Internal Threads

[jee7s]

Originally Posted by Nate Laverdure

Right. Key takeaway: thread strength is not additive. The first couple of threads take more than half of the bolt load, with each successive thread contributing a lesser and lesser amount.

That was the basis of my understanding that extra threads doesn't always mean extra strength as you would expect (e.g. doubling threads does not double strength).

[jee7s]

Originally Posted by bardd

Do you ever tap wood or plastic? Or is that the wrong way to go?
Also, what tools do you use to tap? I don't know any, I'm afraid to say...

There are PEM fasteners for plastic, but I've never used them. Google "PEM nut" and you'll find Penn Engineering's website. There's more info there. Tapping it directly is possible, but it's also weak. For example, there are nylon nuts and bolts available...but those are often for electronics applications. I have heard of them being used in FRC as a weak fastener that's designed to fail in shear, to protect more expensive parts of a manipulator.

If you need metal threads in wood, use a Tee Nut (yes, there's like 8 different meanings to the term). The type you'll want is something like McMaster 90975A053. The center of that nut is threaded for the bolt, and the prongs dig into the wood.

If you do use Tee Nuts in wood, make sure you press them in and then hand tighten a cap screw/washer combination into it to get the prongs to set properly. Otherwise, you risk spinning the nut and gauging the wood. Also, a judicious application of polyurethane glue (like Gorilla Glue) really does a lot to improve the confidence of the connection.

[JamesCH95]

Originally Posted by bardd

I love debates! You get more information than you signed up for.

Again thank you all for the comments.

Do you ever tap wood or plastic? Or is that the wrong way to go?
Also, what tools do you use to tap? I don't know any, I'm afraid to say...

Tapping plastic is fine for lighter-load applications (plastic is very weak compared to most aluminum and steel). Tapping wood is silly, just use a wood screw or a t-nut like this:



One uses a "tap" to cut internal threads, thus the nick-name "tapping". I like ratcheting tap handles to drive the tap, or a ratchet driver with a 12pt socket. Of course, a real tap-handle and guide is the best.

[artdutra04]

Originally Posted by bardd

Do you ever tap wood or plastic? Or is that the wrong way to go?
Also, what tools do you use to tap? I don't know any, I'm afraid to say...

You can tap into plastics, as long as you are using a course thread, have enough engagement, and understand the risk of cross threading and/or tearing out under high load.

Wood cannot be tapped on its own, unless you are using a monstrously huge pitch. It's why wood screws have a TPI in the single digits.

In either case, there are a variety of threaded inserts you can add to either plastic or wood to allow these materials to have threaded holes.

[Jon Stratis]

The proper tool for the proper job.

In some cases, we tap material for the bolts. Generally, this happens when we have thicker material or are using smaller bolts (with closer thread spacing).

In other cases, we use a PEM nut. You have to be careful with these, as they are designed to work with a specific material thickness. If your material is too thick, you won't get a very good grip with them, and they can then cause problems down the road.

In yet other cases, we'll use rivet nuts. These are great for working in the robot frame, where it's generally difficult to get a wrench in there to hold a nut.

Finally, we'll use Nylocks. We much prefer to use a locking nut than a normal one, given how much the robot moves and shakes... we don't want to have to tighten the bolts between every match!

We also use pop rivets. A lot. Generally speaking, major frame parts are bolted together, as are parts designed to be separated. When putting in gussets, shielding, and such we use pop rivets. You don't have to worry about them coming apart (unless used inappropriately), and yet it's pretty easy to take them out with a drill if you need to.

So in short, get a tap set, get some different types of fasteners, and experiment. Even if you decide one type of fastener isn't really useful for your team, everyone will have gained some great experience and knowledge!

[George C]

Originally Posted by artdutra04

Wood cannot be tapped on its own, unless you are using a monstrously huge pitch. It's why wood screws have a TPI in the single digits.

That's very debatable. Wood can certainly be tapped with number, fractional or metric taps. The success and strength depends on the wood. See below.


http://www.leevalley.com/en/hardware...06,41330,40057

http://www.leevalley.com/en/wood/pag...91&cat=1,43000

[DELurker]

Our team is looking at finally using the rivet nut set that we found hiding in the back of a dusty cabinet. However, we also use a lot of self-tapping screws, with pilot holes for the nose to initially sit in. We connect a lot of our robot together with self-tappers.

[Lisa Perez]

For any material that might shear easily (aluminum, plastic, etc.), helicoils are handy. They are comprised of steel, and serve as a reinforcement for whatever threads you might be dealing with.

More information can be found here: http://www2.boellhoff.com/web/centres.nsf/Files/HELICOIL-imperial-GB-0101/$FILE/HELICOIL-imperial-GB-0101.pdf