Internal Threads

[bardd]

First of all, thanks to everyone for the quick replies

Secondly, most of you mentioned Pem nuts. From your posts and the pictures, they seem pretty permanent and hard to remove, and we usually find ourselves making significant changes around day 20...
How easy (or hard) is it to make changes when using this?
How much planning ahead do you need to use them correctly?

[JamesCH95]

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.

Originally Posted by DampRobot

First, I've heard 6 threads, which would make 24 tpi the largest recommendable in quarter inch material.

The number I know is 3 complete turns if the threaded material is as strong as the fastener. Because high-grade aluminum can be about 1/2 as strong as an alloy steel bolt (this is subject to wide variances) 6 turns would suffice.

Now, if the application actually requires the full strength of a selected fastener is a whole different matter...

OP - you might also want to look into rivet nuts, they can be assembled easily and quickly with basic hand tools. I don't want to start a "rivet nut vs pem nut" debate, suffice it to say that they each of their uses, so consider your choice carefully.

Edit: There are also anchor nuts and weld nuts that might work well, depending on your application. Some anchor nuts can be purchased with nylon locking elements and/or floating threads, making them very useful where small adjustments might be needed.

[jee7s]

Originally Posted by JamesCH95

The number I know is 3 complete turns if the threaded material is as strong as the fastener. Because high-grade aluminum can be about 1/2 as strong as an alloy steel bolt (this is subject to wide variances) 6 turns would suffice.

Now, if the application actually requires the full strength of a selected fastener is a whole different matter...

And thus we risk diving into an ongoing debate.

The guidance I have received from a number of sources is that 3 threads is the minimum to get the appropriate geometry for the threads in the hole. If you think about what having a thread or half a thread means...well that's not very reliable.

As for strength, there are definitely places on a robot where the strength of steel is needed, so there steel should be used. But for joints in compression (like say the corners of the frame, or a gusset plate in the corner of a frame), you probably don't need the full strength of steel in the fastener. Most of the time, the load will be carried by the interface between the two pieces being joined, not by the fastener itself (assuming it's properly tightened). Joints in tension are another story, where the load IS carried by the fastener.

And, as I mention, the required strength of the connection is a factor. The design guideline we use on my team is that all of the sheet metal structural connections get PEM nuts, so that we have a steel nut with a steel bolt for strength. Plates 1/4" thick and over are tapped without the PEM nut. We do, however, tap 1/8" aluminum sheet metal for control system mounting holes (jaguars, PDB, sidecars, etc) using #8-32 or #10-32 cap screws.

Regarding fabrication, if you get the correct PEM nut (that would be a self clinching nut in the S, SS, CLS, or CLSS series...each series denotes a different heat treatment or finish), they can be installed using a press or even an impact driver (though I wouldn't recommend the latter). We've used 1/4"-20 fasteners in the past, but the nuts are substantially harder to clinch than the #10 or #8.

There are other very interesting options from PEM. For example, they have PEM studs, which are threaded connections that stick out of the sheet. I find these quite appealing for the aforementioned control system mounts. There are also floating fasteners, which are intended to account for variations in manufacturing. They basically "take up" the tolerance, and allow for a slightly fudged hole to still properly fasten. For example, using a floating fastener is probably a good idea if you are going to bolt down all four corners of the PDB.

Yes, there was a time when I thought lock nuts and cap screws were all there were to fasten things. Then, I did what good engineers do...sought more experienced help. That's lead to a much more enjoyable build and less maintenance at competition...which is good for all.

[JamesCH95]

Originally Posted by jee7s

And thus we risk diving into an ongoing debate.

The guidance I have received from a number of sources is that 3 threads is the minimum to get the appropriate geometry for the threads in the hole. If you think about what having a thread or half a thread means...well that's not very reliable.

Thus why nuts generally have 3-4 threads. Funny how that works

/thread jacking

[Nate Laverdure]

Originally Posted by jee7s

And thus we risk diving into an ongoing debate...

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.

[bardd]

Originally Posted by JamesCH95

I don't want to start a "rivet nut vs pem nut" debate

Originally Posted by jee7s

And thus we risk diving into an ongoing debate.

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...

[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

[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]

Specifically addressing this...

Originally Posted by bardd

...Pem nuts. From your posts and the pictures, they seem pretty permanent and hard to remove, and we usually find ourselves making significant changes around day 20...
How easy (or hard) is it to make changes when using this?
How much planning ahead do you need to use them correctly?

Making changes isn't all that easy, particularly if you're off by less than the diameter of the PEM fastener. When the error is that small, you can't make an appropriate hole to get the fastener to clinch.

So, yes, a PEM nut is pretty permanent. Using them requires planning. But, spending a respectable amount of time in Inventor (or SolidWorks, etc) getting all the constraints and tolerances correct is really all you need. And, if you have to make a change on the fly, it's not much more work than drilling a hole for a nut-bolt connection. You just have to make sure you get it right the first time.

Keep in mind that it's not an all-or-nothing decision. You'll use the same bolts in the PEM nut as you would in a regular hex nut. And, you can always use the hex nuts in some places and PEM nuts in others (just make sure you have the corrrect size hole in the correct location). I'd recommend that you start with PEM applications in good, static, well defined things, like chassis construction before moving on to the potentially more complicated parts.