Not all machine screw threads look alike or work the same way. That’s why there are some general guidelines that product designers should consider when specifying threaded holes and fittings in order to achieve maximum efficiency and cost savings.

Hello again, and welcome. I’m Gordon Styles, the CEO and founder of Star Rapid, and I’ve been involved with rapid prototyping, machining and product development for over 35 years. Join me for another episode of Serious Engineering for serious engineers just like you.

In a previous episode we did a deep dive into the subject of machine screw thread forms from the perspective of a machinist and engineer. It’s absolutely riveting so if you haven’t seen it be sure and do that first, using the handy link over here.

How does this affect your product design?

Now that you’re familiar with the basics of thread forms it’s time to apply that knowledge to your product design decisions. Of course, every project is unique and only you know what your priorities are, but here are a couple of important points to bear in mind.

The Weakest Link

Screws, fixings, nuts, and bolts are very often the weakest link in an assembly, and if they break, it can be anything from a mild inconvenience to a major recall, or even a mass casualty event.

One shocking example was in 2013 when 32 huge bolts sheared off from the new San Francisco – Oakland Bay Bridge. I personally watched the construction of the new bridge and since re-opening I’ve used it many times.

These bolts, maybe better described as tie rods, were installed in 2008. They had not been heat treated properly, were left exposed to the elements for 15 months in situ in the bridge, and most likely suffered from hydrogen degradation. It turned out that the bolts were substandard, and all similar bolts were replaced along with the installation of a harness. The bolts tended to fail at the intersection of the threaded portion with the shaft of the bolt. You can read a full report about this story below.

Fortunately, most of us don’t deal with projects that could potentially kill hundreds of people if our product fails; but some of us do. I would argue that this example has many lessons to be learned.

Look to your ISO Standards

Lesson number one might be that all fixings should be designed to the relevant ISO standard for your industry and application and have the appropriate margin of error built in. This is called the Factor of Safety or FOS. For example, the FOS on structural steel work on a bridge would be between 5 and 7. What does that mean? Well, in layman’s terms that means you make it strong enough to withstand 5 ~ 7 times the stresses and strains that you expect in normal operation.

Lesson Number 2 might be that you must do your best to predict all possible modes of failure. FMEA, or Failure Modes and Effects Analysis, is a systematic way of doing just that. As Edward Murphy used to say – “anything that can go wrong, will go wrong”.

You should also use Finite Element Analysis to model the stresses you are putting on your product and find out if it will survive. It has never been easier to do FEA with products such as Autodesk’s Fusion 360 Simulation. You can literally see where your fixtures are going to break.

I would argue that it is far too dangerous both financially and in terms of human cost, to not use these tools nowadays. There really are no excuses. When you compare the cost of doing FMEA and FEA on a new product against the cost of tooling up a product which subsequently fails, it’s just not worth skipping this step.

How Deep Do You Need To Go?

For internal threads, deeper holes mean more holding force – but only up to a point. In most cases the hole depth should not exceed 2.5X the diameter of the screw or bolt. As thread length increases, more of the holding force is transferred away from the bolt head and to the thread flanks. That means that if the bolt is overtightened, the head will shear off before the screw threads get stripped. A broken bolt head will ruin your whole day so just avoid that if possible.

If you’re not sure how much you need then you may need to calculate the maximum holding forces required, and from there determine how many threads are needed. This is part of FEA that we just discussed.

For blind holes – those that don’t pass all the way through – avoid getting too close to the opposite wall, which can cause it to bulge or distort.

Don’t Be Uptight

When specifying your threads, please ensure that you use good GD&T notation and indicate the type of thread, the depth of the hole and the depth of full thread required. This is important for the machinist who’s going to make your part.

But – and we really can’t stress this enough – don’t try to make every hole location a “critical dimension”. Aaargh! If there’s one thing we wish we could change about the universe it’s the excessive and absolutely unnecessary use of critical dimensions for hole locations.

First, it’s impossible to hold critical tolerances everywhere simultaneously. Trying to do so is a frustrating waste of time and it greatly increases product development costs. But, even more to the point, it’s not needed. Hole locations don’t want to be critical for the simple reason that you need to have some free play in order to wiggle the various screws and bolts into place. Standard tolerances are fine for this, and too tight is simply going to make everyone’s life a misery.

Will Your Part Be Plated or Anodized?
Hole or screw sizes, and their corresponding threads, will get altered slightly if a part is anodized or plated. In the case of anodizing, some raw material is eroded away because of acid etching. In plating the opposite is true, because several layers of thin films get built up on the surface and these can cause thread flanks to jam up.

You can protect internal threads by blocking the holes before the piece is finished, but then of course they will not look the same as the surrounding material. If a threaded fitting is plated, the threads can be chased to restore them to their original condition. In most cases, we have found that anodizing does not substantially affect the performance of thread forms so it’s best not to be worried about that, but do inform your supplier in advance if you want threaded areas to be protected during the finishing process. This is of course especially important if the part is painted, for that will certainly gum up the threads.

We hope that you’ve found this to be useful and can apply our wisdom to your next project. That’s all the time we have for in today’s video. Remember to tap Like, screw the subscribe button, and by all means start a thread below (get it?) if there’s something we failed to cover today.

See you next time with more Serious Engineering for serious engineers.