Thread count considerations when incorporating a fastener into a design

For a few weeks I have been working on a bolt together cnc router design. The thread for that is located here:

http://www.cnczone.com/forums/diy-cn...45116-cnc.html

My question relates to the thread count of the screws holding everything together. If I am looking at a 3/8 inch screw on Mcmaster Carr, I can get it in two flavors, 16 TPI and 24 TPI. I was planning to use the 24 TPI screws since I will be simply drilling and tapping steel tubes to hold a lot of the structure together. Using 24 TPI means that I get more threads inside the screw hole. That started me thinking though about the relationship between the structural integrity of an assembly fastened with 24 TPI screws vs an assembly fastened with 16 TPI screws. What are some of the design considerations that would lend itself to using a particular thread count?

The "thread count" is normally referred to as UNF(fine pitch) or UNC(coarse pitch). Fine pitch has two advantages. More threads in contact and stronger fastener due to greater cross section of the solid area that isn't threaded. Sometimes coarse thread has an advantage such as in castings where the fine threads may crumble in grainy material such as cast grey iron female threads.

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Originally Posted by RICHARD ZASTROW

The "thread count" is normally referred to as UNF(fine pitch) or UNC(coarse pitch). Fine pitch has two advantages. More threads in contact and stronger fastener due to greater cross section of the solid area that isn't threaded. Sometimes coarse thread has an advantage such as in castings where the fine threads may crumble in grainy material such as cast grey iron female threads.

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Thanks for the explanation! So I should be fine with the fine pitch. That is what I needed to hear :-)

a thumbrule says 4 thread the minimal need..
its a very general rule, but works for most case..

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Originally Posted by jerseyguy1996

Thanks for the explanation! So I should be fine with the fine pitch. That is what I needed to hear :-)

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I would have to disagree here. Not that you wouldn't probably be fine with fine thread (no pun intended), but that I would recommend coarse-threaded fasteners unless you have a specific need for fine thread. For structural applications, which is what you are using fasteners for in your CNC build, you will see coarse fasteners 99% of the time. ALL heavy-hex structural bolts are coarse thread.

Fastenal has an excellent go-to technical reference for fasteners, and includes topics like different materials, coatings and heat treatment, joint design, fastener loading, and relevant dimensions. You can download the pdf for free from their website (see link below).

Fastenal Technical Reference Guide PDF

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Thread Series
Three standard thread series in the Unified screw thread system are highly important for fasteners: UNC (coarse), UNF (fine), and 8-UN (8 thread). A chart listing the standards sizes and thread pitches with their respective thread stress areas is located in the appendix.

Below are some of the aspects of fine and coarse threads.

Fine Thread

1. Since they have larger stress areas they are stronger in tension
2. Their larger minor diameters develop higher torsional and transverse shear strengths
3. They can tap better in thin-walled members
4. With their smaller helix angle, they permit closer adjustment accuracy

Coarse Thread

1. Stripping strengths are greater for the same length of engagement
2. Exhibit a better fatigue resistance behavior
3. Less tendency to cross thread
4. Assemble and disassemble more quickly and easily
5. Tap better into brittle materials.
6. Larger thread allowances allow for thicker coatings and platings

Numerous arguments have been made for using both the fine and coarse thread series; however, with the increase in automated assembly processes, bias towards the coarse thread series has developed.

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Strength of Threads
Two fundamentals must be considered when designing a threaded connection.

1. Ensure that these threaded fasteners were manufactured to some current ASTM, ANSI, U.S. Government or other trusted standard.
2. Design bolts to break in tension prior to the female and/or male threads stripping. A broken bolt is an obvious failure. It’s loose. However, when the threads strip prior to the bolt breaking, we may not notice the failure until after the fastener is put into service.

As was shown on page 1, the strength of bolts loaded in tension can be easily determined by the ultimate tensile strength. To determine the amount of force required to break a bolt, we multiply its ultimate tensile strength by its tensile stress area, As. Determining the strength of the threads is more complicated. Since the male threads pull past the female threads, or vice-versa, the threads fail in shear and not in tension. Therefore, the stripping strength depends on the shear strength of the nut and bolt materials.

To determine the force required to strip the threads we must multiply the shear strength by the cross sectional area, which must be sheared. Determining the cross sectional area in which the shear occurs is a problem. Here are three possible alternatives for the failure.

1. The nut material is stronger than the bolt material. In this example, the nut threads will wipe out the bolt threads. The failure will occur at the root of the bolt threads.
2. The bolt material is stronger than the nut material. With this example, the bolt threads will tear out the nut threads. The failure will occur at the root of the nut threads.
3. The nut and bolt are the same material. With this example, both threads will strip simultaneously. This failure will occur at the pitch line.

The tensile strength of most fasteners is a common specification whereas shear strength is not. In order to avoid shearing in the threads, we must insure that the length of engagement between the bolt and nut, or tapped hole, is long enough to provide adequate cross-sectional thread area. The typical failure for both alternative #1 and #2 would be a tensile failure of the bolt provided proper engagement.

With conventional steel nut and bolt materials, a length of engagement of about one nominal diameter of the bolt is typical. A longer thread length engagement will be needed when dealing with tapped holes in soft material.

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After looking through your design/build thread, it looks like you've got a lot of fasteners planned to hold all of that metal together, so I would say neither tensile load failure nor shear failure is your primary concern. So you plan on using 3/8" screws. Assuming you are looking at the "Low-Strength Steel Cap Screws" on McMaster, you'd get around 3,000 lbs of clamping force per fastener. You obviously don't need that much probably for your whole machine, but you also won't actually get that much unless you torqued the fasteners to the proper spec.

