Fastening Elements and Machinability
Updated: May 7, 2021
Almost every machine and other inventions need something to connect their parts, after all, a single element apparatus is very rare. Thereunto, we rely on the so called “Fastening Elements” so that we can fix the parts together and assemble the larger structure. With our AUV, that would not be different: we also make use of the “Fastening Elements” in order to build our structure. The most popular “Fastening Elements” are the best known screws, washers and nuts.
Before we discuss even more about the components above, it’s important to highlight one of the main factors that influence in our choice within those elements: the manufacturing and machinability methods, another subject that will also be covered in this article. The best machinability method depends on the shape of the said part, its material and many other factors that influence equally in what is the best fastening element that we can choose, and by that, those are very well related topics. Thus, for a great project execution, picking the best fastening elements and the best machinability method for the machine are vital.
In this article, within the fastening elements, we will focus on the three most used in our robot. Starting with the screws, we used four different sizes in our design: M3 and M6 (both found in the thrusters supports, for example), M8 (found in the clamps) and M10 (used in the ligaments that connect the aluminum profiles), numbers that represent the diameter of the screw in millimeters. Thus, M6, for example, would indicate a diameter of six millimeters. The main factor to be considered in the choice of this diameter is the material of the object to be drilled: the denser the material or the greater the weight it will have to bear, the greater the diameter of the fastener. In relation to the length of the screw, a good metric is to choose the one with a length three times greater than the thickness of the first part, ensuring that it goes completely through and penetrates the other part in an appropriate manner, establishing its correct fixation on the structure. To exemplify this, if we have a barrier 15 mm thick and we need a screw to drill it, it is vital that this element is at least 45 mm long.
Screws, however, are not only differentiated in length or size of their diameter, but also by their functionality - different screws are designated for different applications. The screw types are varied, for example: The Hexagon bolt, the Auto taper, the Hexagon Internal bolt (also known as Allen screw), the French, the Machine, the Wood and several other types, all differentiated by the shape of their head, body and tip. For example, while the auto taper is the most suitable to be used in drywall, the self-drilling is the most suitable for tiles. This is not always a rule, but it is a fact that some screws better fit certain situations that others often fail to handle. For this, it is good to examine the classification of screws based on their function: there is the Prisoner screw, that has no head or thread at any end; the Pressure screw, fixed by means of the pressure provoked by the screw tips; the through bolt, the most popular, that crosses from side to side the piece that will be united; the non-through bolt which uses the threaded hole in place of the nut. In our project, the most used are the Hexagon bolt type, choosing between the regular “hex” bolt and the Allen type, the vast majority of which are through bolts.
More about the nuts: components almost always associated with screws, we will focus on a special type of nut, even though these are less popular, they are of great relevance to our AUV: theso-called half-cane nut, the most suitable to be used in aluminum profiles, which, in turn, are elements that constitute the frame (structure) of our project. This type of nut is inserted into the profile groove and, thanks to the springs present therein, you can attach it anywhere in the profile groove. The chamfer centers the nut in the groove, thus ensuring an effective support surface. The half cane nut makes the clamping processes simpler and more flexible, providing a high-quality fitting. It can be found in several sizes, however, in our robot the main ones are the M6 and the M8. Other fasteners, such as washers and threads, are less used in our design
As we conclude the fasteners subject, we will address the matter related to the maintenance of these elements – the removal and positioning of these in the structure and techniques to conserve them. To properly keep them safe, we must keep in mind that these components are often found to be in contact with water, which speeds up the rusting process (rusty tools do not work properly). There are several anticorrosive products, however, the most used in our team is the WD-40 Multipurpose Product penetration oil. The WD-40 is not able to remove the rust present in a tool that is already in a very advanced oxidation process, so we perform a periodic cleaning of the fasteners preventive wise. In relation to its withdrawal and positioning, we mainly have the Philips wrench, Allen wrench, Socket Wrench 13 and the aid of male and female soquees.
Now addressing the machining subject, it is important to always take into consideration the machinability degree of the material. Machinability is nothing more than a property that depends on the interaction between the manufacturing process and the characteristics of the part material, that means, the performance of the material when submitted to the different factors of the machining processes, such as cutting speed, cutting fluids, feed, and many more.
Some intrinsic characteristics of the materials favors machinability, such as low values of hardness, mechanical resistance and ductility (ability of a material to deform) and high values of conductivity. However, a low ductility value is accompanied by high hardness, so it is necessary to find a balance between hardness and ductility.
In our AUV, we have aluminum profiles, naval aluminum lids, 1020 steel inner rods and an acrylic main body, and with that in mind, we will enter a more specific field on the machinability of each material mentioned. Starting with the different aluminum alloys that are known to be easily machined, in other words, that have good machinability, the wear of the tool used in the manufacturing process is rarely a problem and, keeping temperatures low, high cutting speeds can be used (which, with the correct tool for its geometry, will result in a good finishing).
Even Though, care must be taken: aluminum deforms about three times more than steel and, therefore, the cutting speed used must be lower than that for machining steel alloys. Also, in the same line of thought, should not use much effort to add its fixing elements.
Now, as for steel alloys, machinability is directly related to hardness (associated with the amount of carbon used): the higher the hardness, the better the machinability. In our rods, we use 1020 steel, which, by nomenclature, we identify that it is a carbon class steel alloy (10XX) with an average carbon percentage of 0.20% (XX20) and is considered a soft, malleable and easy weldability, representing a good machinability steel commonly used in mechanical parts. Despite not being among a range of carbon steel alloys with better machinability (1030 to 1060), this difference can be resolved through cold treatments that promotes strain hardening (a phenomenon that occurs in the metal that generates increased hardness due to plastic deformation)
Finally, we have acrylic, which can be considered a material that can be easily machined, but it does not follow the same rule of splinter formation (chip produced by roughing the material) as the others mentioned above, considering that the reaction to the application of imprecise loads can cause ruptures in the entire length of the material.
Written by Lara Figueiredo and Gabriela Torres.