Quote:
Originally Posted by jee7s
As a general rule of thumb, you need three threads in the material, with at least one exposed thread beyond that. That means that for 1/8" thick material, you need a pitch of at least 24 to properly fasten. That's a lower limit on TPI, and you'd probably want to be higher than that.
I've never used a nut in a piece 1/4" thick or more on a robot. At that thickness, everything gets tapped. At less than that, it depends on the required strength. If the joint is in compression, tapping the material is usually sufficiently strong. Same goes for low load holes, like control system mounts, etc.
If you need extra thread strength in thin material, look into Pem Fasteners. They are pre-threaded insert fasteners, sometimes called "captive nuts" on web sites like McMaster. There's a whole variety of types for different applications. Most of them are designed to be clinched (read pressed) into sheet metal.
As for making the threads, if you plan during design and make the appropriate holes a tap hole for the appropriate screw, then it's actually pretty simple to fabricate. Get handy with a Gun Tap (aka spiral tap) and a properly clutched drill and you can churn out 20 holes a minute. Just make sure you properly lubricate your tap, especially if you are tapping aluminum. Personally, I'd recommend hand tapping any steel.
Speaking for my team, our entire chassis design doesn't have a single nut on it. It's all Pem fasteners (Pem nuts to be exact) and tapped sheet metal. If you watch your tolerances, pay attention to the difference between tap and clearance, use loctite (a drop of 242 now prevents a lot of loosening later) and account for some special considerations on the Pem fasteners (you'll find that in the datasheet)...it's a pretty smooth design with fewer maintenance operations than a nut-bolt paradigm.
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I really love tapping (see my signature)! That said, I have heard some different rules about it. First, I've heard 6 threads, which would make 24 tpi the largest recommendable in quarter inch material. Also, tapping on a CNC mill can work, but can result in a tap getting broken... I would certainly recommend hand tapping for most things rather than going a marginally faster but more risky route.
In regards to PEM nuts, I would be more cautionary. First, some require a special tool to install, so most teams would need to make an investment to get into the system. Additionally, they can get "spun" if installed incorrectly or too much torque is applied. Like all fasteners, they are great for some things, but have their drawbacks. I still prefer rivets for frames and primarily tapping for manipulators.