****
Let’s talk about fasteners because they are big part of machine design.
If you are new to drafting and machine design, you need to become very familiar with screws. You will use them all the time and it will pay off to understand some things about them.
You wouldn’t think something as simple as a screw could have so much information to remember about it.
This is the first in a series on screws in machine design. I am going to get you started in your career with some basic knowledge that will hopefully keep your customers happy with how easy your machines go together and how long they last.
To start, just know that not all screws are alike. … I know you are thinking, of course!
Flat Head Cap Screws (FHCS)
Are different from Socket Head Cap Screws (SHCS).
And wood and drywall screws are different from
Metal screws.
Yes, yes, yes. But I am not talking about that today. I am talking about THREADS.
For instance, 3/8-16 is not the same as a 3/8-24.
The first one has 16 threads per inch. That means if you lay the screw down on a ruler and count the ridges (threads) inside of an inch, you will count 16 ridges. A 3/8-16 means the outside diameter of the thread is 3/8 inch and the number of threads in an inch are 16.
This is what the website I found called “efunda” says about screw threads. (I will give you the link to that educational website later).
“Coarse Thread Series, UNC/UNRC: The coarse thread series UNC/UNRC is the most commonly used thread system used in the majority of screws, bolts, and nuts. It is used for producing threads in low strength materials such as cast iron, mild steel, and softer copper alloys, aluminum etc. The coarse thread is also used for rapid assembly or disassembly.
Fine Thread Series, UNF/UNRF: This is used for applications that require a higher tensile strength than the coarse thread series and where a thin wall is required.
Extra-Fine Thread Series, UNEF/UNREF: This is used when the length of engagement is smaller than the fine-thread series. It is also applicable in all applications where the fine thread can be used.”
At Rentapen, with the size of machines that we usually design, we ALWAYS …. Well, almost always… use coarse thread.
When is fine threads called for?
Purchased Components!
What? You know, the clamps, the flat feet, the stuff in your purchased products CAD library. SOME of those products include screws or screw holes with fine threads. So as a CAD drafter, you have to pay attention! Because a coarse screw won’t work in a fine thread hole and a fine thread screw won’t work in a coarse thread hole.
Not ALL purchased components have fine thread. Most of them that we use, don’t have fine threads. Our own product line, RAPid Tooling Components ™, (blocks and plates used in weld fixtures) use coarse threads. So it is easy to assume the threads will always be coarse. But BE AWARE!
Some purchased products, like the nose of SOME straight-line action clamps or a #10 size screw in a Jergens Flat Foot have fine threads.
SO watch it.
Fine Adjustments!
A fine threaded screw and nut or even extra fine can be used for fine adjustments of locations. We usually don’t do that at Rentapen. We use thin metal shims for our fine adjustments, but your company may be different.
Strength!
The thread grooves aren’t as deep on a fine threaded screw and that makes the shaft thicker and stronger. How much stronger? I found a great PDF that kind of explains the strength of screws and how much stronger a fine threaded screw is over a coarse threaded screw. It’s kind of interesting if you want to peek at it. http://www.kimballmidwest.com/catalog/MarketingText/Actual%20Cap%20Screw%20Strengths.pdf
As a beginning CAD DRAFTER, when you are starting out, you don’t have to stress over strength because you are working under an engineer who will guide you. As you grow in your profession, you will want to know where to find this information.
Less Material Engagement!
Sometimes if you are taping into some sheet metal or thin plate a fine thread or extra fine will provide more threads touching the part than a coarse thread.
For the screw sizes that you normally use (and that depends on the size of machines you design), memorize those coarse thread numbers so when a fine thread shows up in the drawing or specification-sheet of a purchased component, you can recognize it right away.
You don’t want to use a standard coarse socket set screw in the end of a clamp that has a fine thread in it. Your customer will get upset.
The Efunda site is an educational site for learning engineers. It says that fine threads are used in tubing.
“Fine Threads for Thin Wall Tubing in the 27 thread series are used for thin wall tubing in the ¼ to 1 inch nominal size. The minimum recommended length of thread is 1/3 of the nominal diameter + 5 threads (+ 0.185 inch). These are included in the Unified Standard Series.”
I told you earlier I would give you the efunda website address, so here it is:
http://www.efunda.com/DesignStandards/screws/screws_intro.cfm
This information is in inches so far. Many of you will find yourself working in metric and using metric screws. Even when you design in inches, you will sometimes use purchased products with metric screws. Metric screws are called off differently, M2 x .4 for example.
A fun history of this great tool, the screw, can be found at:
http://www.mrmetric.com/articles/A-Brief-History-of-the-Screw/
Other links for great information and references for screws.
http://www.dbroberts.com/dbroberts_fastener_guide.pdf
http://www.americanfastener.com/fasteners/sockets.asp
Enjoy this VIDEO on how Screws are made.
Next time we will chat more about screws in Machine Design.
Until then, have fun exploring the sites listed above.
The Queen
