Visual Difference Among Various Types of Screw Threads

Experienced eyeballs will give you a start...

ie ..from past experience that thread looks like 1/4" UNF

Followed by a Zeus book ,micrometer and thread gauges.

Experience gives the power to the eyes to recognize the correct threads but it is always better to depend on handy tools like thread gauge, caliper etc. Few threads which coincidently have equivalent pitches and poorly machined, can confuse any body at that time matching nut shall be checked for getting the correct size.

I don't have an experienced eye.

One idea would be to invest the small amount of money on a collection of nuts that are labeled according to thread type and size. The nut that your unknown screw fits will give you your answer........every time.

You could even mount them on a board so you don't loose them.

kramarat -- I've encountered at least one metric thread (don't remember which) that was close enough to an American UNF that it almost went through the UNF nut (5/16-24?).

Go-no go gauges are pretty pricey unless you are doing production quantity paying jobs you can bury the cost into.

Were I to take the starving mechanic's approach I think I'd opt for more thread length in the nut, maybe at least two thicknesses. A good hardware store might have those extra long nuts uses to join two threaded rods in a reasonable spread of US threads. Maybe buy a new set of metric taps and before using them for anything else drill and tap some sections of hex bar to make your own gauges. Use 360 brass if you can find it and a tap drill size small enough to get 75-80% thread. And good tapping fluid. And something like your drill press or a hand tapping fixture that keeps the tap in line with the hole. Otherwise the first few threads might be oversized on the pitch diameter.

If you have enough magnification perhaps try looking for the thread root and peak variations as described in Machinery's Handbook. I admit I haven't done anything like that myself. If you're running a job shop keep an eye out for a good deal on a Shadowgraph optical comparator. I've seen them go for a few hundred bucks used. You can buy cheap Chinese binocular microscopes that are about right magnification to examine threads and a lot of other things life tool wear.

Ed Weldon

He wouldn't need a no-go gauge. Thread gauges are less than $30 from McMaster-Carr, and you get over night shipping. Plastic gauge is even cheaper, and a screw pitch gauge even cheaper than that.

And that's an M8 metric that roughly fits the thread of a 5/16-24.

I used to have a screw pitch gauge on my key chain...handy when I worked in a machine shop.

"Roughly" isn't good enough unless you actually like drilling and tapping stripped or broken screws.

I didn't recommend using a 5/16 screw in an M8 hole.

But the fact is, that it DOES roughly fit, and therefore can be mistaken if all you do is try to fit the bolt into the hole, or if you use a nut as a gauge, instead of using an actual thread pitch gauge.

So no, I don't actually like drilling and tapping stripped or broken screws.

Yeah, that's the way I do it, and it's usually at the bins at the hardware store. Doesn't come up very often. If it was something critical, or came up all of the time, I would use one of the more professional methods for sure. Depends on what he's doing. They actually make something that I was thinking of.

http://www.swmanufacturing.com/parts_category.asp?TYP_ID=38

Neat product, kramarat. Many thanks. I may actually buy one of the all metric versions if the price isn't too high. ........... Ed Weldon

I could have used one about 20 years ago. I still wouldn't mind having one around. My Dodge Caravan is frustrating as hell. It's got both metric and standard. The price isn't bad.

M12 x 1.25 vs 1/2 x 20 UNF (12.7 x 1.27) I'd warrant

And no way you'd pick that visually on thread only.

(Or by thread to thread 'daylight' on a pair of short bolts)

(me too caught, obviously)

A little more info for you on thread identification.
http://practicalmaintenance.net/?p=291
http://www.teamonslaught.fsnet.co.uk/threads.htm
ACME is completely different from your standard fastner.

For those interested in ACME threads and Plastic Tooling stories - we had a discussion on "ACME threads and Diamond Dust" at "Skippy and Buzz" -

http://skippyandbuzz.blogspot.com/2010/04/of-acme-threads-and-diamond-dust.html

Good Luck -

Jim

If you can't see the difference between an ACME thread and an ISO thread you must be blind! Even the feel of the thread gives it away.

It's got to be over 35 years since I last cut an ACME thread!

Cutting the nut was a swine of a job!

see my post above - those ACME threads were "designed for the ages" in some industries and still exist -

Jim

One way is look at the head of the bolt. The numbers on them will tell you if it's metric or SAE. On SAE hex bolts there will usually be lines which denote the grade of bolt. One with no lines lowest grade. Metric will have numbers instead of lines. Thread pitch is a different matter. Best to use a thread gauge. Calipers help to determine diameter. Acme the cut leaves the thread flat it does not come to a point.

Are thread gauges baned in your country, or do you want to prove others that you are far experienced about screws?

The rape of a washer woman is reported in the local paper.

"Nut screws washer and bolts."

jt.

"He had delusions of adequacy." - Walter Kerr

This may not be much help to the OP but if you deal with fasteners of different types and sizes on a regular basis you get to the point where you can ID the thread type and size of a fastener by just looking at it. What your brain processes is the relative proportions of diameter, pitch and bolt head size in the case hex head types. This is reinforced by having something of known physical size in the same visual field (even your fingers) to refer back to.

Also one develops a familiarity with the finishes and head shapes of fasteners and associates them with the type of applications in whaich they are used and the thread family they represent. The one important feature that is harder to ID without measuring is the fastener length since that comes in a large number of increments for each thread size and type.

For example if you have a mix of, 3", 3-1/2", 4" and 4-1/2" 1/4-20 cap screws it is hard to score an isolated individual until you place it in the visaul context of the group. Then your brain processes the visual proportions and compares them to the other pieces in the group.

Ed Weldon