Screw it!

As I document assembly instructions for some of my designs, I often get stymied by an alternative to "fastening of two parts by mean of a cylinder, on the outside of which is cut a helical groove". Well, screw it, lets just talk about them, threaded fasteners.

One of the projects early in my career was a simple conversion from British to metric system. The key word is "simple" - all it takes is a few clicks on the 3D modeling / drafting software and voila - we have the drawing converted to metric! That would be the simplest portion, the dimensions. Yes, but would that drawing be actually manufacturable?

Dimensioning is one aspect, making the part manufcturable to people understanding metric dimensioning is another that I will deal with later (mainly the task of converting tolerances and standard sizes).

In this post, I will deal with the one part - translate English threaded fasteners to Metric, machine crews to be more specific. Since we all t=know they done mate with each other, a mere conversion of UNC 4-40 to M3X.5 will not work because of the thread angle and pitch will differ. So we need to go about changing the mated and the mating parts (screw and nut), bith. This leads to a cascading effect, we cannot just stop at one part or we will have a mix of English and Metric in the same assembly. This "simple" project ended up producing a whole new line of products for the European market (Metric followers), where not just the dimensions, but all the CFD and structural analysis, tolerance stack-up had to be redone for the entire line. Though the results were predictably favourable, the effort was far from it. Expected to take one engineer less than a month, I ended up leading an entire team of 4 new college graduates to finish the project in 3 months!

So, all that we did was go about changing all the self-clinch nuts, screws, washers, while maintaining the same type of head and drivers (of course the tools used will also have change to the metric equivalents), in retrospect, it was that simple!