For the most part, as referenced in the above quotes taken directly from the Fastenal Reference, when tapping holes, the primary concern is thread stripout. When tapping a similar material to the bolt/screw, you want one nominal diameter of thread engagement. So, if the threads are perfect, you need 3/8" thick material to screw into. If the threads have any lead or chamfer, you'd really need more than this. Looks like you are using 2" x 2" x 1/4" thk square tubing? It also looks like later in your thread, you've changed to 1/4" screws, but here you say 3/8".

A couple of people mention using a thicker cold-rolled steel bar slid into the tube that is tapped for your fasteners. This is certainly one way to go as it will allow you the full thickness for necessary thread engagement. One drawback to this concept is that if you strip out one hole in the middle of the bar, you either have to leave that fastener out completely, or remove all fasteners, re-tap the hole larger, and then re-assemble everything. This is obviously really time consuming, but only if you have to completely tear it down.

I couldn't find anywhere in the thread where you explained your reasoning behind moving away from through bolts and toward tapped holes. Could you shed some light on that? Bolts or screws and nuts can be matched by material, size and thread pitch, and are available for really cheap. And if you switched to bolts long enough to stick out the other side of your box tubing, they'd now be taking shear load on the nominal diameter as opposed to the minor diameter in the threads (you'd have to buy partially threaded bolts). Another advantage to going this route is that because all of your materials are steel, you wouldn't even need washers. If you are using any aluminum plate, I'd recommend to add in steel washers to prevent bearing failure against the aluminum.

There really are a number of ways to go on this, and you seem to be doing your research and asking lots of questions, so that's good. Personally, if it were me building this, I'd engineer out having to tap all the holes. I prefer bolts and nuts in my day-to-day job for anything steel to steel. Anything aluminum that requires blind tapped holes, I always use helical inserts. If you're concerned about loosening of fasteners due to vibration, nylon locking nuts are fantastic and no more difficult than regular nuts. Anyways, hope this has been somewhat helpful. Good luck. I hope to see some build pics soon on your thread.

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Originally Posted by mkbryant

I couldn't find anywhere in the thread where you explained your reasoning behind moving away from through bolts and toward tapped holes. Could you shed some light on that? Bolts or screws and nuts can be matched by material, size and thread pitch, and are available for really cheap. And if you switched to bolts long enough to stick out the other side of your box tubing, they'd now be taking shear load on the nominal diameter as opposed to the minor diameter in the threads (you'd have to buy partially threaded bolts). Another advantage to going this route is that because all of your materials are steel, you wouldn't even need washers. If you are using any aluminum plate, I'd recommend to add in steel washers to prevent bearing failure against the aluminum.

There really are a number of ways to go on this, and you seem to be doing your research and asking lots of questions, so that's good. Personally, if it were me building this, I'd engineer out having to tap all the holes. I prefer bolts and nuts in my day-to-day job for anything steel to steel. Anything aluminum that requires blind tapped holes, I always use helical inserts. If you're concerned about loosening of fasteners due to vibration, nylon locking nuts are fantastic and no more difficult than regular nuts. Anyways, hope this has been somewhat helpful. Good luck. I hope to see some build pics soon on your thread.

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mkbryant I really appreciate your post. I'm thinking that the rule of thumb of having the thread depth at least the width of the screw diameter is probably designed to insure that the holding power of the threads is equal to the amount of force that the bolt can handle before it fails. In this case I am guessing that the amount of holding power that I need is considerably less than the amount of force required to break the bolt and 1/4 inch of threads should be plenty (but I'm not an engineer so I could be completely wrong).

As far as not carrying the bolts all the way through the tubing, I was convinced by a number of members that passing the bolt all the way through can crush the tubing when you tighten it down. I also liked the idea of just being able to thread the screw right into the tubing. If it strips I can always drill and tap a bigger hole and put in a matching screw for it. Does that sound reasonable or am I way off base here?

if it were pipes tightened in T-shape.. then you can consider to drill trough the pipe, starting with hole that allen head fit trough and other hole is like thread dia, and the crossing pipe has a welded washer close to its end.. inside in the pipe

then that washer can be thicker as much as 8-12 thread or double of bolt dia..

but it works only if holes are relatively small to pipe diameter..

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Originally Posted by jerseyguy1996

mkbryant I really appreciate your post. I'm thinking that the rule of thumb of having the thread depth at least the width of the screw diameter is probably designed to insure that the holding power of the threads is equal to the amount of force that the bolt can handle before it fails. In this case I am guessing that the amount of holding power that I need is considerably less than the amount of force required to break the bolt and 1/4 inch of threads should be plenty (but I'm not an engineer so I could be completely wrong).

As far as not carrying the bolts all the way through the tubing, I was convinced by a number of members that passing the bolt all the way through can crush the tubing when you tighten it down. I also liked the idea of just being able to thread the screw right into the tubing. If it strips I can always drill and tap a bigger hole and put in a matching screw for it. Does that sound reasonable or am I way off base here?

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Honestly if you're not planning (or needing) to tension the fasteners to their full capacity, then it's pretty much about personal preference. So in this case, you're probably OK to drill and tap holes directly into the tubing. Hole depth and thread engagement are not about the screw failing, but about the threads failing on either the screw or hole. If you have improper thread engagement and you tension (or torque) the fastener to it's recommended clamping tension, the threads will strip out. The only way to fail the fastener in pure tension is if you do have proper thread engagement and the tension placed on the fastener exceeds the allowable for the stress area (As) and you stretch the fastener until necking and ultimately failure occur. This will not be an issue with your design.

I would still recommend coarse thread, but you may consider going down to 1/4" fasteners (since total clamping for is not an issue). This will give you 1 nominal D of thread engagement minus any chamfers, deburrs or lead-ins. Coarse thread simply because they're more common and much easier to assemble. I would also recommend a dab of blue loctite to prevent the fasteners from backing out.