A Straight Thread Inspection System should measure all of the vital thread attributes - Functional Thread Size, Lead, Pitch Diameter, Height and Form. By properly inspecting threads, you will minimize process error, verify exact mating and interchangeability and ultimately guarantee your quality. STEP 1: FUNCTIONAL SIZE/DIAMETER After setting the Functional Size Gage, place the fixed lower thread roll onto the external thread. Use lower roll as a pivot and seat the upper roll into flanks of the threads. Sweep gage back and forth and then align side to side noting the measurement. Measure the internal diameter by retracting the lever far enough to place the fixed, lower roll into the flanks of the thread and then carefully release to seat and align the upper roll. Hold the lower arm steady with one hand using it as a pivot, then sweep and align gage accordingly to get the largest reading. Functional gaging takes into account the cumulative effects of Diameter, Lead, Taper, Flank Angle and Form Error. Experience the same functionality of ring and plugs, with the added benefit of accurately inspecting a range of sizes. STEP 2: PITCH DIAMETER After zeroing the Pitch Diameter Gage (“Ball PD Gage”), place the fixed ball contact onto an external thread. Use lower contact as a pivot while carefully seating the moveable contact point ½ pitch up the helix. Sweep back and forth for largest reading. The internal PD gage inspects internal parts – an impossibility with thread wires. After zeroing gage, retract lever and place moveable contact point into thread at the 6 o’clock position. Apply a little pressure and trace the thread to the 12 o’clock position. Release lever and lower the fixed ball into the respective thread. By sweeping the gage, note the largest reading and verify the dimensional size. Improperly sized pitch diameters compromise the load capacity of your threaded components. STEP 3: LEAD Thread Lead is one of the most important thread elements. Zero the Lead Gage on the appropriate setting standard. It’s important to take measurements at locations where the most lead error typically occurs. Check both external and internal lead on the first fully formed thread, about 360 degrees from where the machine tool first touches the part. Inspect the next adjacent thread, and finally take a third measurement on the same axial plane near thread end, close to where the tool pulls out. These multiple readings detect machine tool error and worn lead screws on your lathe. STEP 4: HEIGHT After zeroing the Thread Height Gage on a flat surface, measure the root-to-crest thread height at several locations. Rock the gage front to back and observe the direct reading. Measuring thread height helps monitor and control the depth of cut during manufacturing. It can also detect a broken insert, or be used to calculate the minor or major diameter, when only one is known. Accurate thread height contributes to the interchangeability and integrity of the threaded assembly. STEP 5: FORM Check proper form and pitch by placing the correct Profile Template into the threads. Shine a light behind the template to detect steps or signs of a chipped insert. A template that stands too far out of the threads or allows excess light to shine through could be an indication of lead error. Position the profile to overhang the beginning and ending threads. While sweeping around the thread, feel for burrs or debris. Form provides a complete picture of many thread design aspects including axial alignment and pitch. For more information or to request a Complimentary Straight Thread Inspection Process Poster, contact Gagemaker at info@ gagemaker.com or 713-472-7360.
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