Another Arm for HFM Totem Q250 and Eachine 250 FPV Racer

thingiverse

Update 3/20 Rev C removed one hole. Increased clearance for a 7 inch prop by lowering top line. Update 3/19 I added another revision of the arm with the 4mm pushbutton for the Eachine Racer. Most notable are the changes in the side panels, which hopefully will reduce the chance of breaking at the lower mounting feet. The space between the arms is large enough to fully contain the standard ESC now. The holes were added to reduce weight. Maybe there are too many, maybe more can be tolerated? I didn't include the sketchup file; there may be other things I want to try before finalizing it. I haven't printed it, but here are the settings I would use: .4mm nozzle, .3 layer height 5 bottom layers 5 top layers. 4 perimeters. 0 infill Uncheck the EXTERNAL PERIMETERS FIRST box (in slic3r) so external perimeters print last (unsupported bridges will likely be better) Bed placement: 180, -11.6, 0 The previous, similar version here was over 5 grams lighter than the original (plus spacers). This one will be somewhat heavier. UPDATE 3/17/16 I uploaded a version of the arm that was modified to increase the offset in the top and bottom holes to 9mm. This version is intended for use with larger props. I made the following changes from the original: Increased the width of the landing feet below the motor. Mine are always getting banged up when landing on concrete and rocks. Added a cut through the center of the motor mounting surface so the arm can be changed without having to unsolder anything. Added fillets to the vertical stiffeners in the arm. There is a bad case of stress concentration here; I have had several arms that broke cleanly at these points with a crash that wouldn't have stopped other quads. Added one more perimeter of material around the holes where the threaded bushing inserts. They were a bit thin on two sides w/o infill. Removed some material at the back that (IMO) that contributes to the weight, but not the strength. Printer Brand: RepRap Printer: Prusa I3 Rafts: No Supports: Yes Resolution: 0.4 mm nozzle. 0.3 mm layer height, 4 perimiters min Infill: not required Notes: The following are suggestions; I haven't printed it in ABS yet, only PLA. I used 4 perimeters and zero infill for a first try. If you don't use infill, there will be voids in the center of some layers, but I don't think the strength will be compromised significantly, in most places. With the above settings, 7 meters of 1.75 filament are required including the support material. Before slicing, position the object with the top facing down and tilting at about -12 degrees, depending upon which one you are printing. This puts both ends of the arm close to the bed and permits minimum support structure underneath the arm. My second print was the arm with the increased height. It looked good, and the mounting holes match the factory arms. I only replaced one arm, but all four feet set on a flat surface, unlike the previous sample. It was easier to flex this arm (in PLA) than the original, which, in my experience, breaks upon light impact. Infill and some thickening of the bottom mounting pads might be required as it might not be strong enough as it is. Post-Printing After printing, sand the extrusion flash off the mounting pads. It is a tricky to hold the arm so the surface remains flat and parallel to the top and bottom plates. A block under the free end would help while sanding. Use a soldering iron to gently heat whatever screw you are using before trying to thread it into the arm. You can tell when it cools as it becomes harder to drive the screw. Reheat as necessary and drive the screw within a couple mm of its final position. I used 0.5 inch SAE #4 sheet metal screws. A smaller diameter metric or SAE sheet metal screw would be preferable, if available. After threading, remove the screws and sand the mounting surface to remove the material that was thrown up when the thread was cut. How I Designed This modified the Sketchup file furnished with the original.