The fixture has to hold the fork such that all the critical points are in line with each other for brazing. There are production fixtures available but I’m building electric bikes on a budget so I have to make my own fixtures to save money. The fixture in the photo was built using parts I had around the shop so it was basically free except for the time it took to create.
The front fork presents some interesting alignment issues. There are multiple alignment points that must be as close as possible such that when the wheel is installed it is equidistant between the fork blades and is in line with the steering column. The fork blades must also be parallel to each other so that it is not twisted. The dropouts must be held at the correct height and level and the correct distance apart. It’s like a 3D balancing act. The fixture holds all of these alignment points while the dropouts are brazed. It will also hold the fork blades while brazing them in the crown as well. In my case I was just chopping a pre-built fork so all I had to do was braze the dropouts.
Building fixtures is a great way to learn how to build a frame and fork. It makes you think about the important alignment points. However, even with the best fixture, alignment is not guaranteed. Metal when heated expands and changes structure. This can cause alignment issues during and after brazing or welding. There is no substitute for alignment checking throughout the build process.