Screw Terminology

[Shim]

Hi,
I am new to cnc and trying to build a cnc from plans.

Could someone help shed some light on the following screw terminology from the plans I am working on.

Terminilogy use when refering to BOLTs, TAPs, NUTs, THREADED ROD etc..
10-24, 6-32, 1/4-20
In 10-24, is 10 refering to M10 diameter? What is 24 refering to?

In 1/4-20, is 1/4 refering to 1/4" diameter? What is 20 refering to?

Unfortunately the guys at my local hardware store doesn't seem to know anything except tell me metric in what I should be using.

[Al_The_Man]

This is a US standard and refers to a #10 dia (not metric) and 24tpi.
#6-32 likewise. the last digit is threads/inch.
I find it much simpler to work in metric.
Al.

[JimBoyce]

10-24 (0.190 inch), Use a M5 (0.196 inch)
6-32 (0.138 inch) try a M3 (0.118) or a M4 (0.156)
1/4-20 (0.250), try a M6 (0.236)

[juergenwt]

Shim- If your print calls for a 10-24 or a 6-32 scew than that is what you will have to use. You can not substitute a metric screw. Get yourself a Machinist Handbook or at least look up on google for UNF and UNC threads. Any machinist or anybody selling screws or taps can supply you with a handy chart for the most common US threads. Be prepared - it is much more complicated than metric.

[Glacern]

The first number always refers to the screw diameter, while the second number describes the thread pitch.

Imperial sized screws below 1/4" diameter use a non-intuitive numbering system, with common screws in sizes 2, 4, 6, 8, 10, and 12 (smallest to largest). There are some less common sizes in between, but you typically won't run into those. With metric screws, the first number will always be preceded by an "M". M10 is a 10mm diameter screw.

The second number for imperial threads describes the thread pitch in threads per inch. A 10-24 screw has 24 threads per inch. For metric screws, the thread pitch is described as the distance between threads. Each thread on an M10-1.5 screw is spaced out 1.5mm.

If a third number is used, it describes the length of the screw under the head (in inches for imperial screws and mm for metric screws).

"UNC" stands for Unified National Coarse. Typical UNC screws have "coarse" pitches such as 1/2-13 or 10-24. "UNF" stands for Unified National Fine, with "fine" pitches such as 1/2-20 or 10-32. There's also a "UNEF" Unified National Extra Fine.

Metric screws don't have designations like UNC/UNF, but they do have standard coarse and fine pitches as well, e.g. M10-1.5 coarse pitch and M10-1.25 fine pitch.

-Sol
Glacern Machine Tools

[juergenwt]

Glacern - Good post!
One additional point to be made for metric screw threads.
For the coarsest thread available in any size NO pitch is given i.e. M10. Only time a pitch is given is for FINE Metric Thread i.e. M10x1.25 etc..(ISO/DIN 13)
To make things more complicated - ANSI allows a pitch designation for coarse thread. This seems to be OK at first glance, but in practice it proves to be the wrong decision.
Here is why: The idea was to keep the number of threads to a minimum. The standard coarse thread was to be the one to be used when ever possible. When you had a print showing an M10 thread designation you knew it was the coarsest pitch available and therefor the one size you would be able to get in any store or keep in stock.
Fine thread was for special needs only and as such designated with the pitch.
Now comes ANSI and says it's OK to use the pitch on coarse thread. So now when you look at a print you could see M10x1.5 and if you are like most people in the US and not really familiar with metric threads you will start to wonder if there is a coarser pitch available for M10 (like M10x1.75). If you are a person familiar with metric threads you could than mistake a M10x1.5 designation as a Metric Fine Thread.
If you get a print from somewhere in the world and it shows just the nominal size i.e. M10 - don't call to ask about the pitch.
In most of the rest of the world 95% of all threads are standard coarse and specified as M6. M8, M10, M12 etc.. As I said: Metric Fine Thread is for special uses only.
They just had to screw it up.

[hex173]

metric fine is used for pneumatic and hydraulic pipe couplings/connections, and as said for special occasions.

[garagefela]

I love metric. Tapping drill sizes are so easy to work. Just take away the thread pitch from the size of thread.

Eg M10 x 1.5. Tap drill is 8.5 (10 - 1.5)


Simple