MDF fixtures

MDF

Generally speaking, Medium-Density Fibreboard is a great material to build out a simple structure or prototype. The key aspect is its easy manufacturability and cost. However, when designing the base of a robot, that needs to be sturdy, there are some limitations. Feel free to mix and combine fixture elements such as nuts and bolts. These types of fixtures are a great choice since the forces they would generate as a response of stress would be mainly perpendicular to the material and accross all of its layers. Just as with 3D printed parts, the material is most vulnerable when applying parallel loads to some and not all of the layers of the material. Therefore, adhesive fixtures that act only on the surface are not recommended.

Basic Fixtures

Taking these considerations into account, to design a fixture between 2 MDF parts is simple. In this case, basic types of fixtures refers to a situation in which both parts are perpendicular one from another. Furthermore, there are 4 basic types of contact between 2 MDF parts where some kind of movement is blocked. These are divided depending on where a "plug" type part would act upon the "socket" type part, and whether or not the "socket" part covers both edges of the "plug" part. Generally speaking, the goal of an MDF structure is to block as many degrees of freedom as posible. As you can already tell, this types of fixtures can only go so far on their own. However, when used in coalition with one another, a group of these basic fixtures can do most of the job. Keep in mind that because laser cutting works so well with MDF, tolerances can be neglected. As long as you always take into account the width of the material that you will cut, you can design to your hearts content!

Half-Lap fixtures

A Half-Lap fixture happens when both parts are both "plug" and "socket", and they do not assemble perpendicular from one another, but rather through an axis parallel to both pieces. Both parts have a 'U' shape and these interlock with one another. These are particularly useful because the increased area of contact makes a greater friction force that helps maintain the pieces in place. However the main advantage of this type of fixture is that it locks movement in almost all directions. Therefore, this type of fixture works best in structural beams and as an 'adapter' between 2 pieces.

Mix and Match

If the goal is to use as little nuts and bolts as possible, do not be affraid to make as many components as you like. The limits are your imagination. This is a part of the LARC 2024 chassis of the robot, where the middle beam (transparent) is held in place to the wheel wall (pink outline) by a lock piece (blue) that goes into the "plug" part of the beam and locks the movement parallel to the beam. Since the surface of contact is relatively huge, the friction works so well that it even is a hazzle to disassemble it!