I believe even a #000 screw (not positive on that last one though).

Also, a thread designation is typically followed by thread class. There are six thread classes 1A, 2A, 3A which apply to external threads and 1B, 2B, 3B which apply to internal threads. 1 is a loose fitting thread, for ease of assembly or use in a dirty environment. 2 is the most common thread fit. A slight amount of dirt will not jam it up but too much will. 3 is a close tolerance thread fit meant for precision and clean environments.

[kramarczyk]

Yeah, #000 exists with 120 TPI.

Looking them up on www.McMaster-Carr.com is a fast way to find the basic dimensions. For more details consider picking up a copy of the Machinery's Handbook, now in it's 27th edition. I recommend the large type version. It's got more information than you'll ever need on mechanical stuff if you take the time to look through it. Although, at $60 from amazon it may be better off as a team expense.

[DonRotolo]

Screws come even smaller, #0000 - 160 at Small Parts.They're about $4 each, and made by elves.

So, to elaborate on Tristan's explanation: First measure the bolt's outside diameter, then measure the distance between thread ridges (I usually measure 10 and divide, better accuracy), then measure the length from the bottom of the head to the end.

The diameter won't be exact, but pretty close to the bolt "dimension" - for example a 1/2" bolt might measure just a bit under 0.500. Then threads per inch, then length. Specify the head style and material, and there's little else to describe it by. (Actually, you can classify it further by strength and thread finish. But that's all there is, I think)

After a while, you can tell by looking at it. If you have an extra $10, go to the hardware store and ask to buy one of each size bolt (pick a common length, like 1 inch), as you keep them around you'll get used to the sizes. Then you can sort all those bolts in that old coffee can....

Don

[sanddrag]

Today I was quite impressed to find that one of my local hardware stores carried #0, #1, and even #3 fasteners (in stainless). Usually they'll go #2, #4, etc.
Only one store (OSH) carries #5 though, and the longest they go is 3/4". I've been having a heck of a time finding a #5-40x1" locally. I'll probably just have to order it.

#5 really should be carried by more stores because its major diameter is about .125, perfect to go in a 1/8" bearing.

[Dick Linn]

I saw some company that sold small screws and taps. I recall that one of the 0000 precision taps with 160 threads per inch was around $75. Ouch! Of course, another place had the same size tap for $3.50.

[Qbranch]

Why all the small screws???

Smallest i like to use on machines is 5-40 if i need to push it, 6-32 is as small as i like to go.

On the machinery's handbook note, its a definite must for any machinist or mechanical designer. I use it tons everyday at work. Has everything from horsepower calculations to roller chain sprocket profile construction to threading.... even heat treating and such. The copy i use is old, from the early 80's, its the 26th edition. I don't know how much they can change about it though, has most everything you'd want to know.

-Q

[sciguy125]

Why not get a bunch of nuts of known size and find the one that fits?

[Daniel_LaFleur]

Originally Posted by Qbranch

Why all the small screws???

Smallest i like to use on machines is 5-40 if i need to push it, 6-32 is as small as i like to go.

On the machinery's handbook note, its a definite must for any machinist or mechanical designer. I use it tons everyday at work. Has everything from horsepower calculations to roller chain sprocket profile construction to threading.... even heat treating and such. The copy i use is old, from the early 80's, its the 26th edition. I don't know how much they can change about it though, has most everything you'd want to know.

-Q

Size your screws for their purpose.

If all you are doing is holding down a small piece of sheet matal that is not expected to see any sheer force then using a 2-56 screw is fine, and may even be preferrable considering weight and size requirements.

[DonRotolo]

Originally Posted by Qbranch

Why all the small screws???

Merely because a #0000-160 screw is just so COOL. Other than that, they are a pain, expensive, weak (and as you get older, hard to see...)

Don