3D Printed Calipers

grabcad

Lucas Wallace, University of Victoria.In an effort to bring down the cost of sub-millimeter measuring tools, I've designed a fully 3D printable set of calipers that can be adjusted to account for inaccuracies during the printing process. This allows even low cost FDM printers with standard components to produce an accurate and reliable measuring tool. The top dial rotates one full turn for every millimeter of rail travel, allowing the user to take very small, (<0.1mm) measurements.The main gearbox sits ontop of a rack which meshes with a 4mm diametric pitch, 6 tooth input gear. This gear rotates 1mm for every 1/24th of a rotation, so the dial, (the output) must spin 1 full turn for 1/24th of an input turn. This requires a 24:1 gear ratio between the two. 4 M3 x 30mm bolts hold the gearbox together, and 6 M3 x 15mm bolts are necessary to connect the measuring blades and hold the rail in place. The gearbox ontop of the rack contains the 24:1 gear set, consisting of a large 6:1 gear and two 2:1 gears following it. The gearbox output shaft is then connected to the linear adjustment mechanism underneath the dial, which can vary the final output ratio between 0.9*24:1 and 1.1*24:1. This allows the user to calibrate the reading on the dial against something of known length, like a ruler, to adjust for size inaccuracies in the gears. Please note that this adjustment only accounts for linear inaccuracies, eg. over extruded or under extruded gear profiles.Calibration is reasonably simple. After assembly, find a ruler, or any object of known size, and use the calipers to measure the object. The larger the object the better, as the error in the gearing will accumulate more over longer distances.If the calipers read too large of a value, loosen the M3 bolt in the center of the dial slightly and remeasure. If the value is too small, tighten the M3 bolt. This expands or contracts the diameter of the adjustment gear, altering the overall gear ratio of the calipers. Continue measuring and adjusting the bolt until the reading on the dial is correct. Once the reading is accurate to about 100mm, your set of calipers are now calibrated!Print Suggestions:- All components can be printed with a standard 0.4mm nozzle and PLA. - All gears, including the linear adjustment mechanism, must be printed at a layer height of 0.1mm. All other components can be printed at 0.2mm. - Slow outside wall speeds (< 30mm/s) and low temperatures for your material of choice are suggested to retain tooth accuracy when printing gears.- If gears are too tight after assembly, reduce the flow rate to 99% and reprint.- If there is backlash in the output dial, (it wiggles) try reprinting the gears at 101% flow rate and increasing until backlash disappears. - After printing, make sure to liberally grease the gearbox. - Make sure to move the calipers slowly when measuring, as the interior gears can heat up and deform if spun to quickly.Happy measuring!